Understanding the Requirements for Toll-Quality Voice Chapter 9: Network Requirements and Preparation
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For calls between analog telephones, voice bandwidth is used only on the connection
bewteen the voice switches. For calls involving IP telephones, the bandwidth is required
between the IP phones at the user’s desktop. This means that for IP telephones, network
planning must include provisioning capacity to each IP phone.
When SIP is not enabled, RTP traffic is always sent to UDP port 5004. The source port is
random. When SIP is enabled, both the source and destination ports are random.
9.4.2.4 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 can result in poorer voice quality. Consequently, the ShoreTel 8 system
uses 10-msec voice packets on the LAN, where bandwidth is readily available, and 20-msec
voice packets on 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
implementation of cRTP does not increase the latency or jitter of the voice traffic, since this
can have a negative impact on voice quality. If the router does increase latency or jitter with
cRTP, add this to your overall expected latency and make sure you still have sufficient
performance for acceptable voice quality.
Table 9-5 shows the resulting effective WAN bandwidth. It does not include the overhead
associated with the underlying WAN network protocol, such as HDLC, frame relay, ATM,
and VPN; however, the ShoreTel admission control software computes bandwidth
requirements according to the data in this table and assumes a PPP header-size for
computations.
a. When ADPCM using voice encoding, four bytes are added to the voice data for decoding
purposes.
b. G.729a is supported in 10-msec, 20-msec, and 30-msec packets.
c. Ethernet framing = 14 bytes of Ethernet header, a 4-byte checksum, and 8 bytes of addi-
tional 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).
Linear
Broadband Linear G.711 ADPCM G.729a G.729a
Voice data (20
msec)
640 320 160 80+4a20 30
RTP header 12 12 12 12 12 12
UDP header 8 8 8 8 8 8
IP header 20 20 20 20 20 20
PPP header 5 5 5 5 5 5
Total bytes per
packetb
685 365 205 129 65 75
Table 9-5 WAN Bandwidth—Bytes
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