Residential Cordless (Wireless Networks)

Residential Cordless
Cordless systems are short-range wireless telephone systems that are primarily used in residential applications. Cordless telephones regularly use radio transmitters that have a maximum power level below 10 milliWatts (0.01 Watts). This limits their usable range to 100 meters or less.

The earliest generation of home cordless telephones used a single radio channel that used amplitude modulation. These first generation cordless phones were susceptible to electrical noise (static) from various types of electronic equipment such as florescent lights. The noise encountered when using these phones sometimes created a consumer impression that cordless telephone quality was below standard wired telephone quality. Improved versions of cordless phones that used FM modulation to overcome the electrical noise resulted. As cordless phones became more popular, interference from nearby phones became a problem. In apartment buildings where there were many users of cordless phones in close proximity, the ability to initiate and receive calls could be difficult as radio channels became busy with many users. This led to the development of cordless phones that used multiple radio channels. As voice privacy became more of an issue, cordless phones began to use scrambled voice. Some of these voice privacy systems were analog while a majority of cordless phones that offer voice privacy use digital transmission.

Figure 1 shows the evolution of cordless telephones. Until the mid 1990’s, most cordless telephones were limited to use in a small radio coverage area of their base station that was usually located in the home. That home base station was normally connected to the telephone line of the owner (either residential or a single office telephone line) and they were not intended to serve the general public. To add more value to the use of cordless phones, cordless telephones evolved to allow access to base stations in public locations. Cordless telephones could then be used in the home and in areas that were served by public base stations. The next evolution for cordless telephones was the combination of other types of wireless products and services into the cordless phone. This included the combination of wireless office and cellular telephones into a cordless phone.


Figure 1: Evolution of Cordless Telephone Systems
Most home cordless telephones used frequencies in unlicensed radio frequency bands. Because so many homes operate cordless phones, each manufacturer must build-in circuitry to minimize the interference caused by other cordless devices. The original cordless phones use a very crowded frequency band (around 27 and 49 MHz) utilizing analog radio wave modulation. Recently, cordless telephones have been developed that operate in the 902-928 MHz unlicensed industrial, scientific, and medical (ISM) frequency band.

Residential cordless telephones must automatically coordinate their radio channel access as they operate independently of any type of network control. To coordinate radio channel access and avoid interference to other cordless handsets installed in the vicinity, cordless phones perform radio channel scanning and interference detecting prior to transmitting a signal.

Because cordless telephone systems do not as a rule have a dedicated control channel to provide information, the cordless handset and base station continuously scan all of the available channels (typically 10 to 25 channels). Figure 2 shows the basic cordless telephone coordination process. This diagram shows that when the cordless phone or base station desires to transmit, the unit will choose an unused radio channel and begin to transmit a pilot tone or digital code with a unique identification code to indicate a request for service. The other cordless device (base station or cordless phone) will detect this request for service when it is scanning and its receiver will stop scanning and transmit an acknowledgement to the request for service. After both devices have communicated, conversation can begin. When another nearby base station detects the request for service, it will determine that the message is not intended for it and will not process the call and scanning will continue.


Figure 2: Cordless Telephone System

Wireless PBX | Telecommunications

Wireless PBX
Wireless PBX (wireless office) telephone systems are used in a business environment to provide similar features as a private branch exchange (PBX) with the ability of mobility throughout the office area. The wireless office commonly begins with a specialized wireless private branch exchange (WPBX) that has been adapted for wireless. While more complex than a home cordless telephone, it is not typically as complex as a complete cellular telephone system.

The WPBX telephone radio coverage area is usually within one or more company buildings or on a campus. The more popular WPBX systems use unlicensed frequencies with a protocol available only to the manufacturer of the WPBX. Ordinarily, WPBX telephones cannot be used outside the established campus.

These private WPBX systems use small wall mounted antennas, and like cellular, the space is divided to provide adequate capacity for the expected usage. WPBX telephones, like the one shown in Figure 1, have become commonplace in many hospitals and warehouse environments where the staff is primarily walking around to do their job.


Figure 1: Wireless Office Telephone System
Recent hybrids have been developed whereby the telephone handset has two technologies built into the operation of the phone. When the telephone is inside the WPBX coverage area (preferred) it acts as a private phone; when outside the WPBX coverage area, the phone has the ability to send and receive calls on the public cellular system, incurring airtime charges as any other cellular user.

Satellite (Wireless Networks)

Satellite
Satellite communication systems use of orbiting satellites to relay communications signals from one satellite station to one or several other users. Satellite communication can be divided into categories of fixed satellite service, positioning systems, and mobile satellite communication systems.

There are three basic types of satellite systems: geosynchronous earth orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO). GEO satellites hover at approximately 22,300 miles above the surface of the earth. GEO satellites revolve along with the earth once a day; they appear stationary with respect to the earth. The high-gain antennas used to receive signals from 22 thousand miles away (usually called “dish” antennas) are pointed directly toward the satellite. MEO satellites are located closer to the earth than GEO satellites and do not as a rule require high-gain antennas. This is important as MEO satellites revolve around the earth several times per day and fixed antennas cannot be used. The newest satellite technology being deployed is LEO satellites. LEO satellites are located approximately 450 miles above the surface of the earth. Because these satellites are relatively close to the earth, portable phones with smaller antennas can be used.

Figure 1 shows the different types of satellite communication systems. The GEO satellite system is primarily used for television broadcast services, as their satellites appear stationary above the Earth. MEO and LEO systems are used for mobile communications as they are located much closer to the Earth. However, these satellites continuously move relative to the surface of the Earth.


Figure 1: Satellite Systems
Mobile satellite telephone service allows customers to use specialized satellite mobile telephones to communicate in any part of the world to the PSTN through the use of communication satellites. Commercial communication satellite services began in the mid-1960’s with the establishment of Intelsat, a multinational organization with well over 130 member nations today. An organization known as the Communications Satellite Corporation (COMSAT) also was established in the early-1960’s and became the United States’ representative in Intelsat. These first commercial applications of satellites provided international telephone and television program transmission, primarily between the United States and Europe.

Aircraft Telephones (Wireless Networks)

Aircraft Telephones
Aircraft telephones allow people on an airplane to initiate telephone calls with the public telephone system through connection via land based radio or satellite transmission systems. Recently, some aircraft telephone systems have been upgraded to allow calls to be received on the airplane.

Aircraft telephone systems are ordinarily a hybrid wireless system that is a terrestrial wireless system (land-based) combined with satellite service. The terrestrial system is used to connect telephone calls when the aircraft is above land and is within distance of a ground transmitter. For the terrestrial-based system, the phone handset in the airplane is connected to a transmitter in the plane’s belly that connects the call down to one of the ground antennas located strategically throughout the country. The call is routed to a ground switching station that connects the call to the receiving party.

The satellite system is used mainly over the water, where calls are out of reach of the ground antennas. For the satellite-based system, the phone handset on the plane is connected to an antenna on the top of the plane that connects the signal up to an orbiting satellite. The call is then sent down to earth by the satellite frequencies to its satellite earth station, then to one of the main ground switching stations that routes the call to the PSTN.

Aircraft phone systems normally have handsets in a common area or handsets that are located in the back of passenger seats. If the handset is located in the seat, some aircraft phone systems allow incoming calls. For someone to reach you on an aviation telephone system, the person on the aircraft must first get an telephone access number and temporary identification code by registering with the aviation telephone operator. The person placing the call from the ground dials the access number and enters the temporary identification code and the call will be routed to the aviation telephone.

Figure 1 shows a public aircraft telephone system. This diagram shows that aircraft may be served by terrestrial (land-based) systems or satellite communication systems. In either case, the aircraft communicates with a gateway that links the radio system to the public telephone system.


