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[webrtc-svc] [Question] Difference between SVC and Simulcast (#60)

From: Michał Śledź via GitHub <sysbot+gh@w3.org>
Date: Fri, 07 Jan 2022 14:00:51 +0000
To: public-webrtc-logs@w3.org
Message-ID: <issues.opened-1096342239-1641564050-sysbot+gh@w3.org>
mickel8 has just created a new issue for https://github.com/w3c/webrtc-svc:

== [Question] Difference between SVC and Simulcast ==
Hi, I've read this draft and have a few questions.
Could you please answer them or link some resources where I can read more?

1. Could you please explain me what you mean by writing `single stream simulcast` in [Section 4.2 Negotiation](https://www.w3.org/TR/webrtc-svc/#negotiation)? 
I always thought that simulcast means multiple encodings of the same stream sent as separate RTP streams. Does `single stream simulcast` mean that we can send multiple encodings of the same stream using one RTP stream i.e. using one SSRC?
2. In [Section 6. Scalability modes](https://www.w3.org/TR/webrtc-svc/#scalabilitymodes*), there is 

    For example, VP8 [RFC6386] only supports temporal scalability (e.g. "L1T2", "L1T3"); H.264/SVC [RFC6190],
    which supports both temporal and spatial scalability, only permits transport of simulcast on distinct SSRCs, 
    so that it does not support the "S" modes, where 
    multiple encodings are transported on a single RTP stream.
    But of course, you can send multiple spatial layers with VP8, each layer using separate RTP stream i.e. different SSRC so in my opinion the statement `For example, VP8 [RFC6386] only supports temporal scalability (e.g. "L1T2", "L1T3");` is incorrect.

3. What is the difference between Simulcast and SVC? Is it possible that one codec supports both simulcast and SVC? For example, VP8 supports both simulcast (by multiple encodings of the same stream) and SVC (by temporal layers in each encoding) 

Please view or discuss this issue at https://github.com/w3c/webrtc-svc/issues/60 using your GitHub account

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