1-24
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 1 Overview
What is the Quality of Service Toolset?
Note
In the context of Link-Efficiency Mechanisms, slow-speed links are defined as WAN links with clock
speeds of 768 kbps or less.
Figure 1-15 Fragmentation and Interleaving to Reduce Serialization Delay
LFI belongs to the Layer 2 queueing subsystem and operates on packets that have already exited the
Layer 3 queueing subsystem (LLQ/CBWFQ). As illustrated in
, only packets that are not
assigned to the LLQ are fragmented.
shows the serialization delays of various packet sizes on increasingly faster links.
Table 1-4
Serialization Delays at Various Clocking Speeds
A maximum of 10 ms serialization delay is the recommended target to use for setting fragmentation size,
as this allows adequate time for end-to-end delay required by voice (150-200 ms).
74644
Serialization Delay =
Frame Size
Before
Link Speed
After using LFI Tools
Data
Data
Voice
Data
60 Byte Voice
1500 Byte Data Frame
214 ms Serialization Delay
for 1500 Byte Frame at 56 kbps
Link Speed
Delay at 64
Bytes
Delay at 128
Bytes
Delay at 256
Bytes
Delay at 512
Bytes
Delay at 1024
Bytes
Delay at 1500
Bytes
56kbps
9 ms
18 ms
36 ms
72 ms
144 ms
214 ms
64kbps
8 ms
16 ms
32 ms
64 ms
128 ms
187 ms
128kbps
4 ms
8 ms
16 ms
32 ms
64 ms
93 ms
256kbps
2 ms
4 ms
8 ms
16 ms
32 ms
46 ms
512kbps
1 ms
2 ms
4 ms
8 ms
16 ms
23 ms
768kbps
640 µs
1.28 ms
2.56 ms
5.12 ms
10.4 ms
15 ms