Figure 1: Public Aircraft Telephone System

Land Mobile Radio (LMR)

Land mobile radio (LMR) consists of a wide variety of mobile radio systems, ranging from a simple pair of handheld “walkie-talkies” to digital cellular-like systems. LMR includes radio service between mobile units or between mobile units and a base station.

LMR systems are traditionally private systems that allow communication between a base and several mobile radios. LMR systems can share a single frequency or use dual frequencies. Where LMR systems use a single frequency when mobile radios must wait to talk, this is called a simplex system. To simplify the mobile radio design and increase system efficiency, some LMR systems use two frequencies; one for transmitting and another for receiving. If the radio cannot transmit and receive at the same time, the system is called half duplex. When LMR systems use two frequencies and can transmit and receive at the same time, this is called full duplex. When a company operates an LMR system to provide service to multiple users on a subscription basis (typically to companies), it is called a public land mobile radio system (PLMR).

Figure 1 shows a traditional two-way radio system. In this example, a high power base station (called a “base”) is used to communicate with portable two-way radios. The two-way portable radios can communicate with the base or they can communicate directly with each other.


Figure 1: Traditional Land Mobile Radio System
LMR systems are used by: taxicab companies, conventioneers, police and fire departments, and places where general dispatching for service is a normal course of business communications. SMR radios are regularly designed to be rugged to survive the harsh environment. SMR radios can usually be programmed with a unique code. This code may be an individual code or group code (e.g., pre-designated group of users such as a fire department). This allows all the radios belonging to a group, or a sub-group, to be “paged” by any party in the group. A push-to-talk method is used during the dispatch call (page) or reply. This push-to-talk radio-to-radio communication efficiently utilizes the airwaves because of the bursty (very short transmission time) nature of the information.

Automated land mobile radio systems are divided into two categories; SMR or Enhanced SMR (ESMR). Enhanced land mobile radio systems operate and have similar features to mobile telephone systems.

Wireless Local Loop (WLL)

Wireless Local Loop (WLL)
Wireless local loop (WLL) service refers to the distribution of telephone service from the nearest telephone central office to individual customers via a wireless link. In some cases, it is referred to as “the last mile” in a telephone network. This term is a bit misleading, though, because the coverage area of a WLL system may extend many miles from the central office.

Competitive local exchange carriers (CLEC) are competitors to the incumbent local exchange carriers (ILECS) and are likely to use WLL systems to rapidly deploy competing systems. If CLECs do not use wireless systems, they must either pay the existing phone company for access to the local loop (resale) or dig and install their own wire to the local customers. Many countries, that do not have large wired networks such as the United States, are using wireless local loop as their primary phone system.

Figure 1 shows a wireless local loop system. In this diagram, a central office switch is connected via a fiberoptic cable to radio transmitters located in a residential neighborhoods. Each house that desires to have dial tone service from the WLL service provider has a radio receiver mounted outside with a dial tone converter box. The dial tone converter box changes the radio signal into the dial tone that can be used in standard telephone devices such as answering machines and fax machines. It is also possible for the customer to have one or more wireless (cordless) telephones to use in the house and to use around the residential area where the WLL transmitters are located.


Figure 1: Wireless Local Loop
The most basic service offered by wireless local loop (WLL) system is to provide standard dial tone service known as plain old telephone service (POTS). In addition to the basic services, WLL systems typically offer advanced features such as high-speed data, residential area cordless service, and in some cases, video services. To add value to WLL systems, WLL service providers will likely integrate and bundle standard phone service with other services such as cellular, paging, high speed Internet, or cable service.

WLL systems can provide for single or multiple-line units that connect to one or more standard telephones. The telephone interface devices may include battery back up for use during power outages. Most wireless local loop (WLL) systems provide for both voice and data services. The available data rates for WLL systems vary from 9.6 kbps to over several hundred kbps. WLL systems can be provided on cellular and PCS, private mobile radio, unlicensed cordless, and proprietary wideband systems that operate the 3.4 GHz range.

Wireless Data (Wireless Networks)

Wireless Data
Wireless data systems transfer of digital signals between two data devices via a wireless communication path. Most wireless data services are dedicated to specific types of applications. Vertical wireless data applications (vertical) are very specific solutions, and have continued to win over mass market “horizontal” offerings. Vertical solutions include applications such as utility meter reading or mobile dispatch. Horizontal solutions have mass-market appeal such as wireless e-mail.

The growth of the Internet has also enabled low-cost, standardized access to wireless data networks that is accelerating the growth of the wireless data marketplace. In 2001, almost all the new mobile telephones had wireless Internet access capability.

Figure 1 shows a basic wireless data system. In this example, many types of wireless data devices communicate through a public wireless data system. In the core of the system, there is a switching system. The switching system commonly routes the data between the wireless device and a computer system (such as a company computer). In this diagram, there are more receivers than transmitters. This is required to allow low-power mobile data transmitters to reach the system. Base station transmitters can provide up to 500 Watts effective radiated power (ERP) while portable mobile data devices can usually provide less than 1 Watt of transmitted power.


Figure 1: Wireless Data System
Wireless data get the attention it deserves when a mass-market wireless data application (often called the “Killer App”) is embraced by the public. Here are a few successful vertical wireless data applications:

Wireless data for the electric power, waste water, and natural gas industries. New competition in the utility industry demands the benefits of a wireless data solution for timely customer-focused improvements.

Wireless data for field service personnel. Field service organizations use wireless data to close the gap on a geographic distance to improve customer service, technician productivity, and increased revenues.

Companies with mobile sales forces have increased their productivity and efficiency of personnel by filling out much of their paperwork “on-line”. Sales force access to corporate databases has proven paramount in the new paradigm of doing business the 21st century style.

Paging (Wireless Networks)

Paging
Paging is a method of delivering a message, via a public or private communications system or radio signal, to a person whose exact whereabouts are unknown. Users as a rule carry a small paging receiver that displays a numeric or alphanumeric message displayed on an electronic readout or it could be sent and received as a voice message or other data.

Commercial paging service began in 1949 with the allocation of frequencies exclusively dedicated to one-way signaling services. Subscribers used AM receivers, listened for an operator to announce their number, and then called the service to receive their messages. Selective addressing (the ability to choose one individual pager from the group) was introduced in the mid 1950’s and FM was first used in an experimental paging system in 1960. Pagers with alphanumeric displays made their debut in the early 1990’s. In addition to complete messages that can be sent and stored in these pagers, a number of other services such as stock market and sports score reporting have been developed.

There are 4 basic types of messaging services offered by paging systems: tone, numeric, text (alpha), and voice. Two types of paging systems can deliver these messaging services: one-way and two-way paging. One-way paging systems only allow the sending of messages from the system to the pager. Two-way paging systems allow the confirmation and response of a message from the pager to the system as well.

One-way paging is a process where paging messages (signals) are sent from a radio tower to a pager without a return verification signal. In its simplest form, a one-way paging system can serve up to several hundred thousand numeric paging customers.

Figure 1 shows a one-way paging system. In this diagram, a high-power transmitter broadcasts a paging message to a relatively large geographic area. All pagers that operate on this system listen to all the pages sent, paying close attention for their specific address message. Paging messages are received and processed by a paging center. The paging center receives pages from the local telephone company or it may receive messages from a satellite network. After it receives these messages, they are sent after processing to the high-power paging transmitter by an encoder. The encoder converts the pagers telephone number or identification code entered by the caller to the necessary tones or digital signal to be sent by the paging transmitter.


