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Network design
This section discusses the network design process for IP Telephony. This section focuses on:
LAN issues
IP addressing
IP terminals deployment
WAN
VPN
NAT
LAN issues
This section covers:
General guidelines
VLANS
LAN issues, including speed and duplex, inline power, hubs versus switches, and so on.
General guidelines
Because of the time-sensitive nature of IP Telephony applications, IP Telephony should be
implemented on an entirely switched network. Ethernet collisions, which are a major contributor
to delay and jitter, are virtually eliminated on switched networks. Additionally, the C-LAN,
MedPro, and IP Telephones should be placed on a separate subnetwork or VLAN (that is,
separated from other non-IP Telephony hosts). This separation provides for a cleaner design
where IP Telephony hosts are not subjected to broadcasts from other hosts, and where
troubleshooting is simplified. This separation also provides a routed boundary between the IP
Telephony segments and the rest of the enterprise network, where restrictions can be placed to
prevent unwanted traffic from crossing the boundary. When personal computers are attached to
IP Telephones, the uplink to the Ethernet switch should be a 100-Mbps link, so that there is
more bandwidth to be shared between the telephone and the computer.
Sometimes enterprises are unable to follow these guidelines, and Avaya's solutions can be
made to work in some less-than-ideal circumstances. If IP Telephones will share a subnetwork
with other hosts, the IP Telephones should be placed on a subnetwork of manageable size
(24-bit subnet mask or larger, with 254 hosts or less), with as low a rate of broadcasts as
possible. If the broadcast level is high, remember that 100-Mbps links are less likely to be
Issue 3.4.1 June 2005
283
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