<PAGE>
 
                                                                   EXHIBIT 10.45

                      SIXTH AMENDMENT TO CREDIT AGREEMENT


          THIS SIXTH AMENDMENT TO CREDIT AGREEMENT ("Sixth Amendment") is
                                                     ---------------     
effective as of March  31, 1999, and is entered into by and among P-COM, INC., a
Delaware corporation ("Borrower"); the financial institutions signatory hereto
                       --------                                               
(each individually a "Lender" and collectively the "Lenders"); UNION BANK OF
                      ------                        -------                 
CALIFORNIA, N.A., for itself, as a Lender, and as administrative agent for
Lenders (in such capacity, "Agent"); and BANK OF AMERICA NATIONAL TRUST AND
                            -----                                          
SAVINGS ASSOCIATION (for itself, as a Lender, and as syndication agent for
Lenders (in such capacity, "Syndication Agent").
                            -----------------   

                                 RECITALS
                                 --------

          A.  Borrower, Lenders, Agent, and Syndication Agent have entered into
that certain Credit Agreement dated as of May 15, 1998, as amended
(collectively, the "Credit Agreement"), pursuant to which Agent, Syndication
                    ----------------                                        
Agent, and Lenders agreed to provide financial accommodations to or for the
benefit of Borrower upon the terms and conditions contained therein.  Unless
otherwise defined herein, capitalized terms or matters of construction defined
or established in the Credit Agreement shall be applied herein as defined or
established therein.

          B.  Borrower has requested that Agent, Syndication Agent, and Lenders
make certain amendments to the Credit Agreement, and Agent, Syndication Agent,
and Lenders are willing to do so subject to the terms and conditions of this
Sixth Amendment.

                                 AGREEMENT
                                 ---------

          NOW, THEREFORE, in consideration of the continued performance by
Borrower of its promises and obligations under the Credit Agreement and the
other Loan Documents, and for other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, Borrower, Lenders, Agent, and
Syndication Agent hereby agree as follows:

          1.  Ratification and Incorporation of Credit Agreement and Other Loan
              -----------------------------------------------------------------
Documents.  Except as expressly modified under this Sixth Amendment, (a)
---------                                                               
Borrower hereby acknowledges, confirms, and ratifies all of the terms and
conditions set forth in, and all of its obligations under, the Credit Agreement
and the other Loan Documents, and (b) all of the terms and conditions set forth
in the Credit Agreement and the other Loan Documents are incorporated herein by
this reference as if set forth in full herein.  Without limiting the generality
of the foregoing, each of Borrower and each other Loan Party acknowledges and
agrees that as of March 31, 1999, the sum of (i) the aggregate outstanding
principal amount of all Loans, plus (ii) the aggregate outstanding Letter of
                               ----                                         
Credit Usage was $49,750,000.00.  Each of Borrower and each other Loan Party
represents that it has no offset, defense, counterclaim, dispute or disagreement
of any kind or nature whatsoever with respect to the amount of such Debt.

                                       1
<PAGE>
 
          2.  Amendments to Credit Agreement.
              ------------------------------ 

              a.  The definition of "Applicable Margin" is hereby deleted in its
                                     -----------------                          
entirety and the following is substituted in lieu thereof:

                  "Applicable Margin":  three percent (3%).
                   -----------------                       

              b.  The definition of "Maturity Date" is hereby deleted in its
                                     -------------                          
entirety and the following is substituted in lieu thereof:

                  "Maturity Date":  January 15, 2000.
                   -------------                     

              c.  The definition of "Revolving Commitment" is hereby deleted 
                                     --------------------     
in its entirety and the following is substituted in lieu  thereof:

                  "Revolving Commitment":  (a) $50,000,000, at all times prior
                     --------------------                                       
          to August 15, 1999; (b) $40,000,000, at all times on and after August
          15, 1999, and prior to October 15, 1999; and (c) $30,000,000, at all
          times on and after October 15, 1999, in each case as such amount may
          be further reduced (i) pursuant to Section 2.1(e) and (ii) by the
          amount of any Letters of Credit that expire and are not renewed during
          the period from April 21, 1999, through the Maturity Date.

              d.  The following new definitions are hereby added to the Credit
Agreement in appropriate alphabetical order:

                  "Comerica Receivables Purchase Agreement":  the Receivables
                   ---------------------------------------                   
          Purchase Agreement by and between Borrower and Comerica Bank-
          California dated as of September 30, 1998.

                  "Consolidated Pre-Tax Income":  for any period, the net
                   ---------------------------                           
          income (or loss) prior to (a) income taxes and (b) any non-cash
          extraordinary or non-cash non-recurring losses or gains (but including
          cash losses or gains) for such period of the Borrower and its
          consolidated subsidiaries on a consolidated basis determined in
          accordance with GAAP.