Figure 1: One-Way Paging System
Two-way paging systems allow the paging device to acknowledge and sometimes respond to messages sent by a nearby paging tower. The two-way pager’s low-power transmitter necessitates many receiving antennas being located close together to receive the low-power signal.

Figure 2 shows a high-power transmitter (200-500 Watts) which broadcasts a paging message to a relatively large geographic area and several receiving antennas. The reason for having multiple receiving antennas is that the transmit power level of pagers are much lower than the transmit power level of the paging radio tower. The receiving antennas are very sensitive, capable of receiving the signal from pagers transmitting only 1 watt.


Figure 2: Two-Way Paging System
The number of required receivers for a two-way paging system is dependent on the available transmittal power from the paging and how fast the information is to be transferred. The higher the data transmission rate results in a higher number of required receivers.

The main advantage of two-way paging systems is their ability to require pagers to register their location within the paging system. This allows the paging system to direct pages for a specific pager only to the area near where the pager last registered. This frees up the paging capacity of channels in other geographic areas so paging messages can be sent to other pagers. This is a type of frequency reuse based on geographically separated systems.

Broadcast Television (Wireless Networks)

Broadcast Television
Television broadcasting is the transmission of video and audio to a geographic area that is intended for general reception by the public, funded by commercials or government agencies. Television broadcasters transmit at high power levels from several hundred foot high towers. A high-power television broadcast station can reach over 50 miles.

The standard television system used in the Americas is the National Television Standards Committee (NTSC) system. The first version of this system used 6 MHz RF channels to provide black and white television. The NTSC standard was later modified to allow color television signals to co-exist on the same type of video channel. The television system used in Europe and other parts of the world is phase alternating line (PAL).

The PAL television system was developed in the 1980’s to provide a common television standard in Europe. The PAL system uses 7 or 8 MHz wide radio channels.

Several enhancements have been added to this basic television broadcasting system, including audio stereo sound, additional audio programming channels, very low data rate digital transfer (closed captioning), and ghost canceling.

The NTSC and PAL enhancements are minor when compared to the technological improvements represented by HDTV proposed to provide significantly higher resolution audio and video, as well as data services. A consortium called the Grand Alliance has produced a standard called Grand Alliance HDTV for digital television. The FCC plans to introduce HDTV initially by allowing broadcasters to offer a simulcast of their regular programming, transmitted on UHF television assignments. The period of simulcast will continue for up to 15 years as old broadcast facilities and receivers are phased out. Receivers for the HDTV system will also include the capability to receive and display regular analog broadcasts.

Figure 1 shows a television broadcast system. This television system consists of a television production studio, a high-power transmitter, a communications link between the studio and the transmitter, and network feeds for programming. The production studio controls and mixes the sources of information including videotapes, video studio, computer created images (such as captions), and other video sources. A high-power transmitter broadcasts a single television channel. The television studio is connected to the transmitter by a high bandwidth communications link that can pass video and control signals. This communications link may be a wired (coax) line or a microwave link. Many television stations receive their video source from a television network. This allows a single video source to be relayed to many television transmitters.


Figure 1: Television Broadcast System

Broadcast Radio (Wireless Networks)

Broadcast Radio
Radio broadcasting is the transmission of audio material (called a program) to a geographic area that is intended for general reception by the public, funded by airtime sold between programs.

Amplitude modulation (AM) radio broadcast services have been available for the past 100 years. Most AM radio broadcast systems use relatively low radio frequencies and very narrow radio channel bandwidth to efficiently deliver audio information over large geographic areas. Unfortunately, low frequency used for AM transmission often result in signals that sometimes skip long distances (hundreds of kilometers). This has the potential for interference in distant cities. Amplitude modulation is also easily subject to electrical noise and signal distortion. Recent advancements in AM modulation can allow channel coding for stereo and more reliable (less distorted) radio signals.

To overcome some of the limitations of AM, frequency modulation (FM) was developed. FM transmission is less susceptible to noise and distortion. Unfortunately, most FM broadcast systems use a wider radio channel than AM systems. FM broadcast channels can be up to 20 times the bandwidth of a single AM broadcast channel. The latest advancements in FM broadcasting include conversion from analog to digital and the ability to simultaneously send some additional information (sub-channels) with their audio broadcasts.

The current technology used for FM radio channel broadcast uses less bandwidth than is authorized for transmission. With some modifications to the transmitter, it has been possible for FM broadcast stations to simultaneously send some additional information (sub-channels) with their audio broadcasts. These sub-channels can contain audio or digital information. Sub-channels can be used for data transmission and paging services.

Figure 1 shows a typical radio broadcast system. The radio broadcast system consists of a production studio, a high-power AM or FM transmitter, a communications link between the studio and the transmitter, and network feeds for programming. Radio broadcasting involves the use of various types of information sources called “program sources.” These program sources come from compact discs, tape recordings, soundproof audio studios, remote location sites (such as a van), or other network sources. The production studio controls and mixes the sources of information including audio compact discs, audio studio, audiotape, and other audio sources. A high-power transmitter broadcasts a single radio channel. The studio is connected to the transmitter by a coaxial cable, special leased telephone line (extra high quality), or dedicated radio link. Many radio broadcast stations receive their programming source from a radio broadcast network. This allows a single audio source to be relayed to many radio broadcast transmitters. The diagram also shows how a sub-channel is combined to provide a private audio broadcast service.


Figure 1: Radio Broadcast System
Two separate technologies are being tested to bring digital audio and data services to conventional radio broadcasts. The first incorporates digital data into the conventional FM broadcast by adding the digital data signal to the existing audio signal before FM modulation. The second is a fully digital transmission that is transmitted in addition to the conventional FM. This separate signal is added to the conventional FM signal after the FM modulation. Unlike high definition television (HDTV), these systems do not replace the analog service; they provide additional services and are completely compatible with conventional AM or FM broadcasts. The additional services are available only to those users with a receiver capable of accessing the digital data.

The entry of digital transmission into commercial broadcasting represents a revolution in the types of services that will be available to the public in the near future. Compare the possibilities to the many digital satellite features or the digital programming available with CD players. Imagine pressing one button on the car radio to request only news stations, or your preferred music category.

Digital audio broadcasting (DAB) transmits voice and other information using digital radio transmission. The DAB signal is normally shared with additional digital information on a single digital radio channel.

Cellular and Personal Communication Service (PCS) - (Wireless Networks)

Mobile telephones connect people to the public switched telephone system (PSTN) or to other mobile telephones. Mobile telephone service includes cellular, PCS, specialized and enhanced mobile radio, air-to-ground, marine, and railroad telephone services.

The first mobile telephone system in the United States began in St. Louis, Missouri in 1946. By 1947, more than 25 cities in the United States had mobile telephone service available. The systems used a single high-power transmitter for the base station in the center of a metropolitan area. Coverage was provided for 50 miles or more from the transmitter. These initial systems used a human operator at the base station to manually connect the mobile user with the landline network. In most of these systems, service was very poor because too many customers (called subscribers) shared each radio channel (called loading). It was not uncommon to have busy channels over 50% of the time. Despite this poor service, it revolutionized the definition of telephone service and priority was given to police and ambulance service. The waiting list for mobile phones in some cities was more than 7 years. This type of system was improved many times and the last upgrade, called improved mobile telephone service (IMTS), was introduced in the mid 1960’s. While there may still be some original systems in operation throughout the United States, new equipment for these systems is not currently being produced. It has been replaced with cellular systems.

