It seems cellular telephones are everywhere. The Cellular Telephone Industry Association (CTIA) claims more than 30 million subscribers have a mobile phone, with growth rates approaching fifty percent each year. In 1995 alone more than nine million customers started cellular service. The Federal Communications Commission (FCC) expects well over 50 million cellular subscribers by the year 2000.
The first, and currently dominant, cellular system in the United States is known as Advanced Mobile Phone Service (AMPS). Although the roots of a cell-based communication system go back several decades, in the late 1970's Bell Laboratories proved the viability of the cellular concept with a development network in New Jersey and a test system in Chicago. More than 15 years later this analog cellular service is available in almost every city in the United States.
In defining the initial rules for cellular telephone service, the FCC divided the nation into 306 Metropolitan Service Areas (MSAs) and 428 Rural Service Areas (RSAs), and gave away two operator licenses in each. The existing local telephone company, called the wireline company, received one, and an FCC lottery was held to determine the other, non-wireline license winner.
The key concept in cellular systems is frequency re-use. Prior mobile telephone services in the United States dedicated a single channel to a mobile telephone user across the entire coverage area during a call. When a channel was is use, no other customer could use that channel, no matter where they were located. Since the FCC dedicated only a dozen or so frequencies in each service area the system was often fully loaded while supporting a relatively few number of users. These frequencies, in the 150 MHz and 450 MHz bands, were very crowded, and it was very common at at peak usage times for more than half of all call attempts to fail due to lack of available channels. As late as 1976 there were only 12 channels supporting the entire New York metropolitan area.
The FCC initially allocated two slots for cellular telephone use in the 800 MHz band, one at 825 MHz to 845 MHz, and the other from 870 MHz to 890 MHz. These two 20 MHz slots are divided into 30 kHz wide channels, numbered from 1 to 666. Channels are paired in each of the slots, so a 30 kHz channel in the lower slot corresponds to a 30 kHz channel in the upper slot. The non-wireline company, also known as the A Band carrier, was granted use of channels 1 to 333, and the wireline company, also known as the B Band carrier, was given use of channels 334 to 666. In 1985 the FCC allocated another 10 MHz of spectrum, providing an additional 166 channels, which gave cellular networks a total of 832 channels.
Frequency Range (832 Channel System)
In a cellular system, the service region is divided into much smaller areas called cells, which have a base station at the center. Each base station covers an area anywhere from 1 to 40 miles, but on average 5 to 10 miles. Each base station is configured to handle calls on a small subset of the available channels. That subset will also be served by other base stations, but only those far enough away to avoid interference. Adjacent base stations will not have any channels in common. This re-use pattern allows a much greater number of customers to share the same set of frequencies, since the same channel may be in use in several locations at the same time across the entire service area.
Base stations are linked to a Mobile Telephone Switching Office (MTSO), which connects the base station voice channels to voice trunks in the Public Switched Telephone Network (PSTN). The MTSO also controls the operation of the base station equipment, processing call requests and other support functions for each mobile telephone in the service area.
FULL DUPLEX CHANNELS
The base station transmits on the upper slot of frequencies, and these are referred to as the forward channels, going from base to mobile. The cellular telephones transmit on the lower slot of frequencies, which are referred to as reverse channels, going from mobile to base. By selecting the same channel, a base station and a mobile unit can maintain a full-duplex connection, with the transmitted signals separated by 45 MHz. For example, if a connection is active on channel 452 (a channel assigned to the wireline, or B band carrier), then the base station is transmitting on 883.560 MHz (the forward channel) and receiving on 838.560 MHz (the reverse channel). The mobile telephone in this example is transmitting on 838.560 MHz and receiving on 883.560 MHz.
For Channels 1 to 799
Reverse Frequency = Channel × 0.030 + 825.000
Forward Frequency = Channel × 0.030 + 870.000
For Channels 991 to 1023
Channels in a cellular network are divided into two types, known as voice and control. The bulk of the channels, 395 for each carrier, are assigned to carry the actual voice audio of a conversation, and are referred to as forward voice (FOVC) and reverse voice (REVC) channels. During a conversation the audio is sent analog FM modulated, but when a mobile telephone switches from one cell to another in a process called hand-off, the audio is briefly muted and a burst of digital data is sent from the base to the mobile, indicating the new voice channel to use.
When a base station sends out administrative information it uses a forward control channel (FOCC). When a mobile telephone responds to commands or originates a call, it uses a reverse control channel (ROCC). Twenty-one control channels for each carrier are dedicated to one of two functions: access or paging. Access control channels handle administrative matters related to registering and monitoring mobile telephones using a digital stream of data. The paging channel is a digital stream of system information and telephone call "pages". All mobile telephones, while idle, listen to this paging channel. When a call is placed to a mobile telephone, the cellular system puts the mobile telephone number on the paging channel. If a mobile telephone decodes its own number from the paging channel, it will respond to the incoming call. Not every base station has a paging channel; usually there are only enough paging transmitters to adequately cover the cellular area.
With this as a background, next month we'll cover cellular signals and the procedures a cellular phone goes through to place and receive calls.
Bob Grove's Closing Comments in the September issue elicited a response from Francis Hemming, who writes, in part:
Public Service agencies should stop jumping onto new 800 MHz trunked systems in favour of a PCS-based solution in the near future. Why? Simply because the network diversity offered by PCS is exactly what these agencies need in order to be certain to have adequate communications under all normal and extraordinary conditions.
