UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                   FORM 6-K

                        REPORT OF FOREIGN PRIVATE ISSUER
                       PURSUANT TO RULE 13a-16 OR 15d-16
                     UNDER THE SECURITIES EXCHANGE ACT OF 1934




                       For the month of December , 2006

                                 SPIRENT COMMUNICATIONS plc
     _____________________________________________________________________
                (Translation of registrant's name into English)

  Spirent House, Crawley Business Quarter, Fleming Way, Crawley, West Sussex
                                RH10 9QL, UK.
     _____________________________________________________________________
                    (Address of principal executive offices)


Indicate by check mark whether the registrant files or will file annual reports
under cover Form 20-F or Form 40-F.

                         Form 20-F    X         Form 40-F.....

Indicate by check mark whether the registrant by furnishing the information
contained in this Form is also thereby furnishing the information to the
Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

                               Yes .....        No     X

If "Yes" is marked, indicate below the file number assigned to the registrant
in connection with Rule 12g3-2(b): 82- ________




                          SPIRENT COMMUNICATIONS PLC

                                  (the "Company")

                        NOTIFICATION OF DEEMED INTERESTS
                  BY DIRECTORS, PERSONS DISCHARGING MANAGERIAL
                       RESPONSIBILITY OR CONNECTED PERSONS



The Company was notified on 21 December 2006 that Spirent Sharesave Trust
Limited, the trustee for the Spirent Sharesave Trust (the "Trust") had
transferred Ordinary shares to satisfy the exercise of options under the
Company's Savings Related Share Option Scheme as follows:



No of Shares            Option Price

9,710                     38 pence



Following this transfer the total number of Ordinary shares held by the Trust
is:


1,857,576 (representing 0.21% of the issued share capital of the Company).


The Company's executive directors are potential beneficiaries of the Trust
(together with all other UK employee share option holders) and are, therefore,
treated under the Companies Act 1985 as having a deemed interest in the total
number of shares held by the Trust.  As a result, the directors' deemed interest
in the Trust has been reduced by the above number of shares transferred.



For the avoidance of doubt, the Company confirms that this notification does not
relate to an actual dealing in securities by the executive directors whose
interests in the Company's securities remain unchanged notwithstanding the above
events.





Warren Nash
Spirent Communications plc


 Signatures

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized.

                                               ______MICHAEL ANSCOMBE______

                                                     (Registrant)

Date: 21 December, 2006                    By   ____/s/ Michael Anscombe____

                                                    (Signature)*