Planning and Installation Guide Chapter 9: Network Requirements and Preparation
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toll-quality voice. The ability of your network to deliver this performance will vary based
on the number of simultaneous calls between locations, the voice quality required, and the
other application traffic on the network. Some of the key considerations are:
Bandwidth
Service levels
Addressing
9.3 Advantages of Voice Over IP
Going back to the basics of voice, consider a traditional call over the Public Switched
Telephone Network (PSTN). The PSTN is a circuit-switched network. A telephone call
reserves an end-to-end physical circuit for the duration of the call. This circuit consists of
many subsegments within the PSTN, and a subsequent call between the same two
endpoints may follow a different path. However, for the duration of the call, the circuit is
fully available to that single call.
Packet-switched networks, such as the Internet, do not reserve a circuit between endpoints.
Instead, messages or files are broken into many small packets. These packets may take
different routes from source to destination, traveling along network circuits that are shared
with packets from other sources. These packets travel to the final destination, where they
are reconstructed into the original message or file.
One analogy between circuit-switched and packet-switched networks is that of railway
versus roadway transportation systems. A railway is similar to a circuit-switched network.
The path of the train is essentially reserved, and the whole train travels intact from source
to destination. A roadway, on the other hand, is shared among many smaller units, each
having the intelligence to find its destination. The railway provides a clear end-to-end path,
relatively immune to delays, but at a high overhead cost. The roadway can be used more
efficiently, but it is vulnerable to congestion.
The advantage of circuit-switched networks is that they provide dedicated bandwidth
between endpoints and therefore can easily guarantee a known, consistent quality of
service. Their disadvantage is their poor utilization of network resources, since they
demand a dedicated, separate network relative to the packet-switched network. Conversely,
the advantage of packet-switched networks is that they provide better utilization of
network resources, enable flexible traffic routing, provide a single network to manage,
allow for standard voice and data monitoring tools to be used, allow applications to be
shared over a common network, and enable applications to become more portable—and
this is just the beginning.
9.4 Understanding the Requirements for Toll-Quality
Voice
The ShoreTel system has been designed to deliver the highest possible voice quality. In fact,
third-party testing by Miercom has confirmed that the ShoreTel system provides both low
latency and high voice quality.
With the superior design of the ShoreTel system, all that is needed to achieve toll-quality
voice communications is to deploy the system over a properly designed network
infrastructure. This section provides you with the background to understand the factors
involved in engineering an IP network that is ready for voice communications.
In general, to ensure voice quality on the LAN, the ShoreTel system must be used in a
switched Ethernet network. To ensure voice quality on the WAN, the ShoreTel system
requires that you do the following:
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