SIP Media Management: Early Media vs. Late Media

In a previous blog, I addressed the concepts of early offer and late offer. You are welcome to read the article, SIP Media Management: Early Offer vs. Late Offer, but it all boils down to a simple notion. With early offer you put SDP in the INVITE message and with late offer you put SDP in the ACK. This changes who gets to decide which codec will be used for the session.  The called party decides with early offer and the calling party decides with late offer.

However, this has nothing to do with when media actually starts. Early and late offer simply define when media capabilities are exchanged. With early offer, you can put SDP in the INVITE request, but that doesn’t mean that media will be sent prior to the call being answered.

Early and late media have to do with when media starts to flow. Simply put, early media indicates that media is sent prior to the call being answered and late media indicates that media waits until the call has been answered.

Early and late media actually have nothing to do with early and late offer. You can have late offer and early media and its possible to have early offer and late media.

In order to make sense of this, you need to shift from thinking about SIP and VoIP to that analog telephone back home. For those of you who don’t own a land line, you can think about the phone at grandma’s house. The point is you need to focus on telephony prior to SIP.

When you pick up that home phone, what’s the first thing you hear? Dial tone, of course. The phone company sends dial tone to let you know that the line is working and is ready to accept digits. This is not media. This is simply a friendly, comfort noise that acts like the on/off light on a piece of electronic equipment.

Let’s keep going. You hear the dial tone, you enter the telephone number, and the call is launched. Depending on the number you dialed and the state of the called party you will hear a variety of different sounds.

Ringing is a good thing. That tells you that the called party’s telephone is alerting.  You might also hear a busy signal if the other party is on an existing call.

You might also hear a message telling you that “the number you dialed is no longer in service.” Lastly, it’s possible to hear what we telephone people know of as re-order tone. This “fast busy” is often used to indicate that all circuits are busy and the call cannot be placed.

Whether it’s ring-back or re-order tone, this is media. These sounds are sent from the telephone company to tell you something about your call. If you think about it, how else can they inform you of call progress? Those old analog telephones don’t have displays. So, instead of your eyes, you use your ears to monitor how your call is progressing.

In SIP, we don’t use sounds to indicate call progress. Instead, we have response messages. 180 Ringing tells you that far-end is ringing. 486 Busy Here informs you that the far-end is busy. Your SIP device can do whatever it wants with those messages including playing sounds or displaying messages.   However, if a sound is played it’s a local sound. It is not media sent to the SIP device from either the far-end or the communications system.

So, what happens when you need to interface the SIP world with the non-SIP world? Specifically, how do you deal with a system that uses media for call progress?

You let it.

Early media is simply media that is sent before a call is answered. It’s not the voice of the person you called, but rather system tones, announcements, or any other sound that the phone company wants to send your way. It’s the distinctive ringing you hear when you call a telephone in England. It’s an “all circuits are busy” message. It’s anything you might hear until you hear the called party’s voice.

Early media is typically supported by the use of the 183 Session In Progress response. Unlike a 180 Ringing response, 183 will contain SDP. This SDP is used to establish a media connection that carries those network tones and messages. It will eventually be torn down when the call is answered, but until then, it’s a way for the caller to audibly hear call progress.

I have been talking about early media in terms of the public switched telephone network (PSTN), but early media is also used by some IP PBXs. Why? Because they want to play the same kinds of sounds that the PSTN uses. They want to play announcements and country specific ring-back. Remember, PBXs are rooted in the same TDM world as the PSTN and have adopted much of its behavior.

On an Avaya system, there are a couple of places where you can enable early media. First, you can configure it on the Communication Manager by enabling Direct IP-IP Early Media on the SIP signaling group. Second, the 46xxSettings.txt file contains the parameter Enable_Early_Media. Setting it to 1 instructs the telephone to include SDP in 18x progress messages.

That’s it, folks. If you work with SIP as much as I do, I guarantee that you will run into early media, so it’s important that you are aware of what is happening. It’s not a change in SIP. It’s simply a difference in behavior that creates a bridge from the old world of TDM to the new world of IP telephony.


  1. Erk Musa · · Reply

    Hi Andrew, I’m a fan! Love your blogs! I came across your blogs because I was searching online for possible solutions on the issue I’m facing. We’re currently migrating our telephony infra to SIP and we’re getting a one-way voice path. Here’s the situation:

    – To reach our contact center, customers can either call a short code, long code (mobile number), pilot number (fixed line) and some DIDs (fixed).
    – When the call comes in from a fixed line towards those numbers above, we get voice path on both sides however, when a call originated from a mobile number, we get one voice path (where called party cannot hear the calling party).

    What do you think is the issue?

    1. Thank you, Erk!

      One way audio is often caused by a Codec mismatch or a Firewall/NAT issue. Are the paths different for fixed line or mobile phone? Is the same SBC involved?

