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Cisco Bug: CSCuq62858 - MCU sends multiple TMMBR messages resulting in content channel fluctuati

Last Modified

Sep 17, 2019

Products (15)

  • Cisco TelePresence MCU 4500 Series
  • Cisco TelePresence MCU 4520
  • Cisco TelePresence MCU 4505
  • Cisco TelePresence MCU 5320
  • Cisco TelePresence MCU 4510
  • Cisco TelePresence MCU 4215
  • Cisco TelePresence MCU 4205
  • Cisco TelePresence MCU 4220
  • Cisco TelePresence MCU MSE 8420
  • Cisco TelePresence MCU 4203
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Known Affected Releases

4.5(1.45) 4.5(1.55)

Description (partial)

Symptom: Content from MCU can fluctuate when in Hybrid mode in a conference with TC-endpoints and MXP endpoints.
Condition: MCU is configured with Hybrid mode and outgoing codec is set to H.263+. TC endpoint is the content contributor and the content channel bandwidth is around 512Kbps. MXP endpoint is the content receiver.

The problem happens because TC endpoint connects at a higher bandwidth (>1Mbps) first and sends content using H.263+ codec (TC endpoints send content with H.263+ when the content bandwidth is greater than 512Kbps). MXP endpoint joins at a lower bandwidth (< 1Mbps). Since this is Hybrid mode, the MCU initially starts to send passthrough content from TC endpoint to MXP as both support H.263+

However, since MXP endpoint has joined at a lower bandwidth (so it is only able to receive content at 512Kbps or lower), the content channel from TC is now flow controlled down by MCU. As soon as the bandwidth is reduced from 512Kbps, the TC endpoint now switches to sending content using H.264 codec. But MXP can only receive H.263+ content and therefore the MCU can no longer send passthrough content from TC -> MXP (as there is a codec mismatch). MCU now closes the passthrough content and starts sending transcoded content to the MXP.

Since content is now fully transcoded by the MCU, the bandwidth limitation that was previously applied to TC endpoint, due to MXP's low bandwidth, is no longer necessary. So MCU now decides that the TC endpoint's content bandwidth can be increased back again as it is not sending passthrough content any more. So the MCU now sends a new TMMBR message to flow control up the TC endpoint's content bandwidth.

As soon as the bandwidth now goes over 512Kbps, TC endpoint now starts sending H.263+ content. MCU now again decides that since MXP is also able to receive H.263+, it can send the content as passthrough. So MCU stops sending transcoded content to MXP and switches to passthrough. However, since MXP has connected at a lower bandwidth, the MCU again sends a new instruction to TC endpoint to lower its content bandwidth, which causes it to switch back to H.264 codec. The whole process now repeats, resulting in the MXP endpoint seeing fluctuating content.

This problem therefore has three main causes:
- TC endpoint's algorithm of using different codecs for content channel around 512Kbps
- MXP endpoint joining at a different bandwidth than TC, causing the TC endpoint's content to be flow controlled down around the 512Kbps mark
- The way MCU's Hybrid algorithm works that it will try to do passthrough before transcoding content
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