Cisco High-Density Analog Voice and Fax Network Module
Feature Overview
Normally, the defaults in effect are sufficient for most networks. However, a small playout delay from the jitter buffer can cause lost packets and choppy audio, and a large playout delay can cause unacceptably high overall end-to-end delay.
Note Prior to Cisco IOS Release 12.1(5)T, playout delay was configured in voice-port configuration mode. For Cisco IOS Release 12.1(5)T and later releases, in most cases, playout delay should be configured in dial-peer configuration mode on the VoIP dial peer that is on the receiving end of the voice traffic that is to be buffered. This dial peer senses network conditions and relays them to the DSPs, which adjust the jitter buffer as necessary. When multiple applications are configured on the gateway, playout delay should be configured in dial-peer configuration mode. When there are numerous dial peers to configure, it might be simpler to configure playout delay on a voice port. If there are conflicting playout delay configurations on a voice port and also on a dial peer, the dial peer configuration takes precedence.
Voice Activity Detection Commands
In normal voice conversations, only one person speaks at a time. Today’s circuit-switched telephone networks dedicate a bidirectional, 64-kbps channel for the duration of each conversation, regardless of whether anyone is speaking at the moment. This means that, in a normal voice conversation, at least 50 percent of the bandwidth is wasted when one or both parties are silent. This figure can actually be much higher when normal pauses and breaks in conversation are taken into account.
Packet-switched voice networks, on the other hand, can use this “wasted” bandwidth for other purposes when voice activity detection (VAD) is configured. VAD works by detecting the magnitude of speech in decibels and deciding when to cut off the voice from being framed. VAD has some technological problems, however, which include the following:
•General difficulties in determining when speech ends
•Clipped speech when VAD is slow to detect that speech is beginning again
•Automatic disabling of VAD when conversations take place in noisy surroundings
Benefits
Cost Effective
•Higher density of analog interfaces provides voice and data over a single network
–16 analog voice ports in a single network module (NM)
•Connects existing analog voice telephone sets, fax machines, and key systems using Cisco gateways
Scalability-Modularity and Flexibility
•Eliminates the need for a channel bank
•The network module may be configured to be an FXO, an FXS or a combination module
Serviceability
•Provides remote monitoring, testing, and diagnoses
Cisco IOS Release 12.2(2)XT and 12.2(8)T
