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, the total IP LAN bandwidth usage for each site was 

calculated, and expressed in Erlangs at the bottom of 

. Specifically, the total LAN 

bandwidth usage in Site 1 is 123.5 Erlangs, in Site 2 is 56.6 Erlangs, and in Site 3 is 29.2 
Erlangs. This implies that the average number of bidirectional media streams that are 
simultaneously in use at any given time in Site 1 is 123.5. Analogous statements can also be 
made regarding Sites 2 and 3.
Every media stream across the IP LAN in any of the three sites is assumed to use the 
uncompressed G.711 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. So for the G.711 codec, 

 indicates that each media stream consumes 87.2 kbps of 

IP LAN bandwidth. It may be tempting at this point to simply multiply 87.2 kbps by 123.5 
simultaneous bidirectional media streams, to arrive at the estimate for the overall LAN 
bandwidth needed for Site 1. However, 123.5 is merely the average number of simultaneous 
media streams, and approximately half of the time, there are at least 124 simultaneous media 
streams in use.
In this example, suppose that the goal is to supply enough bandwidth to adequately support the 
media streams at least 99.9% of the time. The standard infinite-server queueing model implies 
that less than 0.1% of the time there are at least 159 simultaneous media streams in the Site 1 
LAN. So, it is sufficient to engineer the LAN bandwidth to support 158 simultaneous media 
streams. Therefore, the Site 1 LAN requires at least (158 simultaneous media streams) x (87.2 
kbps per media stream) = 13.8 Mbps of bandwidth, in each direction. This result, along with the 
analogous results for Sites 2 and 3, are provided in 

, the number of simultaneous media streams for “P001” represents the 99.9th 

percentile for the number of simultaneous unidirectional streams, as determined by applying the 
standard infinite-server queueing model.
A slight variation of the procedure that was used to determine LAN bandwidth in 

 can be used to determine WAN bandwidth. Using compressed RTP (cRTP) is a 

means to conserve bandwidth. Specifically, the use of cRTP reduces the overhead due to IP, 
UDP, and RTP from 40 bytes to between 2 and 4 bytes (4 bytes are assumed for this example). 
Using the PPP overhead of 7 bytes (which would vary if ATM, HDLC, or Frame Relay were 
used) implies a total overhead of 11 bytes (88 bits) in this example. This implies the following 

Simultaneous media streams for 

“P001”

158

81

47

LAN bandwidth (Mbps)

13.8

7.1

4.1