Trunked 800 systems grew out of a need for Public Service agencies to more effectively utilize the spectrum they had, while allowing a number of mobiles to share a common voice channel. Trunked 800 systems have their shortfalls, to be sure, but the network concept they were designed to operate under is one that meshes well with a dispatch-type of operation. PCS networks are not designed to support the one-to-many, highly configurable "task force" groupings that trunked systems offer. This fundamental difference in design will prevent PCS from replacing trunked 800 in the near future.
There is also the problem of local capacity. As an example, a report entitled Metropolitan Washington Area Interoperability, produced this year by the Public Safety Wireless Advisory Committee (PSWAC), estimates that 25 channels (RF communications paths) are required to implement a Mutual Aid Plan for a single major incident (the example they use is the 1982 Air Florida crash in Washington, D.C.). Additional complications or simultaneous disasters will require many more. The final recommendation was "for 100 channels/RF communications paths, in contiguous spectrum and paired for repeater access, be reserved for public safety mutual aid operations, for use by any public safety agency anywhere in the nation."
Local base stations, whether cellular or PCS, simply do not have the capacity to support large numbers of extended calls. The switches also cannot support a common voice path for many simultaneous mobiles.
That being said, some agencies are already using cellular and PCS services for one-to-one communications, where it is feasible to do so. In California, after the 1989 Loma Prieta earthquake and the 1991 Oakland fires, residents were requested to keep cellular calls to a minimum, since emergency crews were using cell phones to keep in contact. It is now common for emergency medical crews to use cellular telephones to contact local hospitals, rather than use crowded medical voice and telemetry channels.
The use of cellular and PCS will continue to grow in the Public Safety sector, but it will not replace common-channel wireless radios, because, as currently designed, PCS cannot do what trunked 800 and similar systems can do.
I must say, however, that looking at the current cellular standard (AMPS), and the political environment that allowed it to be created, there is little hope for a PCS implementation that achieves 10 percent of what's promised. That is because such a system demands standards. I want secure communications without some government holding the codes. I want to be able to activate a GPS receiver in the phone and send my coordinates accurate to within 1 meter to whomever I want. I don't want to send it to everyone and I don't want anyone else finding out without my authorization. I don't think North America is capable of creating and enforcing a set of standards that would allow the potential of PCS to be seen.
As we say in the computer software business, that's what's so nice about standards; there are so many to choose from!
The United States and Europe seem to be trading places as far as standards are concerned. When the FCC began issuing cellular licenses they required each operator to meet specific technical standards, which followed a design worked out by AT&T. Each operator had to provide a set of well-defined services, which guaranteed that any customer's cellular equipment would operate in any service area. Customers could travel coast to coast, through many cellular service regions, and be able to operate in each one.
Europe started down a different path. In the 1970's, several nations proposed and began building a variety of different systems following different, incompatible standards including Total Access Communication System (TACS) in the United Kingdom, Nordic Mobile Telephone (NMT) in Norway, Sweden, Denmark, and Finland, as well as others.
In 1982 representatives of 26 nations agreed to begin the development of a European standard called GSM (Groupe Speciale Mobile, or today, Global System for Mobile Communications), to operate in the 900 Mhz band. The idea was for any GSM phone to be operable in any member country. By 1992 these fully digital networks started to come in to service, and by the end of 1995 were serving more than 12 million customers, and ninety countries had signed the GSM Memorandum of Understanding.
The FCC, however, in auctioning the PCS spectrum, decided to step away from the standards-setting business and stated they would "let the marketplace decide." There are currently no less than seven different standards vying for attention in the PCS marketplace. Sprint Spectrum, covered in this column last month, selected PCS-1900, which is essentially the GSM protocol upbanded to PCS frequencies. Other operators have stated their intention to implement Code Division Multiple Access (CDMA) under Interim Standard IS-95. Dual band/dual mode Digital AMPS (D-AMPS) is being championed by equipment maker Ericsson, as IS-136. Time Division Multiple Access (TDMA), under standard IS-54 is also a contender, as well as DCT-based TDMA, wideband CDMA, and the Personal Access Control System (PACS). A future column will describe these standards in more detail, but for now, suffice to say that customer confusion will be the norm until a winner or two emerge.
In regard to security, there is currently a battle going on in the cryptography world regarding the "government holding the codes," referred to as Key Escrow, otherwise known as Government Access to Keys (GAK). This has not changed under PCS. In Europe, GSM is encrypted with an algorithm, or set of algorithms, referred to as A5. It is rumored that GCHQ (the United Kingdom equivalent to the US National Security Agency) pressured the committee setting GSM standards to use a deliberately weak encryption method. There is no doubt that GCHQ and NSA are capable of decrypting GSM, and it is entirely possible that civilians share in that capability. In the United States this point is moot, at least for law enforcement, as the 1994 Communications Assistance for Law Enforcement Act (CALEA) requires communications providers to give law enforcement agents access and assistance to facilitate eavesdropping. If the police wish to record calls to and from a cellular telephone, they will simply intercept them at the Mobile Telephone Switching Office (MTSO). With the new cellular location reporting requirements, they will also be able to locate the phone to within a few hundred feet, just by watching data at the MTSO.
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Until next month, happy monitoring!
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