Document:

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                                                                   Exhibit 10.23

                                SIPEX CORPORATION

                      2000 NON-QUALIFIED STOCK OPTION PLAN

     1. PURPOSE. This 2000 Non-Qualified Stock Option Plan (the "Plan") is
intended to provide incentives to non-officer employees and consultants of SIPEX
Corporation (the "Company"), and of any present or future parent or subsidiary
of the Company ("Related Entities") by providing them with opportunities to
purchase stock in the Company pursuant to options ("Non-Qualified Options" or
"Options") granted hereunder which do not qualify as "incentive stock options"
("ISOs") under Section 422(b) of the Internal Revenue Code (the "Code"). No
Options may be granted pursuant to this Plan to any officer or director of the
Company or any Related Entity. As used herein, the terms "parent" and
"subsidiary" mean "parent corporation" and "subsidiary corporation" respectively
as those terms are defined in Section 424 of the Code. For purposes of this
Plan, the term "officer" shall be interpreted in accordance with the
interpretations of the National Association of Securities Dealers, Inc. ("NASD")
under the NASD Marketplace Rules.

     2.   ADMINISTRATION OF THE PLAN.

          A. BOARD OR COMMITTEE ADMINISTRATION. The Plan shall be administered
     by the Board of Directors of the Company (the "Board") or, subject to
     paragraph 2(C) (relating to compliance with Section 162(m) of the Code), by
     a committee appointed by the Board (the "Committee"). Hereinafter, all
     references in this Plan to the "Committee" shall mean the Board if no
     Committee has been appointed. Subject to ratification of the grant or
     authorization of each Option by the Board, and subject to the terms of the
     Plan, the Committee shall have the authority to (i) determine to whom, from
     among the class of individuals and entities eligible under paragraph 3 to
     receive Options, Options may be granted; (ii) determine the time or times
     at which Options shall be granted; (iii) determine the option price of
     shares subject to each Option, which price shall not be less than the
     minimum price specified in paragraph 6; (iv) determine (subject to
     paragraph 7) the time or times when each Option shall become exercisable
     and the duration of the exercise period; (v) determine whether restrictions
     such as repurchase options are to be imposed on shares subject to Options
     and the nature of such restrictions, if any, and (vi) interpret the Plan
     and prescribe and rescind rules and regulations relating to it. The
     Committee shall take whatever actions it deems necessary, under Section 422
     of the Code and the regulations promulgated thereunder, to ensure that no
     Option issued hereunder is treated as an ISO. The interpretation and
     construction by the Committee of any provisions of the Plan or of any
     Option granted under it shall be final unless otherwise determined by the
     Board. The Committee may from time to time adopt such rules and regulations
     for carrying out the Plan as it may deem advisable. No member of the Board
     or the Committee shall be liable for any action or determination made in
     good faith with respect to the Plan or any Option granted under it.

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          B. COMMITTEE ACTIONS. The Committee may select one of its members as
     its chairman, and shall hold meetings at such time and places as it may
     determine. A majority of the Committee shall constitute a quorum and acts
     by a majority of the members of the Committee, or acts reduced to or
     approved in writing by a majority of the members of the Committee (if
     consistent with applicable state law), shall constitute the valid acts of
     the Committee. From time to time the Board may increase the size of the
     Committee and appoint additional members thereof, remove members (with or
     without cause) and appoint new members in substitution therefor, fill
     vacancies however caused, or remove all members of the Committee and
     thereafter directly administer the Plan.

          C. PERFORMANCE-BASED COMPENSATION. The Board, in its discretion, may
     take such action as may be necessary to ensure that Options granted under
     the Plan qualify as "qualified performance-based compensation" within the
     meaning of Section 162(m) of the Code and applicable regulations
     promulgated thereunder ("Performance-Based Compensation"). Such action may
     include, in the Board's discretion, some or all of the following (i) if the
     Board determines that Options granted under the Plan generally shall
     constitute Performance-Based Compensation, the Plan shall be administered,
     to the extent required for such Options to constitute Performance-Based
     Compensation, by a Committee consisting solely of two or more "outside
     directors" (as defined in applicable regulations promulgated under Section
     162(m) of the Code), (ii) if any Non-Qualified Options with an exercise
     price less than the fair market value per share of Common Stock are granted
     under the Plan and the Board determines that such Options should constitute
     Performance-Based Compensation, such options shall be made exercisable only
     upon the attainment of a pre-established, objective performance goal
     established by the Committee, and such grant shall be submitted for, and
     shall be contingent upon shareholder approval and (iii) Options granted
     under the Plan may be subject to such other terms and conditions as are
     necessary for compensation recognized in connection with the exercise or
     disposition of such Option or the disposition of Common Stock acquired
     pursuant to such Option, to constitute Performance-Based Compensation.

     3. ELIGIBLE EMPLOYEES AND OTHERS. Non-Qualified Options may be granted to
any non-officer employee or consultant of the Company or any Related Entity. The
Committee may take into consideration a recipient's individual circumstances in
determining whether to grant an Option. The granting of any Option to any
individual or entity shall neither entitle such grantee to, nor disqualify such
grantee from, participation in any other grant of Options.

