<PAGE>

                                                                   EXHIBIT 10.34

                                                                  Execution Copy


                              STANDSTILL AGREEMENT

     This Standstill Agreement (the "Agreement"), dated as of June 1, 1999, is
between Advanced Radio Telecom Corp., a Delaware corporation (the "Company") and
each of the parties listed on Schedule I hereto (the "Purchasers").

     WHEREAS, simultaneously with the execution of this Agreement, the
Purchasers are entering into an agreement to purchase shares of the Company's
Series A and Series B Convertible Preferred Stock (the "Preferred Stock")
pursuant to the Stock Purchase Agreement (the "Purchase Agreement") among the
Company and the Purchasers dated as of the date hereof; and

     WHEREAS, the Company and each of the Purchasers desire to establish in this
Agreement certain conditions of such Purchaser's and such Purchaser's
Affiliates' relationship with Company;

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein and in the Agreement, the parties hereto agree as follows:

                                   ARTICLE I

                                  DEFINITIONS

     SECTION 1.01  Definitions.

          (a) "ACQUISITION PROPOSAL" shall mean a bona fide, written proposal,
     which proposal includes all material terms of a proposed transaction,
     received by the Board of Directors of the Company from any Person or Group
     proposing to enter into a transaction which, if effected, would constitute
     a Change of Control of the Company.

          (b) "AFFILIATE" shall have the meaning given it in Rule 12b-2 under
     the Exchange Act.

          (c) "ASSOCIATE" shall have the meaning given it in Rule 12b-2 under
     the Exchange Act.

          (d) "BENEFICIAL OWNER" shall have the meaning given it in Rule
     13(d)(3) under the Exchange Act; and "Beneficially Own" and "Beneficial
     Ownership" shall apply to securities held by a Beneficial Owner.
<PAGE>

          (e) "CHANGE OF CONTROL" shall mean (1) the acquisition by a Third
     Party of more than 50% of the Company's then outstanding Voting Stock,
     excluding however, a purchase agreement with an underwriter or group of
     underwriters in a registered public offering to the public; (2) the
     consummation of a merger, acquisition, consolidation or reorganization or
     series of such related transactions involving the Company, unless both (x)
     immediately after such transaction or transactions, the stockholders of the
     Company immediately prior to such transaction shall Beneficially Own at
     least 50% of the outstanding Voting Stock of the Company (or, if the
     Company shall not be the surviving company in such merger, consolidation or
     reorganization, the Voting Stock of the surviving corporation issued in
     such transaction in respect of Voting Stock of the Company shall represent
     at least 50% of the Voting Stock of such surviving company), and (y) the
     Company is not subject to an agreement that provides that individuals who
     are directors of the Company immediately prior to such transaction (or
     individuals designated by the Company at or before the closing of such
     transaction) shall constitute less than a majority of the directors of the
     Company (or such surviving company, as the case may be) after the closing
     of such transaction; (3) a change or changes in the membership of the
     Company's Board of Directors which represents a change of a majority or
     more of such membership during any twelve month period (unless such change
     or changes in membership are caused by the actions of the then-existing
     Board of Directors); (4) an Insolvency Proceeding (as defined below); or
     (5) the consummation of a sale of all or substantially all of the Company's
     assets unless immediately after such transaction, the stockholders of the
     Company immediately prior to such transaction shall beneficially own at
     least 50% of the Voting Stock of the acquiring company.

          (f) "EXCHANGE ACT" shall mean the Securities Exchange Act of 1934, as
     amended.

          (g) "GROUP" shall have the meaning provided in Section 13(d)(3) of the
     Exchange Act.

          (h) "INSOLVENCY PROCEEDING" shall mean (1) an assignment for the
     benefit of creditors, (2) the filing by the Company of a petition to have
     the Company adjudged insolvent, bankrupt or seeking a reorganization or
     liquidation under any law relating to bankruptcy, insolvency or
     receivership, (3) an appointment of a receiver or trustee for all or
     substantially all of the assets of the Company unless appointed without the
     Company's consent, in which case if after 60 days such appointment has not
     been vacated or stayed, (4) a public admission in writing of the Company's
     inability to pay its debts as they come due, or (5) the adoption of a plan
     of liquidation or dissolution by the Board of Directors of the Company.

