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ITG Engineering Guidelines Page 127 of
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ITG Trunk 2.0 ISDN Signaling Link (ISL) Description, Installation and Operation
In most corporate environments, the intranet is primarily supporting data and
other services. When planning to offer voice services over the intranet the
technician needs to assess the following:
• Are there existing QoS mechanisms? What kind? The ITG traffic should
take advantage of established mechanisms if possible.
• What is the traffic mix? If the ITG traffic is small compared to data traffic
on the intranet, then IP QoS mechanisms can suffice. If ITG traffic is
significant, data services might be impacted when those mechanisms are
biased toward ITG traffic.
TCP traffic behavior
The majority of corporate intranet traffic is TCP-based. Unlike UDP which
has no flow control, TCP uses a sliding window flow control mechanism.
Under this scheme TCP increases its window size, increasing throughput,
until congestion occurs. Congestion is detected by packet losses, and when
that happens the throughput is quickly throttled down, and the whole cycle
repeats. When multiple TCP sessions flow over few bottleneck links in the
intranet, the flow control algorithm can cause TCP sessions in the network to
throttle at the same time, resulting in a periodic and synchronized surge and
ebb in traffic flows. WAN links would appear to be congested at one time,
and then followed by a period of under-utilization. There are two
consequences:
• poor efficiency of WAN links, and
• ITG traffic streams are unfairly affected
ITG support for TOS field and IP QoS
You can configure the DiffServ/TOS value for Control and Voice packets, if
required, to obtain better QoS over the IP data network (LAN/WAN). Do not
change DiffServ/TOS from default value of 0 unless instructed by the IP
network administrator.
The Type of Service (TOS) byte or Differentiated Service (DiffServ) code
point determine the priority of the control and voice packets in the network
router queues. The values entered in these two boxes must be coordinated
across the entire IP data network. Do not change them arbitrarily.