VoIP is often the first service people think of when QoS is mentioned, although most of the early deployments made do without QoS at all. For the enterprise, VoIP was either restricted to LANs with plentiful bandwidth so that contention issues were rare, or, if allowed out into the WAN, given a separate connection to keep it safe from other traffic. Conversely VoIP was – and still is – extensively used to bypass international telco transit charges, with contention carefully avoided by means of dynamic macro-level routing.
So does VoIP really need QoS? To move into the mainstream and deliver on its promise of low-cost and ubiquitous communications, VoIP must co-exist with other applications. As the supporting VoIP infrastructure matures, the cost of protecting VoIP traffic by over-allocating bandwidth starts to become a serious issue. A VoIP call typically consumes only 100k or so, but without effective QoS it needs much more bandwidth allocated than this to reduce the impact that contention with other traffic has on the loss and delay of the VoIP packets. On a DSL or wireless connection wasting so much bandwidth can be prohibitive. In any case, ensuring that this bandwidth is reserved when other applications are greedily trying to use it requires a basic form of QoS! Using more powerful QoS that can deliver the loss and, particularly, latency bounds that VoIP requires without over-allocation makes a far more efficient use of network resources.
How low the loss and latency must be kept for VoIP packets are across any particular network link depends on two things: how much the VoIP application can tolerate overall (its “budget”); and how much is introduced by the rest of the end-to-end network path. The tolerance of VoIP to loss and latency has improved over time with better codecs and clever jitter and loss concealment techniques, but delay in particular is ultimately constrained by human factors (think of the old intercontinental phone calls that went via satellite!). In the overall mouth-to-ear path a lot of time is taken by the process of sampling and serializing sound, and the speed of light is also a factor, particularly when complex network architectures may make the packet path less than direct!
As video traffic expands to fill every bandwidth upgrade, the days when VoIP could expect to traverse the network unscathed recede ever further into the past. Without effective QoS to allow VoIP to coexist with video and the other traffic, the days when it seemed a good way to talk will recede into the past as well.
Peter Thompson, Chief Scientist.