Specifications

Engineering Guidelines
172
The LAN connection guidelines table also shows the maximum capability of a LAN link assuming
that the link is used purely for voice traffic. If the link is shared with other devices such as PCs,
then some priority mechanism is required to ensure that the voice gets the available bandwidth
when needed. Also, in a busy network with multiple broadcasts, the available bandwidth is
reduced by this percentage. For example, in a network with 10% broadcast traffic (at 10 Mbps),
the 40% available bandwidth is reduced to 30% for a half duplex link, and the number of voice
channels is reduced accordingly.
The ratio from half duplex to full duplex is four (not two) because conversations need both a
talk path and a listen path. For half duplex, both paths share the same physical wire; for full
duplex, both send and receive can occur simultaneously on different wire pairs.
For half duplex, the channel availability is: 10M x 40% / (2 x 100k) = 20 channels.
Only 40% of the bandwidth is available due to collisions and collision avoidance mechanisms.
For full duplex paths, there are no collisions, so usage can double to 80%. Also there are
separate paths for send and receive data, so only half the connection bandwidth is used.
Thus, 10M x 80% / (1 x 100k) = 80 channels.
WAN bandwidth
A WAN link is generally point-to-point between routers and is always a full duplex link. The link
speed for access WAN connections is also slower, which means the number of available voice
channels is reduced. The following table shows the number of voice channels that a 1.5 Mbps
link supports.
When a WAN link is shared with other data devices there are other considerations, including
the introduction of waiting delay. The end device sees this as jitter, resulting in potential packet
loss, and the user experiences degraded voice quality.
Calculations for the number of voice channels are based purely on exclusive use of the link
bandwidth for voice. In reality, other factors similar to those of the LAN connection also come
into play. This becomes much more acute with slower WAN links.
The queuing technique and weightings to the COS or TOS value also become important. For
instance, the use of Expedite Queuing will give better advantage to voice traffic than the simple
Weighted Round Robin technique, which allows even a small percentage of lower priority traffic
under congestion.
Also, consider that if the CIR (Committed Information Rate) is based exclusively on the voice
requirements, additional data above this limit will be marked for “Eligible Discard.” This applies
to all packets, including voice traffic.
Table 51: Voice Channels Supported by a 1.5 Mbps Link
(based on a 20 ms packet rate)
Cable capacity Bandwidth %
Voice channels
G.711
Voice channels
G.729a (x 2.5)
Voice channels
G.722.1
1.5 Mbps without QoS mechanism 40% 6 15 9
1.5 Mbps with QoS mechanism 70% 10 26 16