ss, 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.
Indoor/pica service zone: in-building radio coverage, low to high user densities, medium to high bandwidth requirements (Mbps), very low mobility. Prior to the cellular phone network, base station radio covering a single cell geographic area with a fixed number of channels was the only service available. The cellular phone service divides the service area into cells and assigns a subset of the available channels to any given cell.This way the channels can be reused and interference from neighboring cells is reduced. The system tracks the active mobile unit, delivers calls, and maintains connections as units move between cells (Hand-off: a real-time transfer of a call between radio channels in different cells).
This system is called Advanced Mobile Phone Service (AMPS). Current cellular systems use analog FM technology. However, implementation of digital radio technology is being deployed now. These systems utilize Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) to increase throughput up to ten times the previous analog system.Additionally, end users will access a wider range of telecommunications as the implementation of integrated services digital network (ISDN) principles are utilized.
Personal Communication Services, similar to the current cellular system, will soon be available from the larger telecommunication services, but with reduced price and wider availability. Wireless Advantages Limitations Traditional Cellular no restrictions on length or type of data transmission, national coverage, bill by minute potential line interruptions, congestions in urban areas limited throughput CDPD enhanced technology for data over cellular bill by message size integrated voice and data packet switching error correction techniques lack of applications development not fully developed Dedicated packet switched mobile networks integrated applications and communications no call setup time inherent reliability and security of packet switching coverage not full nationwide limited packet size require specialized modems data only Specialized mobile radio voice and data vehicle based limited coverage Satellite-enabled networks geographic reach expensive equipment and service costs The application of the wireless computing system determines the type of wireless medium system to be employed. Circuit switched or packet switched, both are available through wireless technology and provide connectivity. Circuit switched systems provide a continuous connection established to the destination by the switching system.The most popular examples are the wire-line public switched telephone network (PSTN) and cellular telephones systems. This method of communication can be relatively expensive. If the phone systems offers voice grade bandwidth, then a standard modem can provide speed of 14.
4 Kbps (at the time of this writing). However, if a digital line is provided then higher communication rates can be achieved with more specialized equipment. Packet switched systems provide a delivery system of information packets.The packet contains the data and an address to the destination. Packet switching is far less expensive than circuit switching. Examples would be RAM, ARDIS, and Internet networks. Packet radio networks have been the target of many studies since the military has a vested interest in the communication medium. Concerns such as reliability, throughput optimization and re-routing of packets have been recent topics.
Packet Switched Systems ARDIS RAM Mobile Data Circuit Cellular CDPD Network Capacity 1,300 base stations in approx. 325 metro service area (MSA) 840 base stations in 210 MSAs 8,000 cell sites in 734 metro areas potentially entire cellular network Coverage (cities and towns) 10,700 6,300 NA NA Transmission speed 4.8Kbps. 19.2Kbps upgrade in major metro areas 8Kbps 38.4Kbps to 56Kbps 19.
2Kbps Message capacity 256 bytes 512 bytes NA 114 bytes National roaming completed by mid Sept 94 yes no yes Cellular Digital Packet Data technology (CDPD) utilizes the space between the voice segments on cellular (AMPS) network channels and inserts a data packet. The user pays only for the packet sent as opposed to a cellular circuit switched connection.CDPD cellular communications systems such as the Ubiquity 1000 from PCSI, offer packet burst rate of 19.
2 Kbps with full duplex. This CDPD modem offers the option to use circuit switched cellular, wire line PSTN and voice support. However, in a large urban area with thousands of stations using any packet switching service at current speeds, delay may be unacceptable. Satellite can be used as long distance links within wireless networks. Three major projects have been proposed.
The Teledesic system, composed of 840 low orbit satellites, was proposed by Bill Gates (Microsoft) and Craig McCaw (McCaw Cellular). Second, the Pentagon, solicited a system, using 1,000 smaller satellites, from TRW and Martin Marietta.Both the Teledesic and the Pentagon systems cost around $9 billion. The third system, called Iridium, from Motorola, will use 66 satellites to offer mobile phone service all over the globe. This project will begin this year and the rest in place by 1996. 6 Software Software concerns in a wireless computing environment can be broken into two areas, system and application. 7 System Software Network operating systems must be able to handle the uniqueness of a wireless computer.
