As filed with the Securities and Exchange Commission on January 28,February 26, 2010

Registration No. 333-163252

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

AMENDMENT NO. 35

TO

FORM S-1

REGISTRATION STATEMENT

Under

The Securities Act of 1933

Calix Networks, Inc.

(Exact name of Registrant as specified in its charter)

1035 N. McDowell Boulevard

Petaluma, CA 94954

(707) 766-3000

(Address, including zip code and telephone number, of Registrant’s principal executive offices)

Carl Russo

President and Chief Executive Officer

1035 N. McDowell Boulevard

Petaluma, CA 94954

(707) 766-3000

(Name, address, including zip code and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public:

As soon as practicable after this Registration Statement becomes effective.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act, check the following box.¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the Registration Statement shall become effective on such date as the Securities and Exchange Commission acting pursuant to said Section 8(a), may determine.

The information in this preliminary prospectus is not complete and may be changed. Neither we nor the selling stockholders may sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell these securities and it is not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion

Preliminary Prospectus dated January 28,February 26, 2010

                         Shares

Common Stock

This is an initial public offering of shares of common stock of Calix Networks, Inc.

Calix is offering                      shares of common stock to be sold in the offering. The selling stockholders identified in this prospectus are offering an additional                      shares. Calix will not receive any of the proceeds from the sale of the shares by the selling stockholders.

Prior to this offering, there has been no public market for the common stock. It is currently estimated that the initial public offering price will be between $             and $            . We have applied to have our common stock approved for listing on the New York Stock Exchange under the symbol “CALX.”

See “Risk Factors” on page 8 to read about factors you should consider before buying shares of the common stock.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

To the extent that the underwriters sell more than                      shares of common stock, the underwriters have the option to purchase up to an additional                      shares from the selling stockholders at the initial public offering price less the underwriting discount.

The underwriters expect to deliver the shares against payment in New York, New York on                     , 2010.

Prospectus dated                     , 2010.

RISK FACTORS

Investing in our common stock involves a high degree of risk. Before you invest in our common stock, you should be aware that our business faces numerous financial and market risks, including those described below, as well as general economic and business risks. The following discussion provides information concerning the material risks and uncertainties that we have identified and believe may adversely affect our business, financial condition and results of operations. Before you decide whether to invest in our common stock, you should carefully consider these risks and uncertainties, together with all of the other information included in this prospectus.

Risks Related to Our Business and Industry

Our markets are rapidly changing and we have a limited operating history, which make it difficult to predict our future revenue and plan our expenses appropriately.

We were incorporated in August 1999 and shipped our first product in December 2001. We have a limited operating history and compete in markets characterized by rapid technological change, changing needs of communications service providers, or CSPs, evolving industry standards and frequent introductions of new products and services. We have limited historical data and have had a relatively limited time period in which to implement and evaluate our business strategies as compared to companies with longer operating histories. In addition, we likely will be required to reposition our product and service offerings and introduce new products and services as we encounter rapidly changing CSP requirements and increasing competitive pressures. We may not be successful in doing so in a timely and responsive manner, or at all. As a result, it is difficult to forecast our future revenues and plan our operating expenses appropriately, which also makes it difficult to predict our future operating results.

We have a history of losses and negative cash flow, and we may not be able to generate positive operating income and cash flows in the future.

We have experienced net losses in each year of our existence. For the years ended December 31, 2006, 2007, and 2008 and for the nine months ended September 26, 2009, we incurred net losses of $19.5 million, $24.9 million, $12.9 million and $25.3$22.4 million, respectively. As of September 26,December 31, 2009, we had an accumulated deficit of $394.3$392.2 million.

We expect to continue to incur significant expenses for research and development, sales and marketing, customer support and general and administrative functions as we expand our operations. Given our rapid growth rate and the intense competitive pressures we face, we may be unable to control our operating costs.

We cannot guarantee that we will achieve profitability in the future. Our revenue growth trends in prior periods may not be sustainable. In addition, we will have to generate and sustain significantly increased revenue, while continuing to control our expenses, in order to achieve and then maintain profitability. We may also incur significant losses in the future for a number of reasons, including the risks discussed in this “Risk Factors” section and factors that we cannot anticipate. If we are unable to generate positive operating income and cash flow from operations, our liquidity, results of operations and financial condition will be adversely affected.

Fluctuations in our quarterly and annual operating results may make it difficult to predict our future performance, which could cause our operating results to fall below investor or analyst expectations, which could adversely affect the trading price of our stock.

A number of factors, many of which are outside of our control, may cause or contribute to significant fluctuations in our quarterly and annual operating results. These fluctuations may make

financial planning and forecasting difficult. Comparing our operating results on a period-to-period basis may not be meaningful, and you should not rely on our past results as an indication of our future performance. If our revenue or operating results fall below the expectations of investors or securities analysts, or below any guidance we may provide to the market, the price of our common stock would likely decline. Moreover, we may experience delays in recognizing revenue under applicable revenue recognition rules, particularly from government-funded contracts, such as those funded by the United States Department of Agriculture’s Rural Utility Service, or RUS. The extent of these delays and their impact on our revenues can fluctuate over a given time period depending on the number and size of purchase orders under these contracts during such time period. In addition, unanticipated decreases in our available liquidity due to fluctuating operating results could limit our growth and delay implementation of our expansion plans.

In addition to the other risk factors listed in this “Risk Factors” section, factors that may contribute to the variability of our operating results include:

Ÿ

any obligation to issue performance bonds to satisfy requirements under RUS contracts;

Our business is dependent on the capital spending patterns of CSPs, and any decrease or delay in capital spending by CSPs, in response to recent economic conditions or otherwise, would reduce our revenues and harm our business.

Demand for our products depends on the magnitude and timing of capital spending by CSPs as they construct, expand and upgrade their access networks. For the nine monthsyear ended September 26,December 31, 2009, CenturyLink, Inc. and its predecessors Embarq Corporation and CenturyTel, Inc., which we refer to together as CenturyLink, purchased a significant amount of our access systems and software as a result of an increase in their deployments. However, we cannot anticipate the level of CenturyLink’s purchases in the future. In addition, the recent economic downturn has contributed to a slowdown in telecommunications industry spending, including in the specific geographies and markets in which we operate. In response to reduced consumer spending, challenging capital markets or declining liquidity trends, capital spending for network infrastructure projects of CSPs could be delayed or cancelled. In addition, capital spending is cyclical in our industry and sporadic among individual CSPs, and can change on short notice. As a result, we may not have visibility into changes in spending behavior until nearly the end of a given quarter. CSP spending on network construction, maintenance, expansion and upgrades is also affected by seasonality in their purchasing cycles, reductions in their budgets and delays in their purchasing cycles.

Many factors affecting our results of operations are beyond our control, particularly in the case of large CSP orders and network infrastructure deployments involving multiple vendors and technologies where the achievement of certain thresholds for acceptance is subject to the readiness and performance of the customer or other providers, and changes in customer requirements or installation plans. Further, CSPs may not pursue infrastructure upgrades that require our access systems and software. Infrastructure improvements may be delayed or prevented by a variety of factors including cost, regulatory obstacles, mergers, lack of consumer demand for advanced communications services and alternative approaches to service delivery. Reductions in capital expenditures by CSPs may slow our rate of revenue growth. As a consequence, our results for a particular quarter may be difficult to predict, and our prior results are not necessarily indicative of results likely in future periods.

Government-sponsored programs could impact the timing and buying patterns of CSPs, which may cause fluctuations in our operating results.

Many of our customers are Independent Operating Companies, or IOCs, which have revenues that are particularly dependent upon interstate and intrastate access charges, and federal and state subsidies. The Federal Communications Commission, or FCC, and some states are considering changes to such payments and subsidies, and these changes could reduce IOC revenues. Furthermore, many IOCs use or expect to use, government-supported loan programs or grants, such as RUS loans and grants and the Broadband Stimulus programs under the American Recovery and Reinvestment Act of 2009, or ARRA, to finance capital spending. Changes to these programs could reduce the ability of IOCs to access capital and reduce our revenue opportunities.

