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                                                                    Exhibit 10.5

                              EMPLOYMENT AGREEMENT



         EMPLOYMENT AGREEMENT dated as of March 7, 1999, by and between Castle
Networks, Inc. with its principal place of business at 68 Tadmuck Road,
Westford, Massachusetts 01886 (the "Company"), and Thomas Burkardt, residing at
256 Abbott St., North Andover, MA 01845 (the "Executive").

         WHEREAS, pursuant to an Agreement and Plan of Merger dated as of March
7, 1999 (the "Merger Agreement") among Siemens Corporation, a Delaware
corporation ("Siemens"), Mediator Acquisition Corp., a Delaware corporation
("Acquisition"), and the Company, Acquisition has been merged with and into the
Company (the "Merger") and the Company is the surviving corporation in the
Merger.

         WHEREAS, the Executive's stock and other equity interests in the
Company have been acquired by Siemens in connection with the Merger.

         WHEREAS, the Executive has been employed by the Company, and the
Company wishes to offer continuing employment to the Executive in connection
with the Merger and the Executive wishes to accept such offer, on the terms set
forth below.

         Accordingly, the parties hereto agree as follows:

         1. TERM. The Company hereby employs the Executive, and the Executive
hereby accepts such employment, for an initial term commencing as of the Closing
date (as defined in the merger Agreement) and ending on the third anniversary of
such date, unless sooner terminated in accordance with the provisions of Section
4 or Section 5; with such employment to continue for successive one-year periods
in accordance with the terms of this Agreement (subject to termination as
aforesaid) unless either party notifies the other party in writing prior to 30
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before the expiration of the initial term and each annual renewal thereof (the
period during which the Executive is employed hereunder being hereinafter
referred to as the "Term"). Notwithstanding anything to the contrary contained
in this Agreement, in no event shall the Term commence, or any provision of this
Agreement be effective unless and until the Executive executes a Non-Competition
and Non-Solicitation Agreement and an Employee Patent and Secrecy Agreement
attached hereto as Exhibits A and B, respectively.

         2. DUTIES. During the Term, the Executive shall be employed by the
Company as Executive Vice President and Treasurer of the Company, and as such,
the Executive shall faithfully perform for the Company the duties of said office
and shall perform such other duties of an executive, managerial or
administrative nature consistent with such office as shall be specified and
designated from time to time by the Board of Directors of the Company. The
Executive shall devote substantially all of his business time and effort to the
performance of his duties hereunder.

         3.       COMPENSATION.

                  3.1 BASE SALARY. The Company shall pay the Executive during
the Term a base salary at the rate of $260,000 per annum (the "Annual Salary"),
in accordance with the customary payroll practices of the Company applicable to
senior executives. The Annual Salary shall be reviewed at least annually by the
Compensation Committee of the Board of Directors of the Company, but shall not
be decreased.

                  3.2 BONUS. During the Term, in addition to the Annual Salary,
the Executive shall participate in the Company's Milestone Incentive
Compensation Plan and shall have the opportunity to receive an annual bonus
based on the achievement of certain performance measures. Bonus at 40% as per
phone conversation with Marty Clague.


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                  3.3 BENEFITS - IN GENERAL. The Executive shall be entitled
during the Term to participate in any group life, hospitalization or disability
insurance plans, health programs, retirement plans, fringe benefit programs and
similar benefits that may be available to other senior executives of the Company
generally, on the same terms as such other executives, in each case to the
extent that the Executive is eligible under the terms of such plans or programs,
which plans and programs shall generally provide a level of benefits that is at
least as favorable as the benefits provided by the Company on the date hereof.

                  3.4 EXPENSES. The Company shall pay or reimburse the Executive
for all ordinary and reasonable out-of-pocket expenses actually incurred (and,
in the case of reimbursement, paid) by the Executive during the Term in the
performance of the Executive's services under this Agreement in accordance with
the ordinary policies of the Company with respect to reimbursement of senior
executives for such expenses.

