Networking

How the flexibility of 802.11b saves both time and money

Using wireless networks to bridge buildings


Lucky network administrators get to run cabling in brand new buildings. When linking buildings together, lucky network administrators with big budgets get to run fiber from building to building or maybe even get to invest in a microwave or laser connection. But not all network admins are lucky. For example, what do you do when you have to run new cabling in your 150-year-old building or link multiple buildings together on a shoestring budget? In this Daily Drill Down, I will show you ways in which you can use wireless access points to bridge both floors and buildings together.

Since when does wireless work building-to-building?
Their guidelines state that 802.11b and 802.11a communications are limited in effective range. 802.11b connections are only rated for a maximum of 300 feet, and most 802.11a connections aren’t considered to be effective beyond 60 feet. Of course, the further away you get from the access point, the slower the connection. Therefore, you may think that the usefulness of wireless communications is limited.

These limitations would imply that 802.11x connections can’t handle building-to-building or floor-to-floor connections. However, as described on the O’Reilly Network, 802.11b communications have been possible at distances of up to 10 miles using an antenna featuring an empty Pringles potato chip can. Longer distances (up to 20 miles) have been achieved using either commercially available antennas or something a little more substantial than a Pringles can. With this type of range, it’s very easy to use 802.11b to link networks together within buildings and even in different buildings without having to run fiber optic or potentially expensive T1 lines.

Get to the point
You have two architectural choices to consider when using 802.11b to bridge between networks: point-to-point bridging or point-to-multipoint bridging. Point-to-point bridging means exactly what it sounds like; communications flow from one access point to another when connecting locations. Point-to-multipoint bridging is a little more involved. With point-to-multipoint, one central access point at your main location serves as a connecting point for all other locations/floors.

The difference between point-to-point and point-to-multipoint is analogous to the difference between Thinnet Ethernet and 10Base-T. Like Thinnet, if an access point in a point-to-point configuration fails, communication across the network will be broken, while still allowing the computers connected on either side of the break to talk to each other. In a point-to-multipoint configuration, if the failing point is one of the multiple access points, only that point’s computers will lose communication. However, if the central access point fails, all communication breaks down on the network. Because the cost of 802.11b gear is very reasonable and point-to-multipoint connections create the potential for a complete network-communications failure, it doesn’t make sense to try to bring all the wireless building connections in on the same access point.

If you have no choice but to deploy a point-to-multipoint connection, you must take care to consider bandwidth implications. Your total incoming connections bandwidth can’t exceed the maximum bandwidth of the access point. According to the 802.11b standard, the maximum top speed on a wireless network is 11 megabits per second. Assuming that you have the remote connections locked down to a 1-Mbps connection speed, this means, at most, your bandwidth would allow only 11 simultaneous connections to one access point. At this rate, each connection gets an equal slice of the bandwidth pie available from the central access point.

If you want to use more than 11 connections, you would have to enable Quality of Service (QoS) on your central access point to ensure that each connection would achieve a specific connection speed. Because not all access points have the QoS feature, you must check to see if the feature exists on the access point you’re considering before purchasing it.

If you don’t want to lock down the speed of connections between access points, you can ensure the central access point doesn’t become flooded by allowing it to negotiate the connection speeds on its own. The problem with using this method is that the closer connections will obtain the faster speeds. Because closer access points would obtain higher speeds, this could cause congestion when other connection points have a demand for a higher bandwidth.

Access point placement: Floor-to-floor
Wireless access points work best for spanning between floors when the floors in question are next to each other (i.e., floors 1 and 2). If your organization is in a building where another company has an intervening floor, using wireless access points becomes problematic due to distance problems and potentials for interferences. Look for a common area on each floor where you can place the access points. A janitor or supply closet is a great second choice for access point placement if you don’t have wiring closets on each floor. The locations do not have to be “stacked” one on top of the other, but stacking the access points will cause the signal to be stronger.

Put an access point on one floor and go to the other to see how well you can "hear" the access point on the other floor using a laptop with a wireless card. If you get anything other than a full-strength signal, move the access point to another location. If you can’t accomplish a full-strength signal in any location, you will need to look at some type of directional antenna to boost the signal so that you can punch through the floor and achieve a reliable signal. Once you’ve found the best signal location, place your second access point at that location. Continue on until you have access points placed on all floors of your building.

Don’t forget to check what 802.11B channels are in use on both floors. Make sure the channel you use for between the floors isn’t too close to the channels in use on the floors. If you have channel 2 in use on Floor 1 and channel 5 on Floor 2, using a channel between 2 and 5 wouldn’t be a good idea for in-between the two floors. If you use a channel between 2 and 5, you could experience overlap and interference, which can slow down communications. In this case, you would want to use a higher channel, such as 10 or 11.

Access point placement: Building-to-building
Connecting building-to-building utilizes some of the same concepts as using access points from floor-to-floor. Do a site survey at both locations with a laptop and wireless card to see what channels are in use. Once you can find a clear frequency, you will need to find a location at each building where you have line-of-site to the target building. This may require the use of a tower to get the antenna high enough to get a clear view of the other building.

In a building-to-building connection, using the antenna that comes with the access point probably won’t work. Most antennas that come with access points are typically omnidirectional, which means they send an equal amount of signal in all directions. Because you will be sending data directly from one building to another, a directional antenna will do a much better job. Keep in mind that the higher the db gain figure for the antenna (db is a measurement that indicates how efficiently the antenna is broadcasting the signal), the narrower the beam or signal path coming from the antenna. This narrower beam means that the signal can travel farther before starting to degrade.

However, a narrow beam has one drawback: the higher the gain of the antenna, the more carefully you will need to align the antennas to get the best signal between buildings. In a building-to-building link, you should have an external (outdoor) access point. Outdoor access points are generally a little more expensive than the access points you would use inside the building. If you don’t want to use, or can’t afford, an outside access point, you could also use an internal (indoor) access point in conjunction with a high-gain antenna mounted outside the building.

Regardless of type, make sure the coax cable between the access point and antenna is as short as possible. Because 802.11B uses frequencies in the 2.4-Ghz range, long coax cable runs between the access point and the antenna can cause more signal degradation to occur.

Bridge the gap
Wireless networking solves a lot of problems for network administrators. Sometimes it’s just too difficult to run new wires in a building, or it’s too costly to connect remote buildings together on a campus. In these instances, you can use the flexibility of 802.11b to save you both time and money. Carefully place your access points, and you’ll wonder why you needed cable in the first place.

 

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