     4. STOCK. The stock subject to Options shall be authorized but unissued
shares of common stock of the Company, par value $.01 per share (the "Common
Stock"), or shares of Common Stock reacquired by the Company in any manner. The
aggregate number of shares which may be issued pursuant to the Plan is
1,000,000, subject to adjustment as provided in paragraph 13. If any Option
granted under the Plan shall expire or terminate for any reason without having
been exercised in full or shall cease for any reason to be exercisable in whole
or in part or shall be repurchased by the Company, the shares of Common Stock
subject to such Option shall again be available for grants of Options under the
Plan.

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     No employee or consultant of the Company or any Related Entity may be
granted Options to acquire, in the aggregate, more than 700,000 shares of Common
Stock under the Plan, subject to adjustment as provided in Paragraph 13. If any
Option granted under the Plan shall expire or terminate for any reason without
having been exercised in full or shall cease for any reason to be exercisable in
whole or in part or shall be repurchased by the Company, the shares of Common
Stock subject to such Option shall be included in the determination of the
aggregate number of shares of Common Stock deemed to have been granted to such
employee or consultant under the Plan.

     5.   GRANTING OF OPTIONS. Options may be granted under the Plan at any time
on or after October 31, 2000 and prior to October 31, 2010. The date of grant of
an Option under the Plan will be the date specified by the Committee at the time
it grants the Option; provided, however, that such date shall not be prior to
the date on which the Committee acts to approve the grant.

     6.   MINIMUM OPTION PRICE.

     Subject to paragraph 2(C) (relating to compliance with Section 162(m) of
the Code), the exercise price per share specified in the agreement relating to
each Non-Qualified Option granted under the Plan (the "Agreement"), may not be
less than the fair market value of the Common Stock on the date of grant, but
shall in no event be less than the minimum legal consideration required therefor
under the laws of any jurisdiction in which the Company or its successors in
interest may be organized.

     7.   OPTION DURATION. Subject to earlier termination as provided in other
provisions of this Plan or as specified in the Agreement relating to such
Option, each Option shall expire on the date specified by the Committee, but not
more than ten years from the date of grant.

     8.   EXERCISE OF OPTION. Subject to the other provisions of this Plan, each
Option granted under the Plan shall be exercisable as follows:

          A.   VESTING. The Option shall either be fully exercisable on the date
     of grant or shall become exercisable thereafter in such installments as the
     Committee may specify.

          B.   FULL VESTING OF INSTALLMENTS. Once an installment becomes
     exercisable it shall remain exercisable until expiration or termination of
     the Option, unless otherwise specified by the Committee.

          C.   PARTIAL EXERCISE. Each Option or installment may be exercised at
     any time or from time to time, in whole or in part, for up to the total
     number of shares with respect to which it is then exercisable.

          D.   ACCELERATION OF VESTING. The Committee shall have the right to
     accelerate the date that any installment of any Option becomes exercisable
     despite the fact that the foregoing action may cause the application of
     Sections 280G and 4999 of the Code if a change in control of the Company
     occurs.

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     9.   TERMINATION OF BUSINESS RELATIONSHIP. Each Option may provide that it
shall terminate before its stated expiration date, upon terms specified by the
Committee, if the optionee ceases to be an employee or consultant of the
Company, or of any Related Entity (any such relationship hereinafter referred to
as a "Business Relationship with the Company"). Nothing in the Plan or any
Option granted hereunder shall be deemed to give any optionee the right to
continue his or her Business Relationship with the Company for any period of
time. For purposes of this paragraph 9, a Business Relationship shall be
considered as continuing uninterrupted during any bona fide leave of absence
(such as those attributable to illness, military obligations or governmental
service) provided that the period of such leave does not exceed 90 days or, if
longer, any period during which such optionee's right to reemployment is
guaranteed by statute or by contract. A bona fide leave of absence with the
written approval of the Committee shall not be considered an interruption of a
Business Relationship under this paragraph 9, provided that such written
approval contractually obligates the Company or any Related Entity to continue
the Business Relationship of the optionee after the approved period of absence.
In the event of such an approved leave of absence, vesting shall be suspended
(and the period of the leave of absence shall be added to all vesting dates)
unless otherwise agreed upon.

     10.  DEATH; DISABILITY.

          A. DEATH. Unless otherwise specified by the Committee, if an
     optionee's Business Relationship with the Company terminates by reason of
     death, his or her Option may be exercised, to the extent of the number of
     shares with respect to which such optionee could have exercised it on the
     date of such optionee's death, by such optionee's estate, personal
     representative or beneficiary who has acquired the Option by will or by the
     laws of descent and distribution, at any time prior to the earlier of the
     specified expiration date of the Option or 180 days from the date of death.