          (i) "PERSON" means an individual, corporation, partnership,
     association, trust, unincorporated organization or other entity.

                                      -2-
<PAGE>

          (j) "STOCKHOLDERS AGREEMENT" shall mean the Stockholders Agreement in
     the form attached as Exhibit A, and as amended with the agreement of the
     Company.

          (k) "THIRD PARTY" shall mean any Person (other than any Purchaser and
     its Affiliates and Associates) or Group (other than any Group that includes
     any Purchaser or its Affiliates or Associates).

          (l) "TOTAL VOTING POWER" at any date, with respect to any Person,
     shall mean the total combined Voting Power of all the Voting Stock of such
     Person then outstanding and entitled to vote.

          (m) "TWO-THIRDS IN INTEREST" means, prior to the closing of the
     transactions contemplated by the Purchase Agreement, Purchasers (and
     permitted assignees) who have agreed to purchase more than 66 2/3% of the
     shares of Preferred Stock pursuant to the Purchase Agreement and, on and
     after the closing of the transactions contemplated by the Purchase
     Agreement, Purchasers (and transferees, pursuant to Section 4.01(a) hereof)
     holding more than 66 2/3% of the shares of Preferred Stock purchased
     pursuant to the Purchase Agreement then owned by the Purchasers.

          (n) "VOTING POWER" with respect to any Voting Securities of any Person
     on any date shall mean the voting power in the general election of
     directors of the relevant Person to which such Voting Securities would be
     entitled on such date.

          (o) "VOTING STOCK" of any Person shall mean any securities entitled to
     vote generally in the election of directors of such Person, or any direct
     or indirect rights or options or warrants to acquire any such securities or
     any securities (including, without limitation, the Preferred Stock)
     convertible or exercisable into or exchangeable for such securities,
     whether or not such securities are so convertible, exercisable or
     exchangeable at the time of determination.

                                   ARTICLE II

                                      TERM

     SECTION 2.01  Term.  The term (the "Term") of this Agreement shall commence
on the date hereof and shall continue until the earliest to occur of the
following:

          (a) the fourth anniversary of the Closing under the Purchase
     Agreement;

          (b) the termination of the Purchase Agreement prior to the Closing in
     accordance with its terms; and

                                      -3-
<PAGE>

          (c) the date, following the Closing, on which the Company has no
     further obligations under Section 4.9 of the Purchase Agreement with
     respect to the election to the Company's Board of Directors of any nominee
     of UST or Oak Investment Partners and no nominee of UST or Oak Investment
     Partners designated and elected to the Company's Board of Directors
     pursuant to the provisions of Section 4.9 of the Purchase Agreement is a
     director of the Company
 .
                                  ARTICLE III

                             STANDSTILL PROVISIONS

     SECTION 3.01  Restrictions of Certain Actions.  Each of the Purchasers
hereby severally agrees that during the Term, neither it nor any Affiliate or
Associate of such Purchaser will, singly or as part of a Group, directly or
indirectly:

          (a) acquire or offer, make a proposal or agree to acquire (whether
     publicly or otherwise), in any manner, any material assets of the Company
     or its subsidiaries or any equity securities of the Company or its
     subsidiaries (or Beneficial Ownership thereof), except (1) pursuant to a
     stock split, stock dividend, recapitalization, reclassification or similar
     transaction not effected  pursuant to a violation of this Section 3.01, (2)
     upon the conversion of the Preferred Stock into Common Stock in accordance
     with the Company's Certificate of Incorporation, (3) pursuant to the Rights
     Agreement dated as of June 20, 1997 between the Company and BankBoston,
     N.A. or (4) pursuant to the exercise of rights pursuant to Section 4.16 of
     the Purchase Agreement.