We believe that uncertainties related to Broadband Stimulus programs may be delaying investment decisions by IOCs. In addition, to the extent that our customers do receive grants or loans under these stimulus programs, our customers may be encouraged to accelerate their network development plans and purchase substantial quantities of products, from us or other suppliers, while the programs and funding are in place. Customers may thereafter substantially curtail future purchases of products as ARRA funding

winds down or because all purchases have been completed. Award grants under the Broadband Stimulus programs have been and will be issued between December 2009 and September 2010. Funded projects must be two-thirds complete within two years of the award and complete within three years of the award. Therefore, all funds that are awarded are expected to be expended by September 2013. The revenue recognition guidelines related to the sales of our access systems to CSPs who have received Broadband Stimulus funds may create uncertainties around the timing of our revenue, which could harm our financial results. In addition, any decision by CSPs to reduce capital expenditures caused by changes in government regulations and subsidies would have an adverse effect on our operating results and financial condition.

We face intense competition that could reduce our revenue and adversely affect our financial results.

The market for our products is highly competitive, and we expect competition from both established and new companies to increase. Our competitors include companies such as ADTRAN, Inc., Alcatel-Lucent S.A., Enablence Technologies Inc., Huawei Technologies Co., Ltd., LM Ericsson Telephone Company, or Ericsson, Motorola, Inc., Occam Networks, Inc., Tellabs, Inc. and Zhone Technologies, Inc. Our ability to compete successfully depends on a number of factors, including:

The market for broadband access equipment is dominated primarily by large, established vendors. In addition, some of our competitors have merged, made acquisitions or entered into partnerships or other strategic relationships with one another to offer more comprehensive solutions than they individually had offered. Examples include the merger of Alcatel S.A. with Lucent Technologies, Inc. in November 2006, Ericsson’s acquisitions of Redback Networks Inc. in January 2007 and Entrisphere Inc. in February 2007, and Ciena Corporation’s acquisition of World Wide Packets, Inc. in 2008.2008 and the successful bid to acquire Nortel’s Metro Ethernet Networks business announced in November 2009. We expect this trend to continue as companies attempt to strengthen or maintain their market positions in an evolving industry. Many of our current or potential competitors have longer operating histories, greater name recognition, larger customer bases and significantly greater financial, technical, sales, marketing and other resources than we do and are better positioned to acquire and offer complementary products and services technologies. Many of our competitors have broader product lines and can offer bundled solutions, which may appeal to certain customers. Our competitors may invest additional resources in developing more compelling product offerings. Potential customers may also prefer to purchase from their existing suppliers rather than a new supplier, regardless of product performance or features, because the products that we and our competitors offer require a substantial investment of time and funds to install. In addition, as a result of these transition costs, competition to secure contracts with potential customers is particularly intense. Some of our competitors may offer substantial discounts or rebates to win new customers. If we are forced to reduce prices in order to secure customers, we may be unable to sustain gross margins at desired levels or achieve profitability. Competitive pressures could result in increased pricing pressure, reduced profit margins, increased sales and marketing expenses and failure to increase, or the loss of, market share, any of which could reduce our revenue and adversely affect our financial results.

Product development is costly and if we fail to develop new products or enhancements that meet changing CSP requirements, we could experience lower sales.

Our market is characterized by rapid technological advances, frequent new product introductions, evolving industry standards and unanticipated changes in subscriber requirements. Our future success will depend significantly on our ability to anticipate and adapt to such changes, and to offer, on a timely and cost-effective basis, products and features that meet changing CSP demands and industry standards. We intend to continue making significant investments in developing new products and enhancing the functionality of our existing products.

Developing our products is expensive, complex and involves uncertainties. We may not have sufficient resources to successfully manage lengthy product development cycles. In 20072008 and 2008,2009, our research and development expenses were $44.4$44.3 million, or 23%18% of our revenue, and $44.3$46.1 million, or 18%20% of our revenue, respectively. We believe that we must continue to dedicate a significant amount of resources to our research and development efforts to maintain our competitive position. These investments may take several years to generate positive returns, if ever. In addition, we may experience design, manufacturing, marketing and other difficulties that could delay or prevent the development, introduction or marketing of new products and enhancements. If we fail to meet our development targets, demand for our products will decline.

In addition, the introduction of new or enhanced products also requires that we manage the transition from older products to these new or enhanced products in order to minimize disruption in customer ordering patterns, fulfill ongoing customer commitments and ensure that adequate supplies of new products are available for delivery to meet anticipated customer demand. If we fail to maintain compatibility with other software or equipment found in our customers’ existing and planned networks, we may face substantially reduced demand for our products, which would reduce our revenue opportunities and market share. Moreover, as customers complete infrastructure deployments, they may require greater levels of service and support than we have provided in the past. We may not be able to provide products, services and support to compete effectively for these market opportunities. If we are unable to anticipate and develop new products or enhancements to our existing products on a timely and cost-effective basis, we could experience lower sales which would harm our business.

Our new products are early in their life cycles and are subject to uncertain market demand. If our customers are unwilling to install our products or deploy new services or we are unable to achieve market acceptance of our new products, our business and financial results will be harmed.

Our new products are early in their life cycles and are subject to uncertain market demand. They also may face obstacles in manufacturing, deployment and competitive response. Potential customers may choose not to invest the additional capital required for initial system deployment. In addition, demand for our products is dependent on the success of our customers in deploying and selling services to their subscribers. Our products support a variety of advanced broadband services, such as high-speed Internet, Internet protocol television, mobile broadband, high-definition video and online gaming, and basic voice and data services. If subscriber demand for such services does not grow as expected or declines, or if our customers are unable or unwilling to deploy and market these services, demand for our products may decrease or fail to grow at rates we anticipate. For example, we launched our E5-400 platform family in the fourth quarter of 2008 and have only recently begun to see significant demand.

Our strategy includes developing products for the access network that incorporate Internet protocol and Ethernet technologies. If these technologies are not widely adopted by CSPs for use in their access networks, demand for our products may decrease or not grow. As a result, we may be unable to sell our products to recoup our expenses related to the development of these products and our results of operations would be harmed. We may also be delayed in recognizing revenue related to our new products and related services and may be required to recognize costs and expenses for such products before we can recognize the related revenue.

Our customer base is concentrated, and there are a limited number of potential customers for our products. The loss of any of our key customers, a decrease in purchases by our key customers or our inability to grow our customer base would adversely impact our revenues.

Historically, a large portion of our sales have been to a limited number of customers. For example, for the nine monthsyear ended September 26,December 31, 2009, CenturyLink accounted for 30%38% of our revenue. In 2008, CenturyLink and one other customer accounted for 25% and 11% of our revenue, respectively. In 2007, CenturyLink and another different customer accounted for 22% and 15% of our revenue, respectively. We anticipate that a large portion of our revenues will continue to depend on sales to a limited number of customers. In addition, some larger customers may demand discounts and rebates or desire to purchase their access systems and software from multiple providers. As a result of these factors, our future revenue opportunities may be limited and our margins could be reduced, and our profitability may be adversely impacted. The loss of, or reduction in, orders from any key customer would significantly reduce our revenues and harm our business.

Furthermore, in recent years, the CSP market has undergone substantial consolidation. Industry consolidation generally has negative implications for equipment suppliers, including a reduction in the number of potential customers, a decrease in aggregate capital spending, and greater pricing leverage on the part of CSPs over equipment suppliers. Continued consolidation of the CSP industry, including among the Incumbent Local Exchange Carrier, or ILEC, and IOC customers, who represent a large part of our business, could make it more difficult for us to grow our customer base, increase sales of our products and maintain adequate gross margins.