         4. TERMINATION UPON DEATH OR DISABILITY. If the Executive dies during
the Term, the Term shall terminate as of the date of death, and the obligations
of the Company to or with respect to the Executive shall terminate in their
entirety upon such date except as otherwise provided under this Section 4. If
the Executive becomes disabled for purposes of the long-term disability plan of
the Company for which the Executive is eligible, the Company shall have the
right, to the extent permitted by law, to terminate the employment of the
Executive upon notice in writing to the Executive. Upon termination of
employment due to death or disability, (i) the Executive (or the Executive's
estate or beneficiaries in the case of the death of the Executive) shall be
entitled to receive any Annual Salary and other benefits earned and accrued
under this Agreement prior to the date of termination (and reimbursement under
this Agreement for expenses incurred prior to the date of termination), (ii) the
Executive shall have a non-forfeitable

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right to, and shall be entitled to receive, the consideration payable to the
Executive under the Merger Agreement with respect to which the Executive's
rights were not vested at Closing (as defined in the Merger Agreement) and which
has not yet been paid as of the date of termination, (iii) the Executive (or the
Executive's estate or beneficiaries in the case of the death of the Executive)
shall be entitled to receive all payments that he would have received under the
Company's Milestone Incentive Plan if he had remained employed through the
Performance Period (as defined in the Milestone Incentive Plan) and (iv) the
Executive (or, in the case of his death, his estate and beneficiaries) shall
have no further rights to any other compensation or benefits hereunder on or
after the termination of employment, or any other rights hereunder except as
required by law.

         5.       CERTAIN TERMINATIONS OF EMPLOYMENT.

                  5.1      TERMINATION BY THE COMPANY FOR CAUSE.

                           a.       For purposes of this Agreement, "Cause"
                                    shall mean the Executive's:

                                    (i)     commission  of a felony,  a crime of
moral turpitude, dishonesty, or any crime involving the Company;

                                    (ii) engagement in the performance of his
duties hereunder, or otherwise to the detriment of the Company, in willful
misconduct, willful or gross neglect, fraud, misappropriation or embezzlement;

                                    (iii) failure to follow directions of the
Board of Directors or to follow the Company's written policies and practices,
which failure is not cured within 30 days after written notice thereof is
provided to the Executive;

                                    (iv) breach of either of the agreements
attached hereto as Exhibits A and B, which breach is not cured within 30 days
after written notice thereof is provided to the Executive; or


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                                    (v)     breach in any  material  respect  of
the terms and provisions of this Agreement, which breach is not cured within 30
days after written notice thereof is provided to the Executive; provided that
the Company shall not be permitted to terminate the Executive for Cause pursuant
to Section 5.1(b) except on written notice given to the Executive.

                           b.       The Company may terminate the  Executive's
employment hereunder for Cause. If the Company terminates the Executive for
Cause, (i) the Executive shall receive Annual Salary and other benefits (but, in
all events, and without increasing the Executive's rights under any other
provision hereof, excluding any bonuses not yet paid) earned and accrued under
this Agreement prior to the termination of employment (and reimbursement under
this Agreement for expenses incurred prior to the termination of employment);
and (ii) the Executive shall have no further rights to any other compensation or
benefits hereunder on or after the termination of employment, or any other
rights hereunder.

