Bandwidth Reduction, WAN Optimizers and VoIP PerformanceBandwidth Reduction, WAN Optimizers and VoIP Performance

The WAN optimizer is hardware designed to reduce bandwidth consumption. WAN optimizers are designed for TCP traffic, which dominates the IP network. TCP traffic has a lot of redundancy and can be compressed; it does not have the network performance requirements of VoIP traffic.

The WAN optimizer is hardware designed to reduce bandwidth consumption. WAN optimizers are designed for TCP traffic, which dominates the IP network. TCP traffic has a lot of redundancy and can be compressed; it does not have the network performance requirements of VoIP traffic.However, WAN optimizers have not been designed to deliver the QoS needed by VoIP. Can the WAN optimizer interfere with VoIP performance? YES! So what is the benefit?

Some vendors use the term "accelerate" to describe their optimizer's operation. The goal of the optimizer is to reduce; some vendors say 70% to 95%, the amount of WAN bandwidth necessary to carry the data traffic. Accelerating voice traffic is not possible unless you can make the speaker talk faster. Acceleration is meaningless for voice traffic.

Voice traffic is less tolerant to network performance impairments. Delay/latency is a factor in voice communications. The goal is 150ms latency from mouthpiece to earpiece. As much as half of this latency can be produced by the IP phone and gateways. The remainder (about 80ms) is allotted to the network latency.

Latency occurs in the WAN optimizer as it compresses TCP traffic. The additional compression latency is usually not significantly noticeable to the data users. The additional latency to the voice user can be observed by the talkers and is undesirable and annoying. The additional latency is a factor to consider when implementing WAN optimization.

The VoIP packet rate for a single voice call is small and requires very little bandwidth. An uncompressed voice call using the G.711 standard and a 30ms packet size (30ms of sound), consumes about 80 kbps per call. Even with a number of calls, say 10 calls on a WAN trunk, this is only 800 kbps. If the 30ms packet carries G.729 compressed voice, then only 24 kbps per call is consumed, for a total of 240 kbps. There may be a slight justification for a WAN optimizer compressing the G.711 call, but no advantage to further compressing the G.729 call. So a WAN optimizer will have no appreciable benefit for VoIP traffic bandwidth reduction.

The bandwidth reduction for the data trafic is an indirect benefit. As congestion is reduced on the WAN links, router latency, jitter and packet loss are correspondingly reduced. So a WAN optimizer, by reducing the data bandwidth consumption, will benefit VoIP traffic. It should be placed at the enterprise origination and destination sites before traffic enters the router and WAN. The WAN optimizer will also reduce the enterprise's WAN costs.

Therefore the enterprise should install the WAN optimizer, correct????

The limiting factor for VoIP is QoS and its enforcement. Not all optimizers handle UDP traffic well. UDP traffic, like DHCP, DNS access and TFTP file transfers are not common and produce a very small amount of traffic. VoIP traffic uses UDP packets to carry the speech and SIP signaling. The enterprise MUST look for a WAN optimizer that not only supports VoIP UDP well, but also support VoIP QoS. If not, then the resulting poor treatment of the VoIP packets will negate the benefit of bandwidth and congestion reduction.

An alternative method of bandwidth reduction for VoIP traffic, in addition to voice compression, is header compression. Cisco created this technique and it has become an IP RFC standard, RFC 3096 and later standards. The concept is to reduce the header overhead, since most VoIP packet are quite small. The overhead for VoIP packets can be anywhere between 20% and 80% of the network bandwidth.

The RTP, UDP and IP headers are compressed in this technique. This compression technique works best when there is point-to-point link between the routers. The routers are the header compression devices.

The bandwidth reduction for a G.711 call is from 80 kbps to 67 kbps, a 16% bandwidth reduction. For the G.729 call of 24 kbps, the resulting bandwidth is 11 kbps, a 54% bandwidth reduction. Even though the total bandwidth for a voice call is not large, bandwidth reduction can increase the number of simultaneous callers while also reducing congestion, both of which are benefit the VoIP quality. So definitely apply header compression when it's used in conjunction with compressed voice like G.729.

The WAN optimizer is worth considering, but do not automatically install one. If the WAN optimizer does not support VOIP QoS, then avoid it if you plan to add VoIP traffic to the WAN links.