Specifications

Network Requirements and Preparation
Planning and Installation Guide 9 – 5
Bandwidth in the LAN
For LAN calls using the voice switches, 10 msecs of voice samples are encapsulated in a
Real Time Protocol (RTP) packet before being transmitted onto the LAN. For IP phones
and SoftPhones, 20 msecs of voice samples are encapsulated in an RTP packet before
being transmitted onto the network.
The protocol overhead consists of 12 bytes for the RTP header, 8 bytes for the UDP
header, 20 bytes for the IP header, and 26 bytes for the Ethernet framing. When
ADPCM voice encoding is used, an additional 4 bytes are added to the voice data for
decoding purposes. This yields an effective LAN bandwidth as shown in Table 9-2.
For calls between analog telephones, voice bandwidth is used only on the connection
to the voice switches. For calls involving IP telephones, the bandwidth is required to
the IP phone at the user’s desktop. This means that for IP telephones, network planning
must include provisioning capacity from each IP phone to its desktop connection.
RTP traffic is always sent to UDP port 5004. The source port is random.
Bandwidth in the WAN
Increasing the number of voice samples per packet decreases the bandwidth required
(since the percentage of signaling overhead is reduced); however, it also increases the
latency of the voice call, which results in poorer voice quality. Consequently, the
ShoreTel 6.1 system uses 10-msec voice packets in the LAN, where bandwidth is
readily available, and 20-msec voice packets in the WAN, where bandwidth
conservation is more important. WAN calls are calls made between ShoreTel system
sites.
For WAN calls, routers with RTP Header Compression (cRTP) reduce the 40 bytes in
the IP + UDP + RTP header to 4 bytes. If you want to use cRTP, make sure the router’s
Table 9-2 LAN Bandwidth—Bytes
Linear
Broadband
Linear G.711 ADPCM G.729a
Voice data (10 msec) 320 160 80 40+4
a
20 (20 msec)
b
RTP header 12 12 12 12 12
UDP header 8 8888
IP header 20 20 20 20 20
Ethernet header and
framing
c
26 26 26 26 26
Total bytes per packet
d
386 226 146 110 86 (20 msec)
Bandwidth for voice only
e
256 Kbps 128 Kbps 64 Kbps 32 Kbps 8 Kbps
Bandwidth with overhead 309 Kbps 181 Kbps 117 Kbps 88 Kbps 34 Kbps
a. When ADPCM voice encoding is used, an additional 4 bytes are added to the voice data for
decoding purposes.
b. G.729a is only supported in 20-msec packets in this release.
c. Ethernet framing = 14 bytes of Ethernet header, a 4-byte checksum, and 8 bytes of additional
framing.
d. Voice data bytes per packet = (# bits/sample) x (8 samples/msec) x (10 msecs/packet) / (8 bits/
byte).
e. Bandwidth = (# bytes/10 msecs) x (8 bits/byte).