      1. Erk Musa · ·

        The codec is consistent using G711A and yes the path are the same:

        Mobile Core -> IMS Network -> SBC -> ASM
        Fixed Network -> IMS Network -> SBC -> ASM

        Same SBC is involved. Just an update though:

        So we have a pilot number: 4005xxxx – when called from mobile and fixed line – works perfectly!
        We have a short code: 17x – when called from a fixed line – works fine! When called from a mobile – one way voice path.

        Been trying to get this fixed for the last 2 weeks but no luck.

        We have tried – IP-IP direct media, TDM, enabling/disabling audio hairpinning, early media, late media, with/without codec transcoding – but nothing works.😦

      2. At this point I would be looking at your SBC. It may also be a problem back at the carrier’s SBC. I wish I could be of greater assistance, but it’s hard fixing a difficult problem without being able to actually debug it. Good luck.

  2. Jonathan Els · · Reply

    Hi Andrew

    Perhaps it worth expanding this to include call flows with Wireshark traces highlighting 18X/PRACK signalling, in both SDP and non-SDP scenarios?

    1. I will see what I can do. 🙂

  3. Deepak Rawat · · Reply

    Hi Andrew,

    Is there a way to disable the SDP in 180 ringing from CM. I dont see any field in Avaya CM where we can disable it. Even tried disabling the “Initial IP-IP Direct Media” but no luck.

    Deepak Rawat

    1. Hi Deepak, I don’t know of a way, but I will ask around and if I find something, I will let you know.

  4. Is early media is similar to precondition?

      1. what is the difference?

    1. bhaskarwar · · Reply

      Here is the difference between the two:

      Preconditions require that the participant reserve network resources before continuing with the session. With Preconditions, the chances of a session establishment failure are minimum. Since the participants have already reserved resources needed for the session.

      Early media refers to media (e.g., audio and video) that is exchanged before a particular session is accepted by the called user. An offer/answer exchange that takes place before a final response for the INVITE is sent establishes an “early” media session.

  5. Nice article, it was well written and easy to understand. Thanks for doing your blog!

  6. Ian Smith · · Reply

    Would you elaborate more on SDP in case of early media? I mean, there is an offer in INVITE (with bunch of codecs), than we have early media answer in 183 Session Progress with certain SDP (a specific codec is chosen) and than 200OK comes with final response. Must SDP in 200OK be the same as in 183 (in terms of chosen codec)? What if early media codec is narrowband and final is intended to be wideband – is UPDATE used?

    1. Kyle Dean · · Reply

      Ian, the SDP answer in the 200 OK must match the SDP answer in the 183.

      From RFC 3261:
      o If the initial offer is in an INVITE, the answer MUST be in a
      reliable non-failure message from UAS back to UAC which is
      correlated to that INVITE. For this specification, that is
      only the final 2xx response to that INVITE. That same exact
      answer MAY also be placed in any provisional responses sent
      prior to the answer. The UAC MUST treat the first session
      description it receives as the answer, and MUST ignore any
      session descriptions in subsequent responses to the initial

  7. Hello Andrew,

    thanks a lot for your explanation!


  8. Sonali Kakade · · Reply

    Hello Andrew. I would like to suggest a small change in your blog. You mention that “On an Avaya system, there are a couple of places where you can enable early media. First, you can configure it on the Communication Manager by enabling Direct IP-IP Early Media on the SIP signaling group. Second, the 46xxSettings.txt file contains the parameter Enable_Early_Media. Setting it to 1 instructs the telephone to include SDP in 18x progress messages.”. However it is the other way round. When the Initial IP-IP Direct Media field on the Communication Manager signaling
    group form page 1 is set to ‘y”, Communication Manager sends a “183 Session Progress”
    *without* SDP during an inbound PSTN call that is forwarded to another PSTN call just
    before a 183 is sent with SDP information to the far end.

  9. Quick Question Andrew, Can we consider IVR announcements as early media?

  10. Hello Andrew, I have a confusion

    RFC6337 5.2.5 chapter:
    “When the new offer is sent in response to an offerless (re-)INVITE,
    it should be constructed according to the General Principle for
    Constructing Offers and Answers (Section 5.1 ): all codecs the UA is
    currently willing and able to use should be included, not just the
    ones that were negotiated by previous offer/answer exchanges. The
    same is true for media types — so if UA A initially offered audio
    and video to UA B, and they end up with only audio, and UA B sends an
    offerless (re-)INVITE to UA A, A’s resulting offer should most likely
    re-attempt video, by reusing the zeroed “m=” line used previously.”

    In the above RFC, does ‘zeroed “m=” line’ means non-zero “m= line” with a port which was used previously? Or UA A has to offer with “m=” line as 0

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: