As we explained in Lesson four, wireless computer networks are becoming a major part of the networking market. This is true not only for residences, but also for commercial installations.

PROTOCOLS

The wireless LAN system (802.11), is similar to 802.3 (traditional Ethernet), in that both are designed to support multiple users on a shared system.

In standard Ethernet operations (IEEE 802.3), a special protocol (CSMA/CD) regulates how Ethernet stations detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN.

In a wireless LAN, collision detection is not possible, so 802.11 uses a form of collision avoidance (CSMA/CA). A station wishing to transmit senses the medium (in this case, air). If the station senses no activity, it waits an additional, randomly selected period of time and then transmits if the air is still free. If the packet is received intact, the receiving station issues an acknowledgment frame. If the sending station does not detect the acknowledgment frame, it assumes a collision; after waiting another random amount of time, it retransmits the packets. This adds operations to a wireless LAN that a cabled LAN does not have. For that reason, the wireless LAN will always have a somewhat slower performance than an equivalent cabled LAN.

The 802.11 wireless LAN system uses two spread-spectrum radio techniques. These operate within the 2.4-GHz Industrial, Scientific, and Medical (ISM) band. These frequency bands are recognized by international regulatory agencies in the United States, Europe and Japan, for unlicensed radio operations. As such, 802.11-based products do not require user licensing or special training.

The 802.11 standard specified data rates of 1 and 2 Mbits/sec via radio waves using frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS). FHSS and DSSS are fundamentally different signaling mechanisms and will not interoperate. But 802.11b is based on DSSS, making migration from a 2-Mbit/sec DSSS system to an 11-Mbit/sec system easy. 2-Mbit/sec DSSS systems will be able to coexist with 11-Mbit/sec systems, enabling a smooth transition to the higher-data-rate technology.

SYSTEM LAYOUT

Any LAN application, network operating system or protocol will run on an 802.11-compliant wireless LAN as easily as it will run over Ethernet — just more slowly. Since IEEE 802.11b wireless LANs communicate using radio waves that can penetrate or may reflect around many indoor structures, how much more slowly is primarily determined by the quality of the design.

To establish a wireless LAN, you must install and configure the access points and network-interface cards (NICs). In addition, you will probably need to add cable from the existing network infrastructure to the access points, because they are rarely located near existing outlets. The number of access points depends on the coverage area, number of users and types of services required.

The single most important part of the installation is placement of the access points to ensure proper coverage. Most manufacturers provide a site-survey tool. Place the access points and use the site-survey tool to record signal strength and quality while roving within the intended coverage area. Once the access points are installed, access points and NICs must be configured. Configuration options vary according to the manufacturer. When you are designing a wireless LAN, the cost of the equipment is just one concern; you must also consider installation and maintenance expenses.

Although all 802.11b-compliant products are based on a single standard, the standard offers no guarantee that access points, NICs and other equipment from various manufacturers will interoperate. Talk to your suppliers to be sure that you'll have no problems. In a few years, everything will probably be seamlessly interoperable; but for the moment, make a few phone calls to be sure.

INTEROPERABILITY AND INTERFERENCE

Since users are not likely to walk around with a pocketful of different NICs (network interface cards), manufacturers have formed a group (the Wireless Ethernet Compatibility Alliance, or WECA), to certify cross-vendor interoperability of 802.11b wireless networking products. This group has created tests to certify interoperability and announced the “WiFi” (wireless fidelity) standard, which is an awarded seal for those wireless LAN products that have successfully completed prescribed interoperability testing. The seal provides assurance that products bearing the logo will work together. Many of the industry's leading wireless LAN manufacturers belong to the group.

The WiFi effort is effective to the extent it reaches, but understand its limitations. Not all wireless products are 802.11b-based. For example, both IEEE 802.11 and Bluetooth share common spectrum in the 2.4-GHz ISM band.

With 11-Mbit/sec data rates now possible, IEEE 802.11 DSSS radios can provide a mobile extension to wired networks in large installations, and even completely replace a cabling infrastructure in small and home office environments. Meanwhile, Bluetooth is becoming essential to the mobile worker and business traveler by facilitating e-mail to a laptop using a cellular telephone, synchronization with palmtop devices and access to local printers.

It's inevitable that 802.11b and Bluetooth will bump into each other at some point. Currently, studies are underway to see exactly what the effects will be. But it's known that IEEE 802.11b susceptibility to Bluetooth interference increases as the distance between DSSS wireless nodes and DSSS access points increases. For this widespread acceptance of 802.11b to continue, compatibility among the various wireless standards is imperative. (See Fig. 1 for an example of Bluetooth and 802.11 coverage areas.)

Recently, the IEEE standards board approved a new project within 802.11 to increase the data rate of wireless LANs operating in the 2.4-GHz band from the current 11 Mbits/sec to greater than 20 Mbits/sec.

THE SITE SURVEY

Before a wireless network can be installed, the installer or designer is first required to do a thorough site survey. Do not skip this step. Essentially, setting up a wireless network is the same as setting up a cellular telephone system, except on a smaller scale. If you do not do a thorough site survey, your network may end up with a lot of dead zones where no signal can be transmitted. Elevator shafts, kitchens with metal cabinets, thick concrete walls, steel studs and even large metal fans can seriously restrict the effectiveness of a wireless installation.

A good site survey addresses two primary concerns: First of all, it examines the signal coverage requirements for your site. In other words, you must determine how far the signal goes for each basic service set, or cell. Secondly, the site survey enables you to understand your overall networking and system requirements. Does the existing backbone have adequate bandwidth to handle the overlaid wireless network? Are there sufficient resources to support the wireless capabilities you want?

