Specifications

Engineering Guidelines
208
By modifying the router MTU value to approximately 500, larger packets are divided up and
sent in smaller chunks. The result of this is that there are three times as many opportunities to
send the voice data. Thus the data rate link could be reduced to 300 kbps.
Some packets may not allow MTU to cut them down (video can be one of these). The router
with the lower MTU might reject these packets, effectively denying access. Also, packets where
encryption is used with particular block sizes may also fail to go through a low-MTU connection.
The G.711 CODEC is a linear codec, in that the value represents a specific voice signal level
at that sample time. As such it can handle unexpected variations, or errors, in the values without
impacting voice quality to a high degree. The low bit rate CODECs, including G.729 and G.722.1
encode blocks of samples, and bit rate reduce these values to achieve the reduced bandwidth.
This block is known as a Frame and includes data as well as error correction capability, which
standard G.711 does not include. For G.729 the frame size is 10ms,. For G.722.1 this frame
size is 20ms.
Because the low bit rate CODECs sample data in frames, it is important that a frame is not
“cut” as would happen with an inappropriate MTU setting. Cutting a frame will result in the entire
frame being lost., rather than a few samples. Therefore the packet size and sample rate should
be considered before setting the MTU value. It is possible to send multiple frames per packet,
for example a 20ms packet will consist of two frames at G.729, or a single frame of G.722.1.
The following table highlights the number of bytes needed for Ethernet connections for different
low bit rate CODECs and different packet rates, and hence minimum allowed MTU settings.
Typically the packet rate will be at 20ms, and typically MTU will be limited to a lower value of
576. Under such conditions there will not be an impact on these voice packets. If the MTU is
reduced to non-typical values, then the voice packet sizes in the table should be considered.
Although the data rates above are minimum recommendations, slower speeds can be used,
but these involve links with strict control of priority queuing and may involve physical restrictions,
such as available for PC or phone but not both simultaneously.
For slower links, the recommendation is to reduce the MTU in the routers/gateways to provide
more opportunity for voice traffic. A value of 576 has been found to work well.
Note: Some routers do not function with an MTU as low as 500. Internet specifications
for a reduced packet suggest a lower value limit for MTU of 576.
Table 64: Codec Frame Size and Packet Rates
Codec Packet Rate
10 ms 20 ms 30 ms 40 ms
G.729 102 122 142 162
G.722.1 N/A 162 N.A 242