Voice Level Adjustment, Echo Cancellation | Weed Eater 2600 instruction

Cisco High-Density Analog Voice and Fax Network Module

Feature Overview

Several different types of delay combine to make up the total end-to-end delay associated with voice calls:

•Propagation delay—Amount of time it takes the data to physically travel over the media.

•Handling delay—Amount of time it takes to process data by adding headers, taking samples, forming packets, and so on.

•Queuing delay—Amount of time lost due to congestion.

•Variable delay or jitter—Amount of time that causes the conversation to break and become unintelligible. Jitter is described in detail below.

Note Propagation, handling, and queuing delay are not addressed by voice-portcommands and fall outside the scope of this document.

Voice Level Adjustment

As much as possible, it is desirable to achieve a uniform input decibel level to the packet voice network to limit or eliminate any voice distortion due to incorrect input and output decibel levels. Adjustments to levels may be required by the type of equipment connected to the network or by local country-specific conditions.

Incorrect input or output levels can cause echo, as can an impedance mismatch. Too much input gain can cause clipped or fuzzy voice quality. If the output level is too high at the remote router’s voice port, the local caller hears echo. If the local router’s voice port input decibel level is too high, the remote side hears clipping. If the local router’s voice port input decibel level is too low, or the remote router’s output level is too low, the remote side voice can be distorted at a very low volume and Dual Tone Multi-Frequency (DTMF) may be missed.

Echo Cancellation

Echo is the sound of your own voice reverberating in the telephone receiver while you are talking. When timed properly, echo is not a problem in the conversation; however, if the echo interval exceeds approximately 25 milliseconds, it is distracting. Echo is controlled by echo cancellers.

In the traditional telephony network, echo is generally caused by an impedance mismatch when the

4-wire network is converted to the 2-wire local loop. In voice packet-based networks, echo cancellers are built into the low-bit rate codecs and are operated on each DSP.

Adaptive Jitter Buffering

Delay can cause unnatural starting and stopping of conversations, but variable-length delays (also known as jitter) can cause a conversation to break and become unintelligible. Jitter is not usually a problem with PSTN calls because the bandwidth of calls is fixed and each call has a dedicated circuit for the duration of the call. However, in Voice over IP (VoIP) networks, data traffic might be bursty, and jitter from the packet network can become an issue. Especially during times of network congestion, packets from the same conversation can arrive at different interpacket intervals, disrupting the steady, even delivery needed for voice calls. Cisco voice gateways have built-in jitter buffering to compensate for a certain amount of jitter; the playout-delaycommand can be used to adjust the jitter buffer.

Cisco IOS Release 12.2(2)XT and 12.2(8)T