Cisco 521SG Administration Guide - Page 35

Getting Started Ensuring Voice Quality 1 • Echo-Impedance mismatch between the telephone and the IP telephony gateway phone port can lead to near-end echo. Cisco SPA IP phones have a near-end echo canceller with at least 8 ms tail length to compensate for impedance mismatch. Cisco SPA IP phones implement an echo suppressor with CNG so that any residual echo is not noticeable. • Hardware noise-Certain levels of noise can be coupled into the conversational audio signals because of the hardware design. The source can be ambient noise or 60 Hz noise from the power adaptor. The Cisco hardware design minimizes noise coupling. • End-to-end delay-End-to-end delay does not affect voice quality directly, but is an important factor in determining whether IP phone subscribers can interact normally in a conversation. A reasonable delay should be 50 to 100 ms. End-to-end delay larger than 300 ms is unacceptable to most callers. Cisco SPA IP phones support end-to-end delays well within acceptable thresholds. • Adjustable Audio Frames Per Packet-Allows you to set the number of audio frames contained in one RTP packet. Packets can be adjusted to contain from 1-10 audio frames. Increasing the number of frames decreases the bandwidth utilized, but it also increases delay and can affect voice quality. SIP Publish Signaling Improvements The Cisco SPA IP phones resend the SIP PUBLISH messages with the voice quality report once per 5xx response with a valid Retry-After header. A valid time value in seconds is a positive integer from 0 to 65536. A SIP message with a Retry-After time value of 0 is treated as a 500 Server Internal Error message. A time value less than 0 is ignored. The following is a summary of the 5xx messages with Retry-After header that the phone supports: 5xx SIP Response 500 Server Internal Error 503 Service Unavailable Description An unexpected server condition prevents the fulfillment of request. The server is unavailable due to a temporary overload or maintenance. Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide 34