Our sales cycles can be long and unpredictable, and our sales efforts require considerable time and expense. As a result, our sales are difficult to predict and may vary substantially from quarter to quarter, which may cause our operating results to fluctuate significantly.

The timing of our revenues is difficult to predict. Our sales efforts often involve educating CSPs about the use and benefits of our products. CSPs typically undertake a significant evaluation process, which frequently involves not only our products but also those of our competitors and results in a lengthy sales cycle. We spend substantial time, effort and money in our sales efforts without any assurance that our efforts will produce any sales. In addition, product purchases are frequently subject to budget constraints, multiple approvals and unplanned administrative, processing and other delays. If sales expected from a specific customer for a particular quarter are not realized in that quarter or at all we may not achieve our revenue forecasts and our business could be harmed.

Our focus on CSPs with relatively small networks limits our revenues from sales to any one customer and makes our future operating results difficult to predict.

We currently focus a large portion of our sales efforts on IOCs, cable multiple system operators and selected international CSPs. In general, our current and potential customers generally operate small networks with limited capital expenditure budgets. Accordingly, we believe the potential revenues from the sale of our products to any one of these customers is limited. As a result, we must identify and sell products to new customers each quarter to continue to increase our sales. In addition, the spending patterns of many of our customers are characterized by small and sporadic purchases. As a consequence, we have limited backlog and will likely continue to have limited visibility into future operating results.

We do not have long-term, committed-volume purchase contracts with our customers, and therefore have no guarantee of future revenue from any customer.

Our sales are made predominantly pursuant to purchase orders, and typically we have not entered into long-term, committed-volume purchase contracts with our customers, including our key customers which account for a material portion of our revenues. As a result, any of our customers may cease to

purchase our products at any time. In addition, our customers may attempt to renegotiate the terms of our agreements, including price and quantity. If any of our key customers stop purchasing our access systems and software for any reason, our business and results of operations would be harmed.

Our efforts to increase our sales to larger North American as well as international CSPs may be unsuccessful.

Our sales and marketing efforts have been focused on CSPs in North America. A part of our long-term strategy is to increase sales to larger North American as well as international CSPs. We will be required to devote substantial technical, marketing and sales resources to the pursuit of these CSPs, who have lengthy equipment qualification and sales cycles, without any assurance of generating sales. In particular, sales to these CSPs may require us to upgrade our products to meet more stringent performance criteria, develop new customer-specific features or adapt our product to meet international standards. If we are unable to successfully increase our sales to larger CSPs, our operating results and long-term growth may be negatively impacted.

Our exposure to the credit risks of our customers may make it difficult to collect accounts receivable and could adversely affect our operating results and financial condition.

In the course of our sales to customers, we may encounter difficulty collecting accounts receivable and could be exposed to risks associated with uncollectible accounts receivable. The recent challenging economic conditions have impacted some of our customers’ ability to pay their accounts payable. While we attempt to monitor these situations carefully and attempt to take appropriate measures to collect accounts receivable balances, we have written down accounts receivable and written off doubtful accounts in prior periods and may be unable to avoid accounts receivable write-downs or write-offs of doubtful accounts in the future. Such write-downs or write-offs could negatively affect our operating results for the period in which they occur, and could harm our operating results.

Our gross margin may fluctuate over time and our current level of product gross margins may not be sustainable.

Our current level of product gross margins may not be sustainable and may be adversely affected by numerous factors, including:

Our products must interoperate with many software applications and hardware products found in our customers’ networks. If we are unable to ensure that our products interoperate properly, our business would be harmed.

Our products must interoperate with our customers’ existing and planned networks, which often have varied and complex specifications, utilize multiple protocol standards, software applications and products from multiple vendors and contain multiple generations of products that have been added over time. As a result, we must continually ensure that our products interoperate properly with these existing and planned networks. To meet these requirements, we must undertake development efforts that require substantial capital investment and employee resources. We may not accomplish these development efforts quickly or cost-effectively, if at all. If we fail to maintain compatibility with other software or equipment found in our customers’ existing and planned networks, we may face substantially reduced demand for our products, which would reduce our revenue opportunities and market share.

We have entered into interoperability arrangements with a number of equipment and software vendors for the use or integration of their technology with our products. These arrangements give us access to, and enable interoperability with, various products that we do not otherwise offer. If these relationships fail, we may have to devote substantially more resources to the development of alternative products and processes, and our efforts may not be as effective as the combined solutions under our current arrangements. In some cases, these other vendors are either companies that we compete with directly, or companies that have extensive relationships with our existing and potential customers and may have influence over the purchasing decisions of those customers. Some of our competitors have stronger relationships with some of our existing and potential other vendors and, as a result, our ability to have successful interoperability arrangements with these companies may be harmed. Our failure to establish or maintain key relationships with third-party equipment and software vendors may harm our ability to successfully sell and market our products.

As we do not have manufacturing capabilities, we depend upon a small number of outside contract manufacturers and we do not have supply contracts with these manufacturers. Our operations could be disrupted if we encounter problems with these contract manufacturers.

We do not have internal manufacturing capabilities, and rely upon a small number of contract manufacturers to build our products. In particular, we rely on Flextronics International Ltd. for the manufacture of most of our products. Our reliance on a small number of contract manufacturers makes us vulnerable to possible capacity constraints and reduced control over component availability, delivery schedules, manufacturing yields and costs. We do not have supply contracts with Flextronics or our other manufacturers. Consequently, these manufacturers are not obligated to supply products to us for any specific period, in any specific quantity or at any certain price. In addition, we have limited control over our contract manufacturers’ quality systems and controls, and therefore may not be able to ensure levels of quality manufacture suitable for our customers.

The revenues that Flextronics generates from our orders represent a relatively small percentage of Flextronics’ overall revenues. As a result, fulfilling our orders may not be considered a priority in the event Flextronics is constrained in its ability to fulfill all of its customer obligations in a timely manner. In addition, a substantial part of our manufacturing is done in Flextronics facilities which are located outside of the United States. We believe that the location of these facilities outside of the United States increases supply risk, including the risk of supply interruptions or reductions in manufacturing quality or controls.

If Flextronics or any of our other contract manufacturers were unable or unwilling to continue manufacturing our products in required volumes and at high quality levels, we would have to identify, qualify and select acceptable alternative contract manufacturers. An alternative contract manufacturer may not be available to us when needed or may not be in a position to satisfy our production requirements at commercially reasonable prices and quality. Any significant interruption in manufacturing would require us

to reduce our supply of products to our customers, which in turn would reduce our revenues and harm our relationships with our customers.

We depend on sole source and limited source suppliers for key components and products. If we are unable to source these components on a timely basis, we will not be able to deliver our products to our customers.

We depend on sole source and limited source suppliers for key components of our products. For example, certain of our application-specific integrated circuits processors and resistor networks are purchased from sole source suppliers. We may from time to time enter into original equipment manufacturer, or OEM, or original design manufacturer, or ODM, agreements to manufacture and/or design certain products in order to enable us to offer products into key markets on an accelerated basis. For example, a third party assisted in the design of and manufactures our E5-100 platform family. Any of the sole source and limited source suppliers, OEMs and ODMs upon whom we rely could stop producing our components or products, cease operations or be acquired by, or enter into exclusive arrangements with, our competitors. We generally do not have long-term supply agreements with our suppliers, and our purchase volumes are currently too low for us to be considered a priority customer by most of our suppliers. As a result, most of these suppliers could stop selling to us at commercially reasonable prices, or at all. Any such interruption or delay may force us to seek similar components or products from alternative sources, which may not be available. Switching suppliers, OEMs or ODMs may require that we redesign our products to accommodate new components, and may potentially require us to re-qualify our products with our customers, which would be costly and time-consuming. Any interruption in the supply of sole source or limited source components for our products would adversely affect our ability to meet scheduled product deliveries to our customers, could result in lost revenue or higher expenses and would harm our business.