          (b) make or in any way propose or participate in any "solicitation" of
     "proxies" to vote (as such terms are defined in Rule 14a-1 under the
     Exchange Act), solicit any consent or communicate with or seek to advise or
     influence any Person, other than the Company, with respect to the
     solicitation or voting of any Voting Securities of the Company in
     opposition to any matter that has been recommended by the Board or in favor
     of any matter that has not been approved by the Board, or become a
     "participant" in any "election contest" (as such terms are defined or used
     in Rule 14a-11 under the Exchange Act) with respect to Company except
     pursuant to the Stockholders Agreement.

          (c) form, be a member of, join or encourage the formation of, any
     Group with respect to any Voting Securities of the Company or the
     acquisition of any assets of the Company other than any such Group
     resulting from, and solely to the extent set forth in, the Stockholders
     Agreement;

          (d) deposit any Voting Stock of the Company into a voting trust or
     subject any such Voting Stock to any arrangement or agreement with respect
     to the voting thereof other than the Stockholders Agreement;

                                      -4-
<PAGE>

          (e) seek election to or seek to place a representative on the Board of
     Directors of the Company (except pursuant to Section 4.10 of the Purchase
     Agreement or Section 4.4 of the Certificate of Designation (the
     "Certificate") relating to the Preferred Stock and except for any chief
     executive officer of the Company) or seek the removal of any member of the
     Board of Directors the Company other than pursuant to the Stockholders
     Agreement or the Certificate;

          (f) call or seek to have called any meeting of the stockholders of
     Company other than participation as a director of the Company in calling,
     or seeking to have called, meetings of stockholders generally;

          (g) solicit, seek to effect, negotiate with or provide any information
     to any other party with respect to, or make any statement or proposal,
     whether written or oral, to the Board of Directors of Company or otherwise
     make any public announcement or proposal whatsoever with respect to a
     merger or acquisition of the Company the sale of all or a substantial
     portion of the assets of the Company and its subsidiaries, liquidation of
     the Company, recapitalization of the Company or similar business
     transactions with respect to the Company or take any action which might
     require either party to make a public announcement with respect to any such
     matters (the foregoing shall not limit the Purchaser from discussing any
     Third Party Acquisition Proposal with, or providing any information with
     respect thereto to, the Company or other Purchasers); or

          (h) instigate, encourage or assist, or enter into any discussions or
     arrangements with, any Third Party to do any of the actions described in
     Sections 3.01(a) through (h) (the foregoing shall not limit the Purchaser
     from discussing any Third Party Acquisition Proposal with the Company or
     other Purchasers);

          (i) If any Purchaser or any of its Affiliates or Associates owns or
     acquires any Voting Securities in violation of this Agreement, such Voting
     Securities shall immediately be disposed of to persons who are not
     Affiliates or Associates thereof but only in compliance with the provisions
     of this Section 3.01 and Section 4.01; provided, however, that Company may
     also pursue any other available remedy to which it may be entitled as a
     result of such violation.

Notwithstanding the restrictions contained in this Section 3.01, the Purchasers
shall not be prevented from complying with the requirements of Sections 13(d)
and 16(a) of the Exchange Act and the rules and regulations thereunder, in each
case, as from time to time in effect, or any successor provisions or rules with
respect thereto, or any other applicable law or rule or regulation of any
governmental body.  Notwithstanding the restrictions contained in this Section
3.01, if, prior to the Closing (as defined in the Purchase Agreement), the
Company receives an Acquisition Proposal from a Third Party and the Company (i)
publicly announces that it is considering the Acquisition Proposal or that it is
engaged in discussions with respect to a sale or other transaction involving a
Change of Control, (ii) withdraws or modifies its approval of the Stock Purchase
(as

                                      -5-
<PAGE>

defined in the Purchase Agreement) or (iii) notifies the Purchasers of its
intent to terminate the Purchase Agreement pursuant to Section 6.1 (f) of the
Purchase Agreement (each of the foregoing is a "Limited Trigger") then the
Purchasers shall be free to take any action otherwise prohibited by Sections
3.01(a) (but limited to any acquisition from, offer to, proposal to or agreement
with the Company or its subsidiaries), 3.01(b), 3.01(c), 3.01(g) or 3.01(h) in
contemplation of, and to make, an Acquisition Proposal to be made during the
period from the occurrence of the Limited Trigger until the Company notifies the
Purchasers that the Company has rejected such Acquisition Proposal.

