As Gigabit Ethernet gets cheaper and more popular, many network administrators are also noticing something curious as they test various IP Telephony equipment on these high speed LANs: it usually sounds fine without any QoS configuration applied. This is causing a lot of engineers to choose over-provisioning their LAN over the more grueling effort of a well-thought-out QoS scheme. After all, any network problem can be resolved by simply throwing more bandwidth at it, right?
It's important to remember that although it's fast, Gigabit Ethernet is still fundamentally like Fast Ethernet. The fiber is either lit or it's dark. There's either a voltage on the wire or there isn't. This is where buffers and queues come into play. If a given Fast Ethernet uplink in your network services several Fast Ethernet-connected users, and two of those users each send a 1 Mb burst of data traffic at the same time, half of the packets are going to sit in a buffer momentarily, waiting for their turn even though the Fast Ethernet uplink can handle many times that in a second. Remember that they're not sending data "in a second"; they're sending it in a tiny fraction of a second. While this example isn't going to affect your VOIP quality noticeably, your packets are waiting even though your link will show 2% utilization. The concept of measuring link utilization over a relatively long period of time isn't always practical for predicting VOIP quality.
If your plan is to dramatically overprovision your network so you don't have to worry about it, just remember that the number you really need to pay attention to is not total bandwidth and utilization, but total queue size on each interface in a given path, and your peak queue depth. If you can dramatically over-provision this, then you may be fine. And if your utilization on Gigabit trunks is very low, which it probably is, your codec may be able to compensate for the occasional lost or delayed packet. On the other hand the danger of relying on over-provisioning is that you never know when everyone in the office is going to download a big software patch at the same time, and you're only one poorly coded, web-based ERP implementation away from quadrupling your bandwidth usage. And configuring QoS can protect you during DoS attacks if your voice traffic has guarantees.
One last thing... when you start peering under the hood at things like queues and buffers, it may suddenly become obvious why some switches are more expensive than others.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.