Comfort Noise (RFC 3389) is the noise played in an RTP stream to prevent a user form hearing total silence on the connection. Not all telephony systems have the ability to play music while a party is on hold.  Without Comfort Noise (CN) the party on hold may think the connection dropped.

Starting with ECZ740 the Acme Packet SBC has the ability to inject Comfort Noise into an RTP stream by analyzing the SDP Offer and SDP Answer from both SIP endpoints. CN interworking is triggered on a per realm basis.

In the example below the calling party (Skype for Business) SDP Offer contains CN and is traversing a realm called core which has CN interworking enabled. The SBC forwards SDP to the called party. If the called party SDP Answer contains CN the SBC does not do anything because both endpoints support Comfort Noise. If CN is not present in the PSTN’s SDP Answer the SBC adds CN to the egress SDP Answer (towards Skype for Business) and will take responsibility for injecting CN into the RTP stream.

Example SBC configuration

Example call flow where SBC inserts comfort noise when Skype for Business sends CN during an active call

Comfort Noise state is exchanged with Active-Standby units high availability SBC pair. If there is a mid call failover and the standby becomes active, it will honor the CN message exchange between the called and calling party and generate Comfort Noise.

Additional document may be found in the ECZ740 Configuration Guide which is publicly accessible here.


  1. Robert says:

    Anyone knows if this can affect my acme sbc? I Keep receiving that message in a logfile.

    Mar 6 09:21:30.745 [NPSOFT] [MINOR] 475 Packet drops due to ARP miss on slot 1 port 0, addr
    Mar 6 09:23:30.745 [NPSOFT] [MINOR] 1337 Packet drops due to ARP miss on slot 1 port 0, addr
    Mar 6 09:25:30.745 [NPSOFT] [MINOR] 743 Packet drops due to ARP miss on slot 1 port 0, addr

  2. Bob Nielsen says:


    Do you have any documents on the data that is stored in the ACME on cdr records? on how to retrieve the cdr information?

  3. Mark says:

    There is an Accounting Guide with CDR VSA info, how to configure the SBC, etc.. The SBC sends CDR’s via Radius, FTP, or SFTP. Another cool thing is you can use HMR’s to populate custom VSA info.

  4. HI Mark,

    Do you have the ability to do remote TAC support for a very small network in the USA. We have supported it for the last year, but we have received no calls. It is a small opportunity. Please let me know.ACME SBC.

    Joan Gianiotis

  5. Shyam says:

    Hi Mark,

    Your Blog was very helpful and I have been working on ACME SBC for 3 years. I refer your blog if i have any doubts regarding the SBC ‘s.

    Could you please let me know the upgrade procedure for ACME SBC 3820. Thanks.


    Great Post Mark!

  7. Chris says:

    Hi Mark,

    Thanks for the blog posts, very useful.

    We have a clustered pair of Acme Packet netnet 3820s in two datacentres. We only use them as SBCs between Skype for Business platforms and our PSTN carriers. All calls route inbound and outbound via DC1 with 2nd choice routing to DC2.

    We want to upgrade the firmware on both clusters, DC2 is straight forward, but was wondering if there’s a way to drain the SBCs? We can disable the Session Agents to force calls via DC2 but this disconnects active calls…..


  8. Mark says:

    Hi Chris. Thank you for the comment. If you manually take a session-agent out of service in the config the SBC will not kill the the existing calls. Issuing “show sip session-agents” will let you see how many calls are active. This number will decrease as calls end. New calls will not be setup on a session-agent that has been taken out of service. The SBC will respond with SIP 503 Service Unavailable which tells the SIP device that sent the invite to try another IP (usually an SBC in another datacenter).