Mitel MiVOICE BUSINESS, Инструкция по инженерии

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Engineering Guidelines
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Extensive use of hubs rather than switches also introduces jitter. Hub use for larger networks
and where connections are shared with data devices is not advised.
Use of multiple WAN connections and load-sharing can also introduce jitter due to different
path delays. Ideally, voice should pass down one path or another and may be configurable
based on TOS/DiffServ values.
PACKET LOSS
Packet loss within the network can occur for a number of reasons, mainly congestion of a
connection, where the buffers can overflow and data is lost. Packets may also be discarded at
the gateway or IP phone because the jitter is so variable that the packet arrives too late to be
used for voice. Out-of-sequence packets can also occur over WAN connections. These are like
packets with excessive jitter and can also result in packet discard. Incorrect duplex settings on
LAN connections can also lead to data collisions and packet loss.
Although some packet loss can be handled on an ongoing basis, bursts of packet loss will
become noticeable. A network with 0.1% packet loss over time sounds much different than a
network with the same loss but occurring in bursts of three or more packets.
AVAILABLE BANDWIDTH
If a connection is rated at a particular bandwidth, this does not necessarily mean that all of this
bandwidth is available. Connections between LAN and WAN network devices include a certain
amount of overhead for inter-device traffic, including link termination devices and general
broadcast traffic. A collision in a shared network and guard time between packets also reduces
the available time in which data can be sent, because the data is asynchronous to the
connection. TDM takes care of this through strategies such as framing and clock
synchronization. In summary, the available bandwidth is always less than the connection
bandwidth.
PACKET PRIORITY MECHANISMS
In a network oriented towards data devices, absolute delay is not as important as accuracy.
For voice traffic, however, a certain amount of incorrect or lost information is acceptable, but
information delivered in an untimely manner is not. It is important to ensure that any voice traffic
gets “pushed” to the front of any connection queue. If PC-type data is slightly delayed, this is
less important. There are two similar mechanisms at work to determine priority: 802.1p/Q at
Layer 2 and Diffserv (formerly Type of Service) at Layer 3.
WAN CONNECTIONS
The best Quality of Service is obtained when the customer has control of the external WAN
connections. This can be achieved by using dedicated leased lines between sites, or by
ensuring a guaranteed service-level agreement (SLA) from the external network provider (ISP).
When specifying an SLA it is important that the guaranteed committed information rate (CIR)
is specified and includes a guard band. Data sent in excess of the CIR is likely to be discarded
during congestion periods in order to maintain guarantees on the SLA. It may also be
advantageous to split voice traffic from normal data traffic with different SLAs.