Q

Extending 802.11n wireless LAN and connecting two WLANs

Extending 802.11 WLANs inside a large office can be done by bridging access points together, while outdoor mesh solutions can be used to connect two WLANs between buildings on a small campus.

We have a small business office that is too large to cover with a single 802.11n AP, and we would also like to extend the 802.11n WLAN to a second small building 125 feet away from our main building. How can we extend an 802.11n WLAN to meet these needs?
802.11n WLAN implementation
To read about 802.11n implementation best practices, see Lisa Phifer's Deploying 802.11n AP: Best practices and Understanding 802.11n antennas.

To build a network that delivers solid coverage to both offices, you must solve two problems: First, you must extend

802.11n WLAN coverage provided by your indoor WLAN, and then you must join two indoor WLANs together with a short outdoor wireless link.

There are at least three ways to extend an 802.11n WLAN inside.

1. You could deploy a wireless repeater to extend the reach of your existing AP. Many inexpensive APs can be configured to operate as repeaters. This is rarely a good solution because it cuts available capacity in half. Think of this as talking to someone on the phone who then repeats what you said to a third person standing near them, then waiting for that person's reply to be repeated to you over the phone. Repeaters are easy, but getting anything accomplished takes much longer.

2. Most businesses build larger indoor WLANs by cabling many wireless APs to the same upstream Ethernet switch or WLAN controller. Each AP uses Ethernet to communicate to the switch/controller and 802.11 to communicate to Wi-Fi clients. All APs advertise the same wireless network name (SSID) and together form a single subnet on which clients can easily roam from one AP to another without significant disruption. This can be accomplished with nearly any residential or enterprise AP. However, if you have more than a few APs in one office, choosing a centrally managed enterprise AP will improve control and reduce total cost of operation.

3. Some businesses build indoor WLANs by deploying wireless APs that can communicate with each other over 802.11. These are typically dual-radio APs that dedicate one radio to communicating with clients and a second radio to communicating to other APs. In order to avoid interference, inter-AP communication often occurs over a 5 GHz "backhaul" channel not used by clients. Some inexpensive consumer APs can be configured to communicate with each other over static wireless bridge links using a method called wireless distribution system (WDS). Some enterprise APs use their own wireless mesh management and routing protocols to automatically form dynamic, optimized, self-healing backhaul links. The resulting wireless mesh may allow APs to communicate directly with each other or APs may route traffic through nearby APs to reach more distant APs. However, at least one "root" AP will still need an Ethernet connection leading to the Internet.

Connecting two WLANs

Now for your second problem: Connecting two indoor WLANs with a short outdoor wireless link. As you might imagine, the indoor bridge and mesh solutions described in option 3 above can easily be extended to cover an outdoor distance of just 125 feet. If you are using WDS, just configure a WDS bridge link between APs positioned at the edge of each network, preferably using directional antennas to focus transmissions between them, keeping data rates high. If you are using a proprietary wireless mesh, follow vendor instructions for deploying outdoor mesh nodes. For example, you may dedicate one AP in each building to serve as the "root" for that building's indoor mesh, and then bridge traffic point to point between root APs. Alternatively, your second building may be small enough and close enough that just deploying one mesh AP there and letting it join the indoor mesh in your main building will do the job.

Finally, you may have heard there is a wireless mesh standard being defined by the IEEE. The draft 802.11s standard specifies a Hybrid Wireless Mesh Protocol used to route 802.11 traffic between cooperating mesh stations with some mesh stations also serving as mesh portals to reach a wired network or the Internet. However, 802.11s is not yet completed, so enterprise wireless mesh APs that you buy today use proprietary protocols.

This was first published in July 2010

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