<PAGE>

                                                                    EXHIBIT 10-a
                                 FIRST AMENDMENT
                                       TO
                                    AGREEMENT


                  This First Amendment (this "First Amendment"), dated as of
this 30th day of June, 1999, amends that certain Agreement dated as of June 19,
1999, by and among ADC Telecommunications, Inc. ("ADC") and Saville Systems PLC
("Saville") (the "Agreement"), which Agreement provides for the acquisition of
Saville by ADC in accordance with the terms and conditions set forth therein.

                  WHEREAS, for the avoidance of doubt, the parties desire to
amend the Agreement to clarify an agreed condition thereof and to confirm the
parties' understandings with respect to the matter set forth in this First
Amendment; and

                  WHEREAS, this First Amendment is being made by the parties in
furtherance of their mutual desire to consummate the transactions contemplated
by the Agreement.

                  NOW, THEREFORE, the parties hereto hereby agree as follows:

                  1.       Section 7.2(g) of the Agreement is hereby amended in
its entirety to read as follows:

                  "(g)     POOLING LETTERS. Buyer and the Company shall have
                           received the letters described in the third Recital
                           to this Agreement from Pricewaterhouse Coopers LLP
                           and Arthur Andersen LLP and such letters shall not
                           have been withdrawn, modified or qualified in any
                           material respect as of the Effective Time, as
                           certified by Pricewaterhouse Coopers LLP and Arthur
                           Andersen LLP, respectively, in a writing addressed to
                           their respective addressees and dated as of the
                           Effective Date, and Buyer shall have received the
                           letter of Arthur Andersen LLP, addressed to Buyer and
                           dated as of the Effective Date, stating that, in
                           reliance on the letter and the certification of
                           Pricewaterhouse Coopers LLP described in this
                           paragraph (g) and based on its familiarity with
                           Buyer, the Acquisition will qualify as a
                           pooling-of-interests transaction under Opinion 16."

                  2.       Any capitalized term used herein and not otherwise
defined herein shall have the meaning given to such term in the Agreement.

                  3.       This First Amendment constitutes an amendment of the
Agreement in conformity with and pursuant to the terms of Section 9.6 of the
Agreement. Except as expressly amended herein, all terms set forth in the
Agreement shall continue in full force and effect.

                  4.       The operative terms of this First Amendment may be
inserted into a First Amended and Restated Agreement by the parties and shall
have a date as of the day and year first set forth herein.

                  5.       The internal law, and not the law of conflicts, of
the State of Minnesota will govern all questions concerning the construction,
validity and interpretation of this First Amendment and the performance of the
obligations imposed by this First Amendment.


                  IN WITNESS WHEREOF, the parties have executed this First
Amendment as of the day and year first above written.

ADC TELECOMMUNICATIONS, INC.                                 SAVILLE SYSTEMS PLC


                                      - 1 -
<PAGE>


By       /s/ Robert E. Switz                      By  /s/ John J. Boyle, III
  --------------------------------                    -------------------------
         Robert E. Switz                              John J. Boyle, III
         Senior Vice President and                    Chairman of the Board and
         Chief Financial Officer                      Chief Executive Officer


                                       - 2 -