                  "Indenture":  the Indenture dated as of November 1, 1997,
                   ---------                                               
          between Borrower and State Street Bank and Trust Company of
          California, N.A., as Trustee, relating to Borrower's 4-1/4%
          Convertible Subordinated Notes due 2002.

                  "Purchased Receivables":  all accounts receivable
                   ---------------------                           
          transferred to a Qualified Financial Institution pursuant to a
          Receivables Facility, including 

                                       2
<PAGE>
 
          "Purchase Receivables" (as defined in the Comerica Receivables
          Purchase Agreement).

                    "Third Party Interactives": all Persons with whom Borrower
                     ------------------------                                 
          exchanges data electronically in the ordinary course of business,
          including customers, suppliers, third-party vendors, subcontractors,
          processors-converters, shippers and warehousemen.

                    "Year 2000 Date-Sensitive System/Component": any system
                     -----------------------------------------             
          software, network software, applications software, data base, computer
          file, embedded microchip, firmware or hardware that accepts, creates,
          manipulates, sorts, sequences, calculates, compares or outputs
          calendar-related data accurately; such systems and components shall
          include mainframe computers, file server/client systems, computer
          workstations, routers, hubs, other network-related hardware, and other
          computer-related software, firmware or hardware and information
          processing and delivery systems of any kind and telecommunications
          systems and other communications processors, security systems, alarms,
          elevators and HVAC systems.

               e.  The following new Section 2.5(i) is hereby added to the
                                     --------------                       
Credit Agreement:

                    (i) Cash Collateral.  If any Letters of Credit, whether or
                        ---------------                                       
          not then due and payable, shall for any reason be outstanding on the
          Maturity Date, Borrower shall either (1) provide cash collateral
          therefor in the manner described in the following sentence, (2) cause
          all such Letters of Credit and guaranties thereof to be cancelled and
          returned, or (3) deliver a stand-by letter (or letters) of credit in
          guarantee of such Letters of Credit, which stand-by letter (or
          letters) of credit shall be of like tenor and duration as, and in an
          amount equal to 110% of the aggregate maximum amount then available to
          be drawn under, the Letters of Credit and shall be issued by a Person,
          and shall be subject to such terms and conditions, as are be
          satisfactory to Agent in its sole discretion.  If Borrower elects to
          provide cash collateral, then Borrower will pay to Agent, for the
          benefit of Lenders, cash or cash equivalents acceptable to Agent
          ("Cash Equivalents") in an amount equal to 110% of the maximum amount
          ------------------                                                   
          then available to be drawn under each applicable Letter of Credit
          outstanding.  Such funds or Cash Equivalents shall be held by Agent in
          a cash collateral account (the "Cash Collateral Account") maintained
                                          -----------------------             
          at Agent or another bank or financial institution acceptable to Agent.
          The Cash Collateral Account shall be in the name of Borrower and shall
          be pledged to, and subject to the control of (which control shall be
          exercised by Agent in good faith), Agent, for the benefit of Agent,
          Syndication Agent  and Lenders, in a manner satisfactory to Agent.
          Borrower hereby pledges and grants to Agent, on behalf of Lenders, a
          security interest in all such funds and Cash Equivalents held in the
          Cash Collateral Account from time to time and all

                                       3
<PAGE>
 
          proceeds thereof as security for the payment of all amounts due in
          respect of the Letter of Credit Obligations and other Obligations,
          whether or not then due. This Agreement shall constitute a security
          agreement under applicable law.

               f.  Section 3.2 is hereby deleted in its entirety and the
                   -----------                                          
following is substituted in lieu thereof:

                    3.2  Default Interest.  Notwithstanding anything to the
                         ----------------                                  
          contrary contained in Section 2.3, if (a) an Event of Default or
          Potential Event of Default has occurred and is continuing or (b) all
          or any portion of the principal amount of any of the Loans or any
          interest accrued thereon has not been paid when due (whether at the
          stated maturity, by acceleration or otherwise) then, upon notice to
          Borrower (except as otherwise provided below for an Event of Default
          specified in Section 7.1(g)), the Loans shall bear interest at a per
                                                                           ---
          annum rate that is payable on demand and that is equal to five
          -----                                                         
          percentage points (5%) above the highest rate that would otherwise be
          applicable pursuant to Section 2.3 (such increased rate, the "Default
          Rate") from the date of such Event of Default, Potential Event of
          Default or nonpayment until such Event of Default or Potential Event
          of Default is cured or waived or such nonpayment is paid in full.
          Upon the occurrence and during the continuance of an Event of Default
          specified in Section 7.1(g), the interest rate shall be increased
          automatically to the Default Rate without the necessity of any action
          by Agent or any Lender.

