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as Asynchronous Transfer Mode and IEEE 802.6 Switched Multi-megabit Data

Services; like ATM and 802.6, the RF Adoption Layer includes

segmentation/reassembly functions and Protocol Data Unit generation functions,

and it also includes Forward Error Correction (FEC) generation and verification

functions which substantially increase packet integrity in wireless environments

but adds FEC overhead. ?The RF Hopping Protocol Physical Layer consists of a

transmission convergence sublayer including header generation, RF framing, and

RF hopping protocol functions and the physical- medium-dependent sublayer, in

which the actual characteristics of the RF channel are handled. In the RF

Adoption Layer, a Protocol Data Unit is split into three segments, and two

error-correcting data units are added. The RF Hopping segments, and two

error-correcting data units are added. The RF Hopping Physical Layer builds

special Burst Protocol Data Units out of the data and FEC units and uses

carrier-sense methods borrowed from Ethernet to determine whether an RF Hop

Group is clear for transmission. Each hop group consists of five separate radio

channels. The controller scans hop groups via state-machine operation with four

states: scan, receive, carrier-sense, and transmit. In early tests at Apple, the

hop system showed 80-microsecond hop times, 57-microsecond clock recovery, and a

5-microsecond lapse between the time an empty channel is sensed and transmission

begins. Since each cluster of wireless LANs can use different hop groups,

multiple LANs could operate in the same area without interference. One concern

is whether the overhead for error correction for each packet, which can be as

much as 50% is too high to give the proposal a chance. The safety of those

operating new equipment now plays a larger role in determining the direction of

technological growth now more that ever. Factors under consideration are the

effect of infrared and strong electromagnetic radiation that would pervade the

workplace on the workers. This limits the strength of and communication device

that would be used in accomplishing transmission. For the Personal Computer;The

adapters have a small attached antenna through which they send and receive

network traffic as radio signals. Some wireless products are small boxes that

attach to your PC?s parallel port. In either case, the signals may travel from

PC to PC, forming a wireless peer-to-peer network, or they may travel to a

network server equipped with both wireless and standard Ethernet adapters,

providing notebook users a portable connection to the corporate network. In

either case, wireless LANs can either replace or extend wired networks.

Standards are lacking. Wireless networking is still a technology looking for a

standard, which is why very few wireless products can work with one another.

Each vendor uses a different protocol, radio frequency, or signaling technology.

If wired networks still operated like wireless, you would have to use the same

brand of network interface card throughout your network. Right now you are, for

the most part, tied to whichever brand of wireless LAN you pick. Most of the

products in this comparison listed their wireless protocol as Ethernet carrier

sense multiple access/collision avoidance (CSMA/CA), a variation of standard

Ethernet. Unfortunately, each vendor has put its own spin on CSMA/CA, which

means even their protocols are incompatible. 5 Wireless services As technology

progresses toward smaller, lighter, faster, lower power hardware components,

more computers will become more and more mobile. For space concerns this paper

will exclude any further discussion of the hardware developments toward mobility

except for devices directly related to wireless connectivity such as modems. A

wireless computer is not connected via a wireline and thus has mobility and

convenience. A wireless LAN provides the convenience of eliminating the wires,

yet is not necessarily mobile. (What is mobility?) Mobility is a characteristic

where the wireless computer may connect, loose the physical communication

(possibly due to interference) and reconnect (possibly to another sub-network)

and retain its virtual connections and continue to operate its applications. The

network protocols will be discussed later. (Then, what is portable?) Portable is

defined that the wireless computer may connect, loose the connection and then

re-connect, as well. However, the mobile unit will have to restart if it is

reconnected to another sub-network, requiring that running processes be

shut-down and windows closed. Mobility may be limited by the wireless service

subscribed. Four basic service zones are described: Global/National service

zone: Ubiquitous radio coverage throughout a region, country or the entire

globe, low user densities, and minimal bandwidth requirements. Typically

satellite systems. Mobile service zone: Radio coverage in urban, suburban and

populated rural areas, medium to high user densities, low to medium bandwidth

requirements (tens of Kbps), and high vehicular speed. Cellular (AMPS) system is

a good example. Local/micro service zone: Radio coverage in densely populated

urban areas, shopping malls, and transportation centers. High end user

densities, medium bandwidth requirements, hand-held portable terminals,

low-speed mobility.

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