Voice Compression Channels; Qos - Avaya 4600 Series Installation Manual

IP Phone Installation Manual

Voice Compression Channels

IP Office Voice compression channels are used when a voice call goes between a device on the IP
Office's data network, that is an IP trunk or extension, and a device on the IP Office's TDM telephony
interface, that is a non-IP trunk or extension.
When using Direct Media (the default), calls between IP device (trunks or extensions) do not normally
need a voice compression channel once the call is connected. However they do use a VCM channel for
call signalling tones, music-on-hold, etc.
Voice compression channels are provided by installing a VCM card into the IP Office control unit. These
cards are available in a number of different capacities, ie. number of voice compression channels. The
maximum capacity of an IP Office system depend on the control unit type as follows:
IP Office Control
Unit
Small Office Edition
IP403
IP406 V1
IP406 V2
IP412
The voice compression channels support G.723 (6k3) and G.729a (8k) compression codecs and provide
echo cancellation for high latency circuits. Avaya H.323 IP phones support G.711, G.729a and G.729b,
thus G.729a is normally auto-negotiated when these phones are used on IP Office.
VCM cards exist with the following capacities.
• VCM 5, VCM 10, VCM 20 and VCM 30: These cards support 25ms echo cancellation.
• VCM 4, VCM 8, VCM 16 and VCM 24: These cards support 64ms echo cancellation.

QoS

When transporting voice over low speed links it is possible for normal data packets (1500 byte packets)
to prevent or delay voice packets (typically 67 or 31 bytes) from getting across the link. This can cause a
very unacceptable speech quality.
Thus it is vital that all traffic routers and switches in the network to have some form of Quality of Service
mechanism (QoS). QoS routers are essential to ensure low speech latency and to maintain sufficient
audible quality.
IP Office supports the DiffServ (RFC2474) QoS mechanism. This is based upon using a Type of Service
(ToS) field in the IP packet header. On its WAN interfaces, the IP Office use this to prioritizes voice and
voice signalling packets. It also fragments large data packets and where supported provides VoIP
header compression to minimize the WAN overhead.
Note that the IP Office does not perform QoS for its Ethernet ports including the WAN Ethernet port on
the Small Office Edition.
IP Phone Installation
IP Office 3.1
Voice Compression Channel Capacity
Supplied with either 3 or 16 VCM channels pre-built into the unit. These cannot
be upgraded
Supports a single VCM card with up to 20 channels.
Supports a single VCM card with up to 20 channels.
Supports a single VCM card with up to 30 channels.
Supports any two VCM cards totalling up to 60 channels.
Issue 11c (14th September 2005)
Page 12