Nortel VoIP Gateway, Configuration Manual

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Network engineering 159
Norstar VoIP Gateway Configuration Guide
WAN engineering
Wide Area Network (WAN) links are typically full-duplex links - both talk and listen traffic use
separate channels. For example, a T1 link uses a number of 64 kbit/s (DS0) duplex channels
allowing *64 kbit/s for transmit path and n*64 kbit/s for the receive path.
(WAN links may also be half-duplex.)
Example 1: WAN engineering - voice calls
Consider a site with four IP telephony ports and a full-duplex WAN link using PPP. The preferred
codec is G.729, which uses a voice payload of 20 ms. Silence compression is enabled.
Given the above, what is the peak traffic in kbit/s that IP telephony will put on the WAN?
From the table under “Bandwidth requirements on full duplex links” on page 157 , the following
figure shows the peak transmission rate for G.729 is 24.8 kbit/s per call or 99.2 kbit/s in each
direction for all four calls. In other words, in order to support four G.729 calls, the WAN link must
have at least 99.2 kbit/s of usable bandwidth (in each direction).
The average bandwidth for each call is 12.4 kbit/sec per channel or 49.4 kbit/s for all four calls for
each channel. Low priority data applications can make use of bandwidth made available by silence
suppression.
Figure 45 Peak traffic, WAN link
PPP B/W
No SP
Silence
Suppression
peak
(kbit/s)
peak
(kbit/s)
Avg
(kbit/s)
G.729
(8 kb/s)
10
20 24.8 12.4
30