               g.  The following clause is hereby added at the end of Section
                                                                      -------
6.1(a)(iii):
----------- 

          ; provided, that, for the fiscal quarter ended June 30, 1999, a 
            --------
          Compliance Certificate, shall be furnished to the Agent and each
          Lender on or before July 7, 1999 demonstrating in reasonable detail
          compliance at the end of such quarterly accounting period with the
          restriction contained in Section 6.2(c); and

               h. Section 6.1(b) is hereby amended by replacing the period at
                  ---------------                                        
the end of clause (v) thereof with "; and", and adding the following new clauses
(vi) and (vii) at the end thereof:

                  (vi) as soon as available, but in any event within ten days
          after the end of each month of Borrower, a cash budget prepared by
          Borrower as of the last day of the immediately preceding month for the
          eight week period following the first day of such month that
          incorporates information regarding cash receipts and cash
          disbursements (by major categories) (the "Cash Budget").

                                       4
<PAGE>
 
                   (vii) as soon as available, but in any event
          contemporaneously with their delivery to Comerica Bank-California
          pursuant to the Comerica Receivables Purchase Agreement, copies of all
          receivables agings, reports submitted to credit issuers, and
          accountings for the sale, collection of, and rebates on account of any
          accounts receivable transferred thereunder.

               i.  The following new Section 6.1(p) is hereby added to the
                                     --------------                       
Credit Agreement:

                    (p) Year 2000.  On or prior to September 30, 1999, Borrower
                        ---------                                              
          shall perform all acts reasonably necessary to ensure that Borrower,
          any business in which Borrower holds a substantial interest, and all
          of the customers, suppliers and vendors and other Third Party
          Interactives of Borrower that are material to Borrower's business
          become Year 2000 Compliant, including performing a comprehensive
          review and assessment of all of Borrower's systems and, if necessary,
          adopting a detailed plan for the remediation, monitoring and testing
          of such systems.  For purposes of this Section 6.1(p), the term "Year
          2000 Compliant" shall mean, with respect to Borrower and any other
          Person, that all Year 2000 Date-Sensitive System/Components utilized
          by or material to the business, operations or financial condition of
          such Person will properly perform data sensitive functions before,
          during and after the year 2000.

               j.  Section 6.2(a) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted in lieu thereof:

                    (a) Quick Ratio.  Permit the ratio of Consolidated Quick
                        -----------                                         
          Assets to Consolidated Current Liabilities as of the last day of each
          of the fiscal quarters of Borrower listed below to be less than the
          correlative amount indicated below:
<TABLE>
<CAPTION>
 
               Quarter Ending:         Ratio:
               ---------------         ------    
             <S>                    <C> 
 
               March 31, 1999         0.50 : 1.00
               June 30, 1999          0.65 : 1.00
               September 30, 1999     0.80 : 1.00
               December 31, 1999      1.00 : 1.00.
</TABLE>

               k.  Section 6.2(b) and Section 6.2(e) are hereby deleted in their
                   --------------     --------------                            
entirety.

               l.  Section 6.2(c) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted therefor:

                                       5
<PAGE>
 
                    (c) Consolidated Tangible Net Worth.  Permit Consolidated
                        -------------------------------                      
          Tangible Net Worth (i) as of March 31, 1999, to be less than
          $65,000,000, (ii) as of June 30, 1999, to be less than $71,000,000,
          (iii) as of September 30, 1999, to be less than $70,000,000, and (iv)
          as of December 31, 1999, to be less than $75,000,000; provided, that,
                                                                --------       
          for purposes of this Section 6.2(c), (A) any profit recognized from
          any exchange from time to time by Borrower of certain shares of its
          Common Stock for certain of the Notes issued pursuant to the Indenture
          and other non-cash items shall be excluded from Consolidated Tangible
          Net Worth and (B) preferred stock and warrants issued by Borrower
          shall be treated as equity (and not as Debt).

                   m.    Section 6.2(d) is hereby deleted in its entirety and
                         --------------
the following is substituted therefor:

                    (d) Profitability.  Permit Consolidated Pre-Tax Income of
                        -------------                                        
          the Borrower and its consolidated Subsidiaries to reflect a loss
          exceeding the correlative amount listed below for any fiscal quarter
          of Borrower commencing with the fiscal quarter ended March 31, 1999;
                                                                              
          provided, that any profit recognized from any exchange from time to
          ---------                                                          
          time by Borrower of certain shares of its Common Stock for certain of
          the Notes issued pursuant to the Indenture shall be excluded from
          Consolidated Pre-Tax Income for purposes of this Section 6.2(d):