          B. DISABILITY. Unless otherwise specified by the Committee, if an
     optionee's Business Relationship with the Company terminates by reason of
     such optionee's disability, such optionee shall have the right to exercise
     his or her Option, to the extent of the number of shares with respect to
     which such optionee could otherwise have exercised it on the date his or
     her Business Relationship with the Company terminated, at any time prior to
     the earlier of the specified expiration date of the Option or 180 days from
     the date of the termination of the optionee's Business Relationship with
     the Company. For the purposes of the Plan, the term "disability" shall mean
     "permanent and total disability" as defined in Section 22(e)(3) of the Code
     or any successor statute.

     11.  ASSIGNABILITY. Options shall be assignable or transferable only to the
extent provided in the agreement relating to such option.

     12.  TERMS AND CONDITIONS OF OPTIONS. Options shall be evidenced by
instruments (which need not be identical) in such forms as the Committee may
from time to time approve. Such instruments shall conform to the terms and
conditions set forth in paragraphs 6 through 11 hereof and may contain such
other provisions as the Committee deems advisable which are not inconsistent
with the Plan, including restrictions applicable to shares of Common Stock
issuable

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upon exercise of Options. The Committee may specify that any Option shall be
subject to the restrictions set forth herein or, consistent with paragraph 7, to
such other or additional termination and cancellation provisions as the
Committee may determine. The Committee may from time to time confer authority
and responsibility on one or more of its own members and/or one or more officers
of the Company to execute and deliver such instruments. The proper officers of
the Company are authorized and directed to take any and all action necessary or
advisable from time to time to carry out the terms of such instruments.

     13. ADJUSTMENTS. Upon the occurrence of any of the following events, an
optionee's rights with respect to Options granted to such optionee hereunder
shall be adjusted as hereinafter provided, unless otherwise specifically
provided in the written agreement between the optionee and the Company relating
to such Option:

          A. STOCK DIVIDENDS AND STOCK SPLITS. If the shares of Common Stock
     shall be subdivided or combined into a greater or smaller number of shares
     or if the Company shall issue any shares of Common Stock as a stock
     dividend on its outstanding Common Stock, the number of shares of Common
     Stock deliverable upon the exercise of Options shall be appropriately
     increased or decreased proportionately, and appropriate adjustments shall
     be made in the purchase price per share to reflect such subdivision,
     combination or stock dividend.

          B. CONSOLIDATIONS OR MERGERS. If the Company is to be consolidated
     with or acquired by another entity in a merger or other reorganization in
     which the holders of the outstanding voting stock of the Company
     immediately preceding the consummation of such event, shall, immediately
     following such event, hold, as a group, less than a majority of the voting
     securities of the surviving or successor entity, or in the event of a sale
     of all or substantially all of the Company's assets or otherwise (other
     than a spin-off or similar transaction), or in the event of any other
     acquisition of the business of the Company, or determined by the Board
     (each, an "Acquisition"), the Committee or the board of directors of any
     entity assuming the obligations of the Company hereunder (the "Successor
     Board"), shall, as to outstanding Options, either (i) make appropriate
     provision for the continuation of such Options by substituting on an
     equitable basis for the shares then subject to such Options either (a) the
     consideration payable with respect to the outstanding shares of Common
     Stock in connection with the Acquisition, (b) shares of stock of the
     surviving or successor corporation or (c) such other securities as the
     Successor Board deems appropriate, the fair market value of which shall not
     materially exceed the fair market value of the shares of Common Stock
     subject to such Options immediately preceding the Acquisition; or (ii) upon
     written notice to the optionees, provide that all Options must be
     exercised, to the extent then exercisable or to be exercisable as a result
     of the Acquisition, within a specified number of days of the date of such
     notice, at the end of which period the Options shall terminate; or (iii)
     terminate all Options in exchange for a cash payment equal to the excess of
     the fair market value of the shares subject to such Options (to the extent
     then exercisable or to be exercisable as a result of the Acquisition) over
     the exercise price thereof.

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          C. RECAPITALIZATION OR REORGANIZATION. In the event of a
     recapitalization or reorganization of the Company (other than a transaction
     described in subparagraph B above) pursuant to which securities of the
     Company or of another corporation are issued with respect to the
     outstanding shares of Common Stock, an optionee upon exercising an Option
     shall be entitled to receive for the purchase price paid upon such exercise
     the securities such optionee would have received if such optionee had
     exercised his or her Option prior to such recapitalization or
     reorganization.

          D. DISSOLUTION OR LIQUIDATION. In the event of the proposed
     dissolution or liquidation of the Company, then the Committee shall, as to
     outstanding Options, at its discretion provide, upon written notice to the
     optionees, (i) that all Options must be exercised, to the extent then
     exercisable, within a specified number of days of the date of such notice,
     at the end of which period, the Options shall terminate or (ii) that such
     Options (including those which have not yet vested) shall be exercisable
     within a specified number of days of such notice, at the end of which
     period the Options shall terminate.

