AMPS Specifications

·  See Table 10.2 Blake for carrier A and Carrier B Cellular frequencies

·  Also Refer to Module 5 Course Pack for details

·  Since AMPS is passing away from the scene, it will not be discussed in full details in this course. Only a brief summary will be given since all advance and modern Cellular systems are based on AMPS.

·  AMPS stand for “ Advance Mobile Phone System”. It was the first North American Standard for Cellular Communications and still being used in some rural areas where the carriers have not updated their Networks to modern Protocols

·  Table below lists the major AMPS specs

BS Transmission band (Forward Channels) 869 to 894 MHz 25 MHz BW

Mobile Unit Transmission band (Reverse Channels) 824 to 849 MHz 25 MHz BW

Spacing between Forward and Reverse Channels 45 MHz

Channel Spacing 30 Khz

Number of FDX Voice Channels 790 (395 in each direction)

Number of FDX Control Channels 42 (21 in each direction)

Mobile Unit Maximum power 3W

Cell Size Radius 2 to 20 Km

Modulation, Control Channels FM, 12 KHz peak deviation

Data Transmission Rate FSK, 8KHz Peak deviation

Error Control Coding 10 Kbps

·  In N.A, AMPS uses 2-25 MHz BW, 869 to 894 for BS to Mobile and 824 to 849 for Mobile to BS Communications.

·  These are split into 2 bands to encourage competion between service providers.

·  Each Service Provider is allocated 12.5 MHz, for example, Bell Mobility (a Wireline Provider has 12.5 and Rogers (A Wireless Provider) has 12.5 MHz.

·  Channels are spaced 30 KHz apart giving a total of ______channels to each provider in each direction

·  ______Channels are used to carry calls and ______Channels are used for control

·  Control channels are data channels operating at 10 Kbps

·  The Voice Channels use FM

·  The control information is also sent on voice channels, in bursts of data

·  Frequency Reuse factor of 7 is used to increase capacity with Sectoring and Cell splitting.

Operation of AMPS:

·  Each AMPS capable Cellular phone contains a Numeric Assignment Module (NAM) in its read Only Memory (ROM)

·  Refer to the diagram shown on P111 of your Course pack (Draw the diagram on P5 of your Notes.

·  The NAM contains the telephone number of Mobile (assigned by the service provider) and the serial # of the phone (assigned by the manufacturer of the phone). The Mobile phone # is also called MIN, the Mobile Identification Number.

·  When the phone is turned ON, it transmits its serial number and phone number to the MTSO (Mobile Telephone Switching Office)

·  MTSO is the control computer that connects a Wireless phone call to the PSTN. The MTSO controls the entire system operation, including monitoring calls, Switching, billing and handoffs

·  The MTSO also maintains a database with information about each valid registered mobile phone numbers as well as mobile units that have been reported stolen, and uses the serial number to block out stolen units

·  The MTSO uses the valid subscriber Phone numbers for billing purposes

·  If the phone is used in a remote city, the service is still billed to the users local service provider.

Call Sequence in AMPS:

When a Call is placed, it is processed as follows

  1. The user enters the telephone # of the called party and presses the SEND key
  2. The MTSO receives the # and verifies that the # is valid and that the user is authorized to place the call. Some service providers also require the user to enter a PIN to TX and RX a call
  3. The MTSO sends a message to the mobile , indicating which traffic channel to use for TX and RX
  4. The MTSO sends a ringing signal to the called party. Steps 1 to 4 occur within 10 seconds of initiating a call.
  5. When the called party answers, the MTSO creates a circuit between the calling and called party and initiates billing information.
  6. When one party hangs up, the MTSO releases the circuit, frees the radio channel and completes the billing information.

AMPS Control Channels:

·  AMPS uses 21, 30 KHz channels in Forward (BS to Mobile) and 21 in Reverse (Mobile to BS) directions

·  These channels transmit digital data using FSK

·  In both directions, data are transmitted in frames

·  Fig below shows Forward and Reverse frame structure. Draw fig in the space provided. (draw fig on the board from your notes p6). Note: Module 5 course Pack contains detailed specs. You should familiarize your selves with these specs.

Reverse Control Channel (RCC):

Bit Sync: 30 bits of alternating 1’s and 0’sfor bit synchronization

Word SYNC: 11100010010 = 0712Hex, for word synchronization

7 Bit Digital Color Code (DCC):A unique identifier of a BS used to distinguish transmissions in co-channel cells, and acts as a destination address for a Reverse Control Channel frame.

Data Bits (W1, W2, W3 ……….Wn): Contains control information such as origination, page response, confirmation etc. For reliability, each word is repeated 5 times

Forward Control Channel (FCC):

10 Bit of bit Sync as before

11 Bit Word Sync as before

Data words carry information such as busy, idle, frequency assignment etc. Again messages are repeated to ensure reliability.

Control Information transmitted over voice channels:

·  The mobile unit or the BS can insert a FSK encoded data burst into the voice channels by turning off the voice for about 100 ms

·  These messages are used to exchange urgent information such as change power level or handoffs

Channel Allocation in AMPS:

·  The control channels are used to allocate voice channels to the Mobile

·  When a Mobile is turned ON, it scans the channels to find the strongest, which would be obviously from the closest Cell site.

·  The Cell site then assigns a voice channel to the Mobile

·  While the conversation is in progress, the Cell site adjacent to the one in use monitors the signal strength from the Mobile.

·  When the signal strength in one of the adjacent cell is greater then the one in use, the system transfers the call to this new cell.

·  This transfer is called HANDOFF, and involves a change in frequency mobile is using.

·  A similar procedure takes place for incoming calls.

