Mitel 5560 IPT none Specifications download pdf (Page 215)

Network Configuration Concepts

201

Some carriers may also offer an SLA that honours and provides queuing for incoming (download

to the customer) data as well. There may be an additional charge, but this will provide the added

queuing on the far end of the often bandwidth limited connection between the customer and

the carrier. With the customer providing priority queuing on the outgoing (uplink from the

customer), this link will then have priority queuing at both ends of the connection, to ensure

priority for voice traffic.

If a WAN connection provides both data and voice traffic on a common path, then priority

schemes need to be employed. All IP phones and the 3300 ICP controller use appropriate

Type-of-Service or DiffServ field settings. Priority queuing should be enabled on the end routers,

even if priority is not used within a separate voice network. See the section “Network Priority

Mechanisms” on page 209 for further details.

For more dedicated links, some additional protocols can be used to improve bandwidth usage.

The data in an Ethernet LAN connection includes a data layer for Ethernet and a data layer for

IP. In a WAN connection, the Ethernet layer is not needed. However, other layers are needed

to transport the IP layer and voice data. As a result, certain WAN protocols can use less

bandwidth. These include the more dedicated links such as PPP and compressed PPP.

Transcoding and compression

The terms “transcoding” and “compression” are often used interchangeably. Transcoding is the

changing of voice information from one CODEC type to another. However, most CODEC

devices rely on G.711 as the base entry level. Transcoding from G.729a to G.726 is likely done

through G.711. Compression is simply reducing the amount of data. For voice traffic, this can

be achieved by going from G.711 to G.729a, for example.

Any form of voice compression works by removing a certain amount of information deemed

non-essential. This may include not sending data during silent periods, as well as sending only

the main voice frequency elements rather than the full bandwidth. As a result, some information

is lost. Compressed voice is never as good as uncompressed voice, but the required intelligibility

is maintained. Of the compression CODECs, G.729a has good bandwidth reduction and

maintains good voice quality and intelligibility.

In the LAN environment where bandwidth is plentiful, there is probably little reason to compress

voice, and so G.711 is normally the CODEC of choice. In a WAN environment, where access

bandwidth may be limited, use of the G.729a CODEC can increase the amount of voice traffic

that can be carried on a particular link. In some instances, G.711 is still preferable for voice

quality, but voice traffic will be limited on the link.

Wideband voice

The use of IP and the ability to use bandwidth values that are not directly linked to PSTN BRI

channel limits, allows the use of new CODECs and features.

With Release MCD 5.0, a new wideband audio CODEC has been added to the system capability

and is supported on the IP devices. The new CODEC implemented is G.722.1 and is based on

ITU-T standards. It provides voice capability with a bandwidth of 50Hz to 7kHz, compared to

300-3400Hz for a standard telephony channel.

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Mitel 5560 IPT none Specifications Page 215