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Welcome to the Telcom portion of my site.  In this area I have provided information on Wireless Communications, Digital Trunking Systems, and a wide variety of other topics all to do with Telecommunications. 

  Chapter 1

How Wireless Communications Work

When you make a call on your wireless phone, the message is transmitted by low-energy radio signals to the nearest antenna site which connects with the local phone network. Your call is delivered by the phone lines to the office or home you dialed, or by radio signals to another wireless phone.
Wireless technology uses individual radio frequencies over and over again by dividing a service area into separate geographic zones called cells. Cells can be as small as an individual building (say an airport or arena) or as big as 20 miles across, or any size in between. Each cell is equipped with its own radio transmitter/receiver antenna.

Because the system operates at such a low power, a frequency being used to carry a phone conversation in one cell can be used to carry another conversation in a nearby cell without interference. (this allows much greater capacity than radio systems like Citizens Band (CB) in which all users must try to get their messages on the same limited channels.)

When a customer using a wireless phone - car phone or portable - approaches the boundary of one cell, the wireless network senses that the signal is becoming weak and automatically hands off the call to the antenna in the next cell into which the caller is traveling. When subscribers travel beyond their home geographical area, they can still make wireless calls. The wireless carrier in the area where they are traveling provides the service. This is called roaming.


Each cellular antenna is linked to a mobile switching center (MSC), which connects your wireless call to the local "wired" telephone network. Wireless carriers own MSCs.


The mobile telephone industry is limited to 45 megahertz MHz of spectrum bandwidth, which without frequency-reuse, would limit each cellular carrier to 396 frequencies or voice channels. In order to increase calling capacity, these low power facilities "reuse" frequencies on the radio spectrum. The manner in which providers organize, or "configure," their cells is an important factor in increasing frequency reuse and establishing an area's calling capacity.

  • Analog cellular operates in the 800MHz frequency range and is available across 95 percent of the United States. Analog cellular service sends a voice through the air using continuous radio waves. As the voice signals travel through the air they get weaker with distance. Equipment in the cellular network returns the signal to its original strength, or amplifies it. This technology is the predominant system in use today. The operating system (called the air interface) for analog is called Advanced Mobile Phone Service (AMPS).

  • AMPS stands for advanced mobile phone service. AMPS transmits voices as FM radio signals. The original cellular standard, AMPS is still the most widely used system in the U.S., although digital networks are catching up quickly. A variation on AMPS is narrow-band advanced mobile phone service, or NAMPS, which uses a narrower bandwidth and has greater data capabilities.

  • Digital cellular shares the 800MHz frequency band with analog and is usually available where analog service is offered. In digital transmissions, a conversation is converted into the ones and zeros of computer code. Unlike analog transmissions that are sent out as a continuously varying electrical signal in the shape of a wave, digital transmissions are a combination of on-and-off pulses of electricity. Several incompatible air interfaces are used to implement digital cellular networks, including Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA).

  • CDMA is also known as spread spectrum technology because it uses a low-power signal that is "spread" across a wide bandwidth. With CDMA, a phone call is assigned a code, which identifies it to the correct receiving phone. Using the identifying code and a low-power signal, a large number of calls can be carried simultaneously on the same group of channels.

  • TDMA is a digital air interface technology designed to increase the channel capacity by chopping the signal into pieces and assigning each one to a different time slot, each lasting a fraction of a second. Using TDMA, a single channel can be used to handle simultaneous phone calls.

  • GSM stands for global system for mobile communications. It is a type of time division multiple access (TDMA) digital wireless network that has encryption features. GSM is rapidly being deployed worldwide and is the standard in Europe at 900MHz. In the U.S., carriers are deploying GSM at 1900MHz, making GSM phones sold in the U.S. incompatible with European GSM phones, and vice versa.

  • Personal Communications Service (PCS) is an all-digital service specifically designed for U.S. operations and is available in metropolitan areas. PCS is a term coined by the Federal Communications Commission to describe a digital, two-way, wireless telecommunications system licensed to operate between 1850-1990 MHz, although the FCC's rules describe "PCS" as a broad family of wireless services without reference to spectrum band or technology. PCS is capable of increased call capacity. PCS networks are CDMA, TDMA and global system for mobile communications (GSM).

  • Enhanced Specialized Mobile Radio (ESMR) service is also a digital service, formed by the application of digital systems to traditional dispatch "specialized mobile radio" service spectrum, in the 800 and 900 MHz bands. By aggregating this spectrum, and applying a cellular-like digital network, an ESMR company is able to provide a cellular- or PCS-like voice and data messaging service. NEXTEL is one such company, using Motorola's iDEN (TDMA-based) technology to deliver ESMR services in towns and cities across the U.S.

Satellite systems are another viable type of wireless telecommunications service. Instead of sending and receiving signals from a ground-based antenna, wireless phones will communicate via satellites circling the earth.

  • GeoSynchronous Satellites: Geosynchronous satellites represent yet another way of providing wireless communications. These satellites, located 22,300 miles above the earth, revolve around the earth once each twenty-four hours-the same as the earth itself. Consequently they appear to be stationary. Communications between two places on earth can take place by using these satellites; one frequency band is used for the uplink, and another for the downlink. Such satellite systems are excellent for the transmission of data, but they leave something to be desired for voice communications. This is a result of the vast distance and the time it takes for an electrical signal to make an earth-satellite-earth round trip. That time amounts to one quarter of a second. A reply from the called subscriber takes another quarter of a second, and the resultant half a second is definitely noticeable. Consequently, voice communications is seldom carried via geosynchronous satellites

  • Low Earth Orbit (LEO) satellite system. LEOs are satellites that communicate directly with handheld telephones on earth. Because these satellites are relatively low-less than 900 miles-they move across the sky quite rapidly. In a LEO system the communications equipment on a satellite acts much like the cell site of a cellular system. It catches the call from earth and usually passes it to an earth-based switching system. Because of the speed of the satellite, it is frequently necessary to hand off a particular call to a second satellite just rising over the horizon. This is akin to a cellular system, except that in this case it is the cell site that is moving rather than the subscriber.

How the Wireless Industry Is Structured

There are three major components to the wireless industry. They are the manufacturers, the carriers or service providers, and third-party agents. Manufacturers make the wireless equipment - the telephone handsets - while secondary market manufacturers usually contract with the manufacturers to make the variety of accessories that accompany many wireless telephones. Carriers provide the actual cellular telephone service including the phone number that is assigned to the wireless telephone. Finally, third-party agents are companies, not affiliated with either the manufacturers or the carriers, who sell service and/or telephone equipment.

A common practice in many carrier's retail stores, and also with third-party agents, is to provide a wireless telephone at a reduced price from its retail value as part of new service agreements. The carriers or third-party agents buy these phones from the manufacturers and then resell them to their customers at special reduced rates when new cellular service is activated. The service, however, is independent of the actual wireless telephone. Most manufacturers do not own or operate cellular networks, although some may resell service, and most carriers do not manufacture wireless equipment although some divisions of the parent company to which they belong may make equipment.

There will be more to come.