Exhibit 10.15

MASTER PURCHASE AGREEMENT FOR

TECHNICAL EQUIPMENT AND RELATED

SERVICES

BETWEEN

SPRINT/UNITED MANAGEMENT COMPANY

AND

SYCAMORE NETWORKS, INC.

TABLE OF CONTENTS

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MASTER PURCHASE AGREEMENT FOR TECHNICAL EQUIPMENT

AND RELATED SERVICES

This Master Purchase Agreement for Technical Equipment and Related Services (the ?Agreement?) dated as of April 22, 2004 (?Effective Date?) is between Sprint/United Management Company, a Kansas corporation (?Sprint?), and Sycamore Networks, Inc., a Delaware corporation (?Supplier?).

BACKGROUND

OPERATIVE PROVISIONS

?Acceptance? is defined in Section 9.0 and Schedule D.

?Agreement? refers to this Agreement and its Schedules.

?Backwards Compatibility? means the referenced prior Software Feature Enhancement Revision Level(s) of the applicable Product or System remain fully functional after the integration with the [ * ] Software Feature Enhancement Revision Levels and that after such integration the prior Software Feature Enhancement Revision Level(s) do not lose any functionality and the new Revision Level(s) or Interoperates with all functionalities of the [ * ] Software Feature Enhancement Revision Levels.

?Confidential Information? means information identified in written or oral format by the disclosing party as confidential, trade secret or proprietary information and, if disclosed orally, summarized in written format within [ * ] days of disclosure, including (i) this Agreement and the discussions, negotiations and proposals related to this Agreement, and (ii) any information exchanged in connection with this Agreement concerning the other party?s business including, tangible, intangible, visual, electronic, or written information, such as: (w) trade secrets, (x) financial information and pricing, (y) technical information, such as research, development, procedures, algorithms, data, designs, and know-how, and (z) business information, such as operations, planning, marketing interests, and products, and (iii) that in any event the receiving party should reasonably be expected to judge as confidential, trade secret or proprietary whether, under any of the clauses (i), (ii) or (iii) of this definition, received directly or indirectly from the other party.

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?Control? means: (i) the power to vote 50% or more of the voting interests of an entity; or (ii) ownership of 50% or more of the beneficial interests in income or capital of an entity unless otherwise mutually agreed.

?Deliverable? means any Product, System or Service delivered or to be delivered by Supplier under this Agreement and any applicable Purchase Order.

?Documentation? means all written instructions, manuals, descriptions, and any other documents (i) related to the Deliverables, (ii) necessary for Sprint to support Sprint?s business requirements (such as provisioning, testing, certificates of compliance, operating and troubleshooting) in connection with the Deliverables and (iii) detailed, comprehensive, and prepared in conformance with generally accepted industry standards of professional care, skill, diligence and competence applicable to telecommunications and operational practices similar to Sprint?s.

?Embedded Software? means software that is embedded in hardware and is not intended to be separated from the hardware to function.

?Equipment? means all hardware and other items of personal property as well as Embedded Software, that are provided or to be provided by Supplier under this Agreement, including the Equipment listed in the applicable Schedule and Equipment Modifications and Equipment Feature Enhancements.

?Equipment Feature Enhancement? means (i) feature enhancements that materially improve functionality or performance of Equipment and that Supplier markets as separate commercially available product or (ii) custom developed features for Sprint or another customer of Supplier.

?Equipment Modifications? means any patch, fix, alteration, improvement, correction, revision, release, new version or any other change to the Equipment that is required to address a field affecting change, except for Equipment Feature Enhancements.

?Feature Delivery Date? means the date on which Supplier has agreed that a Technical Annex Deliverable will be made commercially available.

?FRU? means field replaceable unit, for example cards, inter-bay cabling and power supplies, and does not include chassis.

?Full Protection Services? is defined in Schedule O.

?Illicit Code? means any Deliverable containing code that the Supplier intends to use or uses to gain unauthorized in-band access to Sprint systems or networks via call completion or transport device transponders or ports, network or any form of ?back-door? access to Sprint networks. Notwithstanding the above, Supplier shall retain the ability to provide support to Sprint and Sprint Customers on a remote basis.

?Installation Related Materials? are described in a work statement attached to a Purchase Order. Installation Related Materials may include third-party branded equipment, software, or other materials and are not Products. A bill of materials listing the Installation Related Materials will be provided to Sprint at the end of the engineering phase of engineering, furnishing, installation & test services (?EFI&T?).

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?Interexchange Carrier? means a telephone company that is allowed to provide long distance telephone service between LATAs. LATA means local access and transport area (also known as service area) as set forth in the Modified Final Judgment between the United States Department of Justice and AT&T.

?Interoperability? or ?Interoperate? means the Product and/or System may interconnect and successfully operate with other products and systems in accordance with current industry standards and as set forth in the Specification.

?IXC? means Interexchange Carrier (also known as an ?IEC? or ?IC?).

?LEC? means local exchange carrier. Local exchange carrier means any person that is engaged in the provision of telephone exchange service or exchange access.

?Maintenance Services? means the hardware support provided by Sycamore under the terms and conditions contained in Schedule G Section 5 and the prices contained in Schedule J. The Maintenance Service is in addition to the Warranty Services.

?Malicious Software? means any key, node, lock, time-out, ?virus,? ?back door,? trapdoor,? ?booby trap,? ?drop dead device,? ?data scrambling device,? ?Trojan Horse,? means for enabling self-help, restraint, disabling program codes or other functions, whether implemented by electronic, mechanical or other means, which restricts or may restrict use or access to any portion of any Software data or information created by or accessed using the Software. Notwithstanding the above, Supplier shall retain the ability to provide support to Sprint and Sprint Customers on a remote basis.