               March 31, 1999        $(17,000,000)
               June 30, 1999         $(11,000,000)
               September 30, 1999    $( 4,000,000)
               December 31, 1999     $         (1).

               n.  Section 6.2(h) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted therefor:

                    (h) Dividends, Etc.  Declare or pay any dividends, purchase
                        ---------------                                        
          or otherwise acquire for value any of its capital stock now or
          hereafter outstanding, or make any distribution of assets to its
          stockholders as such, or permit any of its Subsidiaries to purchase or
          otherwise acquire for value any stock of the Borrower, except that (i)
          the Borrower may declare and deliver dividends and distributions
          payable in capital stock of the Borrower, (ii) provided no Event of
          Default exists, Borrower may repurchase stock from former employees of
          Borrower in accordance with the terms of repurchase or similar
          agreements between Borrower and such employees; provided, that the
                                                          --------          
          aggregate amount of such stock repurchases shall not exceed $500,000
          in any Fiscal Year, and (iii) Borrower may make redemptions and
          payments permitted under Section 3(k) of the Third Amendment to Credit
          Agreement.

                                       6
<PAGE>
 
               o.  Section 6.2(w) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted therefor:

                    (w) Minimum Consolidated EBITDA.  Permit Consolidated EBITDA
                        ---------------------------                             
          to be less than the correlative amount listed below for any fiscal
          quarter of Borrower commencing with the fiscal quarter ended March 31,
          1999:

               March 31, 1999        $(10,250,000)
               June 30, 1999         $( 4,500,000)
               September 30, 1999    $  2,000,000
               December 31, 1999     $  5,000,000.

               p.  Section 6.2(x) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted therefor:

                    (x) Consolidated Capital Expenditures.  Make, or permit any
                        ---------------------------------                      
          Subsidiary to make, Consolidated Capital Expenditures other than
          Consolidated Capital Expenditures for the Borrower's fiscal quarter
          ending March 31, 1999, and for each fiscal quarter thereafter, not in
          excess of $3,500,000.

               q.  The following new Section 6.2(y), Section 6.2(z) and Section
                                     --------------  --------------     -------
6.2(aa) are hereby added to the Credit Agreement:
-------                                          

                    (y) Purchased Receivables.  Enter into any Receivables
                        ---------------------                             
          Facility or sell any Purchased Receivables to Comerica Bank-California
          or any other Qualified Financial Institution, unless, at Agent's
          request, the underlying documentation for any such Receivables
          Facility provides that, upon any sale of Purchased Receivables to
          Comerica Bank-California or such other Qualified Financial Institution
          that is the provider of a Receivables Facility, the release by Agent
          of its Lien on the Purchased Receivables shall not affect Agent's Lien
          in any proceeds thereof, including any cash, cash equivalents, or
          other proceeds, wherever located; provided, that Agent's Lien on such
                                            --------                           
          proceeds shall be subordinate in priority to the Lien of such
          Qualified Financial Institution.

                    (z) [INTENTIONALLY OMITTED]

                    (aa) Bank Accounts.  No Loan Party shall open any deposit,
                         -------------                                        
          checking, operating or other bank account with any bank or other
          financial institution after April 20, 1999, unless Borrower has taken
          all steps requested by 

                                       7
<PAGE>
 
          Agent to perfect its Lien on, and the cash and cash equivalents held
          in, such account and make other reasonable arrangements with respect
          to such account.

               r.  Section 7.1(c) is hereby deleted in its entirety and the
                   --------------                                          
following is substituted therefor:

                    (c) The Borrower shall fail to perform or observe any term,
          covenant or agreement contained in Sections 3.1, 6.1(a), (b), (c) or
          (p), 6.2(a), (c), (d), (g), (h), (i), (j), (l), (p), (q), (r), (s),
          (t), (u), (v), (w), (x), (y), (z) or (aa) on its part to be performed
          or observed; or

               s.  Exhibit D is hereby deleted and replaced with Exhibit D
                   ---------                                     ---------
hereto.

               t.   Exhibit E is hereby deleted.
                    ---------                   

          3.  Interest Rate Provisions.  Notwithstanding any contrary term or
              ------------------------                                       
provision in the Credit Agreement or the other Loan Documents, at any time after
the Effective Date, all Loans shall be made as Base Rate Loans, all LIBO Rate
Loans shall automatically be converted to Base Rate Loans, and Borrower shall
have no right to convert or continue any Loan as a LIBO Rate Loan.

