Nuts and bolts of a wireless bridge

Putting in a point-to-point wireless connection

When faced with the task of connecting a remote office, the first options that often come to mind are a dedicated circuit (such as T1, T3, or frame relay) or site-to-site VPN. If the remote office building is near the main office, then another option is to lay fiber. However, dedicated circuits are costly and slow, and fiber is even more expensive, but yields faster connections. Site-to-site VPNs can save you money by using less-costly Internet links as the backbone for connection, but there can be QoS, security, and performance issues involved.

Before you invest in any of these technologies, you should consider another solution: A point-to-point wireless connection. Here is a look at the various methods of point-to-point wireless connectivity and how they can be implemented for WAN connections between buildings or across town.

Understanding the technology
When you think of wireless networking the first thing that probably comes to mind is the current Wi-Fi standards: 802.11b, 802.11a and the newest, 802.11g. The popular 11-Mbps, 802.11b standard that is typically used within a building can also be used to link signals outside a building, or building-to-building. In addition to 802.11b, several companies also use proprietary standards for wireless connections.

A typical "indoor" wireless network is made up of one or more access points that allow wireless clients to connect or associate with them. The access point provides the link, or bridge between the wired network and the wireless network. For this reason they are often referred to as a wireless bridge.

Thus, the term wireless bridge can be confusing. When used in the previous statement it refers to a device that connects two networks, a wired network and a wireless network. In the context of this article, we are referring to the application of connecting or "bridging" two wired networks via a wireless connection.

Unlike an access point, a wireless bridge does not connect or associate with wireless clients. It connects to another bridge device to complete the link and join two networks together. Bridges can also be set up to provide multipoint connections enabling several remote sites (B and C) to connect to a main site (A), as shown in Figure A.

Figure A

Bridge antennas are located outdoors, usually mounted to a roof or on a communications tower. Outdoor bridge systems use different style antennas than those required by an indoor access point. An indoor wireless network typically relies on an omni-directional antenna, which distributes its signal in all directions creating a circular coverage pattern. Figure B demonstrates this concept (the black circle in the middle represents the antenna on an indoor system).

Figure B

Outdoor systems use a directional antenna, such as a "yagi" or parabolic dish that focuses its signal in a specific direction, typically at another antenna (Figure C). An outdoor antenna typically has more power, or gain, that allows the signal to travel further than its indoor counterpart.

Figure C

Outdoor wireless considerations
Whether you are connecting two buildings 500 feet apart or five miles apart, one major consideration must be taken into account. That consideration is called line of sight. The buildings must have a location where an antenna can be mounted and "see" the other antenna.

Unlike their indoor counterparts, outdoor units do not pass signals through objects. If a tree, or other physical obstruction is in the way, the signal will probably be attenuated or reduced significantly, causing the bridge connection to fail. If an obstruction is preventing a link, multipoint hops can be installed to by-pass obstacles or extend the range of a link, as shown in Figure D.

Figure D

At a distance beyond six miles, the curve of the earth, referred to as "earth bulge," must be taken into account (this is also seen in the diagram in Figure D). Earth bulge requires the antennas be mounted at higher elevations. Another consideration is the Fresnel zone, an imaginary elliptical path that surrounds the signal path. The Fresnel zone varies with distance and the frequency of the signal. The Fresnel zone must extend above any obstacles, such as trees or tops of buildings located between two points (see Figure E). Additional information on antenna selection and Fresnel zone considerations can be obtained from Cisco's Web site.

Figure E

Several other considerations must be taken into account when designing a wireless link between buildings. The speed of the link is dependent on distance. The further away the two antennas are from each other, the slower the speed. The maximum distance is also dependent on the type of antennas selected. Cisco provides a calculation chart (this is an Excel spreadsheet based on its Aironet 350 series bridge equipment) that assists in selecting the proper antenna and equipment to achieve the desired speed or distance requirements.

