Network Requirements and Preparation
9 ?
6
ShoreTel, Inc.
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 9-3 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.
Latency
Latency is the amount of time it takes for one person?s voice to be sampled, packetized,
sent over the IP network, de-packetized, and replayed to another person. This one-way
delay, from ?mouth-to-ear," must not exceed 100 msecs for toll-quality voice, or 150
msecs for acceptable-quality voice. If the latency is too high, it interrupts the natural
flow of the conversation, causing the two parties to confuse the latency for pauses in
speech. The resulting conversation is reminiscent of international calls over satellite
facilities.
The latency introduced by the ShoreTel 6.1 system can be understood as follows: When
a person talks, the voice is sampled by the ShoreGear voice switch, generating a latency
of 5 msecs. If the call does not traverse switches and is handled completely internally
by the switch, the latency is generated by the basic internal pipeline of the switch. In
this case, the switch samples the voice, processes it, combines it with other voice
streams (switchboard), and then converts it back to audio for output to the phone in
5-msec packets, for a total latency of about 17 msecs.
When the call transfers between switches, the voice is packetized in larger packets?
10-msec for LAN and 20-msec for WAN?to reduce network overhead. The larger
packets take more time to accumulate and convert to RTP before being sent out. On the
receive side, the incoming packets are decoded and placed in the queue for the
switchboard. For a 10-msec packet, this additional send/receive time is approximately
15 msecs, and for a 20-msec packet it is about 25 msecs.
For IP phones, the latency is 20 ms in the LAN and 30ms in the WAN.
Table 9-3 WAN Bandwidth?Bytes
Linear Broadband
Linear
G.711
ADPCM
G.729a
Voice data (20 msec)
640
320
160
80+4
a
20
RTP header
12
12
12
12
12
UDP header
8
8
8
8
8
IP header
20
20
20
20
20
PPP header
5
5
5
5
5
Total bytes per packet
b
685
365
205
129
65
Bandwidth for voice only
c
256 Kbps
128 Kbps 64 Kbps
32 Kbps
8 Kbps
Bandwidth including overhead 284 Kbps
146 Kbps 82 Kbps
52 Kbps
26 Kbps
Bandwidth after cRTP
260 Kbps
132 Kbps 68 Kbps
37 Kbps
12 Kbps
a. When ADPCM voice encoding is used, an additional 4 bytes are added to the voice data for decoding
purposes.
b. Voice data bytes per packet = (# bits/sample) x (8 samples/msec) x (20 msecs/packet) / (8 bits/byte)
c. Bandwidth = (# bytes/20 msecs) x (8 bits/byte)