     SECTION 3.02  Suspension of Restrictions. The limitations provided in
Section 3.01 and Section 4.01 shall immediately be suspended upon the occurrence
of any of the following events.

          (a) any Third Party commences a tender or exchange offer seeking to
     acquire Beneficial Ownership of 50% or more of the outstanding shares of
     Voting Stock, but only if (i) the Company has not within 10 days after
     commencement of such offer (or such longer period as may then be permitted
     under applicable law for the Company's initial recommendation with respect
     to such offer), publicly recommended that such offer not be accepted, or
     (ii) all of the material conditions to such offer relating to the
     elimination or satisfaction of the material defensive provisions
     established by the Company, including any rights plan or similar defensive
     provision of the Company have been satisfied or waived;

          (b) the Company's receipt of an Acquisition Proposal from any Third
     Party but only if the Company has not, within 15 days after such receipt,
     rejected such Acquisition Proposal;

          (c) the occurrence of a Change of Control of the Company;

          (d) the public announcement by the Company that it is "for sale";

          (e) the execution of a definitive agreement which, if consummated,
     would result in a Change of Control of the Company;

          (f) the public announcement by or on behalf of any Person or Group
     (other than the Purchaser and its Affiliates) of the commencement of a bona
     fide proxy or consent solicitation subject to Section 14 of the Exchange
     Act (or any successor provision) to elect or remove a majority of the
     directors of the Company which is not, within 10 days after the
     announcement of such proxy or consent solicitation (or such longer period
     as may then be permitted under applicable law for the Company's initial
     recommendation with respect to such contest if such a period is specified)
     publicly opposed by the Company's Board of Directors and which would, if
     successful, result in a change in the composition of a majority of the
     Board of Directors of the Company; or

                                      -6-
<PAGE>

          (g) the adoption by the Board of Directors of a plan of liquidation or
     dissolution.

     The Company shall provide the Purchaser with prompt written notice of the
occurrence of any of the events set forth in this Section 3.01 or of the receipt
by the Company of an Acquisition Proposal from any Third Party (such notice to
be provided within ten days after receipt thereof, but without disclosing the
terms thereof or the identity of such Third Party).  Upon any (i) withdrawal or
lapsing of any such tender or exchange offer referred to in Section 3.02(a) in
which such Third Party does not acquire more than 15% of the outstanding Voting
Stock of the Company, (ii) withdrawal, rejection or termination of an
Acquisition Proposal referred to in Section 3.02(b), (iii) the public withdrawal
of any "for sale" notice referred to in Section 3.02(d), (iv) the termination of
the agreement referred in Section 3.02(e) without consummation thereof, (v)  the
withdrawal or termination or failure of the solicitation referred to in Section
3.02(f) or (vi) the termination of the plan of liquidation referenced in Section
3.02(g), as the case may be, the limitations provided in Sections 3.01 and 4.01
(except to the extent then suspended as a result of any other event specified in
Section 3.02) shall again be applicable for so long as and only to the extent
provided therein without any extension of the term thereof.

                                   ARTICLE IV

                             TRANSFER RESTRICTIONS

     SECTION 4.01  Permitted Transfers.  During the Term, but only until the
second anniversary of the Closing under the Purchase Agreement, the Purchaser
shall not sell, pledge, hypothecate, assign or otherwise transfer (each a
"Transfer") any Voting Securities of the Company other than the following
Transfers:

          (a) A Transfer to an Affiliate of such Purchaser, provided that such
     Affiliate becomes a party to, and agrees to be bound by, this Agreement;

          (b) A Transfer by partnerships by way of distribution to a limited
     partner or former limited partner of such Purchaser that is not an
     Affiliate of such Purchaser;