                  5.2 TERMINATION BY THE COMPANY WITHOUT CAUSE; TERMINATION BY
THE EXECUTIVE FOR GOOD REASON.

                           a.       The  Company  may  terminate  the
Executive's employment at any time for any reason or no reason. If the Company
terminates the Executive's employment and the termination is not covered by
Section 4 or 5.1 or the Executive terminates service for "Good Reason", (i) the
Executive shall receive Annual Salary and other benefits earned and accrued
under this Agreement and the Company's Milestone Incentive Compensation Plan to
the extent benefits are earned, accrued and payable under the terms of such plan
prior to the termination of employment (and reimbursement under this Agreement
for expenses incurred prior to the termination of employment); (ii) the
Executive shall receive (A) a cash payment equal to 100% of the Executive's
Annual Salary (as in effect immediately before such termination), payable no
later than 15 days after such termination and (B) for a period of 12 months
after termination of employment such continuing coverage under the group health
plan and the

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basic life insurance plan as the Executive was receiving at the time of such
termination of employment at the same cost to the Executive as that paid by
active executive-level employees; (iii) the Executive shall have a
non-forfeitable right to, and shall be entitled to receive, the consideration
payable to the Executive under the Merger Agreement with respect to which the
Executive's rights were not vested at Closing (as defined in the Merger
Agreement) and which has not yet been paid as of the date of termination, (iv)
the Executive shall be entitled to receive all payments that he would have
received under the Company's Milestone Incentive Plan if he had remained
employed through the Performance Period (as defined in the Milestone Incentive
Plan), and (v) the Executive shall have no further rights to any other
compensation or benefits hereunder on or after the termination of employment, or
any other rights hereunder; provided that the Company's obligations with respect
to the payments and benefits provided for in this Section 5.2(a) are conditioned
upon the Executive's execution of a General Release in the standard form used by
the Company. It is expressly understood and agreed that any payment made
pursuant to this Section 5.2(a) shall be in lieu of any other payments that may
otherwise be due to the Executive under any severance or separation plan,
program or policy of the Company.

                           b. For purposes of this Agreement, "Good Reason"
shall mean (i) a breach by the Company in any material respect of the terms and
provisions of this Agreement, which breach is not cured within 30 days after
written notice thereof is provided by the Executive or (ii) the relocation of
the Company's principal place of business, without the consent of the Executive,
by more than 30 miles from such principal place of business on the date hereof.

                           c.       Notwithstanding  clause  (ii)(B) of the
second sentence of Section 5.2(a), (i) nothing herein shall restrict the ability
of the Company to amend or terminate the plans and programs referred to in such
clause (ii)(B) with respect to employees of the Company generally from time to
time in its sole discretion, and (ii) the Company shall in no event be required
to provide any benefits otherwise required by such clause (ii)(B) after such
time as the

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Executive becomes entitled to receive benefits of the same type from another
employer or recipient of the Executive's services (such entitlement being
determined without regard to any individual waivers or other similar
arrangements).

                  5.3      TERMINATION BY THE EXECUTIVE OTHER THAN FOR GOOD
                           REASON.

         The Executive may terminate his employment on at least 30 days' and not
more than 60 days' written notice given to the Company. If the Executive
terminates his employment and the termination is not covered by Section 4 or
5.2, (i) the Executive shall receive Annual Salary and other benefits (but, in
all events, and without increasing the Executive's rights under any other
provision hereof, excluding any bonuses not yet paid) earned and accrued under
this Agreement prior to the termination of employment (and reimbursement under
this Agreement for expenses incurred prior to the termination of employment);
and (ii) the Executive shall have no further rights to any other compensation or
benefits hereunder on or after the termination of employment, or any other
rights hereunder, except as required by law.

         6.       OTHER PROVISIONS.

                  6.1 SEVERABILITY. The Executive acknowledges and agrees that
he has had an opportunity to seek advice of counsel in connection with this
Agreement. If it is determined that any of the provisions of this Agreement, or
any part thereof, is invalid or unenforceable, the remainder of the provisions
of this Agreement shall not thereby be affected and shall be given full effect,
without regard to the invalid portions.

