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IP Telephony network engineering overview
In general, these concerns dictate a hierarchical network that consists of at most three layers
(Table 65: Layers in a hierarchical network
Core
Distribution
Access
Some smaller networks can collapse the functions of several layers into one device.
Table 65: Layers in a hierarchical network
Layer
Core
Distribution
Access
For IP Telephony to work well, WAN links must be properly sized with sufficient bandwidth for
voice and data traffic. Each voice call uses between 6.3 Kbps and 80 Kbps, depending on the
desired codec, quality, and header compression used. G.729, which uses 24 Kbps of
bandwidth, is one of the most promising standards today. Traditional telephone metrics, such as
average call volume, peak volume, and average call length, can be used to size interoffice
bandwidth demands. See
Quality of Service (QoS) also becomes increasingly important with WAN circuits. In this case,
QoS means the classification and the prioritization of voice traffic. Voice traffic must be given
absolute priority through the WAN. If links are not properly sized or queuing strategies are not
properly implemented, the quality and the timeliness of voice and data traffic will be less than
optimal.
278 Avaya Application Solutions IP Telephony Deployment Guide
Description
The core layer is the heart of the network. The purpose of the core
layer is to forward packets as quickly as possible. The core layer
must be designed with high availability in mind. Generally, these
high-availability features include redundant devices, redundant power
supplies, redundant processors, and redundant links. Today, core
interconnections increasingly use Gigabit Ethernet.
The distribution layer links the access layer with the core. The
distribution layer is where QoS feature and access lists are applied.
Generally, Gigabit Ethernet connects to the core, and either Gigabit
Ethernet or 100base-TX/FX links connect the access layer.
Redundancy is important at this layer, but not as important as in the
core.
The access layer connects servers and workstations. Switches at this
layer are smaller, usually 24 to 48 ports. Desktop computers and
workstations are usually connected at 10 Mbps (or 10 Mbps), and
servers are connected at 100 Mbps (or 1 Gbps). Limited redundancy
is used. Some QoS and security features can be implemented in the
access layer.
Traffic engineering
on page 278):
for more information.
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