Voice Compression and Decompression, MC-619

Configuring Subscriber-End Broadband Access Router Features

Voice over IP Operations

•Fax devices—standard Group III and computer-based Group III machines up to 14,400 baud—are supported in Cisco IOS images that support VoIP.

•In general, fax/modem cards are not supported over VoIP links.

Contact your network management, provisioning, or operations team to determine what your network supports.

Voice Compression and Decompression

The Cisco uBR924 cable access router supports the following compression and decompression algorithms (codecs):

•G.711 A-law 64000 bps

•G.711 U-law 64000 bps

•G.723.1 5300 bps

•G.723.1 6300 bps

•G.726 16000 bps

•G.726 24000 bps

•G.726 32000 bps

•G.728 16000 bps

•G.729 Annex A 8000 bps

•G.729 8000 bps (default codec for telephone calls)

Note Because voice transmission is delay-sensitive, a well-engineered network is critical. Fine-tuning your network to adequately support VoIP typically involves a series of protocols and features geared to support QoS.

To achieve acceptable voice quality and reduce network bandwidth usage, several voice processing techniques and services are employed, including echo cancellation, voice compression, voice activity detection (VAD) or silence compression, and dual tone multifrequency (DTMF) tone detection and generation.

The Cisco uBR924 cable access router supports multiple QoS service IDs (SIDs), enabling multiple classes of service on the cable interface. This enables VoIP and data traffic to be treated separately, with all data assigned to a default class of service, while VoIP traffic is assigned to a different class of service. Thus, voice traffic from the Cisco uBR924 telephone ports can take precedence over the data traffic coming from the Ethernet interfaces.

Note Separate class of service (CoS) streams are only available when the Cisco uBR924 is connected to a CMTS that supports multiple classes of service per router. In addition, the router configuration file must specify the use of multiple classes of service.

If the Cisco uBR924 interoperates with a DOCSIS 1.0 CMTS that does not support multiple CoS per router, voice traffic will be transmitted on a best-effort basis along with data traffic. This may cause poorer voice quality and lower data throughput when calls are being made from the router telephone ports.

Cisco IOS Multiservice Applications Configuration Guide