If we fail to forecast our manufacturing requirements accurately and manage our inventory with our contract manufacturers, we could incur additional costs, experience manufacturing delays and lose revenue.

We bear inventory risk under our contract manufacturing arrangements. Lead times for the materials and components that we order through our contract manufacturers vary significantly and depend on numerous factors, including the specific supplier, contract terms and market demand for a component at a given time. Lead times for certain key materials and components incorporated into our products are currently lengthy, requiring us or our contract manufacturers to order materials and components several months in advance of manufacture. If we overestimate our production requirements, our contract manufacturers may purchase excess components and build excess inventory. If our contract manufacturers, at our request, purchase excess components that are unique to our products or build excess products, we could be required to pay for these excess parts or products and recognize related inventory write-down costs. Historically, we have reimbursed our primary contract manufacturer for inventory purchases when our inventory has been rendered obsolete, for example due to manufacturing and engineering change orders resulting from design changes, manufacturing discontinuation of parts by our suppliers, or in cases where inventory levels greatly exceed projected demand. If we experience excess inventory write-downs associated with excess or obsolete inventory, this would have an adverse effect on our gross margins, financial condition and results of operations. We have experienced unanticipated increases in demand from customers which resulted in delayed shipments and variable shipping patterns. If we underestimate our product requirements, our contract manufacturers may have inadequate component inventory, which could interrupt manufacturing of our products and result in delays or cancellation of sales.

If we fail to comply with evolving industry standards, sales of our existing and future products would be adversely affected.

The markets for our products are characterized by a significant number of standards, both domestic and international, which are evolving as new technologies are deployed. Our products must comply with these standards in order to be widely marketable. In some cases, we are compelled to obtain certifications or authorizations before our products can be introduced, marketed or sold. In addition, our ability to expand our international operations and create international market demand for our products may be limited by regulations or standards adopted by other countries that may require us to redesign our existing products or develop new products suitable for sale in those countries. Although we believe our products are currently in compliance with domestic and international standards and regulations in countries in which we currently sell, we may not be able to design our products to comply with evolving standards and regulations in the future. Accordingly, this ongoing evolution of standards may directly affect our ability to market or sell our products. Further, the cost of complying with the evolving standards and regulations, or the failure to obtain timely domestic or foreign regulatory approvals or certification such that we may not be able to sell our products where these standards or regulations apply, would result in lower revenues and lost market share.

We may be unable to successfully expand our international operations. In addition, our international expansion plans, if implemented, will subject us to a variety of risks that may harm our business.

We currently generate almost all of our sales from customers in North America and the Caribbean, and have very limited experience marketing, selling and supporting our products and services outside North America and the Caribbean or managing the administrative aspects of a worldwide operation. While we intend to expand our international operations, we may not be able to create or maintain international market demand for our products. In addition, as we expand our operations internationally, our support organization will face additional challenges including those associated with delivering support, training and documentation in languages other than English. If we invest substantial time and resources to expand our international operations and are unable to do so successfully and in a timely manner, our business, financial condition and results of operations will suffer.

In the course of expanding our international operations and operating overseas, we will be subject to a variety of risks, including:

We may have difficulty managing our growth, which could limit our ability to increase sales.

We have experienced significant growth in sales and operations in recent years. We expect to continue to expand our research and development, sales, marketing and support activities. Our historical

growth has placed, and planned future growth is expected to continue to place, significant demands on our management, as well as our financial and operational resources, to:

If we cannot grow, or fail to manage our growth effectively, we may not be able to execute our business strategies and our business, financial condition and results of operations would be adversely affected.

We may not be able to protect our intellectual property, which could impair our ability to compete effectively.

We depend on certain proprietary technology for our success and ability to compete. As of September 26,December 31, 2009, we held 22 U.S. patents expiring between 2015 and 2026, and had 3032 pending U.S. patent applications. Two of the U.S. patents are also covered by granted international patents, one in five countries and the other in one country.three countries. We currently have no pending international patent applications. We rely on intellectual property laws, as well as nondisclosure agreements, licensing arrangements and confidentiality provisions, to establish and protect our proprietary rights. U.S. patent, copyright and trade secret laws afford us only limited protection, and the laws of some foreign countries do not protect proprietary rights to the same extent. Our pending patent applications may not result in issued patents, and our issued patents may not be enforceable. Any infringement of our proprietary rights could result in significant litigation costs. Further, any failure by us to adequately protect our proprietary rights could result in our competitors offering similar products, resulting in the loss of our competitive advantage and decreased sales.

Despite our efforts to protect our proprietary rights, attempts may be made to copy or reverse engineer aspects of our products or to obtain and use information that we regard as proprietary. Accordingly, we may be unable to protect our proprietary rights against unauthorized third-party copying or use. Furthermore, policing the unauthorized use of our intellectual property would be difficult for us. Litigation may be necessary in the future to enforce our intellectual property rights, to protect our trade secrets or to determine the validity and scope of the proprietary rights of others. Litigation could result in substantial costs and diversion of resources and could harm our business.

We could become subject to litigation regarding intellectual property rights that could harm our business.

We may be subject to intellectual property infringement claims that are costly to defend and could limit our ability to use some technologies in the future. Third parties may assert patent, copyright, trademark or other intellectual property rights to technologies or rights that are important to our business. Such claims may involve patent holding companies or other adverse patent owners who have no relevant product revenue, and therefore our own issued and pending patents may provide little or no deterrence. We have received in the past and expect that in the future we may receive, particularly as a public company, communications from competitors and other companies alleging that we may be infringing their patents, trade secrets or other intellectual property rights and/or offering licenses to such intellectual property or threatening litigation. In addition, we have agreed, and may in the future agree, to indemnify

our customers for any expenses or liabilities resulting from claimed infringements of patents, trademarks or copyrights of third parties. Any claims asserting that our products infringe, or may infringe on, the proprietary rights of third parties, with or without merit, could be time-consuming, resulting in costly litigation and diverting the efforts of our engineering teams and management. These claims could also result in product shipment delays or require us to modify our products or enter into royalty or licensing agreements. Such royalty or licensing agreements, if required, may not be available to us on acceptable terms, if at all.

The quality of our support and services offerings is important to our customers, and if we fail to continue to offer high quality support and services we could lose customers which would harm our business.

Once our products are deployed within our customers’ networks, they depend on our support organization to resolve any issues relating to those products. A high level of support is critical for the successful marketing and sale of our products. If we do not effectively assist our customers in deploying our products, succeed in helping them quickly resolve post-deployment issues or provide effective ongoing support, it could adversely affect our ability to sell our products to existing customers and harm our reputation with potential new customers. As a result, our failure to maintain high quality support and services could result in the loss of customers which would harm our business.

Our products are highly technical and may contain undetected hardware errors or software bugs, which could harm our reputation and adversely affect our business.

Our products are highly technical and, when deployed, are critical to the operation of many networks. Our products have contained and may contain undetected errors, bugs or security vulnerabilities. Some errors in our products may only be discovered after a product has been installed and used by customers, and may in some cases only be detected under certain circumstances or after extended use. Any errors, bugs, defects or security vulnerabilities discovered in our products after commercial release could result in loss of revenues or delay in revenue recognition, loss of customer goodwill and customers and increased service and warranty cost, any of which could adversely affect our business, operating results and financial condition. In addition, we could face claims for product liability, tort or breach of warranty. Our contracts with customers contain provisions relating to warranty disclaimers and liability limitations, which may not be upheld. Defending a lawsuit, regardless of its merit, is costly and may divert management’s attention and adversely affect the market’s perception of us and our products. In addition, if our business liability insurance coverage proves inadequate or future coverage is unavailable on acceptable terms or at all, our business, operating results and financial condition could be adversely impacted.

