Downstream Traffic Management

Configuring the 4300T

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:

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.

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4300T specifications

The Polycom 4300T is an innovative communication device designed for businesses seeking reliable and high-quality audio conferencing solutions. As part of Polycom's renowned lineup of voice communication products, the 4300T delivers superior sound quality, making it an essential tool for any professional environment.

One of the standout features of the Polycom 4300T is its Acoustic Fence technology, which effectively eliminates background noise, allowing participants to focus on the conversation without distractions. This feature is particularly beneficial in open office settings where ambient noise can be a significant barrier to clear communication.

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The 4300T supports both SIP and H.323 protocols, enabling compatibility with a wide range of communication systems and platforms. This flexibility is crucial for organizations that utilize different software and hardware for their communication needs, as it simplifies integration into existing infrastructures.

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