Wireless Cable (Wireless Networks)

Wireless Cable
Wireless Cable is the common term assigned to a radio frequency-based alternative to the cable TV distribution system. An example of wireless cable technologies is multichannel multipoint distribution system (MMDS) or local multichannel distribution system (LMDS). By 1998, there were over 10 million wireless customers throughout the world and over 1.1 million in the United States.

Wireless cable system can simultaneously supply local television channels and high-speed data services. MMDS has a high-speed standardized air interface allowing mass deployment of cable television service by the new unregulated telephone companies. Cable television providers, who have access to most homes, can now provide telephone service.

In 1996, some analog MMDS systems began upgrading to digital service. Through the use of digital video compression, digital transmission allows 5 or 6 times the video channel capacity. In addition to video programming, wireless cable can provide telephone service and data services.

Figure 1 shows that the major component of a wireless cable system is the head-end equipment. The head-end equipment is equivalent to a telephone central office. The head-end building has a satellite connection for cable channels and video players for video on demand. The head-end is linked to base stations (BS) which transmit radio frequency signals for reception. An antenna and receiver in the home converts the microwave radio signals into the standard television channels for use in the home. Like traditional cable systems a set-top box decodes the signal for input to the television. Low frequency wireless cable systems can reach up to approximately 70 miles.


Figure 1: Wireless Cable System
Wireless cable is one of the most economical technologies available for the delivery of pay television service. Wireless cable systems do not require extensive networks of cables and amplifiers, bringing the offered price generally lower than a traditional cable service. To the customer, a wireless cable system operates in the same manner as a traditional cable system. Because wireless signals are transmitted over the air rather than through underground or above-ground cable networks, wireless systems may be less susceptible to outages, offer better signal quality and be less expensive to operate and maintain than traditional cable systems. In conventional coaxial cable distribution networks, the television signal quality declines in strength as it travels along the cables and must be boosted by amplifiers thus introducing distortion into the television signal.

To add security for wireless cable systems, so unauthorized users do not gain access to the system (stealing service), signals from video sources are scrambled with a code. The user must have the code to successfully view the video signals. Like traditional cable systems, wireless cable systems employ “addressable” subscriber authorization technology, which enables the system operator to control centrally the programming available to each individual subscriber, such as a pay-per-view selection

There are two primary methods of providing a communication path back from the end customer to the network operator: a telephone line and wireless. Wireless cable systems have typically only provided wireless downlink service (radio transmission from the system to the customer). Some of the new wireless cable systems now dedicate some of their radio channel capacity to uplink channels (from the customer to the system). Uplink channels allow the customer to select programming sources (such as pay per view) or may allow two-way Internet access.

Wireless Piconets (Wireless Networks)

Wireless Piconets
Wireless piconets are very small networks that connect 5 to 10 devices or allow access to each other. Wireless piconets are primarily personal networks that allow accessories such as printers and headsets and other devices to interconnect without the need for wires. These communication systems allow for the connections to be dynamically added and removed between devices.

A particular form of wireless piconet is called Bluetooth. The Bluetooth system is an industry standard wireless data network. Information and the specifications for Bluetooth can be found at www.Bluetooth.com.

Figure 1 shows Bluetooth devices that have created temporary connections. In this diagram, the personal digital assistant (PDA) device is synchronizing (deleting, changing, and adding) addresses with a laptop computer. The laptop computer is also connected to the Internet through a Bluetooth enabled access node. A mobile phone is also synchronizing its phone book listing with the laptop computer. However, because it is out of direct range of communicating with the laptop, it communicates through the access node. The mobile phone is also communicating with a wireless headset.


Figure 1: Bluetooth Wireless Piconet

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.