User manual

4200 User Manual Edgewater Networks, Inc.
Version 2.2 33
A Closer Look at Traffic Management in the 4200
The traffic management mechanisms provided by the 4200 are designed to ensure
high priority real time voice traffic is processed before lower priority data traffic. At
the same time, bandwidth not in use by voice traffic is made available so that data
traffic can burst up to full line rate making efficient use of WAN bandwidth. Traffic
management mechanisms are applied to traffic in both the upstream (LAN to WAN)
and downstream (WAN to LAN) direction. Each direction is independent of the other
and can support different size priority queues. This is particularly useful in the case
of ADSL where the downstream bandwidth is greater than the upstream bandwidth
and it would be undesirable to limit downstream data traffic to the rate of the slower
upstream link.
Classifying
High priority voice traffic generated by endpoint devices such as IP phone and client
adaptors are identified by their IP address. The user configures these addresses into
a priority list using the traffic shaping section of the 4200 web GUI. As the 4200
processes packets they are marked as either high or low priority based on this
configuration.
Upstream Traffic Management
The 4200 appliance uses a combination of Class Based Queuing and simple classless
queuing to send data in the upstream direction. The Class Based Queue (CBQ)
consists of two priority classes (high and low), a scheduler to decide when packets
need to be sent earlier than others and a traffic shaper to rate limit by delaying
packets before they are sent. Voice traffic is placed in the high priority class and
data traffic is placed in the low priority class. High priority data is sent out at up to
the configured priority data rate and this class is polled before lower priority data to
reduce overall latency for voice traffic. Although preferential treatment is given to
priority data it is bounded so that low priority data is not starved. To smooth bursts
from high speed data links (typically from the LAN Ethernet segment to the WAN)
the 4200 appliance uses a buffer that clocks data out at a rate not exceeding the
maximum amount for the slowest link. Any lasting burst condition will cause packets
to be delayed and then dropped.
Downstream Traffic Management
In the upstream direction (LAN to WAN) it is easy to see how QoS mechanisms can
be applied to traffic being sent by the 4200 to guarantee sufficient bandwidth for
voice traffic. We have control over how packets are handed to the WAN interface.
In the downstream direction (WAN to LAN) we are installed at the CPE end of a
service provider link and have no control over the amount of voice or data traffic
being sent to the WAN interface. How then can we still guarantee the quality of
voice traffic when it is entirely possible for an FTP session, for example, to consume
the vast majority of downstream bandwidth?
Fortunately this is possible by shaping on both the egress LAN and egress WAN ports
of the 4200 appliance and leveraging the congestion avoidance mechanisms built
into TCP to reduce the amount of data traffic on the link. Essentially, data packets
received at a rate that exceeds the configured maximum are delayed (then dropped