                  6.2      DISPUTE RESOLUTION.

                           a.       Any and all claims,  disputes,  and
controversies between the Executive and the Company with respect to
interpretation, construction, breach, enforceability, and/or enforcement of the
terms and provisions of this Agreement (a "Dispute") shall be finally

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resolved as provided in this Section 6.2 by binding arbitration. Arbitration
shall be the exclusive means for determination of all matters as above provided,
and neither party shall otherwise institute any action or proceeding in any
court of law or equity, state or federal, other than respecting enforcement of
the arbitrator's decision or award hereunder. The foregoing shall be a bona fide
defense in any action or proceeding where the matter in dispute was to be
arbitrated or is being arbitrated pursuant to this Agreement.

                           b.       The Company (or its  successor  in interest)
or the Executive shall have the right to submit a Dispute to arbitration, by
delivery to the other, by certified mail, of a written notice and demand for
arbitration of such Dispute. Arbitration shall be by the American Arbitration
Association (the "AAA") in accordance with its Rules applicable to such Disputes
(the "Rules"), by a neutral and impartial arbitrator acceptable to the Company
and the Executive. If such an arbitrator has not been selected by the Company
and the Executive within sixty days after AAA first provides a list of eligible
arbitrators, or within thirty days after the occurrence of a vacancy, a neutral
and impartial arbitrator shall be selected and appointed by the American
Arbitration Association, in accordance with its Rules. Unless otherwise required
under applicable law, the arbitration proceedings shall be conducted in the city
where the principal place of business of the Company is situated at the date of
this Agreement or a city mutually agreed to by the parties, and the procedural
rules of the place of arbitration shall apply. Each party shall be entitled to
be represented by legal counsel.

                           c.       The arbitration  proceedings  (including
discovery and the giving of testimony) shall be conducted in strictest
confidence pursuant to a confidentiality agreement signed by the parties and
devised to protect the confidentiality of and valuable rights of the Company in
the Confidential Company Information (as defined in the Employee Patent and

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Secrecy Agreement) and trade secrets as well as the confidentiality of any other
confidential information included in such proceedings. The arbitrator shall have
the power and authority to make such decisions and awards as he or she deems
appropriate, consistent with applicable law. To the extent applicable law sets
particular requirements for the conduct of such arbitration proceedings, such
as, any with respect to discovery, cross-examination, testimony, or availability
of rights and remedies, the arbitration proceedings shall be conducted in
compliance with those requirements.

                           d.       Subject to applicable  law, the arbitrator
may grant compensatory damages and costs to the prevailing party (but not
punitive or exemplary damages and attorneys' fees and costs related to punitive
or exemplary damages) and injunctions that he or she may deem necessary or
advisable directed to or against a party, including a direction or order
requiring specific performance of any covenant, agreement or provision of this
Agreement as a result of a breach or threatened breach. Any decision or award of
the arbitrator shall be final, binding, and conclusive upon the parties and said
decision and award may be entered as a final judgment in any court of competent
jurisdiction.

                           e.       Subject to the foregoing,  the costs of such
arbitration shall be borne equally by the parties, except that each party shall
bear its own attorneys fees and costs.

                  6.3 NOTICES. Any notice or other communication required or
permitted hereunder shall be in writing and shall be delivered personally,
telegraphed, telexed, sent by facsimile transmission or sent by certified,
registered or express mail, postage prepaid. Any such notice shall be deemed
given when so delivered personally, telegraphed, telexed or sent by facsimile
transmission or, if mailed, five days after the date of deposit in the United
States mails as follows:


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(i)     If to the Company, to:

         Castle Networks, Inc.
         68 Tadmuck Road
         Westford, Massachusetts 01886
         Attention: Board of Directors




         with a copy to:




         Rogers & Wells LLP
         200 Park Avenue
         New York, New York 10166
         Attention: John A. Healy, Esq.



(ii)     If to the Executive, to the address set
         forth above.

         with a copy to:



         Hale and Dorr LLP
         60 State Street
         Boston, Massachusetts 02109
         Attention: Mark G.  Borden, Esq.



Any such person may by notice given in accordance with this Section 6.3 to the
other parties hereto designate another address or person for receipt by such
person of notices hereunder.