Typical distances can range from several hundred feet to 30 to 40 miles, depending on equipment selection and other factors. Cisco's calculation chart mentioned above warns that distances beyond 25 miles can pose difficulties in aligning the antennas.

Wireless bridges operate in the 900-MHz, 2.4-GHz, and 5-GHz frequency ranges. This is referred to as the unlicensed Industrial Scientific Medical (ISM) band.

Licensing may be required
In the US, 802.11b and 802.11g operate at 2.4 GHz, 802.11a operates at 5 GHz, and many cordless phones operate at 900 MHz. Countries outside the United States may require licensing for using wireless equipment.

For U.S. businesses, no FCC license is required to install your link. This sounds like a great thing, until you consider the fact that it means anyone can set up a link without regard to what's already installed nearby. This translates into a potential problem: interference.

An existing installation may interfere with your signal and/or vice versa. All you can do is be aware and realize you may have to relocate an antenna or change to a different channel or frequency. If you hire a contractor, ask that a site survey be completed prior to installation and prepare to resolve any interference issues that may arise with neighboring businesses.

Security concerns
A wireless bridge is based on the same wireless technologies as indoor wireless signals, so it shares the same security concerns. However, there are also additional factors involved that make it more difficult for wireless hackers to intercept the signal.

Since the signal is directional, a hacker would have to have line of sight to the antenna path to intercept the signal. If the antenna were located 100 feet in the air on a tower, the hacker would have to climb the tower, climb atop a nearby building, or use some form of air transportation in order to intercept the signal.

Although these considerations make it difficult for a signal to be intercepted, this is no excuse to leave the link unprotected. WEP is the bare minimum security requirement and admins should also consider additional methods to secure the signal path.

Depending on the distance involved, you may consider consulting a company specializing in wireless bridging installations. Such a company can handle the job of erecting a tower, if necessary, and mounting and aiming the antennas.

For shorter distances, such as between two buildings on the same campus, the project may be tackled without an outside contractor. Wireless bridging "kits" are available from vendors such as 3Com and Proxim. The kits include most, if not all, components required to complete a link. Other vendors provide individual selection guides that aid in selecting the correct components based on your requirements.

Need more speed?
11 Mbps is a fast WAN connection when compared to a single 56K link, or even multiple T1 lines. However, a true 11-Mbps connection is only obtainable at short distances. But what if you want a faster connection? What if you would like to carry voice as well as data? 802.11a and 802.311g have distance limitations, which do not allow them to be used in bridge installations. In calls to both Cisco and 3Com representatives, I confirmed that neither company is offering, or has plans to offer a bridge product based on 802.11a or 802.11g specifications.

Several companies do offer solutions that allow speeds above 11 Mbps and can carry both voice and data. These products are based on proprietary technologies, not the 802.11 wireless standards. Proxim offers a wide range of wireless equipment that ranges in speed from 10 Mbps to 1 Gbps. Speed is only limited by your budget. Most of its network products include a "wayside T1" for voice. Proxim's products are based on its own proprietary standards. This makes the product less susceptible to hacking, because a hacker would have to have a matching radio to decode the transmitted signals. Additional security features are also included with its products.

Higher speed, of course, comes with a price. Proprietary high-speed wireless products are significantly more expensive then their 802.11 counterparts. However, when the cost is compared to the equivalent wired products, such as T3/DS3 service, the devices can pay for themselves over a short period of time.

Wireless links can be a cost effective alternative to conventional "wired" services when considering a connection to a remote office or offices. Once you purchase and deploy the wireless equipment, then the only additional costs are in keeping it in good working order. Therefore, most of the cost is in the initial purchase, but it can save you money over the long run when compared to WAN services with their accompanying monthly fees.

However, the decision to go wireless requires extensive planning and research. This article examined the nuts and bolts involved in making the connections. Next time I'll examine a wireless installation from start to finish and go in-depth to examine the cost savings over a wired connection.

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