          (c) A private Transfer (i) to any "person" (within the meaning of
     Section 13(d)(3) of the Exchange Act), that is not an Affiliate or
     Associate of such Purchaser, which to the knowledge of the Purchaser after
     inquiry beneficially owns or, as a result of such sale or transfer, will
     beneficially own less than ten percent (10%) of the Total Voting Power of
     the Company (a "Permitted Transferee"), provided, that such person will not
     be Permitted Transferee and no such transfer shall be permitted if such
     person, has proposed a business combination or similar transaction with, or
     a Change of Control of the Company or (ii) to an account managed by an
     institutional manager described in Rule 13f-1 of the Exchange Act with
     respect to which the transferred Voting Securities would constitute
     "Section 13(f) securities" within the meaning of Rule 13f-1(c) of the
     Exchange Act; and

                                      -7-
<PAGE>

          (d) a sale to the public (i) pursuant to Rule 144 of the Securities
     Act or (ii) pursuant to the exercise by the Purchasers of their rights
     under the Registration Rights Agreement.

                                   ARTICLE V

                                 MISCELLANEOUS

     SECTION 5.01  Enforcement. Each of the Purchasers, on the one hand, and
Company, on the other, acknowledge and agree that irreparable damage would occur
if any of the provisions of this Agreement were not performed in accordance with
their specific terms or were otherwise breached.  Accordingly, the parties will
be entitled to an injunction or injunctions to prevent breaches of this
Agreement and to enforce specifically its provisions in any court having
jurisdiction, this being in addition to any other remedy to which they may be
entitled at law or in equity.

     SECTION 5.02  Entire Agreement; Waivers. This Agreement, the Purchase
Agreement, the Stockholder Agreement and the Registration Rights Agreement
constitute the entire agreement among the parties hereto pertaining to the
subject matter hereof and thereof and supersede all prior and contemporaneous
agreements, understandings, negotiations and discussions, whether oral or
written, of the parties with respect to such subject matter.  No waiver of any
provision of this Agreement shall be deemed or shall constitute a waiver of any
other provision hereof (whether or not similar), shall constitute a continuing
waiver unless otherwise expressly provided nor shall be effective unless in
writing and executed (i) in the case of a waiver by Company, by the Company and
(ii) in the case of a waiver by the Purchasers, by "Two-Thirds in Interest" of
the Purchasers.

     SECTION 5.03  Amendment or Modification.  The parties hereto may not amend
or modify this Agreement except in such manner as may be agreed upon by a
written instrument executed by the Company and Two-Thirds in Interest of the
Purchasers.

     SECTION 5.04  Successors and Assigns.  All the terms and provisions of this
Agreement shall be binding upon and shall inure to the benefit of the parties
hereto and their respective transferees, successors and assigns (each of which
such transferees, successors and assigns shall be deemed to be a party hereto
for all purposes hereof); provided, however, that (i) neither Company nor any
Purchaser may assign or transfer any of its rights or obligations hereunder
without the prior written consent of the Two-Thirds in Interest of the
Purchasers or the Company, respectively and (ii) no transfer or assignment by
any party shall relieve such party of any of its obligations hereunder.

     SECTION 5.05  Severability.  If any provision of this Agreement is held by
a court of competent jurisdiction to be unenforceable, the remaining provisions
shall remain in full force and effect. It is declared to be the intention of the
parties that they would have executed the remaining

                                      -8-
<PAGE>

provisions without including any that may be declared unenforceable.

     SECTION 5.06  Headings.  Descriptive headings are for convenience only and
will not control or affect the meaning or construction of any provision of this
Agreement.

     SECTION 5.07  Counterparts.  For the convenience of the parties, any number
of counterparts of this Agreement may be executed by the parties, and each such
executed counterpart will be an original instrument.