Our estimates regarding future warranty obligations may change due to product failure rates, shipment volumes, field service obligations and rework costs incurred in correcting product failures. If our estimates change, the liability for warranty obligations may be increased, impacting future cost of goods sold.

Our products are highly complex, and our product development, manufacturing and integration testing may not be adequate to detect all defects, errors, failures and quality issues. Quality or performance problems for products covered under warranty could adversely impact our reputation and negatively affect our operating results and financial position. The development and production of new products with high complexity often involves problems with software, components and manufacturing methods. If significant warranty obligations arise due to reliability or quality issues arising from defects in software, faulty components or manufacturing methods, our operating results and financial position could be negatively impacted by:

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high inventory obsolescence expense;

Our use of open source software could impose limitations on our ability to commercialize our products.

We incorporate open source software into our products. Although we closely monitor our use of open source software, the terms of many open source software licenses have not been interpreted by U.S. courts, and there is a risk that such licenses could be construed in a manner that could impose unanticipated conditions or restrictions on our ability to sell our products. In such event, we could be required to make our proprietary software generally available to third parties, including competitors, at no cost, to seek licenses from third parties in order to continue offering our products, to re-engineer our products or to discontinue the sale of our products in the event re-engineering cannot be accomplished on a timely basis or at all, any of which could adversely affect our revenues and operating expenses.

If we are unable to obtain necessary third-party technology licenses, our ability to develop new products or product enhancements may be impaired.

While our current licenses of third-party technology relate to commercially available off-the-shelf technology, we may in the future be required to license additional technology from third parties to develop new products or product enhancements. These third-party licenses may be unavailable to us on commercially reasonable terms, if at all. Our inability to obtain necessary third-party licenses may force us to obtain substitute technology of lower quality or performance standards or at greater cost, any of which could harm the competitiveness of our products and result in lost revenues.

We may pursue acquisitions, which involve a number of risks. If we are unable to address and resolve these risks successfully, such acquisitions could disrupt our business.

In February 2006, we acquired Optical Solutions, Inc. in order to support the expansion of our product and service offerings. While we do not currently have plans to make an acquisition, we may in the future acquire businesses, products or technologies to expand our product offerings and capabilities, customer base and business. We have evaluated, and expect to continue to evaluate, a wide array of potential strategic transactions. We have limited experience making such acquisitions. Any of these transactions could be material to our financial condition and results of operations. The anticipated benefit of acquisitions may never materialize. In addition, the process of integrating acquired businesses, products or technologies may create unforeseen operating difficulties and expenditures. Some of the areas where we may face acquisition-related risks include:

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difficulties integrating and supporting acquired products or technologies;

Foreign acquisitions would involve risks in addition to those mentioned above, including those related to integration of operations across different cultures and languages, currency risks and the particular economic, political and regulatory risks associated with specific countries. We may not be able to address these risks successfully, or at all, without incurring significant costs, delays or other operating problems. Our inability to address successfully such risks could disrupt our business.

Our obligation to issue performance bonds to satisfy requirements under RUS contracts may negatively impact our working capital and financial condition.

We are often required to issue performance bonds to satisfy requirements under our RUS contracts. The performance bonds generally cover the full amount of the RUS contract. Upon our performance under the contract and acceptance by the customer, the performance bond is released. The time period between issuing the performance bond and its release can be lengthy. We issue letters of credit under our existing credit facility to support these performance bonds. In the event we do not have sufficient capacity under our credit facility to support these bonds, we will have to issue certificates of deposit, which could materially impact our working capital or limit our ability to satisfy such contract requirements. In the event that we are unable to issue such bonds, we may lose business and customers who purchase under RUS contracts. In addition, if we exhaust our credit facility or working capital reserves in issuing such bonds, we may be required to eliminate or curtail expenditures to mitigate the impact on our working capital or financial condition.

Our use of and reliance upon development resources in China may expose us to unanticipated costs or liabilities.

We outsource a portion of our quality assurance and cost reduction engineering to a dedicated team of engineers based in Nanjing, China. We also outsource a portion of our software development to a team of software engineers based in Shenyang, China. Our reliance upon development resources in China may not enable us to achieve meaningful product cost reductions or greater resource efficiency. Further, our development efforts and other operations in China involve significant risks, including:

Difficulties resulting from the factors above and other risks related to our operations in China could expose us to increased expense, impair our development efforts, harm our competitive position and damage our reputation.

Our customers are subject to government regulation, and changes in current or future laws or regulations that negatively impact our customers could harm our business.

The FCC has jurisdiction over all of our U.S. customers. FCC regulatory policies that create disincentives for investment in access network infrastructure or impact the competitive environment in which our customers operate may harm our business. For example, future FCC regulation affecting providers of broadband Internet access services could impede the penetration of our customers into certain markets or affect the prices they may charge in such markets. Furthermore, many of our customers are subject to FCC rate regulation of interstate telecommunications services, and are recipients of federal universal service fund payments, which are intended to subsidize telecommunications services in areas that are expensive to serve. In addition, many of our customers are subject to state regulation of intrastate telecommunications services, including rates for such services, and may also receive funding from state universal service funds. Changes in rate regulations or universal service funding rules, either at the federal or state level, could adversely affect our customers’ revenues and capital spending plans. In addition, various international regulatory bodies have jurisdiction over certain of our non-U.S. customers. Changes in these domestic and international standards, laws and regulations, or judgments in favor of plaintiffs in lawsuits against CSPs based on changed standards, laws and regulations could adversely affect the development of broadband networks and services. This, in turn, could directly or indirectly adversely impact the communications industry in which our customers operate. To the extent our customers are adversely affected by laws or regulations regarding their business, products or service offerings, our business, financial condition and results of operations would suffer.

We may be subject to governmental export and import controls that could subject us to liability or impair our ability to compete in additional international markets.

Our products may be or become subject to U.S. export controls that will restrict our ability to export them outside of the free-trade zones covered by the North American Free Trade Agreement, Central American Free Trade Agreement and other treaties and laws. Therefore, future international shipments of our products may require export licenses or export license exceptions. In addition, the import laws of other countries may limit our ability to distribute our products, or our customers’ ability to buy and use our products, in those countries. Changes in our products or changes in export and import regulations may create delays in the introduction of our products in international markets, prevent our customers with international operations from deploying our products or, in some cases, prevent the export or import of our products to certain countries altogether. Any change in export or import regulations or related legislation, shift in approach to the enforcement or scope of existing regulations, or change in the countries, persons or technologies targeted by such regulations, could negatively impact our ability to sell our products to existing or potential international customers.

If we lose any of our key personnel, or are unable to attract, train and retain qualified personnel, our ability to manage our business and continue our growth would be negatively impacted.

Our success depends, in large part, on the continued contributions of our key management, engineering, sales and marketing personnel, many of whom are highly skilled and would be difficult to replace. None of our senior management or key technical or sales personnel is bound by a written employment contract to remain with us for a specified period. In addition, we do not currently maintain key man life insurance covering our key personnel. If we lose the services of any key personnel, our business, financial condition and results of operations may suffer.

Competition for skilled personnel, particularly those specializing in engineering and sales, is intense. We cannot be certain that we will be successful in attracting and retaining qualified personnel, or that newly hired personnel will function effectively, both individually and as a group. In particular, we must continue to expand our direct sales force, including hiring additional sales managers, to grow our

customer base and increase sales. In addition, if we offer employment to personnel employed by competitors, we may become subject to claims of unfair hiring practices, and incur substantial costs in defending ourselves against these claims, regardless of their merits. If we are unable to effectively recruit, hire and utilize new employees, execution of our business strategy and our ability to react to changing market conditions may be impeded, and our business, financial condition and results of operations may suffer.