          E. ISSUANCES OF SECURITIES. Except as expressly provided herein, no
     issuance by the Company of shares of stock of any class, or securities
     convertible into shares of stock of any class, shall affect, and no
     adjustment by reason thereof shall be made with respect to, the number or
     price of shares subject to Options. No adjustments shall be made for
     dividends paid in cash or in property other than securities of the Company.

          F. FRACTIONAL SHARES. No fractional shares shall be issued under the
     Plan and the optionee shall receive from the Company cash in lieu of such
     fractional shares.

          G. ADJUSTMENTS. Upon the happening of any of the events described in
     subparagraphs A, B or C above, the class and aggregate numbers of shares
     set forth in paragraph 4 hereof that are subject to Options which
     previously have been or subsequently may be granted under the Plan shall
     also be appropriately adjusted to reflect the events described in such
     subparagraphs. The Committee or the Successor Board shall determine the
     specific adjustments to be made under this paragraph 13 and, subject to
     paragraph 2, its determination shall be conclusive.

     14. MEANS OF EXERCISING OPTIONS. An Option (or any part or installment
thereof) shall be exercised by giving written notice to the Company at its
principal office address, or to such transfer agent as the Company shall
designate. Such notice shall identify the Option being exercised and specify the
number of shares as to which such Option is being exercised, accompanied by full
payment of the purchase price therefor either (a) in United States dollars in
cash or by check, (b) at the discretion of the Committee, through delivery of
shares of Common Stock having a fair market value equal as of the date of the
exercise to the cash exercise price of the Option, (c) at the discretion of the
Committee, by delivery of the optionee's personal recourse note bearing interest
payable not less than annually at no less than 100% of the lowest applicable
Federal rate, as defined in Section 1274(d) of the Code, (d) at the discretion
of the Committee and consistent with applicable law, through the delivery of an
assignment to the Company of a sufficient amount of the proceeds from the sale
of the Common Stock acquired upon exercise of

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the Option and an authorization to the broker or selling agent to pay that
amount to the Company, which sale shall be at the participant's direction at the
time of exercise, or (e) at the discretion of the Committee, by any combination
of (a), (b), (c) and (d) above. The holder of an Option shall not have the
rights of a shareholder with respect to the shares covered by such Option until
the date of issuance of a stock certificate to such holder for such shares.
Except as expressly provided above in paragraph 13 with respect to changes in
capitalization and stock dividends, no adjustment shall be made for dividends or
similar rights for which the record date is before the date such stock
certificate is issued.

     15.  TERM AND AMENDMENT OF PLAN. This Plan was adopted by the Board on
October 31, 2000. The Plan shall expire at the end of the day on October 31,
2010 (except as to Options outstanding on that date). The Board may terminate or
amend the Plan in any respect at any time.

     16.  REPRICING. Without the prior approval of the Company's stockholders,
Options issued under the Plan will not be repriced, replaced or regranted
through cancellation or by lowering the Option exercise price of a previously
granted Option.

     17.  APPLICATION OF FUNDS. The proceeds received by the Company from the
sale of shares pursuant to Options granted under the Plan shall be used for
general corporate purposes.

     18.  WITHHOLDING OF ADDITIONAL INCOME TAXES. Each optionee shall pay to the
Company, or make provisions satisfactory to the Company for payment of, any
taxes required by law to be withheld in connection with Options to such optionee
no later than the date of the event creating the tax liability. The Board may
allow optionees to satisfy such tax obligations in whole or in part by
transferring shares of Common Stock, including shares retained from the Option
creating the tax obligation, valued at their fair market value (as determined by
the Board or as determined pursuant to the applicable Agreement). The Company
may, to the extent permitted by law, deduct any such tax obligations from any
payment of any kind otherwise due to an optionee.

     19.  DETERMINATION OF FAIR MARKET VALUE OF COMMON STOCK. Whenever, under
the terms of any option agreement or in administering the Plan, it is necessary
or desirable to determine the fair market value of the Company's Common Stock,
the Committee shall make such determination in accordance with this paragraph.
"Fair Market Value" shall be determined as of the last business day for which
the prices or quotes discussed in this sentence are available prior to the date
such Option is granted and shall mean (i) the average (on that date) of the high
and low prices of the Common Stock on the principal national securities exchange
on which the Common Stock is traded, if the Common Stock is then traded on a
national securities exchange; or (ii) the last reported sale price (on that
date) of the Common Stock on the Nasdaq National Market, if the Common Stock is
not then traded on a national securities exchange; or (iii) the closing bid
price (or average of bid prices) last quoted (on that date) by an established
quotation service for over-the-counter securities, if the Common Stock is not
reported on the Nasdaq National Market. However, if the Common Stock is not
publicly traded at the time an Option is granted under the Plan, "fair market
value" shall be deemed to be the fair value of the Common Stock as determined by
the Committee after taking into consideration all factors which it deems

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appropriate, including, without limitation, recent sale and offer prices of the
Common Stock in private transactions negotiated at arm's length.

     20.  GOVERNMENTAL REGULATION. The Company's obligation to sell and deliver
shares of the Common Stock under this Plan is subject to the approval of any
governmental authority required in connection with the authorization, issuance
or sale of such shares.

