5-4
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 5 QoS in a SOHO Virtual Private Network for IP Telephony
QoS Toolset
Tip
Currently, IOS does not automatically optimize the fragment sizes (which are indirectly defined by the
MLP fragment-delay parameter). Arriving at such optimal sizes requires a fair bit of math, which is well
documented in the Multilink PPP over Frame Relay and ATM white paper (internal).
provides a list of often used PVC speeds and the corresponding fragment delay and interface
bandwidths required to achieve the desired fragment size and delay.
Table 5-1
Common ATM Virtual Circuit Speeds and Maximum Delay and Bandwidths
Traffic Shaping
Traffic shaping is required in SOHO environments to insure that the SP guaranteed rate is not exceeded.
Traffic in excess of the guarantee could result in drops or queuing that would defeat the QoS measures
(LLQ/CBWFQ, and LFI) that have been applied at the edge of the network. To avoid congestion in the
virtual path through the SP network, traffic in the direction of the SP should be rate-limited to the
negotiated SP guaranteed rate.
PVC Speed
(in Kbps)
Fragmentation
Size
(in cells)
PPP Multilink
Fragment Delay
(in msec)
Bandwidth
(in Kbps)
Real Delay
(in msec)
56
2
12
57
13.7
64
2
10
68
12.0
128
4
11
132
12.0
192
6
11
202
12.0
256
7
10
260
10.5
320
9
10
337
10.8
384
11
10
414
11.0
448
12
10
452
10.3
512
14
10
529
10.5
576
16
10
606
10.7
640
17
10
644
10.2
704
19
10
721
10.4
768
21
10
798
10.5