User manual

4300T User Manual Edgewater Networks, Inc.
Version 1.7 41
long-lasting burst condition in low-priority data will cause these packets to be
delayed and, if necessary, dropped.
Downstream Traffic Management
Since the 4300T is the final transmitting device for WAN traffic in the upstream
direction (LAN to WAN) it is easy to see how its QoS mechanisms can be applied to
traffic it is transmitting 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 receiving end of a
service provider link and therefore have no control over the amount of voice or data
traffic being sent to us over the WAN interface. How then can we still guarantee the
quality of in-bound 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 4300T appliance and leveraging the congestion avoidance mechanisms built
into TCP. Essentially, data packets received by the 4300T’s WAN interface at a rate
that exceeds the T1’s bandwidth LESS the bandwidth used for active voice calls are
delayed (then dropped if necessary) before being forwarding on to its LAN interface.
Similarly, data traffic sent back to the 4300T for transmission to the WAN are also
delayed (as described in the above section). This results in the WAN-based devices
following the rules of TCP/IP congestion avoidance and slowing down their transmit
rate. This technique is quite effective in practice, as end stations usually reduce
their transmit rate before VoIP signaling has completed for new call setup.
For example consider the scenario where there are no voice calls over a WAN link
and multiple FTP sessions are consuming all available bandwidth:
1) A new call request is received by the 4300T from the WAN.
2) All signaling messages for the call are classified as voice traffic and therefore
prioritized for transmission to the LAN before servicing the inbound FTP data.
3) RTP traffic (the voice data within an ongoing VoIP session) is similarly
classified as voice traffic and treated with priority.
4) FTP data is buffered (or dropped) by the 4300T and return data, including
the FTP ACKs, are also delayed. This results in a throttling of the transmit
rate by the (far-end) FTP hosts, reducing overall WAN bandwidth
consumption.
Generally, excessive UDP traffic must be shaped in the service provider network, as
UDP does not provide congestion avoidance mechanisms. The exception to this is in
the case of RTP UDP-based messages for voice traffic. Although RTP makes use of
UDP the 4300T appliance is able to provide its own congestion avoidance mechanism
for voice traffic using Call Admission Control (CAC).