     Government regulations may impose reporting or other obligations on the
Company or Related Entities with respect to the Plan. For example, the Company
may be required to file tax information returns reporting the income received by
optionees in connection with the Plan.

     21.  GOVERNING LAW. The validity and construction of the Plan and the
instruments evidencing Options shall be governed by the laws of Massachusetts,
or the laws of any jurisdiction in which the Company or its successors in
interest may be organized.<PAGE>   1

                                                                   EXHIBIT 10.31

                           FIRST AMENDED AND RESTATED

                           CONVERTIBLE PROMISSORY NOTE

                              DUE NOVEMBER 16, 2001

February 8, 2001                                                     $67,000,000

     Unisphere Networks, Inc., a corporation organized under the laws of the
State of Delaware (the "MAKER"), promises to pay to the order of Siemens
Corporation (the "HOLDER"), the principal sum of $67,000,000, plus any other
amounts due under this Convertible Promissory Note (this "NOTE") on November 16,
2001 (the "MATURITY DATE").

     1. INTEREST. Maker acknowledges that Holder has made advances to Maker in
the amount of: (i) $3,750,000, on November 10, 2000 (the "FIRST ADVANCE"), (ii)
$8,000,000, on December 1, 2000 (the "SECOND ADVANCE"), (iii) 2,000,000, on
January 8, 2001 (the "THIRD ADVANCE") and (iv) $28,250,000, on January 10, 2001
(the "FOURTH Advance"); and that Holder will make an advance to Maker in the
amount of $25,000,000 on February 12, 2001 (the "FIFTH ADVANCE"). Maker promises
to pay interest on the outstanding principal amount of this Note for the period
from: (i) in the case of the First Advance, November 16, 2000, (ii) in the case
of the Second Advance, December 1, 2000, (iii) in the case of the Third Advance,
January 8, 2001, (iv) in the case of the Fourth Advance, January 10, 2001, and
(v) in the case of the Fifth Advance, February 12, 2001, until, in each case,
the date upon which this Note is repaid in full or converted as provided herein,
at a rate equal to 7.25% per annum. Interest will be computed on the basis of a
360-day year of twelve 30-day months. Accrued interest shall be payable in
arrears on the date that the principal amount of this Note is paid. Amounts not
paid when due shall bear interest at a rate per annum equal to 2.0% above the
otherwise applicable rate.

     2. PAYMENTS; TAXES; ENFORCEMENT COSTS.

        (a) The Maker will pay all amounts payable with respect to this Note by
crediting before 12:00 Noon, New York time, by bank wire transfer of same day
funds, the Holder's account in any bank in the United States as may be
designated and specified in writing by the Holder.

        (b) All payments in respect of this Note shall be made without set-off,
defense, or counterclaim. Subject to the second sentence of Section 10, if the
Maker is required by law to withhold any amounts (for taxes or otherwise), the
Maker shall gross-up payments under this Note so that the Holder receives the
full amount of the payment as if the Maker was not required to withhold any
amounts.

        (c) The Maker shall pay all costs and expenses, including reasonable
fees and disbursements of counsel, incurred by the Holder in connection with the
enforcement of this Note, including costs of collection.

     3. CONVERSION OF NOTE.

        (a) CONVERSION RIGHT; NUMBER OF SHARES. On the closing date (the "IPO
CLOSING DATE") of a registered public offering by the Maker of its common stock,
par value $.01 per share (the "COMMON STOCK"), this Note shall automatically
convert into shares of the Common Stock. On the earlier to occur of the Maturity
Date and the date, if any, upon which the Holder accelerates the maturity of
this Note pursuant to Section 6, and on any date subsequent to the Maturity Date
or such date of acceleration, as the case may be, the Holder may, at its option,
convert this Note into shares of the Common Stock. Upon any such conversion, the
Holder shall be entitled to receive a number of shares of Common Stock

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equal to (x) the principal amount of this Note plus accrued interest to the date
of conversion divided by (y) if the conversion occurs on the IPO Closing Date, a
number equal to the price per share at which the Common Stock was sold to the
public in the offering (without taking into account any underwriting discount),
and if the conversion occurs on any other date, a number equal to the fair
market value per share of the Common Stock as of such date, determined by the
Holder and the Maker acting together and in good faith.

            If the Holder and the Maker cannot agree on such fair market value
within five business days of the date of conversion, the Holder and the Maker
will engage Credit Suisse First Boston ("CSFB") and UBS Warburg LLC ("UBS") to
select, within ten business days of the date of conversion, a third person that
is an internationally recognized investment bank (the "APPRAISING BANK") to make
such determination. If CSFB is unwilling to so act, the Maker will select, and
if UBS is unwilling to so act, the Holder will select, another internationally
recognized investment bank to select the Appraising Bank. The Holder and the
Maker will then engage the Appraising Bank to determine, within 30 days of the
date of such engagement, the fair market value per share of the Common Stock as
of the date of conversion. The determination by the Appraising Bank shall be
conclusive, absent manifest error. The Holder and the Maker will each pay one
half of the fees and expenses of the Appraising Bank.

