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5-3
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 5 QoS in a SOHO Virtual Private Network for IP Telephony
QoS Toolset
Classification of Voice Signaling Traffic
In a SOHO AVVID network, signaling traffic should be assigned a DSCP PHB label of AF31, an IP
Precedence value of 3, and a CoS marking of 3.
Scheduling
The IOS feature of LLQ/CBWFQ is used in a SOHO environment to schedule the voice bearer and voice
control traffic preferentially over all other types of traffic if there is congestion on the link.
For general recommendations for classification, see
"Scheduling Recommendations" section on
page 1-22
.
Provisioning
In a SOHO/VPN environment there are many aspects to consider when provisioning the CPE device,
such as TX ring sizing, link fragmentation and interleave, traffic shaping, and bandwidth calculation.
TX Ring Sizing
On ATM interfaces, which are the transport medium for many DSL environments, the default depth of
the TX ring is very large. However, a large TX ring and LLQ/CBWFQ do not work well together.
When congestion occurs and LLQ/CBWFQ is engaged (to give voice packets preferential treatment), the
TX ring must be emptied before the first expedited voice packet can get serialized by the physical
interface and be transmitted. If the TX ring is very large, it can take some time to empty before the voice
packets are serviced. This results in very short periods of poor voice quality followed by long period of
excellent voice quality. The solution is to reduce the TX ring size.
Link Fragmentation and Interleave
In DSL environments that use ATM as the transport medium, MLP with fragmentation and interleave is
used as a Layer 2 LFI mechanism. Depending on the underlying (Layer 2) transport, other LFI
mechanisms may be required.
Today's cable deployments use link speeds in excess of 768 Kbps. At link speeds greater than 768 Kbps,
the variation in delay introduced by random-sized packets is not of significant concern and LFI is not
required.
Fragment Sizing for MLPPP over ATM
When ATM is the underlying transport technology in a DSL network, there are 53 byte cells for transport
with 48 bytes of usable payload. Therefore, the fragment size selected must be divisible by 48 with a
remainder of zero so that partial cells are not transmitted. MLPPP with fragmentation and interleave
uses a maximum delay parameter and calculates the fragment size based on the bandwidth of the
interface. Some manipulation of the interface bandwidth statement and max delay statement is required
to arrive at a fragment size that cleanly maps into ATM cells.