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 August, 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

                    ACQUISITION OF IMPERFECT NETWORKS, INC.

London, UK - 10 August 2006: Spirent Communications plc ("Spirent") (LSE: SPT;
NYSE: SPM), a leading communications technology company, today announces that it
has acquired the business of Imperfect Networks, Inc. ("Imperfect Networks"), a
US based developer of security testing solutions. The acquisition will enable
Spirent to deliver enhanced security testing solutions to its customers across a
number of markets.

Spirent is paying an initial  consideration  of $4.0 million  (GBP2.2  million),
payable  in cash  on  completion,  with up to a  further  $4.0  million  (GBP2.2
million) payable depending on the satisfaction of certain  technical  milestones
and  revenues  to  be  achieved  in  2007.   Imperfect   Networks  is  based  in
Massachusetts and has 11 employees.

Spirent has been the exclusive re-seller of Imperfect Networks' ThreatEx product
since 2005 with revenues growing strongly in the first half of this year. When
combined with Spirent's market-leading AvalancheTM product, it provides global
network equipment manufacturers, service providers and enterprises with a
powerful tool for verifying defenses against malicious attacks and helps reduce
the risk of costly network downtime due to network threats such as worms,
viruses and attacks on voice over IP and wireless LAN services. The ThreatEx
business and technology will be integrated into Spirent's Performance Analysis
division.

Anders Gustafsson, Chief Executive, commented:

"The acquisition is in line with our strategy to expand our communications
business, drive profitable growth and deliver shareholder value. Our major
customers are increasingly demanding enhanced security testing solutions.
Combining the ThreatEx technology with our existing solutions will strongly
position us to meet their needs."

                                    - ends -

Enquiries
Anders Gustafsson, Chief
Executive                         Spirent Communications plc +44 (0)1293 767676
Eric Hutchinson, Chief Financial
Officer

Reg Hoare/Katie Hunt/Libby Young  Smithfield                 +44 (0)20 7360 4900

   Photography is available from UPPA (Universal Pictorial Press & Agency) -
                                 www.uppa.co.uk
                             or tel: 020 7421 6000


About Spirent Communications plc

Spirent Communications plc is a leading communications technology company
focused on delivering innovative systems and services to meet the needs of
customers worldwide. We are a global provider of performance analysis and
service assurance solutions that enable the development and deployment of
next-generation networking technologies such as broadband services, Internet
telephony, 3G wireless and web applications and security testing. The Systems
group develops power control systems for specialist electrical vehicles in the
mobility and industrial markets. Further information about Spirent
Communications plc can be found at www.spirent.com.

Spirent Communications plc Ordinary shares are traded on the London Stock
Exchange (ticker: SPT) and on the New York Stock Exchange (ticker: SPM; CUSIP
number: 84856M209) in the form of American Depositary Shares ("ADS"),
represented by American Depositary Receipts, with one ADS representing four
Ordinary shares.

Spirent and the Spirent logo are trademarks or registered trademarks of Spirent
Communications plc. All other trademarks or registered trademarks mentioned
herein are held by their respective companies. All rights reserved.

This press release may contain forward-looking statements that are based on
current expectations or beliefs, as well as assumptions about future events. By
their nature, forward-looking statements are inherently predictive, speculative
and involve risk and uncertainty because they relate to events and depend on
circumstances that will occur in the future. You should not place undue reliance
on these forward-looking statements, which are not a guarantee of future
performance and are subject to factors that could cause our actual results to
differ materially from those expressed or implied by these statements. These
risks include the risks described from time to time in Spirent Communications
plc's Securities and Exchange Commission periodic reports and filings. The
Company undertakes no obligation to update any forward-looking statements
contained in this press release, whether as a result of new information, future
events or otherwise.


                                   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 10 August, 2006                           By   ____/s/ Michael Anscombe____

                                                    (Signature)*