Volatility or lack of performance in our stock price may also affect our ability to attract and retain our key personnel. Our executive officers have become, or will soon become, vested in a substantial amount of shares of common stock or stock options. Employees may be more likely to leave us if the shares they own or the shares underlying their vested options have significantly appreciated in value relative to the original purchase prices of the shares or the exercise prices of the options, or if the exercise prices of the options that they hold are significantly above the market price of our common stock. If we are unable to retain our employees, our business, operating results and financial condition will be harmed.

If we fail to maintain proper and effective internal controls, our ability to produce accurate financial statements on a timely basis could be impaired, which would adversely affect our operating results, our ability to operate our business and our stock price.

Ensuring that we have adequate internal financial and accounting controls and procedures in place to produce accurate financial statements on a timely basis is a costly and time-consuming effort that needs to be re-evaluated frequently. We have in the past discovered, and may in the future discover, areas of our internal financial and accounting controls and procedures that need improvement.

Our management is responsible for establishing and maintaining adequate internal control over financial reporting to provide reasonable assurance regarding the reliability of our financial reporting and the preparation of financial statements for external purposes in accordance with U.S. generally accepted accounting principles. Our management does not expect that our internal control over financial reporting will prevent or detect all error and all fraud. A control system, no matter how well designed and operated, can provide only reasonable, not absolute, assurance that the control system’s objectives will be met. Because of the inherent limitations in all control systems, no evaluation of controls can provide absolute assurance that misstatements due to error or fraud will not occur or that all control issues and instances of fraud, if any, within our company will have been detected.

We expect that we will be required to comply with Section 404 of the Sarbanes-Oxley Act in connection with our annual report on Form 10-K for the year ending December 31, 2011. We are expending significant resources in developing the necessary documentation and testing procedures required by Section 404. We cannot be certain that the actions we are taking to improve our internal controls over financial reporting will be sufficient, or that we will be able to implement our planned processes and procedures in a timely manner. In addition, if we are unable to produce accurate financial statements on a timely basis, investors could lose confidence in the reliability of our financial statements, which could cause the market price of our common stock to decline and make it more difficult for us to finance our operations and growth.

We will incur significant increased costs as a result of operating as a public company, which may adversely affect our operating results and financial condition.

As a public company, we will incur significant accounting, legal and other expenses that we did not incur as a private company. We will incur costs associated with our public company reporting requirements. We also anticipate that we will incur costs associated with corporate governance requirements, including requirements under the Sarbanes-Oxley Act of 2002, as well as rules

implemented by the SEC and the New York Stock Exchange. We expect these rules and regulations to increase our legal and financial compliance costs and to make some activities more time-consuming and costly. Furthermore, these laws and regulations could make it more difficult or more costly for us to obtain certain types of insurance, including director and officer liability insurance, and we may be forced to accept reduced policy limits and coverage or incur substantially higher costs to obtain the same or similar coverage. The impact of these requirements could also make it more difficult for us to attract and retain qualified persons to serve on our board of directors, our board committees or as executive officers. We are currently evaluating and monitoring developments with respect to these rules, and we cannot predict or estimate the amount of additional costs we may incur or the timing of such costs.

New laws and regulations as well as changes to existing laws and regulations affecting public companies, including the provisions of the Sarbanes-Oxley Act of 2002 and rules adopted by the SEC and the New York Stock Exchange, would likely result in increased costs to us as we respond to their requirements.

Risks Related to This Offering and Ownership of Our Common Stock

Our stock price may be volatile, and you may not be able to resell shares of our common stock at or above the price you paid.

Prior to this offering there has been no public market for shares of our common stock, and an active public market for our shares may not develop or be sustained after this offering. We and the representatives of the underwriters will determine the initial public offering price of our common stock through negotiation. This price will not necessarily reflect the price at which investors in the market will be willing to buy and sell our shares following this offering. In addition, the trading price of our common stock following this offering could be highly volatile and could be subject to wide fluctuations in response to various factors, some of which are beyond our control. These factors include those discussed in this “Risk Factors” section of this prospectus and others such as:

In recent years, the stock market in general, and the market for technology companies in particular, has experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to the operating performance of those companies. Broad market and industry factors may seriously affect the market price of our common stock, regardless of our actual operating performance. These fluctuations may be even more pronounced in the trading market for our stock shortly following this offering. In addition, in the past, following periods of volatility in the overall market and the market price of a particular company’s securities, securities class action litigation has often been instituted against these companies. This litigation, if instituted against us, could result in substantial costs and a diversion of our management’s attention and resources.

If securities or industry analysts do not publish research or reports about our business, or if they issue an adverse or misleading opinion regarding our stock, our stock price and trading volume could decline.

The trading market for our common stock will be influenced by the research and reports that industry or securities analysts publish about us or our business. We do not currently have and may never obtain research coverage by securities and industry analysts. If no securities or industry analysts commence coverage of our company, the trading price for our stock would be negatively impacted. In the event we obtain securities or industry analyst coverage, if any of the analysts who cover us issue an adverse or misleading opinion regarding our stock, our stock price would likely decline. If one or more of these analysts cease coverage of our company or fail to publish reports on us regularly, we could lose visibility in the financial markets, which in turn could cause our stock price or trading volume to decline.

Our directors, executive officers and principal stockholders and their respective affiliates will continue to have substantial influence over us after this offering and could delay or prevent a change in corporate control.

After this offering, our directors, executive officers and holders of more than 5% of our common stock, together with their affiliates, will beneficially own, in the aggregate, approximately        % of our outstanding common stock, assuming no exercise of the underwriters’ option to purchase additional shares of our common stock in this offering. As a result, these stockholders, acting together, would have significant influence over the outcome of matters submitted to our stockholders for approval, including the election of directors and any merger, consolidation or sale of all or substantially all of our assets. In addition, these stockholders, acting together, would have significant influence over the management and affairs of our company. Accordingly, this concentration of ownership might harm the market price of our common stock by:

Future sales of shares by existing stockholders could cause our stock price to decline.

If our existing stockholders sell, or indicate an intent to sell, substantial amounts of our common stock in the public market after the 180-day contractual lock-up and other legal restrictions on resale discussed in this prospectus lapse, the trading price of our common stock could decline significantly and could decline below the initial public offering price. Based on shares outstanding as of September 26,December 31, 2009, upon the completion of this offering, we will have outstanding              shares of common stock, assuming no exercise of outstanding options and warrants other than those options or warrants exercised by certain selling stockholders for the purpose of selling shares in this offering. Of these shares,              shares of common stock, plus any shares sold pursuant to the underwriters’ option to purchase additional shares, will be immediately freely tradable, without restriction, in the public market. Goldman, Sachs & Co. and Morgan Stanley & Co. Incorporated may, in their sole discretion, permit our officers, directors, employees and current stockholders to sell shares prior to the expiration of the lock-up agreements.

After the lock-up agreements pertaining to this offering expire and based on shares outstanding as of September 26,December 31, 2009, an additional              shares will be eligible for sale in the public market. In addition, (i) the              shares subject to restricted stock units, (ii) the              shares subject to outstanding options under our 1997 Long-Term Incentive and Stock Option Plan, 2000 Stock Plan and 2002 Stock Plan, (iii) the              shares reserved for future issuance under our 2010 Equity Incentive Award Plan and (iv) the              shares remaining available for issuance under our 2002 Stock Plan, that will become available for issuance under our 2010 Equity Incentive Award Plan, will become eligible for

sale in the public market in the future, subject to certain legal and contractual limitations. If these additional shares are sold, or if it is perceived that they will be sold, in the public market, the price of our common stock could decline substantially.

Purchasers in this offering will experience immediate and substantial dilution in the book value of their investment.

