Cellular Call Processing

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Cellular Call Processing

Enabling Technologies

Frequency synthesis function, the ability of a mobile teleph6ne to dynamically tune itself to any of the frequencies in its assigned spectrum, was shrunk down to the scale of a single integrated circuit (IC). This provided the means to do away with complex networks of crystal oscillators and tuning elements. Microprocessors and microcomputers (i.e., MSCs) allowed complex control functions such as cellular call processing and handling of user keypad functions to be performed within a very small space. Surface mount technology (SMT) applied to discrete circuit elements allowed for the shrinkage of linear circuitry blocks. This resulted in savings in weight and volume. All of these factors came together to not only provide for small, relatively lightweight mobile phones, but to allow wireless switches to perform their functions as well.

Mobile Telephone Specifics

Mobile stations in AMPS systems are categorized according to their maximum allowed ERP (effective radiated power). Each mobile phone can be commanded to a power level within an operating range of levels by the base station; however the mobile may not radiate more power than is allowed for its class. Power control levels are separated by 4 dB each. Note: The capability of the mobile telephone to change power levels is known as control channel mobile attenuation code (CMAC). CMAC describes the maximum power level that a mobile station may use when accessing the wireless system on the reverse control channel (mobile to base). It is set on a per-cell basis, and may be different from cell to cell. The station class mark (SCM) of a mobile phone indicates the maximum ERP that the mobile can deliver as well as the operating frequency range of the phone. Two unique numbers identify each mobile telephone. The mobile identification number (MIN), which is a 34-bit binary encoded representation of the mobile station's 10-digit directory telephone number. This number is programmed into the mobile phone when a subscriber obtains service.


The electronic serial number (LSN), which is a 32-bit binary number that is programmed into the mobile phone in the factory at the time of manufacturing. This 32-bit number can yield over 4 billion combinations. The LSN is unalterable, and uniquely identifies the mobile telephone. The LSN contains an 18-bit serial number and an 8-bit manufacturer code, and is viewed as the electronic fingerprint of the mobile telephone. The remaining 6 bits are reserved for future use.

Mobile-Originated Calls

Once the mobile phone has been powered on in an AMPS system, it monitors the 21 possible control channel frequencies in its assigned band (A or B band) prior to the placement of a call. The purpose of the monitoring is to find the control channel with the strongest signal and lock onto that signal to watch for overhead information and paging information. In a properly engineered wireless system, the strongest signal will come from the nearest cell site. When the user originates a mobile call, the mobile telephone will send in a data message on the reverse control channel to the closest cell site, with the dialed digits entered, the MIN of the phone, and the LSN. This activity will only occur when the busy/idle status (BIS) indicates that the reverse control channel is idle. The base station reverse control channel then receives the origination message and forwards the message (via the cell controller) to the MSC. The MSC receives the origination message, verifies the MIN/LSN combination as valid, and initiates seizure procedures on a PSTN interconnection trunk, assuming the destination of the mobile-originated call is a landline telephone. The MSC then allocates a traffic channel at the cell site where the user is located that will bear the call. The traffic channel is the voice channel (VCH) that carries the actual conversation. The MSC then sends a VCH assignment message to the cell base station with the identity of the VCH. Upon receiving the VCH assignment message, the cell base station VCH keys its transmitter and begins sending supervisory audio tone (SAT) to the mobile phone. Simultaneously the forward control channel sends an assignment order to the mobile station containing the frequency and the SAT of the VCH that will bear the call, and the MIN that identifies the mobile telephone. Note: Maintenance of link continuity between the base station and the mobile phone in a traffic channel (VCH) is accomplished via the SAT tone, which is added to the voice signal prior to the modulation of the RF carrier. See Figure for a step-by-step diagram of mobile-originated call processing.


Figure: Mobile-originated wireless calls: transactional signaling process The mobile telephone receives the assignment order and retunes its frequency synthesizer; then it monitors for the designated SAT. Upon SAT confirmation, the mobile station keys its transmitter and regenerates the SAT to the base station. The cell base station detects the regeneration of the SAT by the mobile station and sends an origination complete message to the MSC. The MSC then connects the PSJN trunk to the VCH trunk. The conversation may now take place, since both the cell base station and the mobile station have reached the same state. Note: Since this is a mobile-originated call, when the MSC is connecting the VCH trunk voice channel) to the PSTN trunk, least-cost routing has already occurred. The MSC has examined the dialed NPA-NXX digit combination, and determined the least expensive route to direct the call.


Mobile-Terminated Calls

Overview

Wireless customers are charged for land-to-mobile calls (calls they did not originate themselves) because the land-to-mobile call still uses considerable resources of the wireless network infrastructure. Land-to-mobile calls use interconnection trunks to access the wireless network as well as cell radio channels, to fully process a land-to-mobile call. To that end, wireless carriers are justified in charging their customers to terminate these calls. This is even easier to understand and justify considering the industry average cost for a wireless base station is anywhere from $250,000 (PCS) to around ($500,000 to $750,000) (cellular).

