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Quality of Service guidelines
318 Avaya Application Solutions IP Telephony Deployment Guide
serialization delay for a variety of packet sizes and line speeds. The formula for determining
serialization delay is:
Because of all these configuration variables, properly implementing QoS on a router is no trivial
task. However, QoS is needed most on the router because most WAN circuits terminate on
routers.
QoS guidelines
There is no all-inclusive rule regarding the implementation of QoS because all networks and
their traffic characteristics are unique. It is good practice to baseline the IP Telephony response
on a network without QoS, and then apply QoS as necessary. Avaya Network Consulting
Services can help with baselining services. Conversely, it is bad practice to enable multiple QoS
features simultaneously, not knowing what effects, if any, each feature is introducing.
Generally, for newer network equipment, best practices involve enabling Layer 3 (DiffServ) QoS
on WAN links traversed by voice. Tag voice and data with DiffServ Code Point 46 (Expedited
Forwarding), and set up a strict priority queue for voice. If voice quality is still not acceptable, or
if QoS is desired for contingencies such as unexpected traffic storms, QoS can then be
implemented on the LAN segments as necessary.
Table 53: Serialization delay matrix
WAN
line
speed
Packet size
64
bytes
128
bytes
256
bytes
512
bytes
1024
bytes
1500
bytes
56 kbps 9 ms 18 ms 36 ms 72 ms 144 ms 214 ms
64 kbps 8 ms 16 ms 32 ms 64 ms 128 ms 187 ms
128 kbps 4 ms 8 ms 16 ms 32 ms 64 ms 93 ms
256 kbps 2 ms 4 ms 8 ms 16 ms 32 ms 46 ms
512 kbps 1 ms 2 ms 4 ms 8 ms 16 ms 23 ms
768 kbps 640 µs 1.28 ms 2.56 ms 5.12 ms 10.24 ms 15 ms
Serialization Delay
Packet Size (bits)
Line Speed
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