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Ultimately, true QoS can extend only as far as your network borders, effectively stopping at your Internet routers. ISPs and Internet backbone providers aren't required to honor your QoS scheme, and you can, in fact, expect them to gleefully ignore it.

So should you implement QoS, or just buy more bandwidth?

The "buy more bandwidth" argument is simply this: Bandwidth is cheap, QoS is complicated. If you have enough capacity, QoS becomes irrelevant. The Internet constantly grows in capacity, and technological advances have continued to increase networking speed.

This argument holds more water on the LAN side than on the WAN side. Gigabit Ethernet is affordable, and most systems have a hard time saturating it, let alone 10GigE. WAN speeds, though, haven't increased at such an affordable pace, and it's commonplace to find your Internet connections saturated at least a few times a day.

QoS capabilities now come bundled free in many infrastructure products, including multipurpose security boxes such as FortiNet's FortiGate, VPN gateways and routers.

The biggest oversight in the unlimited supply model is that if you have extra bandwidth, someone will find a use for it. It's like traffic systems: In the 1930s, New York City built two bridges to ease congestion on its existing span. After the bridges were completed, congestion was just as bad as before the project began, despite the increased capacity.

With the proliferation of file sharing, music trading and P2P, bandwidth should no longer be looked at as limitless. Web pages have become bloated as well.

You can block some frivolous downloads with content filters (assuming your users don't wise up to the fact that content filters can't easily block encrypted HTTPS traffic). However, even traffic that conforms to your acceptable use policy can wreak havoc. A 5-MB attachment sent to a 100-recipient distribution list will do some damage. High-speed users connecting to your Web server may be sucking all the bandwidth away from DSL users. QoS can ensure that all users have an equal experience at your site.

Some organizations run Web site analysis tools against the Web log file. These files can consist of multiple gigabytes, so downloading them by FTP off the Web server may suck up so much bandwidth that there is little left for critical SQL lookups.

Then there's VoIP. We hear from our readers that VoIP pilots are on many IT managers' agendas. If you're among them, you need QoS. Even small periods of bandwidth saturation--just a few seconds' worth--can cause clips in phone conversations. Without a guarantee that VoIP calls will be just as good as POTS calls, getting management to approve a VoIP rollout could be difficult.