Advance+Mobile+Phone+Service+(AMPS)


 * Advance Mobile Phone Service (AMPS) **
 * 1.0  **** Introduction **

The Advanced Mobile Phone System is one of the earliest commercial cellular systems. AMPS technology is currently deployed throughout North America and AMPS-derivative systems are deployed in a majority of worldwide cellular markets. AMPS was invented at Bell Labs and initially deployed in the U.S. in the early 1980's. Ownership of the local cellular service operations was transferred from AT& T to the regional Bell operating companies (RBOCs) at the time of AT& T's divestiture in January, 1984. Other landline telephone service providers, such as GTE, were unaffected by the divestiture and retained their own cellular operations.

Proposed by AT&T in 1971, AMPS is still the standard for analog cellular networks. It was tested in 1978, and in the early 1980s cellular systems based on the standards were installed throughout North America. Although AMPS was not the first system for wireless telephony, the existence of a single set of standards enabled the United States to dominate analog cellular throughout the 1980s. Today, Europe dominates cellular primarily because it is a lower-cost alternative to conventional telephone service. In 1983, AMPS was approved by the Federal Communications Commission (FCC) and first used in Chicago. In order to encourage competition and keep prices low, the U.S. government required the presence of two carriers in every market, known as A and B carriers. One of the carriers was normally the Local Exchange Carrier (LEC); in other words, the local phone company.
 * 2.0  **** History **



Advanced Mobile Phone Service (AMPS) is a standard system for [|analog] signal [|cellular telephone] service. It is based on the initial [|electromagnetic radiation spectrum] allocation for cellular service by the Federal Communications Commission (FCC). AMPS allocates frequency ranges within the 800 and 900 Megahertz (MHz) spectrum to cellular telephone. Each service provider can use half of the 824-849 MHz range for receiving signals from cellular phones and half the 869-894 MHz range for transmitting to cellular phones.
 * 3.0 Features Of AMPS **

** Cellular Concept ** The bands are divided into 30 [|kHz] sub-bands, called //channels//. The receiving channels are called //reverse channels// and the sending channels are called //forward channels//. The division of the spectrum into sub-band channels is achieved by using frequency division multiple access ([|FDMA]). The signals received from a transmitter cover an area called a [|cell]. As a user moves out of the cell's area into an adjacent cell, the user begins to pick up the new cell's signals without any noticeable transition. The signals in the adjacent cell are sent and received on different channels than the previous cell's signals to so that the signals don't interfere with each other. The analog service of AMPS has been updated with [|digital] cellular service by adding to FDMA a further subdivision of each channel using time division multiple access ([|TDMA]). This service is known as digital AMPS ([|D-AMPS]). Although AMPS and D-AMPS originated for the North American cellular telephone market, they are now used worldwide with over 74 million subscribers, according to Ericsson, one of the major cellular phone manufacturers.

 **3.1 AMPS Cellular Operatio** **n**

AMPS cellular operation consists of call origination and call termination procedures, supported by radio resource management and mobility management functions. It is important to remember that AMPS was designed as a voice-only system, which impacts how these processes are handled. Data transmission on AMPS systems is based on this circuit-switched mode of operation and is described in Section 2.8. When an AMPS mobile station powers up, it searches through up to 21 predefined control channels. These control channels are physically no different than AMPS traffic channels, except for how they are used-for control purposes only. Each cell utilizes a forward control channel to continuously broadcast information needed by the mobile station for registration. This information includes the system identification or SID of the MSC, which allows the mobile to know whether it is roaming. AMPS mobile station finds the best forward control channel it can receive (in terms of received signal strength) and announces itself or registers to the serving network via the matching reverse control channel. From that point on, the mobile remains in a passive state tuned to the control channel it selected. When the channel quality degrades (radio resource management determines this), a call event occurs or the mobile crosses a boundary between location areas, [|2.29]  the mobile again signals the network. This receive-only mode reduces the traffic on the reverse control channel, a shared resource. In communicating with the network, the mobile provides two identifiers for registration, call control and validation. The first of these identifiers is the mobile identification number or MIN, which is the programmed handset phone number used to call the subscriber. This programmed identifier is associated with the subscriber and is stored in erasable non-volatile memory in the handset. The second identifier is the electronic serial number or ESN, which is a manufactured characteristic of the mobile unit. This identifier is (in theory) permanent and associated with the physical equipment. It is 32 bits in length, with the first 8 bits identifying the manufacturer. Both the MIN and the ESN are transmitted unencrypted by both the mobile and the network. Simple scanning receivers can be used to capture these values, which has provided many opportunities for fraudulent use of cellular services. Recently the cellular industry has instituted a subscriber-entered personal identification number or PIN as an escalation in the war on cellular fraud. But this measure has proven to be only a temporary complexification for the "bad guys" and in early 1996 cellular service providers began deploying authentication mechanisms.

