How to: Build a Wireless Bridge Using DD-WRT, Part I

Over at Wi-Fi Planet’s monthly Ask the Wi-Fi Guru column, we receive a large helping of reader questions. Over time, some common themes have emerged—problems with wireless signal strength, securing connections, and anything related to the iPhone, for example.

But, even given the intensity of interest in these topics, the single most popular subjects amongst those seeking wireless networking help from Wi-Fi Planet are wireless bridging and wireless repeating.

We published an introductory tutorial on setting up a wireless bridge using the free, open source DD-WRT way back in 2006—which, in Internet years, may as well be 1906. And yet, most of that tutorial is still relevant today and thousands of readers still flock to it every month.

Given the hunger among our readers for more information about DD-WRT and how to create wireless bridges and repeaters with it, we’ve put together this new tutorial series. Consider it “Building a Wireless Bridge, Volume II: Full Throttle.”

Bridge to somewhere

In the real world, a bridge connects two land masses, often separated by water. In wireless networking, a bridge is used to connect two local area networks (or LAN’s) separated by, well, air.

You can think of a wireless router as the center of a single LAN. Every device connected to a single router—whether connected physically by Ethernet cable or wirelessly—is part of the same LAN.

Now suppose you have a device that is not part of the LAN, but you want it to be. The problem is that you don’t have an easy way to connect it.

For example, you might have a printer or a gaming console, such as the Xbox or Wii, which only has a cabled Ethernet connection. If the device is far away or on another floor from your wireless router, running a cable could be complicated.

The solution is a wireless bridge. Using a second wireless router installed with DD-WRT, the router will connect to your primary router and share the network with any connected Ethernet devices (most routers have four built-in Ethernet ports, but you can add one or more external switches with four, eight, or more ports each to expand even further).

In a wireless bridge setup, the devices connected to your secondary router will be part of the same LAN as your primary router, as if every device were connected to your primary router. This means that all machines in the LAN can see each other so that a computer can access a printer or two computers can share files, for example.

Now suppose a different scenario—you want to set up one or more machines with shared Internet access, but you want to “borrow” that Internet access from a primary router, which is elsewhere. In other words, you don’t want to join the primary router’s LAN, you just want to piggyback on its Internet access for a separate LAN.

The solution is a wireless client, which is also a configuration mode in DD-WRT. The principle behind a wireless client is much the same as a wireless bridge, but the wireless client mode creates a second LAN around your secondary router.

This allows you to create separate LANs and firewall traffic from each differently, but devices in one LAN may not be able to see devices in the other.

Most home and small business networks will find the wireless bridge mode more useful than wireless client mode, although for simply extending an Internet connection either mode will work.

The important thing to remember when using either wireless bridge or wireless client modes is that your secondary router is not broadcasting a wireless signal.

In other words, it is receiving a wireless signal from your primary router and sharing that with wired devices. You cannot connect to your secondary router with a Wi-Fi-enabled machine, such as a laptop, printer, or iPod touch.

Repeater repeater

Thanks to DD-WRT, it is possible to create a wireless client or a bridge which also broadcasts a wireless signal. This is called a repeater, and DD-WRT supports two kinds—a wireless repeater and a repeater bridge.

Both modes function like their earlier counterparts. In repeater bridge mode, the secondary router creates a wireless bridge to the primary router, meaning it shares the same LAN.

But in addition to connecting wired devices, it re-broadcasts a wireless signal using a “virtual” radio, allowing both wireless and wired devices to join the primary router’s LAN.

In contrast, wireless repeater mode is akin to wireless client mode, creating a new LAN around the secondary router.

The advantage of repeater mode is obvious: you can effectively extend the range of your primary router’s wireless signal.

But there is also a disadvantage: wireless devices connected to your secondary router will lose half the bandwidth of your LAN.

This means that their network speeds will be slower especially for internal networking, such as file-sharing or streaming media.

Slower wireless speeds from the repeater are a result of how the secondary router must operate: it needs to receive a signal from the primary router and then re-broadcast that signal locally.

But the router has only one radio and so can only do one of these things at a time.

In practice it switches between modes rapidly so that it appears both are happening at once, but in fact by operating in “half-duplex” mode like this, the maximum signal speed is halved. (We will look at one solution in the next installment of this series.)