The assumed initial public offering price of our common stock is substantially higher than the pro forma net tangible book value per share of our common stock outstanding prior to this offering. Therefore, if you purchase our common stock in this offering, you will incur an immediate substantial dilution of $             in net tangible book value per share from the price you paid. In addition, following this offering, purchasers in the offering will have contributed approximately     % of the total consideration paid by stockholders to us to purchase shares of our common stock. In addition, if the underwriters exercise their option to purchase additional shares, outstanding options and warrants are exercised or restricted stock units vest, you will experience further dilution. For a further description of the dilution that you will experience immediately after this offering, see the section titled “Dilution.”

We have broad discretion to determine how to use the funds raised in this offering, and may use them in ways that may not enhance our operating results or the price of our common stock.

Our management will have broad discretion over the use of proceeds from this offering, and we could spend the proceeds from this offering in ways our stockholders may not agree with or that do not yield a favorable return. We intend to use the net proceeds from this offering for working capital, capital expenditures and other general corporate purposes. We may also use a portion of the net proceeds to repay our credit facility or acquire complementary businesses, products or technologies. We have not allocated the net proceeds from this offering for any specific purposes. If we do not invest or apply the proceeds of this offering in ways that improve our operating results, we may fail to achieve expected financial results, which could cause our stock price to decline.

Provisions in our charter documents and under Delaware law could discourage a takeover that stockholders may consider favorable and may lead to entrenchment of management.

Our amended and restated certificate of incorporation and amended and restated bylaws that will be in effect prior to the completion of this offering will contain provisions that could have the effect of delaying or preventing changes in control or changes in our management without the consent of our board of directors. These provisions will include:

ability of our stockholders to force consideration of a proposal or to take action, including the removal of directors; and

We are also subject to certain anti-takeover provisions under Delaware law. Under Delaware law, a corporation may not, in general, engage in a business combination with any holder of 15% or more of its capital stock unless the holder has held the stock for three years or, among other things, the board of directors has approved the transaction. For a description of our capital stock, see the section titled “Description of Capital Stock.”

We may be unable to raise additional capital to fund our future operations, and any future financings or acquisitions could result in substantial dilution to existing stockholders.

We may need to raise additional capital to fund operations in the future. There is no guarantee that we will be able to raise additional equity or debt funding when or if it is required. The terms of any financing, if available, could be unfavorable to us and our stockholders and could result in substantial dilution to the equity and voting interests of our stockholders. Any failure to obtain financing when and as required could force us to curtail our operations which would harm our business.

We do not currently intend to pay dividends on our common stock and, consequently, your ability to achieve a return on your investment will depend on appreciation in the price of our common stock.

We do not currently intend to pay any cash dividends on our common stock for the foreseeable future. We currently intend to invest our future earnings, if any, to fund our growth. Additionally, the terms of our credit facility restrict our ability to pay dividends. Therefore, you are not likely to receive any dividends on your common stock for the foreseeable future.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, particularly in the sections titled “Prospectus Summary,” “Risk Factors,” “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and “Business,” contains forward-looking statements that involve substantial risks and uncertainties. All statements other than statements of historical facts contained in this prospectus, including statements regarding our future financial position, business strategy and plans and objectives of management for future operations, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as “believe,” “may,” “estimate,” “continue,” “anticipate,” “intend,” “should,” “plan,” “expect,” “predict,” “potential,” or the negative of these terms or other similar expressions. We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our financial condition, results of operations, business strategy and financial needs. These forward-looking statements are subject to a number of risks, uncertainties and assumptions described under the section titled “Risk Factors” and elsewhere in this prospectus, regarding, among other things:

These risks are not exhaustive. Other sections of this prospectus may include additional factors that could adversely impact our business and financial performance. Moreover, we operate in a very competitive and rapidly changing environment. New risk factors emerge from time to time and it is not possible for our management to predict all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. We cannot assure you that the events and circumstances reflected in the forward-looking statements will be achieved or occur. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Moreover, neither we nor any other person assume responsibility for the accuracy and completeness of the forward-looking statements. Except as required by law, we undertake no obligation to update publicly any forward-looking statements for any reason after the date of this prospectus to conform these statements to actual results or to changes in our expectations.

You should rely only on the information contained in this prospectus. Neither we nor any of the underwriters have authorized anyone to provide information different from that contained in this prospectus. Neither the delivery of this prospectus, nor sale of common stock, means that information contained in this prospectus is correct after the date of this prospectus. This prospectus is not an offer to sell or solicitation of an offer to buy shares of common stock in any circumstances under which the offer or solicitation is unlawful.

You should read this prospectus and the documents that we reference in this prospectus and have filed as exhibits to the registration statement on Form S-1, of which this prospectus is a part, that we have filed with the SEC with the understanding that our actual future results, levels of activity, performance and achievements may be materially different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

USE OF PROCEEDS

We estimate that the net proceeds we will receive from the offering will be $             million, at an assumed initial public offering price of $             per share, which is the mid-point of the range listed on the cover page of this prospectus, after deducting an assumed underwriting discount and estimated offering expenses payable by us. We will not receive any proceeds from the sale of shares of common stock offered by the selling stockholders.

We currently expect to use the net proceeds from this offering to fund research and development, sales and marketing activities and general and administrative expenses of approximately $25.0 million and capital expenditures of approximately $7.0 million.

We intend to use a portion of the remaining net proceeds from this offering for general corporate purposes, including financing our growth and working capital investments. We may also use a portion of the remaining net proceeds from this offering to repay our existing credit facility or acquire complementary businesses, products or technologies. However, we do not have agreements or commitments for any specific repayments or acquisitions at this time. See the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources” for a description of our loan with Silicon Valley Bank which we may choose to repay with the net proceeds of this offering.

The amount and timing of what we actually spend for these purposes may vary significantly and will depend on a number of factors, including our future revenues and cash generated by operations and the other factors described under the caption “Risk Factors.” We may find it necessary or advisable to use portions of the proceeds for other purposes.

Pending any use, as described above, we plan to invest the net proceeds in a variety of capital preservation instruments, including short- and long-term interest-bearing obligations, direct or guaranteed obligations of the U.S. government, certificates of deposit and money market funds.

DIVIDEND POLICY

We have never declared or paid any cash dividends on our common stock. We currently anticipate that we will retain all of our future earnings for use in the expansion and operation of our business and do not anticipate paying any cash dividends in the foreseeable future. Any future determination to declare cash dividends will be made at the discretion of our board of directors, subject to applicable laws and compliance with certain covenants under our credit facility, which restrict our ability to pay dividends, and will depend on our financial condition, results of operations, capital requirements, general business conditions and other factors that our board of directors may deem relevant.

CAPITALIZATION

The following table presents our cash, cash equivalents and marketable securities and capitalization as of September 26,December 31, 2009:

You should read this table in conjunction with “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our financial statements and related notes included elsewhere in this prospectus.

This table excludes as of September 26,December 31, 2009, the following shares:

DILUTION

Our pro forma net tangible book value as of September 26,December 31, 2009 was $61.3$68.2 million, or $1.28$1.42 per share of common stock. Net tangible book value per share represents the amount of our total tangible assets reduced by the amount of our total liabilities and divided by 48,013,91048,192,195 shares of common stock outstanding after giving effect of the automatic conversion of all outstanding shares of convertible preferred stock into shares of common stock immediately prior to the completion of this offering.

Net tangible book value dilution per share to new investors represents the difference between the amount per share paid by purchasers of shares of common stock in this offering and the pro forma net tangible book value per share of common stock immediately after the completion of this offering. After giving effect to our sale of shares of common stock in this offering at an assumed initial public offering price of $             per share, the mid-point of the range listed on the cover page of this prospectus, and after deducting an assumed underwriting discount and estimated offering expenses payable by us, our pro forma net tangible book value as of September 26,December 31, 2009 would have been, $             million, or $             per share. This represents an immediate increase in net tangible book value of $             per share to existing stockholders and an immediate dilution in net tangible book value of $             per share to investors purchasing common stock in this offering, as illustrated in the following table:

As the underwriters’ option to purchase additional shares of our common stock is solely from the selling stockholders, new investors will not incur additional dilution if such option is exercised.

