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.2.24
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.
To protect the consumer from potentially anti-competitive behavior by the local telephone service providers, government authorities mandated a duopoly structure for the fledgling cellular industry. This duopoly structure for cellular services has been largely imitated by other nations and has resulted in fierce competition between the service providers in many of the 734 markets defined by the FCC.
At the beginning of the cellular industry, local telephone companies (including the RBOCs) were automatically granted one of the two licenses in each market in which they provided wireline service. This is the so-called "B" license, which can be remembered by the initial of the word "Bell".
The second license for each market was initially drawn by lottery and later auctioned. Initially few investors perceived their value-after all, who could compete against the local telco? Some entrepreneurs2.25 quickly recognized the potential of these licenses and obtained as many as possible by buying out other license holders. The early days of cellular are reminiscent of the gold rush days, with pioneers rushing to buy controlling interests from lottery winners. Thus, the "A" side, which can be remembered as "A" for "Alternate," was born.
Early deployments and business deals in the cellular arena were based more on intuition than analysis. An early market analysis conducted at Bell Labs in the late 1960's concluded that the entire nationwide cellular market would peak at about 900 thousand users. Despite analyses of this nature, pioneers were willing to bet that cellular would prove to be popular.
Over time, as the value of cellular licenses were more widely recognized, prices were driven to extreme levels. It became the accepted custom to value licenses on the basis of (potential subscriber) population or "POPS." The price of a cellular market is now evaluated in terms of "dollars per POPS" normalized value, with high water marks in the neighborhood of $ 500 per POPS.2.26
Cellular markets are defined by cellular geographic statistical areas or CGSAs. Of the 734 CGSAs comprising the U.S., 306 are in metropolitan areas and are called metropolitan statistical areas or MSAs. The remaining 428 are called rural statistical areas or RSAs. MSAs are valued more highly because of their greater density of potential subscribers.
The following subsections describe AMPS, the most widely used cellular technology.
The frequencies allocated to AMPS by the FCC range between 824 to 849 MHz in reverse channels (mobile to base) and 869 to 894 MHz in forward channels (base to mobile). As displayed in Table 2.2, they are not contiguous blocks because the initial 40 MHz allocation by the FCC was later extended by 10 MHz when the service's popularity became evident. There are now a total of 416 channels available in each direction, numbered from 1 to 1024 with gaps in the numbering.
Each physical channel is 30 kHz wide and is dedicated to a single mobile station for the duration of the call while the mobile is in the current cell. Each call uses a dedicated forward channel paired with a dedicated reverse channel at a 45 MHz offset. Some of the channel pairs (21 of them) are used for control purposes in the AMPS environment. Analog frequency modulation (FM) with 8 kHz deviation is used in the traffic channels, which convey voice conversations. Binary frequency shift keying (FSK) at 10 kbps-a digital modulation technique-is used in the control channels used for signalling.
AMPS is an analog FM system, with all of the associated ramifications of such a system. AMPS channels are insecure-anyone with a channel scanner can listen to unsuspecting AMPS users.2.27 AMPS channels can suffer from interference, which sounds like static to a user; analog signals suffering from multipath fading cannot be corrected. Finally, AMPS radio resource management is based on signal strength (rather than C/I), which can only be measured indirectly via supervisory audio tones or SAT.
Since no cellular service provider covers the entire country, carriers must provide service to one another's customers for those customers to be able to receive service whenever they are outside of their home area. This capability to receive service while in another service provider's domain is called roaming. Intercarrier business agreements and network to network interoperation (messaging) are essential to support roaming.
The IS-41 standard has provided the technical solution to roaming between networks implemented by different equipment manufacturers. Prior to IS-41, all signalling between systems was proprietary in nature and the roaming capability had to be manually administered. In early years, intercarrier business relationships sometimes abused the customers' need for roaming, with service providers sometimes surprising subscribers with excessive "roaming charges." This has cost the cellular industry much in terms of reputation and customer relations.
Despite these early business foibles and technical incompatibilities, the cellular industry is rapidly moving toward universal service, with more reasonable roaming agreements in place between service providers. Service providers who are extremely competitive with one another in some markets must simultaneously be extremely cooperative with one another in other markets.2.28 This is becoming more important as reduced-size mobile stations encourage wider roaming.
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.
An 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.
