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Traffic engineering
Example 7: LAN bandwidth
In
Example 6: IP bandwidth
calculated, and expressed in Erlangs at the bottom of
(Erlangs) for Example 6: IP bandwidth considerations
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
in each direction, for Example 7: LAN
Table 38: IP LAN bandwidth requirements in each direction, for
bandwidth
Resource
Simultaneous media streams for
"P001"
LAN bandwidth (Mbps)
In
Table
38, 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
LAN bandwidth
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
182 Avaya Application Solutions IP Telephony Deployment Guide
considerations, the total IP LAN bandwidth usage for each site was
Table 37: Bandwidth requirements for media streams
can be used to determine WAN bandwidth. Using compressed RTP (cRTP) is a
Table 34: IP LAN bandwidth usages
on page 179. Specifically, the total LAN
Table 38: IP LAN bandwidth requirements
bandwidth.
Site 1
(Atlanta)
158
13.8
on page 181
Example 7: LAN
Site 2
Site 3
(Boston)
(Cleveland)
81
47
7.1
4.1
Example 7:
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