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IP Telephony network engineering overview
Common issues
Some common negative practices that can severely impact network performance, especially
when using IP Telephony, include:
A flat, non hierarchical network, for example, cascading small workgroup switches
together. This technique quickly results in bottlenecks, because all traffic must flow across
the uplinks at a maximum of 1Gbps, versus traversing switch fabric at speeds up to 256
Gbps. The greater the number of small switches or layers, the greater the number of
uplinks, and the lower the bandwidth for an individual connection. Under a network of this
type, voice performance can quickly degrade to an unacceptable level.
Multiple subnets on a VLAN. A network of this type can have issues with broadcasts,
multicasts, and routing protocol updates. This practice can have a significant negative
impact on voice performance, and complicate troubleshooting.
A hub-based network. Hubs in a network create some interesting challenges for
administrators. It is advisable not to link more than four 10baseT hubs or two 100baseT
hubs together. Also, the collision domain, the number of ports that are connected by hubs
without a switch or router in between, should be kept as low as possible. Finally, the
effective (half-duplex) bandwidth that is available on a shared collision domain is
approximately 35% of the total bandwidth that is available.
Too many access lists. Access lists slow down a router. While access lists are
appropriate for voice networks, care must be taken not to apply them to unnecessary
interfaces. Traffic should be modeled beforehand, and access lists applied only to the
appropriate interface in the appropriate direction, not to all interfaces in all directions.
Avaya recommends caution when using the following:
Network Address Translation (NAT). Most implementations that use IP Telephony
endpoints behind NAT fail because many H.323 messages (the protocol carrying the voice
information) contain multiple instances of the same IP address in a given message, but
NAT is unlikely to find and translate all of them. See
on using NAT with IP Telephony. Avaya products work seamlessly with static NAT
implementation, even if that NAT is not H.323-aware.
Analog dial-up. Be careful in using analog dial-up (56 K) to connect two locations.
Upstream bandwidth is limited to a maximum of 33.6 K, and in most cases is less. This
results in insufficient bandwidth to provide toll-quality voice. Some codecs and network
parameters provide connections that are acceptable, but consider each connection
individually.
Virtual Private Network (VPN). Large delays are inherent in some VPN software products
due to encryption, decryption, and additional encapsulation. Some hardware-based
products, including Avaya VPN products, encrypt at near wire speed, and can be used. In
addition, if the VPN is run over the Internet, sufficient quality for voice cannot be
guaranteed unless delay, jitter, and packet loss are contained within the parameters that
are listed above. See
282 Avaya Application Solutions IP Telephony Deployment Guide
VPN
for more information.
NAT
on page 310 for more information
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