Call Flow During Media Bypass - Skype For Business



Introduction

Media bypass allows an end point to go around the gateway\mediation\SBC to setup media session with next hop (another end point or gateway). Usually, objective of the media bypass is to reduce the number of hops in a media session in order to improve audio quality.

In the world of Skype for Business, Media bypass refers to removing the Mediation Server from the media path whenever possible for calls while signalling still traverses via the Mediation Server.

(Image Courtesy – Microsoft)

Benefits of Media Bypass

Enterprises enjoy following benefits by configuring Media Bypass in an Enterprise Voice (EV) deployment.

  • Improved voice quality by reducing latency
  • Do away with needless mediation processing of RTP packets
  • Decreased potential point of failures
  • Reduced load on Mediation Server

An Example Call Flow

In this example, Media Bypass is enabled in Skype for business topology. A gateway (Mediation peer) is configured to be the next hop of Mediation Server (towards PSTN network).

The mediation peer can be a PSTN gateway, IP-PBX, or Session Border Controller (SBC) at an Internet telephony service provider (ITSP) that is associated with the trunk between the Mediation Server where the call is routed, and the peer is certified by Microsoft.

  • Skype user makes a call
  • Call session established
  • The other user hangs up the call

Let’s look at the SIP verbs to understand this call flow in detail.

Serial Flag Description
1
Invite

Skype user initiates the call. In the INVITE request:

  • The IP address of the Skype user is inserted in the Request-URI field.
  • A unique numeric identifier is designated as Call-ID field.
  • The transaction number within a single call leg is identified in the CSeq field.
  • The media capability that Skype user is ready to receive is specified.
  • The port to receive the RTP data is specified.
2
100 Trying

The Mediation peer sends a SIP 100 Trying response to Mediation Server. In turn, Mediation Server relays 100 Trying to the Skype user. The response indicates that the INVITE request has been received.

3
180 Ringing

The Mediation peer sends a SIP 180 Ringing response to Mediation Server. In turn, Mediation Server relays 180 Ringing to the Skype user. The response indicates that the user is being alerted.

4
200 OK

The Mediation peer sends a SIP 200 OK response to Mediation Server. In turn, Mediation Server relays 200 OK to the Skype user. The response notifies that the connection has been made.

5
RTP

Shows media session being established between the end point and mediation peer.

6
BYE

Other user disconnects the call. BYE relayed to the skype user via mediation server.

7
ACK

Skype user sends termination acknowledgement to the mediation peer via Mediation Server.

Note: The call flow deals with many more things like candidate exchange, early media negotiation etc. I haven’t covered those to stay focussed on media bypass pattern.

Mediation Server Failure

Another Mediation Server will be used if available.

Users wouldn’t be able to make new call if the only Mediation Server is unavailable.

Ongoing calls would continue as Mediation Server plays no role during media packet traversal in such case.

Facts to Consider

It is important to note that additional features (during outgoing call) like call hold, call transfer etc. may not work if only mediation server is unavailable.

Media Bypass is not supported in Cloud Connector Edition (CCE).

Media Bypass and CAC are mutually exclusive, you must be designing the voice deployment accordingly.

Media Bypass cannot be used for Dial-In Conferencing.

Reference Article:

Media Bypass Specification: http://www.google.co.in/patents/US8687640

Media bypass is supported only with products and versions that are listed on the Telephony Infrastructure for Skype for Business Server page at https://technet.microsoft.com/en-us/office/dn947483.aspx



1 Comment

Kiran

Jul 31 2016 9:29PM

Is there a list of media bypass limitations?