Specifications

Engineering Guidelines
168
What is the media bandwidth?
Depending upon how this is measured, this could be simply the payload bandwidth, which is
similar to TDM, or it could be the bandwidth of the packet carried across the network. During
a conversation, this bandwidth is consumed at a constant rate. It may change if the CODEC
includes Voice Activity Detection (VAD) and reduce consumption of bandwidth, but it won’t
exceed a particular level even when network bandwidth is available. This is in contrast to general
TCP data traffic, where bandwidth is consumed to the maximum current capacity of the link.
What is the signalling bandwidth?
The level of signalling is dependent upon call traffic. If there are no phone calls being set up,
then signalling is low (less than 1% of expected media bandwidth). However, setting up a call
uses both voice and signalling bandwidth. In practice, adding 10% to the voice bandwidth for
signalling has been found to be a good rule of thumb that provides sufficient margin.
Table shows typical wire data rates for different protocols and LAN/WAN interfaces.
Note, for example, that a half duplex link uses twice the bandwidth on the connection than a
similar, full duplex connection for the same voice connections. This is because the half duplex
connection is shared with other devices and the repeater on the link retransmits data received
on the received on the receive path for all other devices to hear (it exists on the transmit and
receive cable pairs at the same time).
As the table shows, the physical wire bandwidth required by an IP phone for Ethernet is usually
• G.711 (about 100 kbps at nominal 20ms packet rate)
• G.729a (about 40 kbps at nominal 20ms packet rate)
Under most conditions the default packet rate used by the end devices is 20ms. However when
connecting to other third party products packet rate values may vary from 10ms to 40ms in
10ms steps. Typical packet rates and usage include:
• 10ms (for reduced latency at PSTN gateway)
• 20ms (default IP rate, provides good delay and bandwidth usage efficiency)
• 30ms (reduced packet rate, for example wireless base stations)
• 40ms (limited bandwidth connections where reduced header size and larger packet in-
crease efficiency)
Both LAN (Ethernet) and WAN bandwidth usage is shown in the following tables (“” and “Other
Protocols: On-the-wire Bandwidth”). Often the bandwidth quoted for Ethernet differs between
measurement equipment and in values quoted by different vendors. The values highlighted in
the following tables include all the bits on the wire as would be seen by an oscilloscope. This
includes bits used to delimit the packets and also the inter-packet gap. Although these bits and
Note: Some network analyzers will not monitor the full Ethernet frame, excluding
checksums and synchronization data, and therefore they give a bandwidth somewhere
between wire and IP bandwidth. For the example shown, this would typically be 87.2
kbps, including VLAN.