User's Manual

Avaya Branch Gateway Manager 10.0 Page 359
15-601011 Issue 29r (Friday, November 02, 2012)B5800 Branch Gateway
Configuration Settings: WAN Port
Example: Adjusting the Tc Setting
G.729 VoIP creates a 20 byte packet every 20ms. Adding typical WAN PPP headers results in a 33 byte packet every
20ms.
For a Committed Information Rate (CIR) of 14Kbps, with the Time Constant (Tc) set to 10ms; we can calculate the
Committed Burst size:
Bc = CIR x Tc = 14,000 x 0.01 = 140 bits = 17.5 bytes.
Using 10ms as the Tc, a full G.729 VoIP packet (33 bytes) cannot be sent without exceeding the Bc. The most likely
result is lost packets and jitter.
If the Tc is increased to 20ms:
Bc = CIR x Tc = 14,000 x 0.02 = 280 bits = 35 bytes.
The Bc is now sufficient to carry a full G.729 VoIP packet.
Notes
1.Backup over Frame Relay is not supported when the Frame Link Type is set to RFC1490.
2.When multiple DLCIs are configured, the WAN link LED is switched off if any of those DLCIs is made inactive,
regardless of the state of the other DLCIs. Note also that the WAN link LED is switched on following a reboot even if
one of the DLCIs is inactive. Therefore when multiple DLCIs are used, the WAN link LED cannot be used to determine
the current state of all DLCIs.
3.When the Frame Link Type is set to RFC1490, the WAN link LED is switched on when the WAN cable is attached
regardless other whether being connected to a frame relay network.