Cellular and Personal Communication Service (PCS)
Cellular and PCS mobile telephone systems allow mobile telephones to communicate with each other or to the public telephone system through an interconnected network of radio towers. In early mobile radio-telephone systems, one high-power transmitter served a large geographic area with a limited number of radio channels. Because each radio channel requires a certain frequency bandwidth (radio spectrum) and there is a very limited amount of radio spectrum available, this dramatically limited the number of radio channels that kept the serving capacity of such systems low. For example, in 1976, New York City had only 12 radio channels to support 545 customers and a two-year long waiting list of typically 3,700.

When linked together to cover an entire metro area, the radio coverage areas (called cells) form a cellular structure resembling that of a honeycomb. The cellular systems are designed to have overlap at each cell boarder to enable a “hand-off” (also called a “handover”) from one cell to the next. As a customer (called a subscriber) moves through a cellular or PCS system, the mobile switching center (MSC) coordinates and transfers calls from one cell to another and maintains call continuity.

Figure 1 shows a mobile telephone system. The wireless network connects mobile radios to each other or the public switched telephone network (PSTN) by using radio towers (base stations) that are connected to a mobile switching center (MSC). The mobile switching center can transfer calls to the PSTN.


Figure 1: Mobile Telephone System
When a cellular system is first established, it can effectively serve only a limited number of callers. When that limit is exceeded, callers experience too many system busy signals (known as blocking) and their calls cannot be completed. More callers can be served by adding more cells with smaller coverage areas - that is, by cell splitting. The increased number of smaller cells provides more available radio channels in a given area because it allows radio channels to be reused at closer geographical distances.

There are two basic types of systems: analog and digital. Analog systems typically use FM modulation to transfer voice information and digital systems use some form of phase modulation to transfer digital voice and data information. Although analog systems are capable of providing many of the services that digital systems offer, digital systems offer added flexibility as many of the features can be created by software changes. The trend at the end of the 1990’s was for analog systems to convert to digital systems.

To allow the conversion from analog systems to digital systems, some cellular technologies allow for the use of dual-mode or multi-mode mobile telephones. These telephones are capable of operating on an analog or digital radio channel, depending on availability. Most dual-mode phones prefer to use digital radio channels in the event both are available. This allows them to take advantage of the new features such as short messaging and digital voice quality.

Cellular systems have several key differences that include the radio channel bandwidth, access technology type (FDMA, TDMA, CDMA), data signaling rates of their control channel(s), and power levels. Analog cellular systems have very narrow radio channels that vary from 10 kHz to 30 kHz. Digital systems channel bandwidth ranges from 30 kHz to 1.25 MHz. Access technologies determine how mobile telephones obtain service and how they share each radio channel. The data signaling rates determine how fast messages can be sent on control channels. The RF power level of mobile telephones and how the power level is controlled typically determines how far away the mobile telephone can operate from the base station (radio tower).

Wireless Networks - Technologies

Key enabling technologies for wireless communication include digital modulation, data compression, and digital signal processing.

Digital Modulation
Digital modulation is the process of modifying the amplitude, frequency, or phase of a carrier signal using the discrete states (On and Off) of a digital signal.

When modulating a carrier signal using a digital information signal, this causes rapid changes to the carrier wave. These rapid changes result in the creation of other signals that are usually undesirable. As a result, digital modulation usually includes a process of adjusting the maximum rate of change of the input signal (rounding the digital signal edges) and filtering out some of the unwanted signals that are created during the transition.

Figure 1 shows different forms of digital modulation. This diagram shows ASK modulation that turns the carrier signal on and off with the digital signal. FSK modulation shifts the frequency of the carrier signal according to the on and off levels of the digital information signal. The phase shift modulator changes the phase of the carrier signal in accordance with the digital information signal. This diagram also shows that advanced forms of modulation such as QAM can combine amplitude and phase of digital signals.


Figure 1: Digital Modulation
Data Compression
Data compression is a process that is used encoding information so that fewer data bits of information are required to represent a given amount of data. Compression allows the transmission of more data over a given amount of time and circuit capacity. It also reduces the amount of memory required for data storage.

Access Multiplexing
Access multiplexing is a process used by a communications system to coordinate and allow more than one user to access the communication channels within the system. There are four basic access multiplexing technologies used in wireless systems: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access, (CDMA), and space division multiple access (SDMA). Other forms of access multiplexing (such as voice activity multiplexing) use the fundamentals of these access-multiplexing technologies to operate.

FDMA systems use a process of allowing mobile radios to share radio frequency allocation by dividing up that allocation into separate radio channels where each radio device can communicate on a single radio channel during communication. TDMA systems allow several users to share a single radio channel by dividing the channel into time slots. When a mobile radio communicates with a TDMA system, it is assigned a specific time position on the radio channel. By allow several users to use different time positions (time slots) on a single radio channel, TDMA systems increase their ability to serve multiple users with a limited number of radio channels. Code division multiple access (CDMA), a form of spread spectrum communication. CDMA is a method of spreading information signals (typically digital signals) so the frequency bandwidth of the radio channel is much larger than the original information bandwidth.

Some systems coordinate system access on the same radio channels that are used for communication and other systems use a separate (dedicated) control channel. When using a control channel to coordinate access to the system, it is called an access control channel. The access control channel coordinates the random requests for service that is received from users (mobile radios) in the system. The control channel may also transfer identification information that allows the system to determine if the user is authorized to receive access to the system.

Figure 2 shows the common types of channel-multiplexing technologies used in wireless systems. This diagram shows that FDMA systems have multiple communication channels and each user on the system occupies an entire channel. TDMA systems dynamically assign users to one or more time slots on each radio channel. CDMA systems assign users a unique spreading code to minimize the interference receive and cause with other users. SDMA systems focus radio energy to the geographic area where specific users are operating.


Figure 2: Channel Multiplexing

Wireless Networks - Market Growth

In 2001, approximately 1 in 8 people in the world were using mobile telephones. The growth of some vertical wireless data markets is over 80% per year.

Mobile Telephone Service
By 2001, there were 781 million mobile telephone subscribers in the world. Figure 1 shows the recent trend in subscribership to mobile telephone services. Some of the key drivers for continual growth include lower monthly cost of service and pre-paid wireless services. Pre-paid wireless service allows customers with bad or damaged credit to forego the normal credit check required with wireless service and pay for their service before they use it. Many of the new wireless subscribers have credit challenges.


Figure 1: Mobile Telephone Wireless Growth.
Source: GSM MOU

Data Networks
With the demand for high data rate communications solutions, paralleling interest in the Internet, (fueled by easy-to-use application software, its wide array of text, graphics, video and audio content), wireless data market growth has increased substantially. The availability of Internet services over wireless radio channels will be a critical factor in determining overall market growth.

To date, most wireless data applications are non-human in nature. These include applications such as monitoring wireless parking meters, vending machines, and environmental concerns among others. Human access includes the ability to access data available on the Internet, private intranets, new services, and e-mail. The Internet, for example, is being used by businesses for building interactive branding via communication with customers, advertising products and services, publishing product specifications; and acting as a source for point-of-sale applications.

Market growth for wide area wireless data communications services is in the early stages, primarily because wireless data is not yet capable of providing high data transfer rates at a cost comparable to fiber optic cable or wired connectivity. However, the overall market growth of the wireless data market is up. In 1997, there was over 21% growth for circuit switched data (primarily cellular data) and over 89% growth for packet data (ARDIS, RAM, CDPD, and Ricochet).

Wireless Local Area Network (WLAN)

Wireless Local Area Network (WLAN)
The most common fixed wireless application is the popular wireless local area network (WLAN) replacing common connections previously made by cable. Cable provides an excellent transmission media and supports data rates in the tens and hundreds of millions of bits per second. There are applications, however, that cannot use cable or are prohibitively expensive if cable is used.