     SECTION 5.08  Notices.  Any notices or other communications required or
permitted hereunder shall be sufficiently given if in writing (including
telecopy or similar teletransmission), addressed as follows:

                  If to the Company, to:

                  Advanced Radio Telecom Corp.
                  500 108th Avenue NE, Suite 2600
                  Bellevue, WA  98004
                  Attn: Thomas M. Walker, Esq., General Counsel
                  Telecopier No: (425) 990-1642 or (425) 688-0703
                  Telephone No:  (425) 990-1669

                  with a copy to:

                  Ropes & Gray
                  One International Place
                  Boston, MA  02110
                  Attn: Mary E. Weber, Esq.
                  Telecopier No: (617) 951-7050
                  Telephone No: (617) 951-7391

          (a) if to the Purchasers, to such address listed on Schedule I to the
     Purchase Agreement, with a copy to:

                  Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP
                  155 Constitution Drive
                  Menlo Park, CA  94025
                  Attn:  Brooks Stough, Esq.
                  Telecopier No:  (650) 321-2800
                  Telephone No:  (650) 463-5370

                  O'Melveny & Myers, LLP
                  1999 Avenue of the Stars

                                      -9-
<PAGE>

                  Los Angeles, California  90067-6035
                  Attn:  Steven Grossman, Esq.
                  Telecopier No: (310) 246-6779
                  Telephone No:  (310) 553-6700

Unless otherwise specified herein, such notices or other communications shall be
deemed received (a) in the case of any notice or communication sent other than
by mail, on the date actually delivered to such address (evidenced, in the case
of delivery by overnight courier, by confirmation of delivery from the overnight
courier service making such delivery, and in the case of a telecopy, by receipt
of a transmission confirmation form or the addressee's confirmation of receipt),
or (b) in the case of any notice or communication sent by mail, three business
days after being sent, if sent by registered or certified mail, with first-class
postage prepaid.  Each of the parties hereto shall be entitled to specify a
different address by giving notice as aforesaid to each of the other parties
hereto.

     SECTION 5.09   Governing Law.  This Agreement shall be governed by and
construed in accordance with the domestic substantive law of the State of New
York, without giving effect to any choice or conflict of law provision or rule
that would cause the application of the law of any other jurisdiction.

     SECTION 5.10  Termination.  This Agreement will terminate at the end of the
Term or earlier upon the written approval of the Company and Two-Thirds in
Interest of the Purchasers.

     SECTION 5.11  Fiduciary Duties.  Notwithstanding the restrictions set forth
herein, any director of the Company may exercise his fiduciary duties in his
capacity as a director with respect to the Company, as opposed to taking action
with respect to the direct or indirect ownership of any Voting Stock, and no
such exercise of fiduciary duties shall be deemed to be a breach of or a
violation of the restrictions set forth herein, and none of the Purchasers shall
have any liability hereunder for any such exercise of fiduciary duties by such
director in his capacity as a director of the Company.



                    [Remainder of Page Intentionally Blank]

                                      -10-
<PAGE>

                                                          [Standstill Agreement]

     IN WITNESS WHEREOF, the Company and the Purchasers have caused this
Agreement to be executed as of the date first above written by their respective
officers thereunto duly authorized.



The Company:                  ADVANCED RADIO TELECOM CORP.


                              By: /s/ Henry C. Hirsch
                                  -------------------------------------------
                                  Name: Henry C. Hirsch
                                  Title:   Chairman and CEO


The Purchasers:               U.S. TELESOURCE, INC.

                              By: /s/ Marc B. Weisberg
                                  -------------------------------------------
                                  Name: Marc B. Weisberg
                                  Title:


                              OAK INVESTMENT PARTNERS VIII, LIMITED PARTNERSHIP

                              By:  Oak Associates VIII, LLC, General Partner

                              /s/ Bandel L. Carano
                              -----------------------------------------------
                              By:  Bandel L. Carano, Managing Member


                              OAK VIII AFFILIATE FUND, LIMITED PARTNERSHIP

                              By:  Oak VIII Affiliates, LLC, General Partner

                              /s/ Bandel L. Carano
                              ----------------------------------------------
                              By:  Bandel L. Carano, Managing Member