When an AMPS mobile is not engaged in a call, it monitors the forward control (paging) channel. A call attempt directed at the mobile (i.e., to the MIN assigned to the mobile) is received by the mobile as a page on the control channel. The page is repeated several seconds later, in case the mobile was temporarily in an RF "hole" or otherwise unable to receive the first page. The time interval between pages is short to minimize the ringing delay experienced by the originator of the call. The mobile responds to the page via the reverse control channel and awaits the traffic channel assignment. The mobile response is also repeated, in case the initial response collided with another mobile on the reverse control channel or suffered from bad RF conditions. When the mobile receives the traffic channel assignment from the network (the MSC via the base station), it proceeds to that channel and produces an audible ringing tone for the subscriber. From this point forward, all further signalling between the system and the mobile is conducted in-band.

AMPS channels are controlled by the MSC. The traffic channel assignment process was described in the preceding subsections. However, there are other aspects of RRM, including power control and handoff. Power control is handled by monitoring the received signal strength of the reverse channel at the base station, which in turn passes this and other channel quality information to the MSC. The MSC evaluates this data, including a trend analysis, to determine whether the mobile should increase or reduce its power level or be handed-off to another cell. AMPS defines eight power levels in 4 dB steps. This power level control is a means of controlling the local access point to the network for a mobile station. Cell handoff is handled in a BCHO manner, as discussed in Section 2.4. The system controls handoffs by transmitting the SAT in-band on the forward channel. This tone is filtered by the mobile-it is outside the range of the audible channel-before reaching the subscriber's ear, and is reflected back to the system in-band on the reverse channel. The base station filters the reflected SAT and evaluates the quality of the reflected tone. The base station forwards SAT quality information on to the MSC. Based on RSS and SAT data, the MSC determines whether or not to initiate cell handoff procedures. Cell handoff procedures include having neighboring cells' base stations monitor the mobile's reverse channel and evaluating the received signal strength. If another base station "hears" the mobile better than the current base station, the mobile is instructed to move to a new channel pair via a "blank and burst" message transmitted in-band by the base station. The mobile then tunes its RF transceivers to the channel pair instructed. All of these steps are orchestrated by the MSC. The "blank and burst" message sounds like static to the ear of the subscriber and is momentarily disruptive to the conversation taking place. It is also highly destructive of any data transfer which could be occurring at that point in time via modems on the cellular channel. This is one of the reasons that cellular has historically been a harsh environment for mobile data users. Intelligent algorithms are used to prevent unnecessary and premature handoffs, especially for non-moving mobiles, mobiles located in poor in-cell coverage areas, mobiles traveling along the border between cells and situations in which no cellular channels are available beyond the cell's boundary.


 * 4.0 Conclusion **

It was a first-generation technology, using FDMA which meant each cell site would transmit on different frequencies, allowing many cell sites to be build near each other. However it had the disadvantage that each site did not have much capacity for carrying calls. It also had a poor security system which allowed people to steal a phone's serial code to use for making illegal calls. It was later replaced by the newer Digital TDMA system which brought improved security as well as increased capacity.