Figure 1 shows product that are typically used in a WLAN system. This WLAN system includes radio access ports and extension ports. The extension ports shown in figure 1 are PCMCIA cards that plug into a laptop computer. These extension ports communicate via radio-to-radio access ports. The radio access ports convert the WLAN radio signal back into computer network signals (such as Ethernet or token ring).


Figure 1: Wireless Local Area Network (WLAN)
Other WLAN applications, like point-of-sale terminals in the ever-changing retail environment make wireless access more cost-effective than cabled access. Mobile inventory scanning in warehouses tie WLANs to a wireless scanner. Some building architectures make cable installation prohibitively expensive, WLANs are well suited for these types of applications.

Often, Infrared (IR) light energy is used for point-to-point computer connections, because IR cannot pass through walls, ceilings, or floors. This is considered an advantage because it enhances the security of a WLAN link and decreases interference between other nearby WLANs.

Wireless Networks - Radios

Wireless networks are composed of radios, radio towers or base stations, interconnection systems, and network management and information systems.

Radios
Radios may be fixed in location (such as a television) or may be mobile (such as a cellular telephone). Some radios may only communicate in one direction (typically a receiver) or may have two-way capability. When a single radio has both a transmitter and receiver contained in the same unit, it is called a transceiver.

Figure 1 shows a block diagram of a mobile radio transceiver. In this diagram, sound is converted to an electrical signal by a microphone. The audio signal is processed (filtered and adjusted) and is sent to a modulator. The modulator creates a modulated RF signal using the audio signal. The modulated signal is supplied to an RF amplifier that increases the level of the RF signal and supplies it to the antenna for radio transmission. This mobile radio simultaneously receives another RF signal on a different frequency to allow the listening of the other person while talking. The received RF signal is then boosted by the receiver to a level acceptable for the demodulator assembly. The demodulator extracts the audio signal and the audio signal is amplified so it can create sound from the speaker.


Figure 1: Mobile Radio Block Diagram
Radio Towers and Transmitter Equipment
Radio towers are poles, guided towers, or free standing constructed grids that raise one or more antennas to a height that increases the range of a transmitted signal. Radio towers can vary in height from about 20 feet to more than 300 feet. A single radio tower may host several antenna systems that include paging, microwave, or cellular systems. Radio towers are located strategically around the city to provide radio signal coverage to specific areas. At the base of the towers are electronic control rooms that contain the components to operate the radio portion of the communications system.

Radio towers and their associated radio equipment (e.g., base station) may include one or more antennas, transmitters, receivers (for two-way systems), system controllers, communication links, and power supplies. Transmitters provide the high level RF power that is supplied to the antenna. For broadcast systems, the amount of transmitter power can exceed 50,000 Watts. Receivers boost and demodulate incoming RF signals from mobile radios. If a base station contains receivers, it is typical to use one or more different antennas for the receivers. Controllers coordinate the overall operation of the base station and coordinate the alarm monitoring of electronic assemblies. Communication links allow a command location (such as a television studio or a telephone switching center) to control and exchange information with the base station. Base station radio equipment requires power supplies. Most base stations contain primary and backup power supplies. A battery typically maintains operation when primary power is interrupted. A generator may also be included to allow operation during extended power outages.

Figure 2 shows a typical radio base station block diagram that is used in a mobile telephone system. This diagram shows that the base station holds the radio transceiver (transmitter and receiver assemblies) that is part of the radio tower (cell site). This diagram also shows that one antenna is used for transmitting and two antennas are used for receiving (for improved reception). This base station also contains a backup battery that is maintained at full charge so radio communications will not be interrupted in the event AC power is lost.



Figure 2: Radio Tower and Base Station Equipment
Switching Facilities
Switching facilities are typically used in two-way mobile communication systems to allow the connection of mobile radios to other radios in the system or to the public telephone network. When used in a cellular system, the switching system is typically called a mobile switching center (MSC). The MSC, just like a local telephone company, processes requests for service from mobile radios (subscribers) and routes the calls to other destinations.

Figure 3 illustrates a wireless switching system basic functional components. These include: communication line interfaces, a switch, a customer database, system and communication controllers, primary and backup (batteries) power, and the software to interface and control the radio tower’s and base station (BS) it is connected to.


Figure 3: Wireless Switching System Block Diagram
Interconnection to Other Networks
Wireless systems may be connected to other networks. Broadcast wireless systems are connected to media sources (such as audio or video programs) via satellite links while cellular networks may be interconnected to the public telephone network. Interconnection involves the physical and software connection of network equipment or communications systems to the facilities of another network such as the public telephone network. Government agencies such as the Federal Communications Commission (FCC) or Department of Communications (DOC) regulate interconnection of wireless systems to the public telephone networks to ensure reliable operation.

Customer Databases
Customer databases are computer storage devices (typically a computer hard disk) that hold service authorization and feature preferences of customers. For wireless systems that allow the customer to operate in other territories, a home (local) database is used. Each wireless subscriber has a real-time user profile in the database that is typically called the home location register (HLR). The HLR identifies the current location of the mobile radio, the most likely place for the mobile to be, or the last location the subscriber was active. The MSC system controller uses this information to route calls to the appropriate radio tower for call completion. If the wireless user is not in a predetermined “home” range of the MSC, the mobile will register back through to the home signaling system to its home location register (HLR) for profile information.

When customers use the wireless services of systems outside of their home area, their information is transferred to a database in that system called the visitor location register (VLR). The VLR is part of a wireless network (typically cellular or PCS) that holds the subscription and other information about visiting subscribers that are authorized to use the wireless network.

System Security
In some wireless networks, access to system services requires validation of the customer’s identity. These systems may use an authentication center (AUC) to store and process secret data to stop fraudulent calls or prohibit access to other paid for subscription services.

Wireless phones transmit some of their identification information over the public airwaves when they attempt to access the system. Thieves may try and intercept this information and copy (clone) the identification information that would allow them to make phone calls that would be billed to the other telephone. To prevent this unauthorized duplication of identification information, an authentication process can be used that uses secret keys to validate access information.

During the authentication process, code keys are created from secret codes that are stored in both the mobile radio and in the system. Along with basic identification information, these keys are exchanged during each system access attempt. The secret codes are not transmitted. Because the system and the mobile radio have the secret keys, both the mobile phone and the system can validate that the code information is correct. If the codes do not match, the system should not allow the call to be processed. New codes are created during each access attempt to prevent copying of the codes and immediately attempting access.

Wireless Networks: Radio Frequency (RF)

Radio Frequency (RF)
The radio frequency spectrum is divided into frequency bands that are authorized for use in specific geographic regions. Globally, the International Telecommunications Union (ITU) specifies the typical use for radio frequency bands. Within each country, government agencies create and enforce the rules for which specific types of systems and services are used in specific frequency bands and which companies will be able (will be licensed) to own and operate these systems.

Licensing
The national government is responsible for dividing the available frequency bands for licensing to users and regulates what the frequencies may be used for. The legal right-to-use of this public resource is controlled by rules and licensing of very specific frequencies, a range of frequencies or a block of sub-divided channels at a given frequency or frequency range.

For example, the frequencies allocated for FM radio must be used for the purpose licensed; that is a combination of music or news and public information. FM radio stations are not licensed to broadcast a secret “Morse-code” to a following of undercover militia! Neither can a “Paging Service” use one or all of their frequency channels to broadcast radio. However, with the recent deregulation of telecommunications services, wireless service providers are now permitted to offer many new types of services provided they can fulfill their basic licensing requirements.

To prevent unwanted interference from radio devices, the reckless use of transmitting energy or information on our public airwaves according to publicly published rules or licenses will violate federal law. Such transmissions are subject to prosecution or suspension of the radio operator’s license.

