Tuesday, January 11, 2011

History of GSM

First Networks were MTS (Mobile Telephone Service)

· Pre cellular radio service

· There had to be operator assisted at all times on both sides ie. If you called from a landline, the operator would answer and transfer the call to another operator who would transfer the call to the recipient

· Channels are prone to network congestion & Interference


· IMTS (Improved Mobile Telephone Service)

· Links PSTNs( Public switched Telephone network) commonly known as Landlines

· No need for an operator

· Used only for voice calls

· Analog Mode

· Most common IMTS phone was the Motorola TLD 1100

· It uses two circuit boards to perform channel scanning and digit decoding

· All logic is performed with transistors

· In a given city, one IMTS base station channel is marked Idle by the transmission of a steady 2000 Hz "idle" tone. Mobiles would scan the available frequencies and lock on to the channel transmitting the idle tone.

· When a call is placed to a mobile, the idle tone would change to 1800 Hz "channel seize" tone, and the 7 digit mobile number (3 digits of area code and 4 digits of subscriber number) would be sent out as rotary dial pulses, switching between 2000 and 1800 Hz to represent digits.

· Any mobile recognizing that the call was for someone else would resume scanning for marked idle tone, while the called mobile would then transmit 2150 Hz "guard" tone back to the base station. This would also initiate ringing at the mobile, and when the mobile subscriber picked up the phone, 1633 Hz "connect" tone would be sent back to the base station to indicate answer supervision and the voice path would be cut through. When the mobile hung up, a burst of alternating 1336 "disconnect" and 1800 Hz "seize" tones would be sent to allow the base station to service another call.

· Mobiles would originate calls by sending a burst of connect tone, to which the base station responded with a burst of seize tone. The mobile would then respond with its identification, consisting of its area code and last four digits of the phone number sent at 20 pulses per second, just as in inward dialing but with the addition of rudimentary parity checking. Digits are formed with a pulsetrain of alternating tones, either connect and silence (for odd digits) or connect and guard (for even digits). When the base station received the calling party's identification, it would send dialtone to the mobile. The user would then use the rotary dial, which would send the dialed digits as an alternating 10 pps pulse train (originally, directly formed by the rotary dial) of connect and guard tones.

1 G:

· AMPS ( Advanced Mobile Phone system)

· Single Frequency was used for a single user

· Similar to 0G but it uses more computing power to select frequencies, hand off to PSTNS, handle billing and Call set-up