<PAGE>

                                                          [Standstill Agreement]

                              MERITECH CAPITAL PARTNERS

                              By:
                                 ---------------------------
                                  Name:
                                  Title:


<PAGE>

                                                          [Standstill Agreement]

                              ACCEL VI L.P.
                              BY:  ACCEL VI ASSOCIATES L.L.C.
                              ITS GENERAL PARTNER

                              By: /s/ G. Carter Sednaoui
                                 --------------------------------------------
                                 Managing Member


                              ACCEL INTERNET FUND II L.P.
                              BY:  ACCEL INTERNET FUND II ASSOCIATES L.L.C.
                              ITS GENERAL PARTNER

                              By: /s/ G. Carter Sednaoui
                                 --------------------------------------------
                                 Managing Member


                              ACCEL KEIRETSU VI L.P.
                              BY:  ACCEL KEIRETSU VI ASSOCIATES L.L.C.
                              ITS GENERAL PARTNER

                              By: /s/ G. Carter Sednaoui
                                 --------------------------------------------
                                 Managing Member


                              ACCEL INVESTORS '98 L.P.

                              By: /s/ G. Carter Sednaoui
                                 -------------------------------------------
                                 General Partner


<PAGE>

                                                          [Standstill Agreement]



                              BRENTWOOD ASSOCIATES IX, L.P.

                              By:  Brentwood IX Ventures, L.L.C.
                              Its General Partners

                              By: /s/ John L. Walecka
                                  -------------------------------------------
                                 Name:  John L. Walecka
                                 Title:  Managing Member


                              BRENTWOOD AFFILIATES FUND IX, L.P.

                              By:  Brentwood IX Ventures, L.L.C.
                              Its General Partner

                              By: /s/ John L. Walecka
                                  -------------------------------------------
                                 Name:  John L. Walecka
                                 Title:  Managing Member


<PAGE>

                                                          [Standstill Agreement]

                              WORLDVIEW TECHNOLOGY PARTNERS II, L.P.
                              By: Worldview Capital II, L.P., General Partner
                              By: Worldview Equity I, L.L.C., General Partner

                              By: /s/ James Wei
                                  -------------------------------------------
                                  Name:  James Wei
                                  Title:  Member


                              WORLDVIEW TECHNOLOGY INTERNATIONAL II, L.P.
                              By: Worldview Capital II, L.P., General Partner
                              By: Worldview Equity I, L.L.C., General Partner

                              By: /s/ James Wei
                                  --------------------------------------------
                                  Name:  James Wei
                                  Title:  Member


                              WORLDVIEW STRATEGIC PARTNERS II, L.P.
                              By: Worldview Capital II, L.P., General Partner
                              By: Worldview Equity I, L.L.C., General Partner

                              By: /s/ James Wei
                                  -------------------------------------------
                                  Name:  James Wei
                                  Title:  Member


<PAGE>

                                                          [Standstill Agreement]

                              BESSEMER VENTURE PARTNERS IV L.P.
                              By: Deer IV & Co. LLC, General Partner

                              By: /s/ Robert H. Buescher
                                  -------------------------------------------
                                  Name:  Robert H. Buescher
                                  Title:  Manager


                              BESSEC VENTURES IV L.P.
                              By: Deer IV & Co. LLC, General Partner

                              By: /s/ Robert H. Buescher
                                  -------------------------------------------
                                  Name:  Robert H. Buescher
                                  Title:  Manager


                              COVE VENTURES, LLC
                              By: Cove Road Associates, LLC, Managing Member

                              By: /s/ Robert Goodman
                                  -------------------------------------------
                                  Name:  Robert Goodman
                                  Title:  Managing Member



<PAGE>
                                                          [Standstill Agreement]


                              ADAMS CAPITAL MANAGEMENT, L.P.
                              By: ACM Capital Partners II, L.P., General Partner
                              By: Joel P. Adams, General Partner

                              By: /s/ Joel P. Adams
                                  -------------------------------------------
                                  Name:  Joel P. Adams
                                  Title:  General Partner