Polycom 4300T Benutzerhandbuch

 Configuring the 4300T

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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 long-lasting burst condition in low-priority data 

will cause these packets to be delayed and, if necessary, dropped.  

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:

A new call request is received by the 4300T from the WAN. 

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.

RTP traffic (the voice data within an ongoing VoIP session) is similarly 

classified as voice traffic and treated with priority.

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.