        (b) MANNER OF EXERCISE. In order to convert this Note, the Holder shall
surrender this Note to the Maker, accompanied by written notice to the Maker
(the "CONVERSION NOTICE") that the Holder elects to convert this Note or, in the
case of a conversion on the IPO Closing Date, that this Note has automatically
converted. The Conversion Notice shall also state the name or names, together
with address or addresses, in which the certificate or certificates for shares
of Common Stock which shall be issuable on such conversion shall be issued. As
promptly as practicable after the surrender of this Note, as aforesaid, the
Maker shall issue and shall deliver to the Holder a certificate or certificates
for the number of full shares of Common Stock issuable upon the conversion of
this Note in accordance with the provisions hereof, and any fractional interest
in respect of a share of Common Stock arising upon such conversion shall be
settled as provided below. Any conversion shall be deemed to have been effected
(i) immediately prior to the close of business on the date on which this Note
shall have been surrendered and the Conversion Notice received by the Maker as
aforesaid or (ii) in the case of a conversion on the IPO Closing Date, on the
IPO Closing Date, and the person or persons in whose name or names any
certificate or certificates for shares of Common Stock shall be issuable upon
such conversion shall be deemed to have become the holder or holders of record
of the shares represented thereby at such time.

        (c) PAYMENT IN LIEU OF FRACTIONAL SHARES. No fractional shares of Common
Stock shall be issued upon conversion of this Note. In lieu of issuing a
fractional share of Common Stock, the Maker shall pay to the Holder cash in an
amount equal to the product of the price per share applicable to the conversion
and a fraction representing the portion of a share of Common Stock that the
Maker would have otherwise issued upon such conversion.

        (d) TAXES UPON CONVERSION. The Maker will pay any and all documentary
stamp or similar issue or transfer taxes payable in respect of the issue or
delivery of shares of Common Stock upon conversion of this Note.

     4. REPRESENTATIONS AND WARRANTIES OF MAKER. The Maker represents and
warrants to the Holder that:

        (a) The Maker is a corporation duly organized, validly existing and in
good standing under the laws of the State of Delaware. The Maker has the
corporate power and authority to execute, deliver and carry out its obligations
under this Note. The Maker has taken all necessary action to authorize the
execution and delivery of this Note and the performance of its obligations
hereunder. This

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Note has been executed and delivered by the Maker and constitutes the legal,
valid and binding obligation of the Maker enforceable in accordance with its
terms.

        (b) The execution and delivery of this Note and the performance by the
Maker of its obligations hereunder does not and will not violate any existing
law or regulation or any decree of any court or governmental agency applicable
to the Maker, the certificate of incorporation or by-laws of the Maker or any
agreement or undertaking to which the Maker is a party or by which it or its
properties are bound.

        (c) The Maker is in compliance with all applicable laws, rules,
regulations and orders of any governmental authority, except for such instances
of non-compliance which, individually or in the aggregate, could not reasonably
be expected to have a material adverse effect on the condition (financial or
otherwise), business, operations or properties of the Maker (a "MATERIAL ADVERSE
EFFECT").

        (d) Each share of Common Stock to be issued upon conversion of this
Note, when issued, will be duly authorized, validly issued, fully paid,
nonassessable, free of any Lien or encumbrance and not subject to any preemptive
or similar right. "LIEN" means any mortgage, pledge, security interest,
encumbrance, lien or charge of any kind (including any conditional sale or other
title retention agreement or lease in the nature thereof).

     5. COVENANTS OF MAKER. The Maker covenants and agrees that, until its
obligations under this Note have been satisfied in full:

        (a) The Maker will maintain its corporate existence, rights, franchises,
privileges and good standing.

        (b) The Maker will not, without the prior written consent of the Holder,
dissolve, liquidate, or merge or consolidate with any other person or transfer
all or substantially all of its assets to any other entity; provided that the
Maker or any of its subsidiaries may merge with Broadsoft, Inc., a Delaware
corporation.

        (c) The Maker will comply with the requirements of all applicable laws,
rules, regulations of any governmental authority, except for instances of
non-compliance that, individually or in the aggregate, could not reasonably be
expected to have a Material Adverse Effect.

        (d) The Maker will use the proceeds of this Note only to meet its cash
requirements arising in the ordinary course of business.

        (e) The Maker will not, without the prior written consent of the Holder,
(i) declare or pay any dividend or make any distribution on or in respect of its
Capital Stock or similar payment to the direct or indirect holders of its
Capital Stock; (ii) purchase, redeem, retire or otherwise acquire for value any
of its Capital Stock; or (iii) make any investment in, or loan to, any person;
PROVIDED that the Maker may (x) make loans to employees to be used by such
employees to exercise options granted by the Maker to such employees in July,
September and November of 1999 and February of 2000 if such loans have been
approved by the Maker's Board of Directors and do not exceed, in the aggregate,
an amount equal to $14,000,000 and (y) repurchase shares of Common Stock from
employees, if approved by the Maker's Board of Directors. "CAPITAL STOCK" means,
with respect to any person, shares, interests, rights to purchase, warrants,
options, participations or other equivalents of or interests in (however
designated) equity of such person, including preferred stock.