·  A mobile periodically identifies itself to the system whenever it is turned ON. Thus the System has a good idea of its current location

·  Paging Signals are sent out on control channels and the Mobile responds

·  The phone is instructed to ring, and once it is answered, the system assigns it a voice channel

·  After that, the system follows the Mobile as it moves from one cell to the next

Example:

A vehicle travels through a Cellular System at 100 Km/h. Approximately, how often will hand off occur if the cell radius is (a) 10 Km (b) 500m

Solution: Speed in meters/s = 100 Km/h ´ 1000m/Km ¸ 3600 s/h = 27.8m/s

(a) Time between handoffs for a radius of 10Km:

Diameter of the Cell = 2 ´ 10Km = 20 Km

Time between handoffs t = d/v = 20´ 1000m/27.8m/s = 719 seconds, approximately every 12 minutes

(b)  Time between handoffs for a cell radius of 500 m. Calculate? Ans: every 36 seconds

AMPS Control System: Performs following functions

·  Keeps track of Mobile phones

·  Determines which phones have been turned on and their locations

·  Keeps track of telephone numbers for authentication and billing

·  Detects and prevents fraudulent use.

·  Sets up calls in both directions (from Cell to Mobile and vice versa)

·  Transfers calls from cell to cell as required

·  An advance Control system should also allow fax, email and internet surfing.

Voice and Control Channels: (ST-Signaling Tones and SAT Supervisory Audio Tones): See P128 of the course Pack)

·  Voice channels are mostly used for voice transmission, however, sometimes control signals are sent over the voice channels during conversation using either (a) in-channel, out of band signaling (meaning tones above the voice frequency range) or (b) BLANK and BURST signaling, during which voice signals are muted for 100 ms

·  These signals are FSK with 8 KHz deviation (16KHz total shift), Manchester encoded, at 10 Kbps rate

·  These control signals, sent during conversation are used for emergency messages such as change power level or handoffs

·  Each message is sent 5 times with hamming Error correction code to minimize errors

·  Other control signals are used for call set/termination and call control

·  One of the serious drawbacks of AMP is the total lack of security. There is no encryption and all the data coding is publicly available.

Mobile and Base Identification:

·  Each Mobile has 2 unique Numbers.

·  The Mobile Identification Number (MIN) stored in the Number Assignment Module (NAM)

·  The MIN is the 10 digit phone number of the Mobile, Area code + 7 digit local #, stored as a 34 bit Binary number, assigned by the service provider

·  The NAM is a programmable memory module

·  Usually a phone is registered with either service provider A or Service provider B and has only one MIN.

·  It can operate on the other system (called Roaming) provided there is an agreement between 2 systems

·  The 2nd Identification number is the factory assigned Electronic Serial Number (ESN), which Identifies the manufacturer of the Mobile

·  This ESN is not supposed to be alterable by the customer without making the phone inoperable. However, an expert technician with the right tools can reprogram it.

·  The combination of the MIN and ESN is used by the Service Provider to bill the right person and detect fraudulent use.

·  The Mobile also has a # called the Station Class mark (SCM), which identifies its maximum power level as follows:

Class I Mobile + 6 dBW (4 W)

Class II Transportable +2 dBW (1.6W)

Class III Portable -2 dBW (600mW)

·  The maximum power level is specified as ERP (Effective radiated Power)

·  Mobile transmitted Power is controlled by the BS in 4 dB increments, with the lowest level being –22dbW (6.3mW)

·  The purpose is to reduce interference with the adjacent cells by using as little power as possible.

·  The Cellular System itself also has an ID called System Identification Number (SID) to enable the mobile to find out if it is communicating with its home system or it is Roaming.

·  A Digital Color Code (DCC) is associated with each cell site to detect if co-channel interference. When a mobile detects a change in DCC without a change in frequency it means co-channel Interference

Turning ON a Mobile Phone:

·  When a cell phone is turned ON, it identifies itself to the Network and scans all the control channels for its designated system (A or B) and finds the strongest.

·  It also looks for the System ID (SID) to determine whether it is Roaming.

·  If it doesn’t find this information in 3 Seconds, it tries the next strongest control channel.

·  Once it receives the System information, it turns to the strongest paging channel to look for any incoming calls. Paging channel carry the information about the incoming calls to the Mobile.

·  Only the System transmits on the Forward channel (from the BS to the Mobile). Any Mobile can transmit on the reverse channels.

·  After checking that the Reverse channel is free, the Mobile now transmits its MIN and ESN to the station to let the system know its identity, location and the willingness to call someone.

·  If the Mobile loses the signal or moves to another cell, the whole procedure is repeated again.

·  System also periodically polls the mobiles to see which are still ON and active.

·  In the IDLE condition, the Mobile continues to check the control channels from the cell site at least every 46.3 ms to determine if the signal is still available, and that it is from the same system and that there are no incoming calls.

Initiating a Call:

·  If the Mobile wants to place a call, it keys in a phone Number and presses SEND on the Reverse channel. This message also includes the MIN and the ESN.

·  The Cell Site passes this information to MTSO

·  The MTSO checks its database for the validity and authorization of the calling party using MIN and ESN, and if finds valid, authorizes the call.

·  The cell site than sends a message to the Mobile on the forward channel, indicating which voice channel to use, DCC and CMAC (Control Module Attenuation Code- which sets the power level to be used)

·  This power level can be changes by the BS as necessary during the call by means of a control message on the forward channel by sending a data burst).

·  The cell site then sends a continuous sine wave called SAT (Supervisory Audio Tone) on the voice channel to the Mobile to confirm correct connection. The Mobile returns the SAT back to the Station to confirm the connection.

·  The SAT frequencies are above the voice band, 5970Hz, 6000Hz, and 6030Hz.

·  After this handshaking, the actual conversation begins.