Mitel MiVOICE BUSINESS, Инструкция по инженерии

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Network Configuration Concepts
203
GENERAL GUIDELINES FOR QUALITY OF SERVICE
The main issues that affect system installation and user perceptions are
• Quality of service (voice quality during the call)
• Availability of the service (setting up and clearing voice connections or signalling)
The challenge is to engineer the network to ensure that these quality requirements are met.
With TDM, this is possible by providing dedicated connections to the desk. With IP, the network
may have to share connections with other devices, such as PCs. The requirements of the PC
and an IP phone differ: PCs need to send data as quickly as possible using all available
bandwidth, but IP phones must send and receive limited data on a very regular basis with little
variation (jitter).
In summary, the challenge is to place connection-oriented devices into a connectionless
environment and still maintain the expected operation.
The sections below discuss several concepts related to Quality of Service.
BASIC CONCEPTS
The sections below describe some areas that affect installation and briefly explain their
importance.
DELAY
As delay increases in a conversation it becomes increasingly difficult to sustain normal two-way
communication. Such a conversation rapidly changes from an interactive exchange to an “over
to you” radio-style conversation. The delay is noticeable at a 150 ms to 200 ms delay, and is
radio-style by a 400 ms delay. The phones and gateway in the controller introduce some
necessary delay. These guidelines identify the delays that can be tolerated to ensure that voice
quality is maintained.
ECHO
Echo generally results from poor termination of a PSTN line or acoustic feedback. When delay
is short, echo is usually not heard due to the level of local sidetone. But as delay is introduced,
this echo becomes noticeable. To counteract this, the gateway device includes echo
cancellation up to 64 ms looking towards the PSTN. The IP phone includes echo-suppression
to remove acoustic echo.
JITTER
Jitter is the variation in delay that can occur in networks. The major source of jitter is serialization
delay, which occurs when a packet cannot be sent at the ideal time because another packet is
already being sent on the same connection. The result is that the packet must wait. For
high-speed links, a maximum packet size of about 1500 bytes is sent in microseconds, so jitter
is negligible. However, for slower WAN connections, such as a Frame Relay connection, the
delay becomes significant.