When performing the site survey, you should use the tools and equipment provided by your wireless vendor. If you are installing, designing or selling a wireless LAN for the first time, it would be a good idea to have specialist come in and walk you through the process. Afterward, do it yourself.

There are six major areas to consider when designing the site:

Future use. The customer may want to expand or modify the system at some later date. Build in plenty of excess capacity if at all possible.

Coverage. Define the area in which you want wireless coverage, and its characteristics. Indoors or outdoors? Do you need coverage to be isolated to certain spaces? A wireless access point's typical range is spherical, and each area will generally need four access points, with cells overlapping by about 30% to achieve optimum coverage. Access point and antenna ranges differ by vendor specification; make sure you know the ranges for your system. (See Fig. 2.)

Capacity. How many users are you expecting? What sorts of applications will they run? What types of equipment do you plan to use? These factors will affect the number of access points you need. Site capacity can be increased by adding more access points. While wireless LANs operating at 1 to 2 Mbits/sec are available, you should try to stay with the 802.11b rate of 11 Mb/s; this allows the user to transfer high-quality video signals in real time, which is soon to be an important application. These higher data rates have a much shorter range, however, and such installations generally require more access points. In some cases, it may be better to use the slower speeds and wider service areas.

Interference. What are your potential sources of interference now and in the future? These can include sensitive equipment, previously installed systems and the upcoming Bluetooth initiatives for very-short-range mobile communications. In outdoor installations, moving vehicles such as trucks, planes or other equipment may also be large enough to temporarily block signals. In such difficult locations, you may need to place access points at very high locations for vertical signal penetration. Or, access points may be placed at opposite sides of the yard, so that if one is shadowed, the signal may still be transmitted through the other. You should also examine potential antenna performance patterns to decide on an omnidirectional or directional antenna. Interference avoidance design requires a combination of sufficient signal strength, the appropriate technology and intelligent access point and antenna placement.

Connectivity and power requirements. What are your environment's networking constraints? All your networks must be based on the same standards, usually Ethernet. One cannot assume the current environment will be able to handle the increased workload, and you don't want 40 new access points suddenly over-straining your system. You should also calculate your installation based on the network device with the least range, so that every device is sure to transmit clearly. Connecting to the wired infrastructure may also require different cabling alternatives. Wireless access points are typically installed in ceilings for better coverage, which means that AC power and data cabling will need to be run in some difficult locations.

Cost and ease of installation. Performing a site survey provides a realistic understanding of your wireless installation's cost before you sign a contract. Perhaps the site has unusually high interference issues to resolve, or capacity is greater than you anticipated. Based on a good site survey, you can put together a reasonable cost estimate, so there are no surprises on any side of the transaction. Trying to build a wireless LAN in stages is a complex undertaking. Many installers have found themselves in the midst of a communications disaster as sessions drop out and cell locations are lost. Because wireless systems simply fit in on top of existing environments, however, you can anticipate limited work interruptions during the installation process.

Setting up temporary antennas and access points. This process includes assessing every part of the site with a software survey tool installed on a handheld computer or laptop that monitors the signal and identifies failure zones. Such site-survey tools measure performance between access points, identify sources of interference and help determine access-point placement.

Site surveys differ in their complexity and level of effort, based on technology and space. Small facilities may not require one at all, and you can often use only product specifications and a good set of blueprints. However, larger installations are another story. The site survey offers a level of security in an arena in which, unlike wired networks, there are many variables to consider and few fixed rules. A thoughtful, accurate and effective assessment of coverage and system requirements will make your wireless LAN installation more straightforward and less expensive, while laying the groundwork for ongoing expansion. The most important task when installing a wireless network is to do a site survey.

INSTALLATION AND DESIGN CONSIDERATIONS

Following are several installation tips to remember.

  • There are three typical interfaces between the hardwired and the wireless network: cable modem, DSL and simple dial-up. For cable modems, the jack should be placed in the center of the facility. DSL and dial-up are apt to be somewhat simpler, since the interface can be to any RJ-11 telephone jack, and the jack can be installed wherever telephone wires run.

  • The standard wireless network is the 11 Mb/s 802.11b system. As mentioned earlier, this system has a smaller radius of coverage than slower systems. Be familiar with it and its related standards.

  • Begin with small, simple systems, then move on to larger and more complex systems. Have your vendor's representatives show up on the job site at first, and help you through the process.

  • Most wireless LAN products for offices cover a cell about 300 feet in diameter. The residential products are rated at 150 feet, or enough to cover a typical 2,500-square-foot home. Ratings should be more conservative in the home, because an average home has more sources of interference.

  • Unforeseen problems do arise. For example, wireless LAN devices typically operate in the 2.4 GHz band, which is unlicensed. Other devices can interfere. It's important to check noise and signal-to-noise ratios and to re-set channels to different frequencies if there is interference. Wireless networks distribute their signals in concentric circles; it's quite possible to have an installation that's 11Mb inside the first 50-foot circle, but only 5 Mb at 150 feet and 2 Mb at 400 feet. If certain users demand full signal speeds, make sure they are close to the Access Points. This is shown in Fig. 3.

INSTALLATION: COMMON MISTAKES

Probably the most common error in wireless networking (as in most telecommunication work) is the improper installation of cable connectors. In particular, the coaxial connections to the antennas seem to be a problem area. That's nice, because it is an easy problem to fix, and to avoid. (Train your people well.)

Improper installation of data communications interconnections is probably next. This includes both mechanical connections, and using incompatible products or protocols. Again, train your people well. Have the manufacturer's rep come in and spend a few hours with your people.

Another common mistake is simply not following the manufacturer's specifications. Remember, this is sensitive equipment, and if not installed correctly, it will not work. This work is far removed from the old telephone company days when connections were made under the rule of, “If they're touching, they're talking.”

To be continued next month.