          4.  Restructure Fee.  Borrower hereby agrees to pay to Agent, for the
              ---------------                                                  
ratable benefit of Lenders, a restructure fee (the "Restructure Fee") payable as
                                                    ---------------             
follows:  (a) $250,000, payable on April 21, 1999; and (b) $750,000, payable on
the earlier of (i) July 10, 1999, and (ii) the date on which Borrower receives
the Net Proceeds of the first Equity Issuance by Borrower after the Effective
Date (as defined in Section 9 below).  The Restructure Fee has been fully earned
                    ---------                                                   
on the date hereof and shall be non-refundable when paid.  Any failure by
Borrower to comply with the obligations in this Section 4 shall constitute an
                                                ---------                    
Event of Default.

          5.  Plan.  Borrower shall (a) on or before May 30, 1999, enter into an
              ----                                                              
agreement with an investment banking firm, which firm shall be acceptable to
Agent in Agent's reasonable discretion, to provide a plan that, among other
things, examines Borrower's alternatives for permanently paying, on or prior to
the Maturity Date, all of the Debt owed to Lenders under the Credit Agreement
and the other Loan Documents, and (b) on or before July 30, 1999, provide to
each Lender such plan, in form and substance acceptable to Agent.  Borrower and
each other Loan Party authorizes Agent to communicate directly with the
investment banking firm engaged by Borrower to prepare the plan described in the
preceding sentence and authorizes and shall instruct such firm to disclose and
make available to Agent any and all documents, schedules and information
relating to (i) Borrower's alternatives for permanently paying, on or before the
Maturity Date, all of  the Debt owed to Lenders under the Credit Agreement and
other Loan Documents or (ii) the business, financial condition, and other
affairs of any Loan Party; provided, that prior to the occurrence and
                           --------                                  
continuance of a Potential Event of Default or an Event of Default, any such
communication by Agent with such firm shall 

                                       8
<PAGE>
 
be in the presence of an officer or other representative of Borrower. Any
failure by Borrower to comply with the obligations in this Section 5 shall
                                                           --------- 
constitute an Event of Default.


          6.  Cash Sweep Mechanics.  Upon Borrower's receipt of written notice
              --------------------                                            
from Agent, which notice may be provided by Agent to Borrower at any time after
(a) the occurrence of a Potential Event of Default or Event of Default, (b)
Agent reasonably believes based upon information available to it that a
Potential Event of Default or an Event of Default is likely to occur, (c) Agent
reasonably believes that an event or circumstance which is likely to have a
Material Adverse Effect has occurred, or (d) Agent reasonably has grounds to
deem itself to be insecure, each Loan Party shall establish blocked, lock box,
or pledged account arrangements with each bank or other financial institution at
which such Loan Party maintains a deposit, checking, operating, or other bank
account and enter into blocked, lock box, or pledged account agreements with
each bank or other financial institution at which such Loan Party maintains a
deposit, checking, operating, or other bank account and enter into blocked,
lockbox, or pledged account agreements on terms reasonably satisfactory to Agent
and shall cause all cash, checks, notes, drafts, and other similar items and all
other receipts to be deposited into such lock boxes or blocked accounts.  Any
failure by any Loan Party to comply with the obligations in this Section 6 shall
                                                                 ---------      
constitute an Event of Default.

          7.  Purchased Receivables.  The parties hereto acknowledge and agree
              ---------------------                                           
that any release by Agent of its Lien on Purchased Receivables that was made
prior to the Effective Date shall not be deemed to include, and Agent shall
continue to hold a Lien on, all of Borrower's cash, cash equivalents, and
deposit accounts.

          8.  Delivery of Amended Schedules; Other Matters.
              -------------------------------------------- 

              a.  On or before April 30, 1999, Borrower shall deliver to Agent
amended schedules to the Credit Agreement that have been updated as of the
Effective Date to reflect any matters that would have been required to be set
forth or described therein if such matters existed or occurred as of May 15,
1998, and such matters have not previously been disclosed in an updated version
of any such schedule that was delivered to Agent subsequent to May 15, 1998.
Failure to deliver such amended schedules on or before such date shall
constitute an Event of Default.

              b. On or before May 4, 1999, Borrower shall deliver the following
to Agent:

                 (i) a schedule, in form and detail satisfactory to Agent, of
     all Debt of each Loan Party, as well as a listing of scheduled payments for
     all such Debt;

                 (ii) (A) a list of all banks and other financial institutions
     at which each Loan Party (other than Borrower) maintains depository and
     other accounts, including the name and address of each depository and a
     description and number of the

                                       9
<PAGE>
 
     account, together with the name in which such account is held, and (B)
     executed notice letters under California Uniform Commercial Code Section
     9302(1)(g) to all of the holders of the depository and other accounts of
     the Loan Parties (other than Borrower);

                  (iii) executed notice letters under California Uniform
     Commercial Code Section 9302(1)(i) to all of the issuers of any Loan
     Party's insurance policies; and

                  (iv) amendments to any of the (A) Security Agreement, (B)
     Subsidiary Pledge and Security Agreement, (C) the Intellectual Property
     Security Agreement, or (D) any Subsidiary Intellectual Property Agreement,
     if any such amendments are necessary based on updated disclosure in the
     schedules delivered pursuant to Section 8(a) of this Sixth Amendment.
                                     -------------                        

          Failure to deliver any of the matters set forth above on or before May
4, 1999, shall constitute an Event of Default.

