Calculating Bandwidth for Video Calls

A few weeks ago I wrote a blog on determining the bandwidth used by voice calls in Calculating Bandwidth for SIP Trunks. Now, while voice is an extremely important aspect of SIP communications, the beauty of SIP is that it isn’t limited to strictly voice. On my various PC and smart phone clients I do voice, presence, instant message, and video.

While communications forms such as instant message and presence do consume some network bandwidth, it’s extremely low and neither is of a real-time nature. However, video is very real-time in nature and will typically consume far more data than even the most data intensive wide band voice codec.

Video codecs have a number of distinguishing characteristics, but in terms of bandwidth utilization we are concerned with two major factors – resolution and frame rate. Resolution is expressed by the pixel height and pixel width of the rendered image. Frame rate is expressed in image frames per second. Clearly, more pixels sent more often produces the best image. This, of course, leads to greater bandwidth consumption and possibly less video calls on your network.

It is my experience that the following four video codes are the ones you will most often encounter with video calls – Divx, H.263, H.263+, and H.264/MPEG-4. Each of these codes offer different resolution and frame rate values that ultimately create different quality experiences and bandwidth requirements.

For DviX codecs you will commonly find the following variations:

Client Setting	Resolution	Frames / sec	Quality	Bandwidth (kbps)	Average (kbps)
Very low bandwidth	160 x 120	2	Very low	10 – 20	15
Low bandwidth	160 x 120	10	Low	60 – 120	80
Medium bandwidth	320 x 240	10	Medium	150 – 300	200
High bandwidth	352 x 288	15	High	400 – 800	600

For H.263, H.263+, and H.264/MPEG-4, it looks like this:

Client Setting	Resolution	Frames / sec	Quality	Bandwidth (kbps)
Very low bandwidth	176 x 144	2	Very low	10
Low bandwidth	176 x 144	10	Low	64
Medium bandwidth	352 x 288	10	Medium	192
High bandwidth	352 x 288	15	High	512
Very high bandwidth	640 x 480	30	Very high	768

At this point you need to determine the number of simultaneous video calls and the quality level of those calls. Unlike your voice traffic, these numbers might not be as easy to determine. Video is still a relatively new form of communication and you may not have the empirical knowledge required to do a full analysis. This means that you will have to take your best guess and be prepared to add or remove bandwidth as users become more comfortable with making and receiving video calls.

However, I will take a stab at a few estimates that may form the basis of your best guess approach.

Expected Video Sessions:

Subscribers	Sessions
10	8
25	12
50	17
75	23
100	27
250	54
500	95
750	134
1000	171
1250	210

Your number may vary quite a bit from these. The important thing is to do the best job you can in determining the number of sessions.

Next, you need to translate those sessions into bandwidth requirements. Given the above data I come up with the following video requirements in Mbit/sec.

Sessions	Very Low	Low	Medium	High	Very High
8	0.1	.6	1.6	4.8	6.1
12	0.2	1.0	2.4	7.2	9.2
17	0.3	1.4	3.4	10.2	13.1
23	0.3	1.8	4.6	13.8	17.7
27	0.4	2.2	5.4	16.2	20.7
54	0.8	4.3	10.8	32.4	41.5
95	1.4	7.6	19.0	57.0	73.0
134	2.0	11	27.0	80.0	103.0
171	2.6	14	34.0	103.0	131.0
210	3.2	17	42.0	126.0	161.0

To determine your particular bandwidth needs, I suggest the following:

Estimate the total number of SIP video subscribers that will be using each video quality level.
From this estimate, determine the number of video sessions using the above table.
Using the bandwidth requirements table, determine the bandwidth for each expected codec. Sum those numbers.
Note. Rather than trying to estimate for each video codec type, pick an average codec and multiply by the expected number of video sessions.

You then need to ensure that your switches, routers, queues, and uplinks are sized to meet the expected video traffic. Also, make sure that you apply the proper QoS settings to ensure the best video experience possible.

I hope this helps you understand what you need in terms of bandwidth for your video roll-out. It’s important to realize that video is not going away and the demand for quality, well-behaved video calls will be growing every year. Do it right and your users will be happy. Fail to plan and configure your network appropriately and you risk a help desk nightmare.