Frequency Allocation Charting
There are thousands of wireless applications that are assigned to many different frequency bands. The selection of the assigned frequency bands is determined by a variety of factors including the radio propagation characteristics and the availability of radio channel frequencies at the time.

Because most of the frequencies have already been assigned to licensees, a new assignment of frequencies typically requires existing licensees or users to stop using a band. These users are typically shifted to another band. This process is called re-allocation.

Historically, major re-allocations are done in the higher frequencies to avoid congestion. This has advantages and disadvantages. The radio frequency (RF) devices employed within the newer systems are subject to more loss based on distance. This requires closer distances, increasing the total number radio sites to cover the same area previously covered by radio devices at a lower frequency. However, the higher frequencies tend to penetrate buildings more readily and the antennas involved are physically smaller - both important attributes for systems that seek to reach 100% of the available population.

RF Channels and Bandwidth
An RF channel is a communication link that use radio signals to transfer information between two (or more) points. To transfer this information, a radio wave (typically called a radio carrier) is modulated (modified) within an authorized frequency band to carry the information. The modulation of the radio wave forces the radio frequency to shift above and below the reference (center) frequency. Typically, the more the modification of frequency, the more information can be carried on the radio wave. This results in RF channels typically defined by their frequency and bandwidth allocation.

Bandwidth allocation is the frequency width of a radio channel in Hertz (high and low limits) that can be modulated to transfer information. The amount type of information being sent determines the amount of bandwidth used and the method of modulation used to impose the information on the radio signal.

A government regulation agency (the FCC in the United States) defines a total frequency range (upper and lower frequency limits) that a radio service provider can use to transmit information. In some systems (such as AM or FM radio station broadcasting), this is a single radio channel. For other systems (such as cellular, PCS, or PCN), this is a range of frequencies that can be sub divided into smaller radio channels as determined by the radio carrier. When the allocated frequency range is further subdivided into smaller allowable bands, these subdivided areas are referred to as channels.

Mobility and Fixed Wireless
Most applications use wireless to allow mobile service. However, many fixed applications of wireless are practical. There is a general data transmission rate tradeoff between mobile and fixed wireless systems. Mobile wireless systems have a relatively low data transfer rate (typically below 28 kbps) while fixed wireless systems can have data transfer rates that exceed 45 Mbps. The primary advantage of fixed wireless service is the ability to focus radio transmissions to a particular direction or region. This typically reduces interference to and from other radios and increases the capacity (data transfer rate) available to the fixed wireless device. The basic types of fixed wireless systems in use include wireless computer networks, competing wireless television systems, and wireless local telephone service.

Future Enhancements: 10 Gigabit Ethernet (10 GE), LAN Telephony, Storage Area Networks (SANs)

Future Enhancements
Future enhancements for data communication networks include increased data transmission speed, LAN telephony, and storage area networks (SANs).

10 Gigabit Ethernet (10 GE)
10 Gigabit Ethernet (10 GE) is a data communication system that combines Ethernet technology with fiberoptic cable transmission to provide data communication transmission at 10 Gbps (10,000 Mbps). The specifications for 10 GE are being developed by the Gigabit Ethernet Alliance. The Gigabit Ethernet Alliance is a group of companies that was formed in January 2000.

LAN Telephony
LAN telephony (sometimes called TeLANophy) use LAN systems to transport voice communications. LAN telephone technology is a merging of packetized voice with the high-speed data transmission ability of LAN systems. The ability to share data networks with voice systems offers significant cost reduction for telephone services.

LAN telephone system consists of LAN telephones, a data network, a LAN call processing system, and a voice gateway to the PSTN. LAN telephones convert audio into digitized packets that are transferred on the LAN to the call processing computer (CTI system). Each LAN telephone has its own network data address that is related to its telephone number or extension number.

LAN telephones can be integrated into computers or they can be standard along telephones that use LAN protocols that communicate with the systems. In 2001, there were several manufacturers producing IP telephones.

Storage Area Networks (SANs)
Storage area networks (SANs) distribute data and other information to multiple storage devices that are interconnected by data networks. SANs allow for the sharing of resources and pooling of information in the form of shared files at both the server level and the client (individual PC) level. Storage area networks (SAN’s) provide fault-tolerant operation through the use redundant data storage in multiple locations. If a failure occurs in one data storage device, other redundant data storage devices may automatically be used as the backup source of information.

Services : Internet Service Provider (ISP) & Virtual Private Networks (VPN’s)

Services
Some of the key services provided by data network operators include Internet service provider (ISP) and virtual private networks (VPNs).

Internet Service Provider (ISP)
Internet service provider is a company that provides an end user with data communication service that allows them to connect to the Internet. An ISP purchases a high-speed link to the Internet and divides up the data transmission to allow many more users to connect to the Internet. Internet service providers provide a gateway between end-users and the Internet. For this service, an ISP usually charges a monthly access fee and may charge for the amount of time or amount of data transferred during the billing period.

Virtual Private Networks (VPN’s)
Virtual private networks (VPN) network operators provide data connections to companies to allow interconnection of data networks. Companies use VPN to create MANs or WANs.

The best examples of VPN’s today are ATM and frame relay networks that connect multiple client sites on what appears to be dedicated circuits. In these networks, data is routed through the VPN network using routing algorithms that transfer data based on congestion and priorities. Because of the speed and fault-tolerance of the VPN provider network, the client company operates as if the inter-site connections were dedicated circuits.

Data Communications Systems: ATM 25, Phoneline Networking, Universal Serial Bus (USB),

Asynchronous Transfer Mode 25 (ATM 25)
ATM 25 is a 25 Mbps version of the asynchronous transfer mode (ATM) system. ATM technology is relatively complex when compared to Ethernet and token ring systems. As a result, the use of standard ATM technology in LAN systems has been limited. However, a 25 Mbps version of the ATM standard was developed for PDN LANs. The capability of ATM systems to simultaneously provide multiple communication channels with varying levels of quality of service (QoS) make it advantageous for use in multimedia systems. ATM 25 technology is used to provide digital video and Internet access through the use of ATM in digital subscriber line (DSL) and cable modem systems.

Phoneline Networking
In the late 1990’s, the home phoneline network alliance (HomePNA) developed a specification that allows home computers and data devices (such as network printers) to interconnect via standard home telephone wiring. In the first generation of phoneline networking, data rates of 1 Mbps were achieved but recently data transmission rates of 10 Mbps have been demonstrated. The Phoneline Network uses special NIC’s that send and receive high frequency signals that do not interfere with standard telephone service. To connect a phoneline network to a DSL connection, a phoneline bridge must be used.

Universal Serial Bus (USB)
Universal serial bus (USB) is a short distance data communication interface (typically, only a few meters) that now comes standard on most personal computers. The USB was designed to replace the older slower UART data communications port. USB ports permit data transmission speeds up to 12 Mbps. Most computers that were manufactured in 2001 included a universal serial bus (USB) connector. The USB data bus can also connect up to 10 devices to the same bus using a low cost hub device. USB lines can only extend for a few feet from the computer.

FireWire
FireWire is a short distance data communications interface (up to approximately 5 meters) that is based on industry standard IEEE-1394. FireWire can transmit at speeds up to 400 Mbps and can support up to 63 devices per bus. Firewire provides for isochronous (repetitive streaming data format) that allows it to transfer audio and video signals.

Data Communications Systems : Internet

The Internet is a public data network that interconnects private and government computers. The Internet transfers data from point-to-point by packets that use Internet protocol (IP). Each transmitted packet in the Internet finds its way through the network switching through nodes (computers). Each node in the Internet forwards received packets to another location (another node) that is closer to its destination. Each node contains routing tables that provide packet-forwarding information. The Internet was designed to allow continuous data communication in the event some parts of the network were disabled. The world wide web (WWW) is an application on the Internet that allows users to graphically navigate through computers that are connected to the Internet.