          9.  Conditions to Effectiveness.  This Sixth Amendment shall become
              ---------------------------                                    
effective on March 31, 1999 (the "Effective Date"), only upon satisfaction of
                                  --------------                             
each of the following conditions:

              a.  receipt by Agent of this Sixth Amendment duly executed by
Borrower, Bank of America National Trust and Savings Association, as Syndication
Agent and Lender, and Union Bank of California, N.A., as Agent and Lender;

              b.  receipt by Agent of the attached Guarantor Consents 
(individually, a "Guarantor Consent" and collectively, the "Guarantor 
                  -----------------                         ---------
Consents"), duly executed by each Guarantor;
--------
              c.  receipt by Agent of copies of resolutions, in form and
substance satisfactory to Agent and its counsel, of the Board of Directors or
other authorizing documents of Borrower, authorizing the execution and delivery
of this Amendment;

              d.  receipt by Agent of (i) a list of all banks and other
financial institutions at which Borrower maintains depository and other
accounts, including the name and address of each depository and a description
and number of the account, together with the name in which such account is held,
and (ii) executed notice letters under California Uniform Commercial Code
Section 9302(1)(g) to certain of the holders of the Borrower's depository and
other accounts;

              e.  receipt by Agent of a copy of the Collateral Schedule
referred to in the Comerica Receivables Purchase Agreement; and

                                       10
<PAGE>
 
               f.  the absence of any Potential Events of Default or Events of
Default (assuming effectiveness of this Sixth Amendment as of March 31, 1999).

          10.  Representations and Warranties.  In order to induce the Lenders
               ------------------------------                                 
to enter into this Sixth Amendment, Borrower represents and warrants to Agent,
Syndication Agent, and Lenders that the following statements are true, correct
and complete as of the effective date of this Sixth Amendment:

               a.  Corporate Power and Authority.  Borrower has all requisite
                   -----------------------------                             
corporate power and authority to enter into this Sixth Amendment and to carry
out the transactions contemplated by, and perform its obligations under, the
Credit Agreement as amended by this Sixth Amendment (the "Amended Agreement").
                                                          -----------------    
The Certificate of Incorporation and Bylaws of Borrower have not been amended
since the copies previously delivered to Agent, Syndication Agent, and Lenders.

               b.  Authorization of Agreements.  The execution and delivery of
                   ---------------------------   
this Sixth Amendment and the performance by Borrower of the Amended Agreement
have been duly authorized by all necessary corporate action on the part of
Borrower.

               c.  No Conflict.  The execution and delivery by Borrower of 
                   -----------   
this Sixth Amendment do not and will not contravene (i) any law or any
governmental rule or regulation applicable to Borrower, except to the extend not
resulting in a Material Adverse Effect, (ii) the Certificate of Incorporation or
Bylaws of Borrower, (iii) any order, judgment or decree of any court or other
agency of government binding on Borrower, or (iv) any material agreement or
instrument binding on Borrower, except to the extent not resulting in a Material
Adverse Effect.

                d.  Governmental Consents.  The execution and delivery by 
                    ---------------------   
Borrower of this Sixth Amendment and the performance by Borrower of the Amended
Agreement do not and will not require any registration with, consent or approval
of, or notice to, or other action to, with or by, any federal, state or other
governmental authority or regulatory body (except routine reports required
pursuant to the Securities Exchange Act of 1934, as amended (as such act is
applicable to any Loan Party), which reports will be made in the ordinary course
of business).

                e.  Binding Obligation.  This Sixth Amendment and the Amended
                    ------------------                                       
Agreement have been duly executed and delivered by Borrower and are the binding
obligations of Borrower, enforceable against Borrower in accordance with their
respective terms, except in each case as such enforceability may be limited by
bankruptcy, insolvency, reorganization, liquidation, moratorium or other similar
laws and equitable principles relating to or affecting creditors' rights.

                f.  Incorporation of Representations and Warranties From Credit
                    -----------------------------------------------------------
Agreement.  The representations and warranties contained in Section 5.1 of the
---------                                                   -----------       
Credit Agreement are correct in all material respects on and as of the effective
date of this Sixth Amendment as 

                                       11
<PAGE>
 
though made on and as of such date, except (a) to the extent that a particular
representation or warranty by its terms expressly applies only to an earlier
date, (b) to the extent that Borrower or any other Loan Party, as applicable,
has previously advised Agent in writing as contemplated under the Credit
Agreement, or (c) for matters that are to be the subject of updated disclosure
in the schedules to be delivered pursuant to Section 8(a) of this Sixth
Amendment.

               g. Absence of Default. After giving effect to this Sixth
                  ------------------                                      
Amendment, no event has occurred and is continuing or will result from the
consummation of the transactions contemplated by this Sixth Amendment that would
constitute an Event of Default or a Potential Event of Default.

