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table of WAN bandwidths, 

page 222, which assumes the use of cRTP:

This table can be used in the WAN bandwidth calculation for the system in 

.

, the total IP WAN bandwidth usage between each 

pair of sites was calculated, and expressed in Erlangs at the bottom of 

. Specifically, the 

total WAN bandwidth usage between Sites 1 and 2 is 24.0 Erlangs, between Sites 1 and 3 is 
10.0 Erlangs, and between Sites 2 and 3 is 4.0 Erlangs. This implies that the average number of 
media streams simultaneously in use at any given time between Sites 1 and 2 is 24. Analogous 
statements can also be made regarding WAN traffic between each of the other two pairs of 
sites.
Every media stream across the IP WAN, between any pair of sites, is assumed to use the 
compressed G.729 codec, since bandwidth is relatively inexpensive within a private LAN, as 
opposed to a public WAN. Assume, for the sake of this example, a standard IP packet size of 20 
ms. For the G.729 codec, 

 indicates that each (unidirectional) media stream consumes 

12.4 kbps of IP WAN bandwidth. Similar to the case in 

, 24 is the 

average number of simultaneous bidirectional media streams. As in 

, 

the bandwidth is sized to a “GOS” of P001 (“GOS” in this context is actually a pseudo-GOS; true 
GOS is associated with a fixed number of channels, as is typical of circuit-switched systems). 
The standard infinite-server queueing model implies that less than 0.1% of the time there is at 
least 40 simultaneous media streams between Sites 1 and 2. So, it is sufficient to engineer the 
WAN bandwidth between those two sites to support 39 simultaneous media streams. Therefore, 
the WAN between Sites 1 and 2 requires at least (39 simultaneous media streams) x (12.4 kbps 
per media stream) = 484 kbps of bandwidth. This result, along with the analogous results for the 
WAN traffic between the other two pairs of sites, are provided in 

10

72.8

16.8

20

68.4

12.4

30

66.9

10.9

60

65.5

9.5