Specifications

Network Requirements and Preparation
9 – 8 ShoreTel, Inc.
Packet Loss
Lost packets can occur on the IP network for any number of reasons. Packet loss above
1% begins to adversely affect voice quality. To help reduce this problem, the ShoreGear
voice switches have a feature called lost packet concealment. When there is no voice
sample to be played, the last sample available is replayed to the receiving party at a
reduced level. This is repeated until a nominal level is reached, effectively reducing the
clicking and popping associated with low levels of packet loss.
NOTE Fax and modem calls demand essentially zero packet loss to avoid missing lines
on fax calls and to avoid dropped modem calls. In addition, fax and modem
calls, when detected, may change to a higher-rate codec.
Summary of the Network Requirements
Table 9-5 summarizes the network requirements for bandwidth, latency, jitter, and
packet loss.
Impact of Long Network Outages
The ShoreTel 6.1 system is a completely distributed system in which each ShoreGear
voice switch provides all call control functionality for inbound and outbound calls, as
well as features such as transfer, conference, pickup, and trunk selection. When there is
a long network outage, the switches will detect the problem and run isolated from the
switches that can no longer be reached.
In the ShoreTel 6.1 system, switches communicate every 30 seconds and disconnect
when there is no response after 60 seconds.
Bandwidth Management
In addition to the network requirements discussed above, bandwidth management
techniques need to be deployed to ensure that real-time voice data is not affected by
bursts or high amounts of data traffic.
Table 9-5 Network Requirements
Parameter Requirement
Bandwidth With ADPCM and no RTP Header Compression: 52 Kbps per call
With G.729a and no RTP Header Compression: 26 Kbps per call
With G.711 and no RTP Header Compression: 82 Kbps per call
NOTE: If your network uses VPN, bandwidth use is affected.
Latency and jitter for toll
quality
< 100 msecs total
100 msecs less 42 msecs allocated for the ShoreTel 6.1 system yields a
58 msec budget for the network.
When G.729a encoding is used, 100 msecs less 62 msecs allocation for
the ShoreTel 6.1 system yields a 38 msec budget for the network.
Latency and jitter for
acceptable quality
< 150 msecs total
150 msecs less 42 msecs allocated for the ShoreTel 6.1 system yields a
108 msec budget for the network.
When G.729a encoding is used, 150 msecs less 62 msecs allocated for
the ShoreTel 6.1 system yields an 88 msec budget for the network.
Packet loss < 1% for voice calls, and no packet loss for fax and modem calls