          11.  Release.
               ------- 

               a.  Each Loan Party acknowledges that Agent, Syndication Agent,
and Lenders would not enter into this Sixth Amendment without the Loan Parties'
assurance that each Loan Party has no claims against Agent, Syndication Agent,
or Lenders, or any of such parties' shareholders, officers, directors,
employees, agents, or attorneys arising out of or related to the Loan Documents
or any other agreement between any Loan Party and any Lender. Except for the
obligations arising hereafter under the Amended Agreement, each Loan Party, for
itself and on behalf of its successors, assigns, and present and future
shareholders, officers, directors, employees, agents, and attorneys, hereby
remises, releases and forever discharges each of Agent, Syndication Agent, and
each Lender and their respective present and former shareholders, officers,
directors, employees, agents, attorneys, successors and assigns from any and all
claims, rights, actions, causes of action, suits, liabilities, defenses, damages
and costs that both (a) exist or may exist as of the Effective Date and (b)
arise from or are otherwise related to the Credit Agreement or the other Loan
Documents, any transaction contemplated thereby, the administration of the Loans
and other financial accommodations made thereunder, the collateral security
given in connection therewith, or any related discussions or negotiations, in
each case whether known or unknown, suspected or unsuspected. Each Loan Party
waives the provisions of California Civil Code section 1542, which states:

          A general release does not extend to claims which the creditor does
          not know or suspect to exist in his favor at the time of executing the
          release, which if known by him must have materially affected his
          settlement with the debtor.

               b.  The provisions, waivers and releases set forth in this
section are binding upon each Loan Party and each Loan Party's shareholders,
agents, employees, assigns and successors in interest. The provisions, waivers
and releases of this section shall inure to the benefit of each Lender, Agent,
Syndication Agent, and each such party's shareholders, agents, employees,
officers, directors, assigns and successors in interest as parties to the Credit
Agreement.

                                       12
<PAGE>
 
               c.  The provisions of this section shall survive payment in full
of the obligations, full performance of all the terms of this Sixth Agreement
and the Loan Documents, or Agent's or any Lender's actions to exercise any
remedy available under the Loan Documents or otherwise.

               d.  Each Loan Party warrants and represents that each such Loan
Party is the sole and lawful owner of all right, title and interest in and to
all of the claims released hereby and each such Loan Party has not heretofore
voluntarily, by operation of law or otherwise, assigned or transferred or
purported to assign or transfer to any Person any such claim or any portion
thereof. Each Loan Party shall indemnify and hold harmless each Lender, Agent,
and Syndication Agent, from and against any claim, demand, damage, debt,
liability (including payment of reasonable attorneys' fees and costs actually
incurred whether or not litigation is commenced) based on or arising out of any
assignment or transfer.

          12.  Entire Agreement.  This Sixth Amendment, together with the Credit
               ----------------                                                 
Agreement and the other Loan Documents, is the entire agreement between the
parties hereto with respect to the subject matter hereof.  This Sixth Amendment
supersedes all prior and contemporaneous oral and written agreements and
discussions with respect to the subject matter hereof.

          13.  Miscellaneous.
               ------------- 

               a.  Counterparts.  This Sixth Amendment may be executed in 
                   ------------   
identical counterpart copies, each of which shall be an original, but all of
which shall constitute one and the same agreement; signature pages may be
detached from multiple separate counterparts and attached to a single
counterpart so that all signature pages are physically attached to the same
document. Delivery of an executed counterpart of a signature page to this Sixth
Amendment by facsimile transmission shall be effective as delivery of a manually
executed counterpart thereof.

               b.  Headings.  Section headings used herein are for convenience
                   --------   
of reference only, are not part of this Sixth Amendment, and are not to be taken
into consideration in interpreting this Sixth Amendment.

               c.  Recitals.  The recitals set forth at the beginning of this 
                   --------   
Sixth Amendment are true and correct, and such recitals are incorporated into
and are a part of this Sixth Amendment.

               d.  Governing Law.  THIS SIXTH AMENDMENT SHALL BE GOVERNED BY, 
                   -------------   
AND CONSTRUED AND ENFORCED IN ACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF
CALIFORNIA APPLICABLE TO CONTRACTS MADE AND PERFORMED IN SUCH STATE, WITHOUT
REGARD TO THE PRINCIPLES THEREOF REGARDING CONFLICT OF LAWS.