Autonomous Mobile Registration

Autonomous mobile registration plays an integral part in the processing of mobile- terminated wireless calls. Also known simply as mobile registration, or registration, it is the process where, by design, all wireless phones continually transmit their MIN/FSN combination over the air to the nearest cell site every 5 to 15 minutes so that the MSC will know the mobile user's location in the event of an incoming call. Mobile registra-tion is the mobile phone's way of continuously saying "Here I am" to the wireless system. There are more steps involved in the processing of land4o-mobile calls when compared to the processing of mobile4o-land calls. This is mainly because of the paging function and the alerting function.

Call Processing (Mobile-Terminated)

Similar to a mobile-originated call, the mobile station continuously monitors the 21 control channel frequencies in its assigned band (A or B band) and locks onto the strongest control channel. Scenario: A person sitting at home decides to call a friend and they initiate a call over the PSTN by dialing the 10-digit number (MIN) assigned to the mobile station. The PSTN routes the call to the appropriate interconnection associated with the MIN, and on to the MSC. The MSC checks its database for a list of the last 10 cell sites where the mobile station last registered and broadcasts a page to all those base stations. The base station where the mobile is located receives the page message and transmits it to the mobile phone on the forward control channel The mobile telephone receives the page message while monitoring the control channel.


The base station forwards the page response to the MSC. The MSC receives the page response message, verifies the MINIESN combination, and allocates a VCH (voice channel) at the cell site that will bear the call. The MSC then sends a VCH assignment message to the base station with the identity of the VCH (i.e., channel set/frequency assignment). Upon receiving the VCH assignment message, the base station VCH keys its transmitter and begins sending the SAT to the mobile phone. At the same time, the control channel sends an assignment order to the mobile telephone that contains the frequency and SAT of the VCH that will bear the call. The mobile telephone receives the assignment order and retunes its frequency synthesizer and monitors for the designated SAT. Upon SAT confirmation, the mobile station keys its transmitter and regenerates the SAT to the cell base station. The cell base station detects the regeneration of the SAT by the mobile station and sends an alert order to the mobile telephone, which causes the mobile phone to ring. The mobile station confirms the alert status by continuously gating the signaling tone into the reverse VCH signal. At this time, and after detecting the presence of the signaling tone from the mobile station, the cell base station sends a message to the MSC indicating that the mobile station has successfully arrived on the VCH and is alerting. When the wireless customer goes off hook, the mobile telephone removes the signaling tone from the reverse VCH signal. The base station detects the absence of the signaling tone and sends a message to the MSC indicating that the mobile telephone has gone off hook. Conversation may now take place, since both the base station and mobile telephone have reached the same state. See Figure for a step-by-step diagram of mobile4erminated call processing.


Figure: Mobile-terminated wireless calls: transactional signaling process (applies to land-to-mobile and mobile-to-mobile calls.

Call Handoff

Assume a wireless customer has placed a call from his phone to a friend across town, and he is in the middle of a conversation. Scenario: The cell where the user is currently located, base station 1, detects that the reverse voice channel signal strength justifies the consideration of a handoff This is determined via the received signal strength indicator (RSSI) at the cell base station.


Note: Wireless technicians manually set The RSSI threshold parameter that determines when a handoff is justified. However it is at a threshold that is well above a power level that would allow a call-in-progress to be dropped from the system. Base station 1 then sends a handoff request to the MSC containing information relevant to the mobile telephone (i.e., power class, current power level) and its current reverse voice channel signal strength. The MSC receives the handoff request and determines which cells are adjacent to base station 1. In this example, base station 2 is one of several adjacent cells, and it is the one cell receiving the strongest signal from the mobile. Therefore, the MSC sends a handoff measurement request to base station 2. Base station 2 receives the handoff measurement request and uses its locating receiver to determine the suitability of a handoff The locating receiver will tune to the reverse VCH frequency the mobile telephone is currently using, and make signal strength measurements on all (or some) antennas. If the measurements exceed a certain criteria for handoff, then base station 2 will send a handoff measurement response to the MSC. The MSC receives the handoff measurement response and chooses a VCH for base station 2 to use to support the call. The MSC then sends the VCH assignment message to base station 2 Base station 2 receives the VCH assignment message, keys the VCH transmitter (transceiver), and sends the VCH assignment confirmation message to the MSC. The MSC receives the VCH assignment confirmation message from base station 2 and sends the handoff order message to base station 1 containing the frequency and SAT of the VCH in base station 2 Base station 1 transmits the handoff order to the mobile telephone over the forward VCH. The mobile telephone hears the handoff order, confirms the order by gating" 50 ms of signaling tone into the reverse VCH signal, and reprograms its frequency synthesizer for the VCH in base station 2. The mobile telephone then looks for base station 2's VCH SAT and, having found the SAT, regenerates it on the reverse VCH to base station 2. Base station 2 detects the regeneration of the SAT on the reverse VCH and sends a handoff OK message to the MSC. The MSC then sends a release source channel message to base station 1, which causes the forward VCH to be dekeyed, making the channel available for use by another subscriber in base station l's territory. The call handoff is now complete, and the conversation is now being carried over a VCH at base station 2. See Figure for a step-by-step diagram of call handoff processing.


Figure: Call handoff: transactional signaling process

Documents

Cellular Call Processing