Cisco Cisco IOS Software Release 12.2(2)XK
Voice and Quality of Service Features for ADSL on Cisco 2600 and 3600 Series Routers
Feature Overview
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Cisco IOS Release 12.2(2)XK
Link Latency
The following existing Cisco IOS link latency feature is now supported on ADSL:
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Selectable Transmission Ring
The transmission ring (tx ring) is the first-in, first-out (FIFO) buffer used to hold frames before
transmission at the digital subscriber line (DSL) driver level. The tx ring defines the maximum number
of packets that can wait for transmission at Layer 2.
transmission at the digital subscriber line (DSL) driver level. The tx ring defines the maximum number
of packets that can wait for transmission at Layer 2.
The tx ring complements the ability of LLQ to minimize jitter and latency of voice packets. For
maximum voice quality, a low tx ring setting should be used. For maximum data throughput, a high tx
ring setting should be used.
maximum voice quality, a low tx ring setting should be used. For maximum data throughput, a high tx
ring setting should be used.
You can configure the size of the tx ring for each permanent virtual circuit (PVC). The default value is
24. The value can be changed to 3. (The only permitted values are 24 and 3.) A tx ring setting of 3 is
required for latency-critical traffic. For example, when the tx ring limit is configured as 3 and LLQ is
configured on the PVC, the worst case delay for a voice packet is the time required to transmit three data
packets. When the buffering is reduced by configuring the tx ring limit, the delay experienced by voice
packets is reduced by a combination of the tx ring and LLQ mechanism.
24. The value can be changed to 3. (The only permitted values are 24 and 3.) A tx ring setting of 3 is
required for latency-critical traffic. For example, when the tx ring limit is configured as 3 and LLQ is
configured on the PVC, the worst case delay for a voice packet is the time required to transmit three data
packets. When the buffering is reduced by configuring the tx ring limit, the delay experienced by voice
packets is reduced by a combination of the tx ring and LLQ mechanism.
Note
The size of the tx ring buffer is measured in packets, not bits.
Other (IP QoS)
The following IP QoS features are now supported on ADSL:
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Local Policy Routing
For information about local policy routing (LPR), refer to the following documents:
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Configuring IP Routing Protocol—Independent Features
” in the Cisco IOS IP
Configuration Guide, Release 12.2
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Configuring IP Routing Protocols
” in the Router Products Configuration Guide
For information about policy-based routing (PBR), refer to the following document:
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Quality of Service Overview
” in the Cisco IOS Quality of Service Solutions
Configuration Guide, Release 12.2