FRTS - Committed Burst (Bc) - Correct value to support VoIP

[dvaggalis]

I have seen several articles that give different values for BC to support VoIP, what is the correct formula? Does it make a diffence if your circuit is a 56K, T1 or something in between?

Make Bc small so that Tc (shaping interval) is small (Tc = Bc/CIR)

The minimum Tc value is 10 ms, which is ideal for voice. With a small Tc value, there is no risk of large packets using all the shaping credits. Large Tc values can lead to large gaps between packets sent because the traffic shaper waits an entire Tc period to build up additional credits to send the next frame. Making Bc = 1000 bits is usually a low enough value to force the router to use the minimum Tc of 10ms. This setting should not affect data throughput.

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a00800d6788.shtml

Note: When FRTS is enabled, the Frame Relay Committed Burst (Bc) value (in bits) should be configured to a maximum of 1/100th of the CIR value (in bits per second). This configuration ensures that the FRTS token bucket interval (Bc/CIR) does not exceed 10 Ms, and that voice packets are serviced promptly.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b75c7.html

[pacameron]

Set Bc to be 1/100th of the CIR. This creates 100 seperate shaping intervals per second, which is optimal for voice as a normal codec (G729 or 711) sends 50 packets per second, so essentially you get a steady stream of voice/data/voice/data/voice etc so the delay is minimised, and jitter is reduced.

So -

if CIR=64K, then Bc=64000/100 = 640

if CIR=256K, then Bc=256000/100 = 2560

etc ...