                  6.4 ENTIRE AGREEMENT. Other than with respect to the
agreements attached hereto as Exhibits A and B entered into by the Company and
the Executive dated as of the date hereof, as they may be amended from time to
time, which agreements shall survive in their entirety without regard to this
Agreement, this Agreement contains the entire agreement between the parties with
respect to the subject matter hereof and supersedes all prior agreements,
written or oral, with respect thereto.

                  6.5 WAIVERS AND AMENDMENTS. This Agreement may be amended,
superseded, canceled, renewed or extended, and the terms hereof may be waived,
only by a

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written instrument signed by the parties or, in the case of a waiver, by the
party waiving compliance. No delay on the part of any party in exercising any
right, power or privilege hereunder shall operate as a waiver thereof, nor shall
any waiver on the part of any party of any such right, power or privilege nor
any single or partial exercise of any such right, power or privilege, preclude
any other or further exercise thereof or the exercise of any other such right,
power or privilege.

                  6.6 GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS
WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF LAW.

                  6.7 ASSIGNMENT. This Agreement, and the Executive's rights and
obligations hereunder, may not be assigned by the Executive; any purported
assignment by the Executive in violation hereof shall be null and void. In the
event of any sale, transfer or other disposition of all or substantially all of
the Company's assets or business, whether by merger, consolidation or otherwise,
the Company may assign this Agreement and its rights hereunder.

                  6.8 WITHHOLDING. The Company shall be entitled to withhold
from any payments or deemed payments any amount of tax withholding required by
law.

                  6.9 BINDING EFFECT. This Agreement shall be binding upon and
inure to the benefit of the parties and their respective successors, permitted
assigns, heirs, executors and legal representatives.

                  6.10 COUNTERPARTS. This Agreement may be executed by the
parties hereto in separate counterparts, each of which when so executed and
delivered shall be an original but all such counterparts together shall
constitute one and the same instrument. Each counterpart may consist of two
copies hereof each signed by one of the parties hereto.


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                  6.11 SURVIVAL. Anything contained in this Agreement to the
contrary notwithstanding, the provisions of Sections 6.2, 6.3 and 6.8, and the
other provisions of this Section 6 (to the extent necessary to effectuate the
survival of Sections 6.2, 6.3 and 6.8), shall survive termination of this
Agreement and any termination of the Executive's employment hereunder.

                  6.12 NON-DISPARAGEMENT. While Executive's non-competition
obligations under Exhibit A are in effect, the Executive shall not publish any
statement or make any statement under circumstances reasonably likely to become
public that is critical of the Company or any of its affiliates, or in any way
adversely affecting or otherwise maligning the Company's business or the
reputation of the Company or any of its affiliates.

                  6.13 EXISTING AGREEMENTS. The Executive represents to the
Company that he is not subject or a party to any employment or consulting
agreement, non-competition covenant or other agreement, covenant or
understanding which might prohibit him from executing this Agreement or limit
his ability to fulfill his responsibilities hereunder.

                  6.14 HEADINGS. The headings in this Agreement are for
reference only and shall not affect the interpretation of this Agreement.

                  6.15 EFFECTIVE DATE. Notwithstanding, anything to the contrary
herein, this Agreement shall become effective only upon the Effective Time (as
defined in the Merger Agreement), and this Agreement shall automatically be void
upon any termination of the Merger Agreement prior to the Effective Time.

         IN WITNESS WHEREOF, the parties hereto have signed their names as of
the day and year first above written.

                                            CASTLE NETWORKS, INC.



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                                            By:/s/ Stephen R. Kelly
                                               _______________________________

                                            Name:   Stephen R. Kelly
                                                 _____________________________


                                            Title:  Executive Vice President
                                                    and Treasurer
                                                  ____________________________


                                            By:  /s/ Thomas M. Burkardt
                                               _______________________________
                                                              Executive



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