?Net Price? means the final price paid by Sprint and Sprint Affiliates after all discounts are applied.

?Network Services? are the services provided by Sprint or a Sprint Affiliate to Sprint Customers, which services may include, but are not limited to, the following: (a) access to the Internet, (b) data and voice transmission and (c) telecommunications services related to such access and transmission, including managed network services whereby Sprint or a Sprint Affiliate manages network elements belonging to Sprint or a Sprint Affiliate, but located at the premises of a Sprint Customer in conjunction with Sprint?s or a Sprint Affiliate?s providing services to the Sprint Customer.

?Purchase Order(s)? means any written purchase order for Deliverables issued by Sprint to Supplier under this Agreement.

?Product? means the collective reference to Equipment and Software.

?Resolution? means the satisfactory conclusion of a service request. Resolution can be one of the following occurrences: (a) the identified problem has been resolved; or (b) a documented action plan containing the solution and timeframe for delivery. Resolution may include Supplier providing one or more interim patches or workarounds. The provision of a mutually acceptable patch(s) or workaround(s) shall reduce the severity level of the case.

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?Revision Level? means, with respect to any Product or System, any change from the immediately preceding version, including, any Software Upgrade, Software Feature Enhancement, Equipment Modifications and Equipment Feature Enhancement.

?Services? means any services related to the Products or System that Supplier may offer, such as Warranty Services, Software Support Plan, other Software support, installation services and training services.

?Software? means the computer software programs provided or to be provided by Supplier under this Agreement, including the Software listed in the applicable Schedule, any Software Upgrade, Software Feature Enhancement, Embedded Software and any related Documentation.

?Software Feature Enhancement? means (i) feature enhancements that materially improve functionality or performance of Software and that Supplier markets as a separate commercially available product or (ii) custom developed features for Sprint or another customer of Supplier. A Software Feature Enhancement in Supplier?s Revision Level numbering convention is denominated by the second character of its numbering system. For example in the Revision Level 6.2.1, the ?Software Feature Enhancement? number is ?2.?

?Software Support Plan? means the software support provided by Sycamore under the terms and conditions contained in Schedule G Section 6 and the prices contained in Schedule J. The Software Support Plan is in addition to the Software Warranty.

?Software Upgrade? means any commercially available upgrade, enhancement, modification, patch, fix, alteration, improvement, correction, revision, release, new version or any other change to the Software or Documentation, except for Software Feature Enhancements. A Software Upgrade in Supplier?s Revision Level numbering convention is denominated by the third character of its numbering system. For example in the Revision Level 6.2.1, the ?Software Upgrade? number is ?1.?

?Specification(s)? means the technical requirements and associated performance standards set forth in Schedule A.

?Sprint Affiliate? means (i) any entity, directly or indirectly, Controlling, Controlled by or under common Control with Sprint; and (ii) any entity that is listed in Schedule K, which may be amended from time to time as mutually agreed; and (iii) any entity to which any Sprint Affiliate as defined in clause (i) or (ii) of this definition is required by law or regulation to provide services or products and (iv) as the Parties otherwise mutually agree.

?Sprint Customer? means the entity to which Sprint or a Sprint Affiliate provides managed Network Services through use of the Products.

?Sprint Routing Guide? is attached as Schedule H.

?Supplier Personnel? means any employees, subcontractors or agents of Supplier who perform Services, act on Supplier?s behalf or are paid by Supplier in connection with this Agreement.

?Supplier Personnel Compensation? means wages, salaries, fringe benefits and other compensation, including contributions to any employee benefit, medical or savings plan and all payroll taxes, unemployment compensation benefits, including withholding obligations.

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?System? means an SN16000 MC, SN16000 SC or SN3000 or other Sycamore Product that may be added to this Agreement by mutual written consent.

?Technical Annex? means Schedule A.

?Technical Annex Deliverable? means any Equipment or Software required to gain compliance with a Specification identified in the applicable Schedule as a ?Future Deliverable? or ?Delayed Closure.?

?Unmitigated Vulnerabilities? means any Deliverables (i) containing items listed by Carnegie Mellon CERT^? Coordination Center (www.cert.org), (ii) containing items listed in the Mitre Common Vulnerabilities and Exposures List (www.cve.mitre.org), or (iii) that must be configured in a manner inconsistent with due diligence or industry-accepted best practices such that the Supplier is only able to provide contracted features or functionality under this Agreement with Deliverables configured in a manner susceptible to exploitation.

?Warranty Service(s)? means the services with respect to the Products and Systems further described in this Agreement and in Schedule G.

This Agreement sets forth general terms and conditions that apply to any Purchase Order Sprint or a Sprint Affiliate may issue to Supplier for Deliverables. Purchase Orders may be issued to Supplier for use by Sprint or Sprint Customers. Each Purchase Order specifically incorporates the terms of this Agreement.

All references to ?Sprint? refer equally to Sprint or the Sprint Affiliate issuing a Purchase Order under this Agreement.

[ * ] Sprint?s issuance of a Purchase Order is Sprint?s offer to pay for Deliverables and is conditioned upon Supplier?s acceptance of the Purchase Order, in each case in accordance with this Agreement and the applicable Purchase Order.

Sprint will provide, on a [ * ] basis, a [ * ] forecast for Deliverables to Supplier to assist Supplier in planning in accordance with the appropriate Schedule. Any forecast will not be a Purchase Order or otherwise considered a commitment by Sprint. Each forecast will be provided to Supplier?s North American Sales Vice President or an assigned representative during the [ * ] program review or as otherwise mutually agreed.