Routers, Switches and Wi-Fi, Oh My!

By Steven J. Vaughan-Nichols

May 16, 2003

Let’s get straight to the point; if all you’re doing is sharing your Wi-Fi connection with your roommates or neighbors, you need to buy a wireless switch like a fish needs to buy a bicycle. But, if you’re running Wi-Fi in a business, you probably are going to need to get a wireless switch. And, the bigger your business is, the more likely it is that a wireless switch will be a necessity rather than a luxury.

That’s because switches gives network administrators the kind of centralized control they need to manage and troubleshoot large networks. In wired networks, switches are vital for network administration. The rise of wireless switching, there are more than a dozen networking vendors entering the area is a sure sign that Wi-Fi has arrived in business networking.

Switch/Router Basics

So what is a switch anyway and what makes it more special than a router? The technical difference is that switches operate at ISO layer 2, the data link layer, while Routers operate at ISO layer 3, the network layer. That really tells you a lot doesn’t it?

Practically speaking, when data comes into a router, whether it’s wired or unwired, it travels over what amounts to a party-line connection. Before any device on that line can send data it has to wait for a break in all the other TCP/IP data calls before it can start chatting. Of course, it’s not only possible, it’s likely that multiple devices can try to chat at one time. The same thing happens on a router or a hub when people do this at a party–they can’t communicate.

The solution is the same one that people use: when two or more devices are trying to talk at once they have what’s called a collision. Then, the devices stop talking and wait for a variable amount of time before trying to talk again. Then, whoever speaks first gets to talk. If they, or other devices, collide again, they again wait a variable amount of time that will be twice as long as the first attempt, this is known as exponential back off, before trying to get a word in again. Technically speaking, networks like Wi-Fi and the Internet, use Carrier Sense Multiple Access/Collision Detection.

As network traffic increases, two problems emerge with the router approach. The first is the logical one that as the number of devices increases network demand, each one gets less and less of a share of the total available bandwidth. At the same time, as network load increases, efficiency begins to drop as collisions and the delays they cause begin to take their toll.

A switch, on the other hand, doesn’t use the party-line style of networking where every device shares the network. Instead, switches connect devices directly with each other at the data link level.

What that means is when a device talks on a switch, its first Ethernet packet are examined for its destination device’s 48-bit address Media Access Controller address. Once the address is found, and matched with the switch’s map of MACs and ports, the packet is switched to its proper port and the network conversation begins. If the switch doesn’t recognize a MAC, it checks all its ports until it find the right one, updates its MAC address map, and forwards the message along.

What this means for us is that laptop A can talk to server C while desktop B is simultaneously talking to laptop D. The end result: overall network performance increases as each device can now operate at full speed instead of waiting for the network chatter to quiet down. In addition, switched devices can transmit and receive at the same time — this is called full-duplex — thus delivering even higher performance.

Switch and Wi-Fi

Unfortunately, on a Wi-Fi network, switches don’t deliver as much of a performance benefit as they do on a wired LAN. After all, 802.11a has twelve effective data channels, and 802.11b only has three, that can be used at a given moment. Thus, with Wi-Fi, you can’t guarantee that a switched connection will always be available so you still need to deal with CSMA/CD overhead anyway.

So why bother since switches are much more expensive than routers? The one word answer is management.

Besides making networks effectively faster, switches also enabled network administrators to manage their networks. Until now, Wi-Fi networks have been noxiously hard to administer. With the advent of switches, which give managers a centralized access point control, now administrators can run, scale and secure their wireless LANs.

As it is now (unless you have a proprietary solution), a network administrator has to go to each access point to set up authentication, encryption, change services set identifiers (SSID)s and so on. With a switch, you can do those things from one console to multiple access points at once. Besides saving time, centralized management also makes it less unlikely that a typing error will lead to a network access mess.

But, a switch isn’t a management solution in and of itself. A switch may come with the basics above and say Simple Network Management Protocol, but just how much management functionality a specific switch delivers depends on its software.

Unlike most Wi-Fi routers, though, switches are actually small computers in their own right. So, for example, Aruba Networks provides a software suite, AirOS, with different levels so that you can use 802.1x, add a VPN and firewall or automatically check and stop unauthorized, the so-called rogue, access points.

Some switches, like those from Legra, will also support third party management programs like Funk Software’s 802.1x and RADIUS user authentication suite.

What to Look For

At this point though, even though there are a flood of switch announcements, few switches have actually been released. So it’s hard to say what buyers will find in switches.

There are a few basics to consider. SNMP is the lingua franca of network management software. Any switch you buy should support it.

The switch should also be software upgradeable. Wi-Fi standards change faster than runway models. You want a switch that can support Wi-Fi Protected Access (WPA) today and 802.11i tomorrow without a great deal of fuss.

You’ll also want a switch that uses open standards and an open operating system. It’s purely practical. You don’t want to be locked into a proprietary switch where the only software you can use on it comes from the vendor at fat prices. Or, for example, you have to buy a virtual private network and firewall together when all you want is a VPN because you’re perfectly happy with the firewall you’ve already got.

You’d better brace yourself for the prices, too. A minimal switch will run you close to a thousand dollars. Once you add in additional software and the access points needed, you can expect your corporate card to take several thousands dollars worth of damage. Buying an enterprise switch isn’t like buying the cheapest wireless unit at Best Buy or CompUSA.

But then switches aren’t meant for small networks. They’re meant for large networks where the time and management savings will more than make up for the initial cost of the switch. Switches are not for everyone, but for those who need them they’ll be a God-send.

Okay, sounds good right? Except there’s about three different architectures all claiming to be “the one true switch”! How do we find out which is right? Join us at the 802.11 Planet Conference & Expo, June 25 – 27, 2003 at the World Trade Center Boston in Boston, MA. We have a panel we think of as the WLAN switch cage-match called New Architectures: Switched WLANs. It sounds more polite than it is likely to be.