Real-time multi-user transcoding for push to talk over cellular (PoC)

1. Real-time multi-user transcoding for push to talk over cellular (PoC) Stéphane Coulombe

2. Why transcoding for PoC? • Interoperability is hurting wireless multimedia applications: • Browsing, MMS, content delivery & sharing, etc • Main causes: • Different device capabilities and products’ evolution • Transcoding is a must: • Fixes interoperability issues • Enables products’ evolution without ‘breaking’ the service • New formats (H.264, etc.), increased bitrate & resolution, etc • Situation is not different with PoC

3. Outline • What is PoC? • What is the problem? • How can we fix it? • Conclusions and future work

4. What is PoC? • PoC service allows mobile users to create group sessions: • Voice and data communications • 1-to-1 or 1-to-many sessions • PoC service feels like walkie-talkie • Dedicated «talk» button • Short talk bursts are copied from the one who talks to the others

5. What is PoC? • PoC service is built on top of a SIP/IP core (e.g. 3GPP or 3GPP2 IMS) • Participating PoC function (PPF) • Provides PoC session handling (policy enforcement for PoC sessions) • Relays TBC messages between the PoC client and the CPF. • May also relay RTP media between the PoC client and the CPF. • Controlling PoC function (CPF) • Centralized PoC session mgmt: • RTP media distribution • copies of RTP packets • Talk Burst Control (TBC) • controls who talks • Policy enforcement • who can participate

6. What is the problem? • Interoperability is again the problem: • 3GPP mandates AMR speech codec (+AMR wideband if 16kHz supported) • 3GPP2 mandates EVRC speech codec • Therefore 3GPP and 3GPP2 terminals won’t be able to communicate • More problems will emerge with video and products’ evolution • PoC standard (v1.0 and v2.0) mention that transcoding is needed: • No means for achieving transcoding is provided • PoC v2.0 introduces the PoC Interworking Function • It may, among other things, perform transcoding • But its realization is outside the scope of the OMA • Bottom line: PoC has interoperability issues without a solution

7. What is the problem? Currently, the session setup obviously fails

8. How can we fix it? • Issues to address: • Control plane adaptation • SIP INVITES must reflect transcoding capabilities of the system • Media transcoding • Media packets must travel through a transcoder when needed • Efficiency: • We want to minimize the number of transcoding operations • Multi-user session support • Sessions contains many users • Sessions are dynamic (people leave and join, talker changes) • Compatibility with existing Terminals • The solution must not require any change to mobile terminals • The network-side will have to deal with the problem!

9. How can we fix it? Solution 1: Transcoding distributed among the various PPFs • Must be performed at each receiving PPF • This is how a back to back user agent (B2BUA) solution would be implemented • This is sub-optimal from processing perspective • The same transcoding may be replicated more than once (for each user)

10. How can we fix it? Solution 2: Transcoding centralized at the CPF • The CPF has two main responsibilities with respect to enabling transcoding: 1. Manage session control operations: • Setup: • fix codec incompatibilities, • manage transcoding server’s (TS) operations • Update: update transcoding operations dynamically • Different speaker • Usersleaving and joining the session 2. Manage the flow of media streams between users: • Media streams (RTP packets) have to flow through a TS

11. How can we fix it? Solution 2: Transcoding centralized at the CPF 1. Manage session control operations: Setup • Enhance SIP INVITE’s SDP with codecs supported by the TS • Use TS’s IP address and ports in SIP INVITEs for routing

12. How can we fix it? Solution 2: Transcoding centralized at the CPF 1. Manage session control operations: Setup

13. How can we fix it? Solution 2: Transcoding centralized at the CPF 1. Manage session control operations: Update • TS session must beupdated as new membersjoin or leave • TS session isupdatedbased on whotalks (TS ops + routing)

14. How can we fix it? Solution 2: Transcoding centralized at the CPF 2. Manage the flow of media streams between users: • Manage flow of TBC (who speaks) and speech packets • 2 options: • All the media packets arrive at the CPF • speech packets forwarded to TS, TS transcodes media and returns it to CPF or sends it directly to destination • All the media packets arrive at the TS • TBC packets forwarded to CPF, processed and returned to TS or sent directly to destination • From a scalability and general performance perspective, this option is more efficient

15. How can we fix it? Solution 2: Transcoding centralized at the CPF 2. Manage the flow of media streams between users: • 1. All the media packets arrive at the TS • 2. All the media packets arrive at the CPF

16. How can we fix it? • There are actually more details to consider which can’t be shown here… • Precise modifications to SDP messages • Codecs and IP addresses & ports • Precise messaging flow and content for: • Session setup • Session update • Request for talking • TBC and speech packets • Precise operations taking place in the TS • More details in the paper and in the references

17. Conclusions and future work • We proposed a solution to fix the problem!!! • Transparent for all PoC clients • Only requires changes to PoC servers • Compatible with existing PoC specifications • Works for all PoC group session scenarios: • 1 to 1, 1 to many, 1 to many to 1, ad hoc, etc. • Scalable by offloading work to various TSs • Extensible to other SIP/SDP-based real-time multi-party applic. • Allows minimization of the processing required for transcoding • Can perform transcoding once for many destinations • Allows transcoding to be customized for each user • Select a distinct AMR bitrate for every PoC client

18. Conclusions and future work • Future research topics • Investigate a proxy-based transcoding solution • not requiring any change to PoC servers or to PoC clients already deployed. • Study the problem of optimally selecting TSs in a distributed configuration • based on transmission and processing costs over the network. • Trade-off between quality and processing cost.

19. Thank you for your attention.Questions? • Contact: Stéphane Coulombe Department of Software and IT Engineering   École de technologie supérieure Montréal, Canada Phone : (514) 396-8407 e-mail: stephane.coulombe@etsmtl.ca   • The author would like to thank Vantrix (www.vantrix.com) corporation for supporting this research.