Exhibit 10.1

** NOTE: CERTAIN MATERIAL HAS BEEN OMITTED PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT AND SUCH MATERIAL HAS BEEN FILED SEPARATELY WITH THE SEC.

AMENDMENT FOUR TO INTEGRATED  NETWORK SOLUTION PURCHASE AGREEMENT

THIS AMENDMENT FOUR TO INTEGRATED NETWORK SOLUTION PURCHASE AGREEMENT(this "Amendment
Four"), is effective as of August 21, 2002 (the "Effective Date"), by and between Genuity Solutions Inc., with a
principal business address at 225 Presidential Way, Woburn, Massachusetts 01801 ("Genuity"), and Allegiance Telecom Company Worldwide, with a principal
business address at 9201 N. Central Expressway, Dallas, Texas 75231 ("Allegiance") (collectively, the
"Parties"), which amends that certain Integrated Network Solution Purchase Agreement by and between the Parties dated July 24, 2000, as amended
from time to time (the "Agreement").

WHEREAS,as of the date hereof, the Parties have verbally agreed to, and Allegiance has implemented, Ports in excess of the Cumulative
Required Capacity and in advance of the dates set forth in the Implementation Plan set forth in the Agreement; and

WHEREAS,the Parties desire to memorialize their verbal agreement and further amend the Agreement to include, without limitation,
(i) a new Minimum Required Capacity**; (ii) a revised Purchase Price Payment Schedule; and (iii) an updated and amended
Termination Schedule.

NOW, THEREFORE,in consideration of the mutual covenants and promises contained herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the Parties agree to the foregoing and as follows:

1. Amendments.

a.Section 2.4(a).





b.Section 2.5(b).Section 2.5(b) shall be amended by adding the following sentence at the end of
such section:



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c.Section 3.1(a).The Purchase Price payment schedule set forth in Section 3.1(a) of the
Agreement is deleted in its entirety and replaced with the Purchase Price payment schedule attached hereto asAttachment B.

d.Schedule 10.6.The termination value schedule set forth in Schedule 10.6 of the Agreement is
deleted in its entirety and replaced with the termination value schedule attached hereto asAttachment C.

2. **

3.Effect of Amendments.Capitalized terms used herein but not defined herein
shall have the meanings ascribed to them in the Agreement. All terms of this Amendment Four shall supersede any conflicting terms set forth in the Agreement. Any terms of the Agreement not expressly
amended or replaced herein shall remain in full force and effect.

Signature Page Follows

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IN WITNESS WHEREOF,the Parties have caused this Amendment Four to be executed by their duly authorized representatives.

|GENUITY SOLUTIONS INC. |  | ALLEGIANCE TELECOM COMPANY WORLDWIDE
|---------------------- |  | ------------------------------------
|By:                    |  | /s/COLIN P. BEASLEY                 
|Name:                  |  | Colin P. Beasley                    
|Title:                 |  | SVP, Network Planningand Engineering
|Date:                  |  | August 21, 2002                     

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ATTACHMENT A    IMPLEMENTATION PLAN (SECTION 2.4 (a))

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ATTACHMENT B    PURCHASE PRICE PAYMENT SCHEDULE (SECTION 3.1(a))

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|Total Payments |  | $563,010,886

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ATTACHMENT C    TERMINATION VALUE (SCHEDULE 10.6)

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