Accelerator products can help enterprises a lot in addressing the performance requirements of all enterprise applications including voip. First, Accelerators change the economics of wide area networking by squeezing an average of 100% - 400% more bandwidth with peaks of 1000% depending on traffic mix. This frees up link bandwidth to support high quality voip services - and it does it without expensive WAN upgrades.

It is also worthy to note that Accelerators do not actually use lossy compression schemes that might degrade voice quality and they add less than one millisecond of latency. In fact, the Accelerator’s compression actually reduces end-to-end latency by reducing serialization delays on WAN links. For example, it takes 125 ms to serialize a 1,000 byte packet on a 64 kbps link, but if an Accelerator increases the effective bandwidth by 4X to 256 kbps, the serialization delay is reduced by a factor of four to 31 ms. The following formula can be used to calculate the serialization delay for any combination of packet size and link speed:

Packet Size (in bytes) x 8 / Speed (in kbps) = Serialization Delay (in ms)

In addition to freeing up the bandwidth normally consumed by data applications, Accelerators are able to reduce WAN bandwidth requirements for different voip codecs. In fact, tests have shown that Accelerators reduced G.711 bandwidth requirements by 20% and G.729 by 70%. As a result, WAN links can carry more simultaneous voice calls and the performance of other applications may also be improved.

Accelerators solve increased jitter and latency caused by large data packets over slow WAN links by fragmenting large data packets and injecting voip packets at regular intervals. This feature allows voip and data to co-exist even on branch office WAN links. For example, normally, a voip packet “stuck” behind a 1,500 byte packet on a 64kbps lin will be delayed by 188ms.

Using the Accelerator’s packet fragmentation will result in the data packet being reduced in size (accelerated - say from 1500 bytes to 500b bytes) and then fragmented into smaller data packets (say - 2 packets of 250 bytes each). In this case, the latency for the voip packet will go down from 188 ms to 31ms! In addition to increasing WAN capacity for both data and voip while reducing latency and jitter, Accelerators also manage WAN bandwidth to ensure that critical applications like voip get the bandwidth they need.

Accelerators include an Instant QoS feature that prioritizes application access to WAN bandwidth. Without such priorization, the additional effective bandwidth provided by Accelerators could be consumed by aggressive, non-critical applications such as file sharing. Accelerator’s AppView feature provides graphical visibility for all application traffic sharing a link. AppView can be used to monitor WAN utilization and to plan future capacity requirements.

And finally, voip accelerators have a set of data integrity features that are designed to stop the packet loss that can degrade voice quality. A flow control mechanism reduces packet loss caused by link congestion and a packet recovery feature ensures that any lost packets are transparently recovered at the link level before they can cause voice quality problems.

Jim Francisto