The mobile originates a call (following its owner's depression of the "send" button) via the reverse control channel in the cell the mobile is currently located in. The mobile "knows" which control channel to use by information broadcast by the network on its selected forward control (paging) channel.
Access to the reverse control channel is by a CSMA2.30 -type scheme. The mobile simply transmits its request (which includes information about the subscriber such as the MIN and ESN) and "listens" on the forward channel for its subsequent channel assignment. The base station forwards the request to the MSC.
After validating the mobile (i.e., does the subscriber pay their bill and do we have a business/roaming agreement with the subscriber's home service provider?) via the HLR and the VLR, the MSC selects a traffic channel pair for the mobile. If no channels are available, the MSC simply rejects the request (which results in the mobile producing the annoying "fast busy" audible signal).
If the MSC grants a channel for the subscriber, it must then connect the call through to the destination. This is done via standard telephony procedures-the MSC simply appears to be a private branch exchange (PBX)2.31 to the PSTN. The channel grant message is relayed to the mobile via the forward control channel.
The mobile then tunes its transmitter and receiver to the assigned traffic channel pair for the duration of the call. Call and power control from this point forward are handled in-band on the AMPS traffic channel assigned to the mobile.
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" message2.32 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.
Mobility management in AMPS networks appears to be based on the engineering assumption that most calls are originated by the mobile and seems optimized for mobiles that are usually in their home area.
Intelligent paging algorithms are employed by AMPS to reduce the collective forward bandwidth required for a page. The paging algorithm starts by paging in only a small area, based on where the mobile usually receives service (the home area) or perhaps where the mobile was last registered (i.e., the location area).2.33 When a mobile receives a page, it responds and proceeds to the assigned traffic channel pair.
AMPS mobility management has been greatly enhanced by IS-41, which defines the standard for interoperation between networks. Mobility management is handled by databases known as the home location register or HLR and the visiting location register or VLR.
The purpose of the HLR is to track the location of (the VLR serving) a mobile. The HLR also contains information about each of the mobiles associated with a home area (e.g., is the mobile allowed to originate an international call, etc.). This database logically unites data describing both the subscriber and the subscriber's equipment into a single service profile. Because this "permanent" information is critical to serving customers, the HLR function is typically supported by multiple distributed fault-tolerant computers.
The purpose of the VLR is to track all mobile stations currently roaming in the local MSC coverage area. The VLR contains the service profile of each roaming mobile as well as other information necessary for calls terminating at the mobile station. Because the VLR function is so closely aligned with an operating MSC, it is typically collocated with or part of that MSC. Since the information it contains is of a temporary nature, fault-tolerance is less critical than for the HLR.
Because base stations periodically broadcast the SID and location area identifiers on the forward control channel, the mobile station knows immediately when it has roamed into another system or location area. An option in IS-41, known as autonomous registration, allows the mobile to register to the host MSC. This registration is forwarded by the MSC to the VLR. Another IS-41 message is then used by the VLR to notify the HLR that it is currently hosting the mobile.
The HLR passes necessary service profile information to the VLR in another IS-41 message, enabling the host system to provide service to the mobile station. Information such as whether or not the mobile is allowed to originate international calls is contained in this service profile. Since the HLR now knows the location of the mobile, more efficient paging can be used for mobile-terminated calls. The HLR also notifies any other VLR which had been previously hosting the mobile to deregister the mobile.
A mobile-terminated call attempt is always first directed to the mobile station's HLR by the gateway MSC first contacted from the PSTN. The HLR is responsible for contacting the current serving system, obtaining a temporary local directory number (TLDN) from the serving system, and transferring the call to the TLDN. The serving system is responsible for the connection between the TLDN and the roaming mobile station.
IS-41 supports uninterrupted voice services while the mobile station moves between MSCs. This is equivalent to maintaining a session between a mobile data device and another host while the mobile host is in motion between areas. Because trunk connections are used to carry the voice traffic, the concatenated trunk length could grow as the mobile moves about while the conversation is active (i.e., the mobile "grows a tail"). This is usually not a significant problem because most conversations are only a few minutes in duration and even fast-moving vehicles covers only so much ground in a typical call period.
HLRs and VLRs are not affected by cell handoffs, only by wider-scale mobility (between MSCs). Mobiles reregister every time they cross boundaries separating location areas. These location areas are clusters of cells large enough to minimize the number of re-registration messages (on the contended reverse control channel) while also minimizing the number of paging channels involved in a page.