The Internet is a network of networks. Although these networks communicate with each other using many different languages (protocols), they all agree to transport data within their network according to a common Internet communication language called transmission control protocol/Internet protocol (TCP/IP). TCP/IP is a set of protocols developed by the U.S. Department of Defense (US DOC) that facilitate the interconnection of dissimilar computer systems across networks. The TCP protocol coordinates the overall flow of data during a data communication session between points (nodes) in the Internet.

IP is an addressing structure that allows packets of data to be routed (re-directed) as they migrate through different networks to reach their ultimate destination. Each network receives packets of data in a format that is compatible with the Internet (IP address followed by control and data information) and they encapsulate (place the whole Internet data message into their own data packet format (including the IP address and control information). This allows IP data packets (called “datagrams”) to be sent through the network regardless of their actual length or format.

Figure 1 shows that the Internet is the network of networks and it communicates using the universal protocol language TCP/IP. This diagram shows a user who is sending email through the Internet. In this diagram, the application is email. The data from the email is divided into packets and given sequence number by TCP protocol. The destination address is appended to each packet by the IP layer. The IP packets are then sent through an Ethernet LAN by encapsulating the IP datagram within the Ethernet data packet. When the data packet is extracted from the Ethernet, it is placed on the E1 transmission line. When the IP data packet reaches the ATM network, it is subdivided into very small 53 byte data packets that travel through the ATM network. When the ATM packets reach their destination in the ATM network, the original IP datagram is recreated and transferred via the T1 communication line. The T1 communication line interfaces to another Ethernet data network. This Ethernet data network encapsulates the IP datagram and forwards it on to the NIC of the receiving computer. The NIC of the receiving computer removes the IP address and reassembles the IP data packets to form the original email message.


Figure 1: Internet Data Routing

Data Communications Systems : Fiber Distributed Data Interface (FDDI)

Fiber distributed data interface (FDDI) is a computer network protocol that uses fiber optic cable as the transmission medium to provide high-speed data transmission service to LANs. FDDI is a token protocol. The basic transmission rate of FDDI is 100 Mbps. FDDI is commonly used as a backbone network that interconnects several LANs within a company.

The FDDI specification is IEEE 802.2 and FDDI data transmission speed range from 100 to 200 Mbps. 1000 Mbps and higher FDDI speeds are in development.

FDDI is a LAN architecture that is based on redundant fiber rings that transmit in opposite directions. One of the rings is the primary ring and the other ring is the secondary ring. When the primary ring ceases to be operational (such as a cut cable) the network reconfigures itself (called “self-healing”) and it reconfigures the secondary ring as the primary ring.

Both single mode fiber and multimode fiber cable systems can be used with FDDI. Multimode fibers have a wider optical bandwidth transmission capability. However, this introduces distortion and limits the maximum distance for multimode fiber systems to about 2 kilometers. Single mode fiber systems have maximum range of approximately 60 km.

FDDI is a token passing architecture differing from token ring in that while a station has a token it can transmit as many frames as possible before the token expires. Because of this, there can be multiple frames on the ring at any time.

The interconnection devices in a FDDI network include a dual attached concentrator (DAC) and dual attached station (DAS). These devices remove and insert data to the FDDI ring. Each of these devices has dual transmission capability. If the fiber ring is cut, they can automatically redirect data onto its other channel (the secondary ring).

The DAC is a concentrator the converts the optical data on the FDDI system into another format that can be used to connect to other data networks. This allows one FDDI network node to connect to many other data communication devices.

Figure 1 shows FDDI system that uses dual rings that transmit data in opposite directions. This diagram shows one dual attached station (DAS) and a dual attached concentrator (DAC). The DAS receives and forwards the token to the mainframe computer. The DAC receives and token and coordinates its distribution to multiple data devices that are connected to it.


Figure 1: Fiber Distributed Data Interface (FDDI)

Data Communications Systems: Token Ring

Token Ring
Token ring is a LAN system developed by IBM that passes a token to each computer connected to the network. Holding of the token permits the computer to transmit data. The token ring specification is IEEE 802.5 and token ring data transmission speed range from 4 Mbps or 16 Mbps. 100 Mbps and higher token ring speeds are in development.

Token ring networks are non-contention based systems, as each computer connected via the token ring network must have received and hold a token before it can transmit. This ensures computers will not transmit data at the same time. Token ring systems provide an efficient control system when many computers are interconnected with each other. This is the reason token ring systems will not see data traffic degradation when many new users are added compared to Ethernet systems. However, passing tokens does add overhead (additional control messages) that reduces the overall data transmission bandwidth of the system.

The token ring LAN architecture was invented by IBM and touted to be the standard for clients of IBM mainframes who sought to replace aging 3270 terminals with LAN’s. IBM also developed cabling standards along with hub-like devices called multi-station access units (MAU’s). The original MAU’s formed a star network with the client PC’s and simulated the ring internally. The PC’s were connected to the MAU via IBM category type 1, 2, or 3 cable.

Figure 1 shows a typical token ring LAN. This diagram shows that the network is logically setup in a ring and each computer in the token ring network must receive a token before it can transmit. Since the token is relatively small compared to the packets of data that are sent, the token can rapidly move from computer to computer. When a computer receives a token, it can transmit data for a limited amount of time before it is required to forward the token.


Figure 1: Token Ring

Data Communications Systems: Ethernet

Ethernet
Ethernet is a packet-switching transmission protocol that is primarily used in LANs. Ethernet is often characterized by its data transmission rate and type of transmission medium (e.g., twisted pair is T and fiber is F). Ethernet systems in 1972 operated at 1 Mbps. In 1992, Ethernet progressed to 10 Mbps data transfer speed (called 10BaseT). In 2001, Ethernet data transfer rates included 100 Mbps (100BaseT) and 1 Gbps (1000Base T). In the year 2000, 10 Gigabit fiber Ethernet prototypes had been demonstrated.

Ethernet can be provided on twisted pair, coaxial cable, wireless, or fiber cable. In 2001, the common wired connections for Ethernet was 10 Mbps or 100 Mbps. 100 Mbps Ethernet (100BaseT) systems are also called “Fast Ethernet.” Ethernet systems that can transmit at 1 Gbps (1 Gbps = 1 thousand Mbps) or more, are called “Gigabit Ethernet (GE).” Wireless Ethernet have data transmission rates that are usually limited from 2 Mbps to 11 Mbps.

Wired Ethernet conforms to IEEE 802.3 standards and wireless Ethernet conforms to 802.11. IEEE 802.3 standard and uses carrier sense multiple access with collision detection (CSMA/CD) media access control (MAC).

Ethernet is the older than token ring and is based on linear bus technology. Originally installed using RG-6/8 coaxial cable (called “thicknet”), it was used for high-speed bus applications to interconnect mainframes and mini-computers. With the growth of personal computer (PC) workstations in the 80’s and early 90’s, a new wiring strategy was implemented using thinner RG-58 coaxial cable (called “thinnet”). In the mid-90’s newer twisted pair standards were set and higher speeds were achieved. 10 Mbps (10BaseT) became achievable on Category 3 unshielded twisted pair (UTP) wire.

Because Ethernet systems can use different cabling systems (e.g., twisted pair and coax), network interface cards (NICs) must contain a connector that is compatible with the cabling systems. Some NIC cards come with multiple connectors. The different types of connectors include:

DB-15 AUI connector for thicknet, 10Base5

BNC coaxial connector for thinnet, 10Base2

RJ-45 for twisted pair, 10BaseT or 100BaseT.

