Some Wireless Technologies
Wireless Transmission Media
Networks that use unguided transmissions to transport data
and protocols have some to be known as wireless. It is important to
remember that the name wireless is really misleading. The vast
majority of purported wireless LANs still have some wiring for connections
someplace in the LAN. It is probably reasonable to classify wireless
LANs by identifying the following four characteristics.
The four main wireless interconnective technologies we will discuss are
Spread Spectrum Radio. The FCC has allocated the 902 - 928MHz and 2.4 - 2.4835GHz bands of the electromagnetic spectrum for industrial, scientific and medical (ISM) use. They are referred to as the ISM bands. The uses of these bands is largely unregulated except to establish guidelines for electrical and electronic devices that use these bands.
Largely analogous to the ISM bands are the U - NII bands for Unlicensed National Information Infrastructure bands at 5 GHz. These are relatively uncrowded with other uses. Unfortunately, while these bands have higher bandwidth associated with their higher frequency, they are also more frangible and attenuate about twice as fast as the ISM bands.
Now for some definitions that relate to different strategies to make use of these bands. Baseband transmissions use the entire available bandwidth as a single channel. The same signal is sent across the entire band completely filling the available "pipe". Broadband transmissions channelize the available bandwidth into multiple, smaller channels where each smaller channel supports a different signal transmission. Frequency hopping also subdivides the bandwidth into multiple channels that are each used one-at-a-time to support signal transmissions. But with frequency hopping the signal hops from channel to channel at a predetermined rate and in a predetermined sequence.
Frequency hopping has several advantages
In contrast, direct sequence uses the available channels sequentially. Each of these options has its advantages and disadvantages.
Fortunately, these ISM bands provide sufficient bandwidth to compete with wire based LANs. They also tend to be relatively inexpensive since bands are not licensed and manufacturers can provide hardware for less money than they can for dedicated frequency products.
Unfortunately, radio signals are not capable to full-duplex communication on a single frequency, you are either sending or receiving, but not both simultaneously. For example, think of walkie-talkies and the push to talk necessities. This and other complexities reduce the overall effective throughputs rate to 2Mbps in Ethernet compatible networking. Effective throughputs in wire based Ethernets is 5 to 5.5 Mbps. Thus, radio based connectivity could easily be a relative bottleneck in current networking environments. In addition, the absence of FCC licensing to give dedicated frequencies requires limiting wattage so that reach is somewhere between 600 and 800 feet. There are also other issues of competition for this range within zones that may greatly reduce its effectiveness.
Single Band Radio. Single band radio is essentially the opposite of spread spectrum and uses a single channel. This signal is usually sent in the microwave range which are actually high frequency radio waves. Waves at the lower end of the microwave range behave much like radio waves, at the high end they behave much more like light.
To use a dedicated frequency you need FCC licensing. This technology was pioneered by Motorola who obtained exclusive rights to the 18 - 19 GHz band for all the major metropolitan markets in the US. Motorola acts as an agent to the FCC for any customers wanting to use this technology.
Firms that make sue of this maintain their existing wire based LAN backbones, hubs and software drivers. Transmission voltage is approximately 25 milliwatts which is too low to cause health concerns in metropolitan areas. The wattage, in conjunction with the relatively frangible microwave signal, limits the effective range to 140 feet of open air and 40 feet with sheetrock wall obstructions.
The gross bandwidth is approximately 15Mbps. Modifying this due to typical Ethernet considerations reduces the effective yield to around 5.5Mbps, comparable to wired Ethernet LANs.
Unfortunately, since the signal is within such a focused range it isn't as easy to make it secure as the more spread spectrum signals.
Infrared. The infrared spectrum operates between the visible part of the electromagnetic spectrum and the shortest microwaves. Infrared is actually a form of light which cannot penetrate opaque solids but does reflect off them.
Infrared can be used in direct and/or diffuse form. Transfer speeds range from 4 Mbps to 16 Mbps.
Infrared networking requires a transceiver in both communicating devices and may also require synchronization software. Some operating systems have built-in infrared support.
Direct infrared is typically used in household electronics with remote control devices which must be pointed at the device to be controlled. This is the same when applied to LANs. There has to be a line-of-sight connection.
Diffuse infrared scatters omnidirectionally. The intent is to bounce signals off ceilings and walls so that the transmit/receive device doesn't need to be in line-of-sight.
Infrared communications are not capable of penetrating even the least dense of opaque solids and thus don't require FCC regulation. But the FCC establishes specifications about the devices using the infrared spectrum. Lin-of-sight communications are severely limiting in most environments, particularly offices. Thus in offices, diffuse infrared is necessarily preferred to direct. But considering the highly frangible nature of the medium the effective range is extremely small, less than 100 feet. In addition the throughput rates are extremely modest.
Laser. You can think of laser-light amplification by stimulated emission of radiation communications within a LAN as single mode fiber optic communication without the fiber optic. A laser outputs a coherent electromagnetic field in which all of the waves are the same frequency and aligned in phase. A phase is a fraction of a complete cycle that has elapsed and it is measured from a specific point of reference. Different lasers operate at different frequencies and produce beams of different wavelengths.
Their cost precludes its use on a station by station basis. It's better when applied in ways similar to direct infrared since they are line of sight devices. Essentially, it is best used to interconnect access units that are connected in some other way to stations. They are best when mounted away from people, as near to the ceiling as possible. This makes them much less likely to cause eye injuries or to be disrupted. They can also be used to bridge gaps such as parking lots. This sort of implementation usually costs less than paying for routers and leased lines.
Some Wireless Gadgets
|Some Wireless WANs.
Sometimes it is impossible or inconvenient to connect WAN or LAN sites
over wire. Wireless solutions can also be convenient when users need
to be on the move. Unfortunately, wireless solutions work best for
communicating small amounts of data.
Some of the major wireless technologies are
It is important to remember that all of these are quite slow in comparison to wired approaches, providing no more than 14,400 bps. Digital cellular which offers a maximum of only 64 kbps. We now present a brief survey of some other issues associated with wireless connectivity.