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used) implies a total overhead of 11 bytes (88 bits) in this example. This implies the following
table of WAN bandwidths,
assumes the use of cRTP:
This table can be used in the WAN bandwidth calculation for the system in
bandwidth
considerations.
Example 8: WAN bandwidth
In
Example 6: IP bandwidth
pair of sites was calculated, and expressed in Erlangs at the bottom of
bandwidth usages (Erlangs) for Example 6: IP bandwidth considerations
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,
page 183 indicates that each (unidirectional) media stream consumes 12.4 kbps of IP WAN
bandwidth. Similar to the case in
simultaneous bidirectional media streams. As in
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
requirements in each direction, for Example 8: WAN bandwidth
Table 39: IP WAN bandwidth requirements for media
Table 39: IP WAN bandwidth requirements for media streams
Packet "size"
(ms)
10
20
30
60
considerations, the total IP WAN bandwidth usage between each
Table 39: IP WAN bandwidth requirements for media streams
Example 7: LAN
G.711 (kbps)
G.729 (kbps)
72.8
16.8
68.4
12.4
66.9
10.9
65.5
9.5
bandwidth, 24 is the average number of
Example 7: LAN
bandwidth, the bandwidth is
Table 40: IP WAN bandwidth
on page 184.
Resource sizing
streams, which
Example 6: IP
Table 35: IP WAN
on page 179.
on
Issue 3.4.1 June 2005
183
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