User manual

4300T User Manual Edgewater Networks, Inc.
Version 1.7 40
H. Press Submit.
A Closer Look at Traffic Management in the 4300T
The traffic management mechanisms provided by the 4300T 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.
Classifying
High priority voice traffic generated by endpoint devices such as IP phone and client
adaptors are automatically identified by their registration with the EdgeMarc’s VoIP
Application Layer Gateway. Other VoIP devices (not making use of the ALG) can be
defined as high-priority by their IP address. The user configures these addresses
into the priority list in the Traffic Shaper section of the 4300T web GUI.
As the 4300T processes packets they are identified as either high or low priority
based on this configuration. Packets identified as high priority are marked as such in
the TOS bits of their IP header, allowing prioritization by downstream routers. The
TOS field is set to 12 hexadecimal “minimize delay and maximize throughput” This
value overwrites any prior value.
Upstream Traffic Management
The 4300T 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, and a traffic shaper to rate-limit by delaying packets before
they are sent. Each of these is described in more detail below.
Priority classes:
Voice traffic is placed in the high-priority queue and data traffic is placed in the low-
priority queue. The IP header TOS field of packets in the high-priority queue is set to
“minimize delay and maximize throughput”.
Scheduler:
High-priority data is polled before low priority data, thereby minimizing the latency
for voice traffic. High-priority data is allowed to use up to 85% of the total WAN
bandwidth. Although preferential treatment is given to high-priority data, 15% of
the WAN link is always reserved so that low-priority data is not starved.
High priority data is polled before lower priority data to reduce overall latency for
voice traffic.
Traffic shaper:
To smooth bursts from high speed data links (typically from the LAN Ethernet
heading to the WAN) the 4300T appliance uses a buffer that clocks data out at rates
not exceeding automatically-calculated maximums. Low-priority data is clocked out
at the WAN link’s full rate LESS the bandwidth currently being used for high-priority
(ie voice) data. High-priority data is clocked out at the WAN’s full link rate. Any