        (f) The Maker will, as soon as practicable after the date hereof,
reserve and keep available, free from preemptive rights, out of the aggregate of
its authorized but unissued shares of

                                       3

<PAGE>   4

Common Stock or its issued shares of Common Stock held in its treasury, or both,
for the purpose of effecting conversions of this Note, the full number of shares
of Common Stock deliverable upon the conversion of this Note. Prior to the
delivery of any shares of Common Stock upon conversion of this Note, the Maker
shall comply with all laws and regulations requiring the registration of such
securities with, or any approval of or consent to the delivery thereof by, any
governmental authority, unless an exemption from registration is available.

        (g) The Maker will ensure that all shares of Common Stock delivered upon
conversion of this Note will upon delivery be duly and validly issued, fully
paid and nonassessable, free of all Liens as defined below and charges and not
subject to any preemptive or similar right.

        (h) The Maker will not, and will not permit any of its subsidiaries to,
directly or indirectly, create, incur, assume or suffer to exist any Lien upon
any of its property, assets or revenues, whether now owned or hereafter
acquired, unless, prior to or simultaneously with the creation, incurrence or
assumption of such Lien, the Maker causes its obligations under this Note to be
secured on terms that are at least as favorable as to the beneficiary of such
Lien.

     6. DEFAULTS AND REMEDIES. If an Event of Default (as defined below) occurs
and is continuing, the Holder may declare this Note to be immediately due and
payable without presentment, demand, protest or notice of any other kind, all of
which the Maker expressly waives; PROVIDED that if an Event of Default set forth
in (a) or (b) below occurs, then this Note shall become immediately due and
payable without any action by the Holder (and the Maker waives presentment,
demand, protest or notice of any other kind in connection therewith). Upon the
occurrence of an Event of Default, the Holder shall be entitled to exercise any
remedies available at law.

     Each of the following events shall be an "Event of Default:"

        (a) the Maker commences a voluntary case or other proceeding seeking
liquidation, reorganization or other relief with respect to itself or its debts
under any bankruptcy, insolvency or other similar law now or hereafter in effect
or seeking the appointment of a trustee, receiver, liquidation, custodian or
other similar official of it or any substantial part of its property, or
consents to any such relief or to the appointment of or taking possession by any
such official in an involuntary case or other proceeding commenced against it,
or makes a general assignment for the benefit of creditors, or fails generally
to pay its debts as they become due, or takes any action to authorize any of the
foregoing; or

        (b) an involuntary case or other proceeding is commenced against the
Maker seeking liquidation, reorganization or other relief with respect to it or
its debts under any bankruptcy, insolvency or other similar law now or hereafter
in effect or seeking the appointment of a trustee, receiver, liquidator,
custodian or other similar official of it or any substantial part of its
property or an order for relief is entered against the Maker under any such law,
and such involuntary case or order relief or other proceeding remains
undismissed and unstayed for a period of 45 days; or

        (c) a final judgment or order for the payment of money in excess of
$5,000,000 or its equivalent in other currencies is rendered against the Maker,
and the Maker does not discharge the same or provide for its discharge in
accordance with its terms, or procure a stay of execution thereof, within 45
days after the date of entry thereof and within such 45-day period (or such
longer period during which execution of such judgment is stayed) appeal
therefrom and cause the execution thereof to be stayed during such appeal; or

        (d) the Maker shall repudiate any of its obligations under this Note; or

                                       4

<PAGE>   5

        (e) any representation or warranty made by the Maker shall be inaccurate
in any material respect at the time such representation or warranty is made; or

        (f) the Maker shall fail to comply with any provision in Section 5; or

        (g) the Maker shall fail to comply with any other covenant contained
herein, and such failure is not rectified or cured within 30 days after notice
from the Holder or after the Maker acquires knowledge of such failure; or

        (h) the Maker shall default in the payment when due of any amount owing
under an instrument or agreement evidencing Indebtedness (as defined below) of
the Maker in excess of $20,000,000 or shall fail to perform any other obligation
under any such instrument if the effect of such failure is to cause or permit
the acceleration of such Indebtedness.

     "INDEBTEDNESS" means, with respect to any person:

          (i) the principal of and premium (if any) in respect of indebtedness
     of such person for money borrowed and indebtedness evidenced by notes,
     indentures, bonds, other similar instruments for the payment of which such
     person is responsible or liable;

          (ii) capital lease obligations of such person;

          (iii) obligations of such person issued or assumed as the deferred
     purchase price of property or services (other than trade payables incurred
     in the ordinary course of business);

          (iv) obligations of such person for the reimbursement of any obligor
     on any letter of credit or similar credit transaction;

          (v) all obligations under or in respect of interest rate protection or
     other hedging agreements;

          (vi) all obligations of the type referred to in clauses (i) through
     (v) above of third parties secured by any lien on any property or asset of
     such person;

          (vii) Indebtedness secured by any lien existing on property acquired
     by such person subject to such lien, whether or not the Indebtedness
     secured thereby shall have been assumed; and

          (viii) guarantees, endorsements and other obligations, whether or not
     contingent, in respect of, or agreements to purchase or otherwise acquire,
     Indebtedness of other persons.

