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Configuring Subscriber-End Broadband Access Router Features
Voice over IP Operations
MC-619
Cisco IOS Multiservice Applications Configuration Guide
• 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.