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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 1 Overview
Why is Quality of Service Required for AVVID?
A more accurate method for provisioning VoIP is to include the Layer 2 overhead, which includes:
preambles, headers, flags, CRCs, and ATM cell-padding. The amount of overhead depends on the
technology used:
·
802.1Q Ethernet adds 32 bytes of Layer 2 overhead.
·
Point-to-point protocol (PPP) adds 12 bytes of Layer 2 overhead.
·
Multilink PPP (MLP) adds 13 bytes of Layer 2 overhead.
·
Frame Relay adds 8 bytes of Layer 2 overhead.
·
ATM adds varying amounts of overhead. depending on the cell-padding requirements.
When Layer 2 overhead is included in the bandwidth calculations, the VoIP call requirements translate
to the figures shown in
.
Table 1-2
Voice Bandwidth (with Layer 2 overhead)
Bandwidth requirements for voice traffic range from 21 kbps to 106 kbps, depending on the CODEC,
the sampling rate, and the Layer 2 media used.
shows the bandwidth requirements of a G.711 voice call (64 kbps) over Frame-Relay media.
Figure 1-2
G.711 Pulse-Code Modulation (PCM) Voice-Call Over Frame-Relay
Voice Control Traffic
In centralized call processing designs, the IP phones use a TCP control connection to communicate with
the Cisco CallManager. If there is not enough bandwidth provisioned for these lightweight control
connections, the user might be adversely affected. For example, let's look at the Delay to Dial-Tone
(DTT) time periods. When an IP phone goes off-hook, it "asks" the CallManager what to do. The
CallManager instructs the IP phone to play a Dial-Tone. If control traffic is dropped or delayed within
the network, the user will not get the Dial-Tone played out. This same logic applies to all signaling
traffic for gateways and phones.
For Cisco IP phones, the control traffic required is approximately 150 bps per phone (not including
layer 2 overhead).
Bandwidth
Consumption
802.1Q
Ethernet PPP MLP Frame-Relay
ATM
G.711 at 50 pps
93 kbps
84 kbps
86 kbps
84 kbps
106 kbps
G.711 at 33 pps
83 kbps
77 kbps
78 kbps
77 kbps
84 kbps
G.729A at 50 pps
37 kbps
28 kbps
30 kbps
28 kbps
43 kbps
G.729A at 33 pps
27 kbps
21 kbps
22 kbps
21 kbps
28 kbps
Single PCM VoIP Call
84Kbps
64Kbps
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