Video Bandwidth Considerations

Unlike voice, video uses a variety of packet sizes and packet rates to support a single video stream. Most video codecs take advantage of what can loosely be called "prediction," by sending an encoded video frame (large packet), followed by a series of vectors describing changes to the previous frame (small packet). Although this type of algorithm greatly reduces the required bandwidth, it does cause video streams to use a dynamic packet rate, with a range of packet sizes. Also the actual average bandwidth required for a section of video depends on the complexity and amount of movement in the video. Table 1-22 lists four popular video codecs and the required bandwidth ranges for each: Table 1-22 Video Codecs and Required Bandwidth

Video Codec

Required Range

MPEG-1

500 to 1500 kbps

MPEG-2

1.5 to 10 Mbps

MPEG-4

28.8 to 400 kbps

H.261

100 to 400 kbps

Different codecs simply provide different tradeoffs for quality and bandwidth, and many different codecs are needed to support applications of all types. For instance, MPEG includes several standards that were created for different types of applications. ITU H.261 provides a video standard for video conferencing, which works well when the callers do not move around too much! If you have ever been on a video conference and watched someone who used their hands a lot to talk, for instance, you might have seen jerky arm movements. All these codecs operate with dynamic bandwidth, and with different-sized packets. Figure 1-29 shows a packet distribution for percentages of packets at various sizes in an H.261 conference call.

Figure 1-29 Packet Size Distributions in a Video Conference Using H.261

100 Tl

100 Tl

65- 129- 257- 513- 1025128 256 512 10241518

65- 129- 257- 513- 1025128 256 512 10241518

As mentioned earlier, video flows vary both the size of packets sent and the packet rate. In a high-quality video conference that might average 768 kbps of bandwidth, for example, the packet rates might vary from as low as 35 pps to as many as 120 pps. Some QoS tool configuration options might be affected by not only the bandwidth required (kbps), but also by the packet rate (pps). Remember, queue sizes are measured in packets; so to avoid tail drop, a queue holding video traffic may need a much larger queue size than a queue holding voice. Table 1-23 summarizes some of the key bandwidth differences between voice and video traffic.

Table 1-23 Voice and Video Bandwidth Contrasted

Feature

Voice

Video

Number of flows in each direction

1

2 (1 audio, 1 video)

Packet sizes

Static, based on codec

Variable

Packet rate

Constant (isochronous)

Variable

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