The following table summarizes on an as adjusted pro forma basis as of September 26,December 31, 2009:

The above discussion and tables are based on 48,013,91048,192,195 shares of common stock issued and outstanding as of September 26,December 31, 2009, and excludes:

Sales by the selling stockholders in this offering will cause the number of shares held by existing stockholders to be reduced to             shares or    % of the total number of shares of our common stock outstanding after this offering. If the underwriters exercise their option to purchase additional shares in full, the number of shares held by the existing stockholders after this offering would be reduced to    % of the total number of shares of our common stock outstanding after this offering, and the number of shares held by new investors would increase to             shares or    % of the total number of shares of our common stock outstanding after this offering.

SELECTED FINANCIAL DATA

The following selected financial data should be read in conjunction with the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the financial statements and related notes included in this prospectus. The selected financial data included in this section is not intended to replace the financial statements and related notes in this prospectus.

We derived the statements of operations data for the nine months ended September 27, 2008 and September 26, 2009 and the balance sheet data as of September 26, 2009 from our unaudited financial statements and related notes, which are included elsewhere in this prospectus. We derived the statements of operations data for the years ended December 31, 2006, 2007, 2008 and 20082009 and the balance sheet data as of December 31, 20072008 and 20082009 from our audited financial statements and related notes, which are included elsewhere in this prospectus. We derived the statements of operations data for the years ended December 31, 20042005 and 20052006 and the balance sheet data as of December 31, 2004, 2005, 2006 and 20062007 from our audited financial statements and related notes which are not included in this prospectus. Historical results for any prior period are not necessarily indicative of future results for any period.

MANAGEMENT’S DISCUSSION AND ANALYSIS OF

FINANCIAL CONDITION AND RESULTS OF OPERATIONS

You should read the following discussion of our financial condition and results of operations in conjunction with the financial statements and the notes thereto included elsewhere in this prospectus. The following discussion contains forward-looking statements that reflect our plans, estimates and beliefs. Our actual results could differ materially from those discussed in the forward-looking statements. Factors that could cause or contribute to these differences include those discussed below and elsewhere in this prospectus, particularly in the section titled “Risk Factors.”

Overview

We are a leading provider in North America of broadband communications access systems and software for copper- and fiber- based network architectures that enable communications service providers, or CSPs, to connect to their residential and business subscribers. We develop and sell carrier-class hardware and software products, which we refer to as our Unified Access Infrastructure portfolio, that are designed to enhance and transform CSP access networks to meet the changing demands of subscribers rapidly and cost-effectively. Our Unified Access Infrastructure portfolio consists of our two core platforms, our C-Series multiservice, multiprotocol access platform, or C-Series platform, and our E-Series Ethernet service access platforms, or E-Series platforms, along with our complementary P-Series optical network terminals, or ONTs, and our Calix Management System, or CMS, network management software. We also offer installation, training, post-sales software support and extended warranty services. To date, service revenue has comprised an insignificant portion of our revenue.

We were founded in August 1999. In December 2001, we shipped our first C-Series platform, developed to support delivery of voice, data and video services over copper- and fiber-based network architectures. In 2003, we shipped products to our one hundredth customer. In 2006, we acquired Optical Solutions, Inc., or OSI, and integrated the engineering resources and technology acquired in the OSI transaction to develop our current suite of ONTs. We began shipping our ONTs and our E-Series platforms, developed to deliver advanced Internet protocol-based services, in 2006 and 2007, respectively. Our C-Series and E-Series platforms consist of electromechanical equipment such as cabinets and shelves, interchangeable line cards and integrated software that are designed to deliver specific features and functionality. We outsource the manufacturing of our products. Our CMS is server-based network management software that oversees and manages multiple C-Series and E-Series networks.

We have traditionally targeted CSPs which own, build and upgrade their own access networks and which also value strong relationships with their access systemsystems and software suppliers. We market our access systems and software to CSPs in North America, the Caribbean and Latin America through our direct sales force. To date, our customers have included large CSPs in North America, such as CenturyTel, Inc. and Embarq Corporation (which merged to form CenturyLink, Inc., or CenturyLink, as of July 1, 2009), Windstream Corp. and TDS Telecommunications Corporation. In addition, we also target over 1,000 other CSPs in North America, some of which receive government funds in the form of loans, loan guarantees and grants from the U.S. Department of Agriculture’s Rural Utilities Services, or RUS. We also have customers in international markets including the Bahamas, Barbados, Dominican Republic, Jamaica and Trinidad and Tobago. We also target new entrants to the access services market who are building their own access networks, including cable multiple systems operators, or MSOs, and municipalities. As of September 26,December 31, 2009, we have shipped over six million ports of our Unified Access Infrastructure portfolio to more than 500 North American and international customers, whose networks serve over 32 million subscriber lines in total. Our customers include 13 of the 20 largest U.S. Incumbent Local Exchange Carriers, or ILECs.

Our revenue has increased from $89.3$133.5 million for 20042005 to $250.5$232.9 million for 2008 and was $144.6 million for the nine months ended September 26, 2009. Continued revenue growth will depend on our

ability to continue to sell our access systems and software to existing customers and to attract new customers, including in particular, those customers in the large CSP and international markets. Since our inception we have incurred significant losses, and as of September 26,December 31, 2009, we had an accumulated deficit of $394.3$392.2 million. Our net loss was $19.5 million, $24.9 million, and $12.9 million and $22.4 million for 2006, 2007, 2008 and 2008,2009, respectively. For the nine months ended September 26, 2009, our net loss was $25.3 million.

Basis of Presentation

Revenue

We derive our revenue primarily from sales of our hardware products and related software. We generally recognize revenue only after all products in an order have been delivered and accepted, and title has been transferred to the customer. In certain cases, our products are sold along with services, which include installation, training, post-sales software support and/or extended warranty services. To date, service revenue has comprised an insignificant portion of our revenue, and we have not reported service revenue separately from product revenue in our financial statements. As of September 26,December 31, 2009, our revenue deferrals related to arrangements that have been partially delivered, RUS contracts that include installation services, special customer arrangements and ratably recognized services totaled $39.5$36.5 million. Large orders that are scheduled to ship over multiple reporting periods will be deferred and not recognized until the period of final delivery. Where substantive acceptance provisions are specified in an arrangement or extended return rights exist, revenue is deferred until all acceptance criteria have been met or the extended return rights expire. The timing of deferred revenue recognition may cause significant fluctuations in our revenue and operating results from period to period.

Cost of Revenue

Our cost of revenue is comprised of the following:

Gross Profit

Our gross profit and gross margin have been, and will likely be, impacted by several factors, including changes in customer mix, changes in the mix of products demanded, changes in our product costs and changes in pricing due to competitive pressure. Changes in these factors could have a material impact on our future average selling prices and unit costs. Also, the timing of deferred revenue recognition and related deferred costs can have a material impact on our gross profit and gross margin results. The timing of recognition and the relative size of these arrangements could cause large fluctuations in our gross profit from period to period.

Operating Expenses

Operating expenses consist primarily of research and development, sales and marketing and general and administrative expenses and are recognized as incurred. Personnel-related costs, which include stock-

based compensation expense, are the most significant component of each of these expense categories. We expect to continue to hire new employees in order to support our anticipated growth and status as a public company. In any particular period, the timing of additional hires could materially affect our operating expenses, both in absolute dollars and as a percentage of revenue. We anticipate that our operating expenses will increase in absolute dollar amounts but will decline as a percentage of revenue over time.