The maximum distance between devices in an Ethernet network is determined by the type of cable selected and performance of the NIC. Figure 1 shows different types of Ethernet LAN systems and the approximate distances devices can be connected together in these networks. Thicknet Ethernet uses a low loss coaxial cable to provide up to 500 meters of interconnection without the need for repeaters. Thinnet systems use a relatively thin coaxial cable systems and the typical signal loss in this cable restricts the maximum distance to approximately 185 meters. 100 BaseT systems use category 5 UTP cable and the maximum distance is approximately 100 meters.


Figure 1: Ethernet

Technologies: Routers, Gateways, Firewall

Routers
A router is a device that directs (routes) data from one path to another in a network. Routers base their switching information on one or more information parameters of the data messages. These parameters may include availability of a transmission path or communications channel, destination address contained within a packet, maximum allowable amount of transmission delay a packet can accept, along with other key parameters. Routers that connect data paths between different types of networks are sometimes called gateways.

Routers provide some of the same functionality as network switches. Their primary function is to provide a path for each routable packet to its destination. When a router is initially installed into a network, it begins its life by requesting a data network address. Using this data network address, it sends messages to nearby routers and begins to store address connections of routers that are located around it. Routers regularly exchange their connection information (lists of devices it is connected to) with nearby routers to help them keep the latest packet routing information.

A router can make decisions on where to forward packets dependent on a variety of factors including the maximum distance or packet priority. Distance vector routing and link state routing allow the router to select paths that match the needs of the data that is being sent through it.

Routers may also have fixed routing tables that are manually programmed by the network administrator. These static routing tables may be inflexible, however the use of static routing ensures other router’s that may have corrupt routing tables does not change the table.

Figure 1 shows a how a router can dynamically forward packets toward their destination. This diagram shows that a router contains a routing table (database) that dynamically changes. This diagram shows a router with address 100 is connected to two other routers with addresses 800 and 900. Each of these routers periodically exchanges information allowing them to build routing tables that allow them to forward packets they receive. This diagram shows that when router 100 receives a packet for a device number 952, it will forward the packet to router 900. Router 900 will then receive that packet and forward it on to another router that will help that packet reach its destination.


Figure 9.11: Router

Gateways
Gateways are devices that enable information to be exchanged between two dissimilar computer systems or data networks. A gateway reformats data and protocols in such a way that the two systems or networks can communicate. Gateways can convert packets between dissimilar networks.

Figure 2 shows how a gateway can convert large packets from a FDDI into very small packets in an ATM network. Not only does the gateway have to divide the packets, it must also convert the addresses and control messages into formats that can be understood on both networks.


Figure 2: Gateway

Firewall
A firewall is a device or software program that runs on a computer that provides protection from external network intruders by inhibiting the transfer of unauthorized packets and by allowing through packets that meet safe criteria. There are various processes that can be used by firewalls to determine which packets are authorized and packets that should be rejected (not forwarded).

Because firewalls can use many different types of analysis to determine packets that will be rejected, they can be complicated to setup. If a firewall is not setup correctly, it can cause problems for users that are sending and expected return packets that may be blocked by the firewall. Because firewalls process and analyze information, this process requires additional time and this can slow down network data transfer and response time.

Figure 3 shows how a firewall works. This diagram shows that a user with address 201 is communicating through a firewall with address 301 to an external computer that is connected to the Internet with address 401. When user 201 sends a packet to the Internet requesting a communications session with computer 401, the packet first passes through the firewall and the firewall notes that computer 201 has requested a communication session, what the port number is, and sequence number of the packet. When packets are received back from computer 401, they are actually addressed to the firewall 301. Firewall 301 analyzes the address and other information in the data packet and determines that it is an expected response to the session computer 201 has initiated. Other packets that are received by the firewall that do not contain the correct session and sequence number will be rejected.


Figure 3: Firewall

Firewalls are also appropriate for small office and home office (SOHO) applications. There are low-cost software packages and hardware equipment that offer a moderate level of increased security. They cannot stop all hackers, but they will stop some of them.

Technologies: Data Modems, Hub, Bridge

Data Modems
Data modems are devices that convert signals between analog and digital formats for transfer to other lines. Data modems are used to transfer data signals over conventional analog telephone lines. The term modem also may refer to a device or circuit that converts analog signals from one frequency band to another.

A point-to-point analog data circuit requires a modem at each end to transfer digital signals. The type of modems used on each end must be compatible due to encoding and decoding processes. Analog communication lines are restricted to audio bandwidth of 300 Hz to 3300 HZ. To communicate digital data and control signals, the modems vary the frequency of the carrier in each direction based on an agreed to algorithm for encoding bits.

Figure 1 shows a modem with its functional responsibilities listed. From the DTE (serial interface RS 232-C) to the line the modem performs a digital-to-analog conversion and from the line to the DTE an analog-to-digital conversion.


Figure 1: Data Modem

Digital Service Unit (DSU)/Channel Service Unit (CSU)
DSU/CSU’s are the digital equivalent of the analog modem and are translation codecs (COde and DECode) coupled with a network termination interface (NTI). DSU/CSU’s operate only in a digital environment. DSUs/CSUs work together to reformat and channelize digital signals for transmission on multiple channel lines.

Hub
A hub is a communication device that distributes communication to several devices in a network through the re-broadcasting of data that it has received from one (or more) of the devices connected to it. A hub generally is a simple device that re-distributes data messages to multiple receivers. However, hubs can include switching functional and multi-point routing connection and other advanced system control functions. Hubs can be passive or active. Passive hubs simply re-direct (re-broadcast) data it receives. Active hubs both receive and regenerate the data it receives.

Figure 9.9 shows an Ethernet hub. This diagram shows that one of the computers has sent a data message to the hub on its transmit lines. The hub receives the data from the device and rebroadcasts the information on all of its transmit lines, including the line that the data was received on. The hub’s receiver and transmit lines are reversed from the computers. This allows the computers that are connected to the hub to hear the information on their receive lines. The sending computer uses the echo of its own information as confirmation the hub has successfully received and retransmitted its information. This indicates that no collision has occurred with other computers that may have transmitted information at the same time.


Figure 9.9: Hub

Bridge
A bridge is a data communication device that connects two or more segments of data communication networks by forwarding packets between them. The bridge operates as a physical connector and buffer between similar types of networks.

Bridges extend the reach of the LAN from one segment to another. Bridges have memory that allows them to store and forward packets. Bridges are protocol independent as the do not perform protocol adaptations.

Bridges contain a packet address-forwarding table (routing table) that they use to determine if the packets should be forwarded between networks. The packet-forwarding table contained within the bridge can be initially programmed or learned by the bridge. A self-learning bridge can monitor packet traffic in the network to continually update its packet-forwarding table

Bridges primarily operate at the physical layer and link layers of the OSI reference model. A bridge receives packets from one network, review the address of the packet to determine if it should be routed to the other network(s) it is connected to, and retransmits the packet following the standard protocol rules for the systems it is connected to.

Figure 3 shows the basic operation of a bridge that is connecting 3 segments of a LAN network. Segment 1 of the LAN has addresses 101 through 103, segment 2 of the LAN has addresses 201 through 203, and segment 3 of the LAN has addresses 301 through 303. The table contained in the bridge indicates the address ranges that should be forwarded to specific ports. This diagram shows a packet that is received from LAN segment 3 that contains the address 102 will be forwarded to LAN segment 1. When a data packet from computer 303 contains the address 301, the bridge will receive the packet but the bridge will ignore (not forward) the packet.


Figure 3: Bridge