EXHIBIT 10.15 CAIS, INC. 1232 22nd Street, N.W. Washington, D.C. 20037 Tel. (202) 463-8500 Fax (202) 463-7190 April 4, 1997 Mr. David D. Goodman Inline Connection Corporation 730 N. Danville Street Arlington, VA 22201 Dear David: CAIS, Inc. hereby exercises its Option Pursuant to Section 1.6 of the November 5, 1996 Agreement for Cooperative Use of Communication Patents among Inline Connection Corporation, you in your individual capacity, and CAIS, Inc. , as amended by letter agreement dated February 28, 1997 (the "Agreement"). Enclosed is a check for $22,500 payable to Inline Connection Corporation. As we agreed today, an additional $20,000, which represents the balance of the $50,000 Option exercise payment as provided in Section 1.6 of the Agreement (after credit for the $7,500 advance previously paid to Inline, as set forth in paragraph 2 of the February 28 amendment) will be paid to Inline by no later than May 4, 1997. Please indicate your consent to this payment arrangement by signing below individually and on behalf of Inline. We appreciate your cooperation, and look forward to a continued mutually successful relationship with you and Inline. Sincerely, CAIS, Inc. By: /s/ Ulysses G. Auger, II --------------------------- Ulysses G. Auger, II, President encl. Agreed to: Agreed to: Inline Connection Corporation By: /s/ David D. Goodman David D. Goodman ---------------------- ----------------- David D. Goodman, President David D. Goodman, in his individual capacity 4/4/97 4/4/97 ------------------- ------------------ Date Date