December 14, 2000, 8:55 AM — Quality of service has emerged as a salient architectural element missing from
today's IP-based backbone networks. Yet it's clear that, so far, the lack of QoS has
not dimmed IP's prospects for global domination. So how seriously should you take
warnings that, without a workable standard for QoS very soon, prospects for converging
IP-based data and voice traffic are doomed?
Chances are that right now you don't need QoS on your LAN. If you're like most of
your contemporaries, you're still operating mutually exclusive voice and data networks.
The bulk of your switched voice traffic is sent to an interexchange carrier, to which
you pay something like four to seven cents per minute per call for delivery nationwide.
But you may have also noticed that, in any given week, a few more of the Fortune 500
are announcing big-time migration plans towards convergence. Do they know something
that you don't?
Not necessarily. Most of these enterprises are planning to achieve convergence via
voice over IP, and VoIP does not require any special QoS as long as the IP backbone is
well behaved -- that is, as long as there is no congestion anywhere along the transport
route that would lead to buffering.
VoIP uses an efficient mechanism called a jitter buffer to adjust each
realtime VoIP stream to accommodate slight temporal variations in transmission. Packets
may skew in time by 30 or 50 or even 100 milliseconds due to the dynamic nature of IP
packet transport. The jitter buffer realigns and resynchronizes the packets.
However, if you hit a router interface that is congested -- such as a 100 Mbps Fast
Ethernet LAN funneling traffic onto a T1 WAN link -- the result will be buffering (or
dropped packets). Buffering will add at least 100 milliseconds of one-way delay to VoIP
packets, and consequently will lead to those packets being discarded at the receiving
If you want IP convergence without standard QoS, your choices are either to avoid
congestion and buffering -- easier for enterprise networks than for carriers and
service providers -- or accept that, from time to time, there may be congestion and
buffering, and VoIP quality will drop whenever that happens.
But those aren't your only choices. You can also implement one of the current
nonstandard forms of QoS on your IP backbone. At Mier Communications, we've tested
various IP prioritization techniques and products. For example, Cisco's latest IOS
offers new features that implement prioritization policies -- sort of -- in the form of
access control lists. And numerous standalone-device vendors, like Packeteer, offer
boxes that effectively allocate bandwidth to match user-specified traffic classes, as
well as rearrange packet streams on the fly so that high-priority packets always go
ahead of others.