Nortel VoIP Gateway, Configuration Manual, Page: 163 / 220

Further network analysis 163
Norstar VoIP Gateway Configuration Guide
Implementing the network, LAN engineering
To minimize the number of router hops between the systems, connect the gateways to the intranet.
Ensure that there is enough bandwidth on the WAN links shorter routes. Place the gateway and the
LAN router near the WAN backbone. This prevents division of the constant bit-rate IP telephony
traffic from bursty LAN traffic, and makes easier the end-to-end Quality of Service engineering
for packet delay, jitter and packet loss.
Further network analysis
This section describes how to examine the sources of delay and error in the intranet. It also
discusses several methods for reducing one-way delay and packet loss.
The key methods are:
• “Components of delay” on page 163
• “Reducing link delay” on page 164
• “Reducing hop count” on page 164
• “Routing issues” on page 165
Components of delay
End-to-end delay is the result of many delay components. The major components of delay are:
• Propagation delay: Propagation delay is the result of the distance and the medium of links
moved across. Within a country, the one-way propagation delay over terrestrial lines is under
18 ms. Within the U.S., the propagation delay from coast-to-coast is under 40 ms. To estimate
the propagation delay of long-haul and trans-oceanic circuits, use the rule of thumb of 1 ms per
100 terrestrial miles.
If a circuit goes through a satellite system, estimate each hop between earth stations adds 260
ms to the propagation delay.
• Serialization delay: The serialization delay is the time it takes to transmit the voice packet one
bit at a time over a WAN link. The serialization delay depends on the voice packet size and the
link bandwidth, and is the result of the following formula:
serialization delay in ms = 8(IP packet size in bytes/link bandwidth in kbit/s)
• Queuing delay: The queuing delay is the time it takes for a packet to wait in the transmission
queue of the link before it is serialized. On a link where packets are processed in a first-come
first-served order, the average queuing time is in milliseconds and is the result of the following
formula:
queuing time in ms = 8(average IP packet size in bytes/(1-p)(link bandwidth in kbit/s))
The average size of intranet packets carried over WAN links generally is between 250 and 500
bytes. Queueing delays can be important for links with bandwidth under 512 kbit/s, while with
higher speed links they can allow higher utilization levels.