Advanced operating systems utilizing distributed technology must be adapted to the specific communication media.The advancement of technology has provided that even mobile computer systems the size of notebooks are capable of internetworking as a host in global networks. Mobile host protocols compatible with TPC/IP have been developed to allow continuous network connectivity where ever the host may be. Due to the unpredictable nature of wireless connections, even operating systems may have to be written to provide support services for mobile network. The WIN*OS, a micro kernel for a wireless-compatible operating system, was developed to “support concurrent and composable objects and coordinated communication among groups of objects through a process of agreements.
” 8 Application Software Application software concerns in the wireless computing environment vary depending on the type of application and wireless medium used. For example, E-mail software must know how to communicate with the packet switched network as compared to the traditional cellular network. Software developer kits (SDK) and application programmers interfaces (API) are usually available by the service provider. Remote access software allows the remote user to connect to a host workstation to view the screen and control the keyboard as if the user was there.The data does not have to be communicated to the remote user and thus allows processing locally. Carbon copy and PC anywhere are among the programs which provide remote access for microcomputers. High baud rate is needed especially when a graphical user interface (GUI) is used.
9 Wireless Local Area Networks (WLAN) WLAN offers the same features as a wireline LAN but without the wires. Coverage can range from a room to a building to a “campus” (wide-spread, multi-building).Both stationary desktop systems and mobile notebook computers can connect using specialized wireless LAN adapter cards. Another configuration allows wireless additions to current networks. Wireless Hubs have been developed which bridge the wireless units into the wireline network. As mentioned before, during the recent natural disasters in California, the Federal Emergency Management Agency (FEMA) set up field offices with WLAN very quickly. Here is a great exampleof how WLAN can be used: An ETHERNET connection over a radio link provided data from a low-power PC in a buoy to a PC on a ship.
The system provided a megabyte/sec data rate for four days while guaranteeing error-free delivery of data. Even more incredible is the MBARI acoustic LAN. Since under water, radio waves travel only a few feet but sound waves can travel for miles, the acoustic LAN uses the better carrier of wireless data signals. The acoustic LAN has two 5Kbps data channels and two slow-speed command channels. The LAN is used to communicate with tilt meters and buoys.
Personal Data Assistants (PDA) are the new handheld computers which also have wireless options. Using a pen-based GUI operating system, the applications are accessed from local storage. Fax, data and voice can be transferred to and from the PDA via cellular phone system.
The AT&T EO can run a program called Gnosis which when also loaded on a remote server host will allow the user to search for documents and have them downloaded in minutes including graphics. Even though all these nifty devices such as radio modems and PDAs are developed and marketed, a recent study of mobile professionals shows that currently relatively few spend time far from their desks. In fact, only 13 percent of mobile users spend time outside their metro area and just 1 percent outside the country. As the technology becomes more common place, more users will find themselves moving further out of their wired areas and into the wireless field.10 Security Security becomes essential in wireless computing.
Especially since the data is broadcast to the receiving unit. International Standards Organization (ISO) has published security services, which provide for secure data ad computer systems on standard wire line networks. However, these must be modified to meet the needs of mobile users and systems. Data encryption and two possible solutions include exchanging security information between a small number of entities, or even more complex involving an information center. Infrared offers the least problem of security due fact that stations must be in the line-of-sight and the limited area of coverage, usually one room. Spread spectrum RF transmissions spread the data over a range of frequencies making interception extremely difficult.Also, low power limits the coverage area, although the signal will penetrate walls.
Cellular phone networks offer no security of their own. Even though listening to these transmissions has been made unlawful, the signals can be overheard by a radio scanner. Data encryption is left up to the connecting unit. Packet radio offers inherent data security by scrambling the data packets. Clipper chip will replace the digital encryption statndard (DES). The Clipper chip boasts to be 16 million times stronger with 80-bits as compared to the old DES, which has a 56-bit binary key.
This chip will be used in many communication products, especially wireless. The Department of Justice and AT&T will be installing them in their telephone products. The controversy about these chips stems from the fact that they are programmed with a back door.
The government can, with a court order, access the chip and monitor the communication. 11 Conclusion In the relatively short time of the Information Revolution, the world has seen several technologies, first introduced as “convenient”, become “essential” the basic structure of the modern lifestyle. The automobile, telephone, and the refrigerator are easy examples to cite. The wireless revolution will transform another “convenience” to a necessity.