A Brief Introduction to Wireless...

... and why the very reason you want to use it is the very reason it won't work!!!

WiFi is most often thought of as a convenient means of connecting a laptop PC to the Internet at the coffee shop. However, the availability of high gain antennas in a variety of configurations significantly extends the potential commercial use of this wireless technology. Wireless networks can represent significant cost savings especially when faced with installing new wiring in an old building.

The wireless device is simple in appearance. But don't be fooled. It requires no less planning than a wired network, and potentially more planning. A substantial amount of literature is available on the subject of wireless networking, and Control Solutions encourages you to spend some money at the book store. Only a very brief summary of key points is provided here.

And why won't it work?

The most desirable place to use wireless is on retrofit jobs in old buildings where running network wire where you need it is a seemingly impossible (or at least expensive) job. The biggest problem is too much steel and concrete in the way. But that is the very reason wireless won't work. Its penetration is poor. While data rates are higher at higher frequencies (thus the popularity), the higher the frequency is, the lower the penetration is.

So why sell WiFi? The antenna you see pictured on our products may be useful in a limited number of cases. But mostly it just makes a nice looking picture, and is legal without licensing. If you really need to get serious about wireless, unscrew that antenna, and put some coax cable between the controller and the antenna. You may even need to consider a power booster (but consult a radio technician about keeping it legal).

Wireless requires wire, just less of it. Now for some useful terminology...

Access Point

An access point is the wireless equivalent of the 10BaseT hub or 100BaseT switch. Wireless devices do not communicate directly with each other in a managed wireless network. They communicate via the access point. A popular access point product is the Linksys WRT54GS broadband router. In addition to providing the "hub" for the wireless devices, it provides a wireless to wired router. The WRT54GS also includes a DHCP server. At some point, wireless devices interface to wire, and this is the box that does the job.

Infrastructure vs. Ad-hoc

There are two basic topologies of wireless networks: Infrastructure and Ad-hoc. Other terms for "infrastructure" are managed or hosted. Other terms for "ad-hoc" are unmanaged or peer to peer. The simple definition of ad-hoc is that it has no access point. The advantage of ad-hoc is that some small number of devices can talk directly to each other. The disadvantage of ad-hoc is that security options are more limited, and network management must be done more explicitly (e.g., by assigning static IP addresses, etc.).

SSID

The service set identifier (SSID) was originally created as a minimal security measure. It is not currently regarded as a security measure since it is analogous to a user name without any password. Moreover, this "user name" or SSID is beaconed (transmitted) by the access point for anyone to grab onto and join your network. In search mode, a PC will "listen" for all the SSID's it can hear, and let you pick any one of them to "join" as your network. Although not very secure, the SSID is required in order to connect to any wireless network.

WEP and WPA

In simple terms, SSID is minimum security, WEP is medium security, and WPA is high security. Control Solutions WiFi products support all of these. The entire wireless network must be set up for the same set of security options (or at the very least, the access point must be configured to meet the minimum requirements of the selected Control Solutions product's WiFi security).

WEP (wired equivalent privacy) provides encryption of transmitted data that results in security equivalent to a wired network. Its weaknesses include having a static key that must be manually distributed, and no user authentication.

WPA (WiFi protected access) includes user authentication, dynamic encryption key distribution, and strong TKIP encryption. It originally was not as widely supported as WEP simply because it is relatively new (first WPA products available in 2003).

For more information on wireless security, consult the references noted below.

Range

Indoor range is almost impossible to predict due to the nature and variation in building materials, building layout, furniture and equipment within the building, and people. As a general rule, every increase of 9 dB roughly doubles the coverage area. Outdoor range is often more line of sight. As a general rule for outdoor line of sight, every increase of 6 dB doubles the distance.

Line of sight is always better for wireless performance. If that is not possible, limit the obstructions as much as possible. For example, if the device will be enclosed in a metal cabinet, you will need to use a bulkhead connector to mount the antenna on the outside of the cabinet. A variety of antenna configurations are available, including some that don't look like antennas at all (the "smoke detector" antenna for example).

Signals will generally not penetrate steel or concrete walls or floors. Trees and leaves are also obstructions at 802.11 frequencies. Metal studs in walls, aluminum siding, pipes and wiring in the walls, etc., are also obstructions that are common for indoor settings.

Number of Devices

The maximum number of wireless clients is not unlike wired access. Every client has its own IP address, so address space is no different than wired networks. However, the wireless access point devices are typically limited to 32 wireless clients per access point.

Just like wired networks, too many busy users on the same network will cause congestion and hence poor response. The added complication with wireless is that everybody, including your next door neighbors, shares the same air space and effectively the same "wire" for all of the networks combined. To see what is going on in your air space, you need wireless survey software such as the program available at www.netstumbler.com or CommView for WiFi from TamoSoft at www.tamos.com. NetStumbler is "freeware" but only locates access points. CommView for WiFi is a full featured packet sniffer and will capture packets between two WiFi devices talking to each other in ad hoc configuration (no access point involved).

Antenna Gain

The standard antenna shipped with Control Solutions WiFi products will go a reasonable distance (and is FCC certified for use with the device). You can buy larger antennas, as well as power amplifiers, from commercial wireless communications distributors. It will take little effort and no planning to greatly exceed FCC legal limits. It is the responsibility of the installer to make sure the resulting system either meets limits for unlicensed use, or has a site license from the FCC.

Coax cable inserted between the device and its antenna will subtract from the gain. Refer to cable specifications for attenuation specifications. Something near 19 dB per 100 feet is typical. This means it takes less than 100 feet of coax cable to end up with zero net power out of the Control Solutions WiFi devices. (A power booster is needed to go further via coax.)

Control Solutions' WiFi transmit power is 16 dBm and the standard antenna has a gain of 2 dBi. The EIRP (effective isotropic radiated power) is approximately 58 mW. The EIRP is the value used by regulatory agencies to determine radio transmission power limits. In Europe, this limit is 100mW on the ISM band (2.4 GHz). In the US, the FCC limit is 4W on the ISM band.

The formula for calculating EIRP in watts is mW = 10^(dBm/10) where dBm is the sum of transmitter power and antenna gain. Thus with our transmitter at 16 dBm, if we were to choose a 20 dBi antenna, the system would be right at the FCC limit of 4 watts. There is loss in cable, and loss per connector, between the transmitter and antenna. These losses are in dB, and would be subtracted from the total dBm before converting to EIRP.

References

A good introduction to wireless networking may be found on Microsoft's web site right here: Wireless LAN Technologies and Windows XP.

There are several good books on wireless networking. A good novice to intermediate book is "Jeff Duntemann's Wi-Fi Guide" (ISBN 1-932111-88-3 Paraglyph Press).