     7. NO WAIVERS BY HOLDER. No failure on the part of the Holder to exercise,
and no delay in exercising, any right, power or remedy hereunder shall operate
as a waiver thereof, nor shall any single or partial exercise by the Holder of
any right, power, or remedy hereunder preclude any other or further exercise
thereof or the exercise of any right, power or remedy by the Holder. No
modification or waiver of any provision of this Note or consent to any departure
herefrom shall in any event be effective unless the same shall be in writing and
signed by the Holder, and then such waiver or consent shall be effective only in
the specific instance and for the purpose given. No notice or demand on the
Maker in any case shall, of itself, entitle the Maker to any other or further
notice or demand in similar or other circumstances.

                                       5

<PAGE>   6

     8. INDEMNIFICATION. The Maker will indemnify the Holder against, and on
demand reimburse the Holder and each of its officers, directors, employees and
agents (collectively, the "INDEMNIFIED PARTIES") for, any and all liabilities,
obligations, losses, damages, penalties, taxes, actions, judgments, costs,
expenses or disbursements of any kind or nature whatsoever, which may at any
time be imposed on, incurred by or asserted against any Indemnified Party
directly relating to or arising out of this Note; PROVIDED that the Maker shall
not be liable for any of the foregoing in respect of an Indemnified Party to the
extent they arise from the gross negligence or willful misconduct of such
Indemnified Party.

     9. WAIVERS BY MAKER. The Maker hereby expressly waives presentment for
payment, demand for payment, notice of dishonor, protest, notice of protest,
notice of nonpayment, and all lack of diligence or delays in collection or
enforcement of this Note.

     10. TRANSFERS AND ASSIGNMENTS. This Note may not be assigned by the Maker
and any purported assignment shall be null and void. The Holder may not transfer
this Note without the consent of the Maker; PROVIDED that the Holder may
transfer this Note to any affiliate without the Maker's consent; PROVIDED,
FURTHER that the Maker shall not be required to pay a greater amount under the
second sentence of Section 2(b), as a result of such a transfer to an affiliate
without the Maker's consent, than the Maker would have been required to pay
under the second sentence of Section 2(b) in the absence of such a transfer.
Following any transfer of this Note, the transferee shall be deemed to be the
"Holder" hereunder. "AFFILIATE" means any person or legal entity directly or
indirectly controlling, controlled by or under common control with such
specified person or legal entity.

     11. VALIDITY. If any part of this Note is contrary to, prohibited by or
deemed invalid under any applicable law or regulation, such provision shall be
inapplicable and deemed omitted to the extent so contrary, prohibited or
invalid, but the remainder hereof shall not be invalidated thereby and shall be
given full force and effect as far as possible. No provision of this Note shall
require the payment, or permit the collection, of interest in excess of the
highest rate permitted by law.

     12. NOTICES. All notices, consents, request, demands, offers, and other
communications required or permitted to be given pursuant to this Note shall be
in writing and shall be considered properly given and received when personally
delivered to the Maker or when sent by facsimile or by overnight courier, or
four (4) business days after being sent by certified mail, return receipt
requested, in a sealed envelope, with postage prepaid, addressed to: Unisphere
Networks, Inc., One Executive Drive, Chelmsford, MA, 01824, Attn: James A.
Dolce, Jr., Tel. (978) 848-0300, Fax (978) 848-0399; PROVIDED that any notice
sent by facsimile shall be promptly followed by a copy of such notice sent by
mail or overnight courier in the manner described herein.

     13. AMENDMENTS AND WAIVERS. Any provision of this Note may be amended or
waived if, but only if, such amendment or waiver is in writing and signed by the
Maker and the Holder.

     14. GOVERNING LAW. THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

                                       6

<PAGE>   7

     15. SUBMISSION TO JURISDICTION. The Maker hereby submits to the
jurisdiction of the United States District Court for the Southern District of
New York and of any New York state court sitting in New York City, with respect
to any action, suit or proceeding brought against it arising out of or relating
to this Note and the transactions contemplated hereby. The Maker hereby
irrevocably waives, to the fullest extent permitted by applicable law, any
objection which it may now or hereafter have to the laying of the venue of any
such action, suit or proceeding brought in such a court and any claim that any
such action, suit or proceeding brought in such a court has been brought in an
inconvenient forum.

                                         Unisphere Networks, Inc.

                                         By: /s/ Mark Nasiff
                                            ------------------------------
                                            Name: Mark Nasiff
                                            Title: Vice President of Finance
                                                   and Administration

                                       7

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