                                       13
<PAGE>
 
          e.  Effect.  Upon the effectiveness of this Sixth Amendment, from and
              ------                                                           
after the date hereof, each reference in the Credit Agreement to "this
Agreement," "hereunder," "hereof," or words of like import shall mean and be a
reference to the Credit Agreement as amended hereby and each reference in the
other Loan Documents to the Credit Agreement, "thereunder," "thereof," or words
of like import shall mean and be a reference to the Credit Agreement as amended
hereby.

          f.  No Novation.  Except as expressly provided in this Sixth
              -----------                                             
Amendment, the execution, delivery, and effectiveness of this Sixth Amendment
shall not (i) limit, impair, constitute a waiver of, or otherwise affect any
right, power, or remedy of Agent or any Lender under the Credit Agreement or any
other Loan Document, (ii) constitute a waiver of any provision in the Credit
Agreement or in any of the other Loan Documents, or (iii) alter, modify, amend,
or in any way affect any of the terms, conditions, obligations, covenants, or
agreements contained in the Credit Agreement, all of which are ratified and
affirmed in all respects and shall continue in full force and effect.

          g.  Conflict of Terms.  In the event of any inconsistency between the
              -----------------                                                
provisions of this Sixth Amendment and any provision of the Credit Agreement,
the terms and provisions of this Sixth Amendment shall govern and control.



                 [Remainder of page intentionally left blank.]

                                       14
<PAGE>
 
          IN WITNESS WHEREOF, this Sixth Amendment to Amended and Restated Loan
and Security Agreement has been duly executed as of the date first written
above.

                              P-COM, INC.


                              By: /s/ Michael Sophie
                                  -----------------------------

                              Name: Michael Sophie
                                    ---------------------------

                              Title: Chief Financial Officer
                                    ---------------------------

                              UNION BANK OF CALIFORNIA, N.A.,
                              as Agent and a Lender


                              By: /s/ Patricia Lee
                                  -----------------------------

                              Name: Patricia Lee
                                    ---------------------------

                              Title: Vice President
                                    ---------------------------

                              BANK OF AMERICA NATIONAL TRUST AND SAVINGS
                              ASSOCIATION, as Syndication Agent and a Lender

                              By: /s/ M. Duncan McDuffie
                                  -----------------------------

                              Name: M. Duncan McDuffie
                                    ---------------------------

                              Title: Vice President
                                    ---------------------------

                                       15
<PAGE>
 
                                 GUARANTOR CONSENTS

          Each of the undersigned hereby (i) ratifies and reaffirms, as of the
date hereof, all of the provisions of that certain Subsidiary Guaranty in favor
of Agent dated as of May 15, 1998 (the "Guaranty"), (ii) acknowledges receipt of
                                        --------                                
a copy of the Sixth Amendment to Credit Agreement effective as of March 31, 1999
(the "Sixth Amendment"), (iii) consents to all of the provisions of the Sixth
      ---------------                                                        
Amendment, and (iv) acknowledges and agrees that nothing contained in the Sixth
Amendment in any way affects the validity and enforceability of the Guaranty.


Effective as of March 31, 1999  CONTROL RESOURCES CORPORATION

                                By: /s/ Warren T. Lazarow
                                    -----------------------------

                                Name: Warren T. Lazarow
                                      ---------------------------
  
                                Title: Secretary
                                      ---------------------------


Effective as of March 31, 1999  P-COM NETWORK SERVICES, INC.

                                By: /s/ Warren T. Lazarow
                                    -----------------------------

                                Name: Warren T. Lazarow
                                      ---------------------------

                                Title: Secretary
                                      ---------------------------


Effective as of March 31, 1999  P-COM FINANCE CORPORATION

                                By: /s/ Warren T. Lazarow
                                    -----------------------------

                                Name: Warren T. Lazarow
                                      ---------------------------

                                Title: Secretary
                                      ---------------------------

Effective as of March 31, 1999  P-COM UNITED KINGDOM, INC.

                                By: /s/ Warren T. Lazarow
                                    -----------------------------

                                Name: Warren T. Lazarow
                                      ---------------------------

                                Title: Secretary
                                      ---------------------------


Effective as of March 31, 1999  TELEMATICS, INC.

                                By: /s/ Warren T. Lazarow
                                    -----------------------------

                                Name: Warren T. Lazarow
                                      ---------------------------

                                Title: Secretary
                                      ---------------------------

                                       16