Video resolution requirements

The video quality of a meeting varies depending on the available network bandwidth and on the number of participants. The Sametime® client can receive a maximum of six video streams. So depending on the number of participants in the conference, Sametime clients receive and send remote video streams that have different resolution, based on the bandwidth that is available.

Bandwidth requirement for a downlink from the Video MCU to the client

Higher video resolution requires more bandwidth to produce the video. And the higher the bandwidth, the better the video quality. Sametime automatically adapts to the available bandwidth and reduces the bit-rate to a certain threshold for each chosen video resolution. However, the result may be a loss of quality if the available bandwidth becomes too low.

For example, if there are ten participants in the conference, the client can receive six remote video streams. With a line rate of 1024kbps, two streams will be at 180p@30fps (frames per second), and four will be at 180p@7.5fps. Tables 1-6 show the downlink resolution in frames per second (fps), based on the available bandwidth.

Table 1. Video Resolution for Six Video Streams
Bit Rate Downlink Resolution Total Displayed Videos
1920kbps 180p@30fps x 6 6
1024kbps 180p@30fps x 2 +

180p@15fps x 4

 6
768kbps 180p@15fps x 4 +

180p@7.5fps x 2

 6
512kbps 180p@15fps x 5 5
384kbps 180p@15fps x 1 +

180p@7.5fps x 2

 3
256kbps 180p@7.5fps x 2 2
Table 2. Video Resolution for Five Video Streams
Bit Rate Downlink Resolution
1920kbps 180p@30fps x 5
1024kbps 180p@30fps x 4 +

180p@15fps x 1
768kbps 180p@15fps x 5
512kbps 180p@7.5fps x 5
384kbps 180p@15fps x 1 +

180p@7.5fps x 2
256kbps 180p@7.5fps x 2
Table 3. Video Resolution for Four Video Streams
Bit Rate Downlink Resolution
1920kbps 360p@30fps x 4
1024kbps 180p@30fps x 4
768kbps 360p@15fps x 1 +

180p@15fps x 3
512kbps 180p@7.5fps x 4
384kbps 180p@15fps x 1 +

180p@7.5fps x 2
256kbps 180p@7.5fps x 2
Table 4. Video Resolution for Three Video Streams
Bit Rate Downlink Resolution
1920kbps 360p@30fps x 3
1024kbps 360p@30fps x 2 +

360p@15fps x 1
768kbps 180p@30fps x 3
512kbps 180p@15fps x 3
384kbps 180p@15fps x 1 +

180p@7.5fps x 2
256kbps 180p@7.5fps x 2
Table 5. Video Resolution for Two Video Streams
Bit Rate Downlink Resolution
1920kbps 360p@30fps x 2
1024kbps 360p@30fps x 2
768kbps 360p@30fps x 1 +

360p@15fps x 1
512kbps 180p@30fps x 2
384kbps 180p@15fps x 2
256kbps 180p@7.5fps x 2
Table 6. Video Resolution for One Video Stream
Bit Rate Downlink Resolution
1920kbps 720p@30fps x 1
1024kbps 720p@30fps x 1
768kbps 360p@30fps x 1
512kbps 360p@30fps x 1
384kbps 360p@30fps x 1
256kbps 360p@30fps x 1

Bandwidth requirement for an uplink from the client to the Video MCU

The Sametime client uses three temporal layers to stream video at different frame rates for each of the spatial resolutions of 180p, 360p and 720p. Table 7 shows the different speeds for each temporal layer.

Table 7. Temporal Layer Bandwidth Resolution
Temporal Layer Frames per Second 180p Resolution 360p Resolution 720p Resolution
Base layer 7.5fps 86kbps 173kbps 346kbps
First layer 15fps 128kbps 256kbps 512kbps
Second layer 30fps 192kbps 384kbps 768kbps

The Sametime client uses this temporal scalability to send multiple temporal layers of each resolution based on bandwidth available. As shown in Table 8, if 1024kbps are available, the client can send three streams of 180p@30fps, 360p@15fps, and 720p@15fps. However, to save bandwidth, the client sends a stream only if there is at least one remote client in the conference receiving it. So, for example, if no remote client is receiving 720p@15fps, then it is not sent. Only the streams 180p@30fps and 360p@15fps are sent.

Table 8. Uplink Resolutions
Bit Rate Uplink Resolution
1920kbps 180p@30fps + 360p@30fps + 720p@30fps
1024kbps 180p@30fps + 360p@15fps + 720p@15fps
768kbps 180p@30fps + 360p@30fps
512kbps 180p@30fps + 360p@15fps
384kbps 180p@15fps + 270p@15fps
256kbps 180p@30fps
128kbps 180p@7.5fps