Polytechnic University1 GSM MAC Original by: Ahmed Ibrahim, Chang Wu Ma, Danny Mangra, EL604, Fall 2001; Modified by Prof. M. Veeraraghavan Architecture Frequency bands Spectrum efficiency Traffic and control frames Speech coding and data rates Control channels Signaling

Polytechnic University2 Architecture of the GSM network

Polytechnic University3 Frequency bands Frequency band: Uplink: MHz, Downlink: MHz Frequency range: 50 MHz (25 MHz Up, 25 MHz Down) Carrier spacing: 200 KHz (but time shared bet. 8 subscribers) Duplex distance: 45 MHz (FDD)

Polytechnic University4 Frequency bands Number of carriers: 25 MHz/200KHz =124 Users/carrier: 8 The reverse channel is retarded by 3 time slots relative to the forward

Polytechnic University5 Frequency bands One or more carrier frequencies are assigned to each BS Eight time slots are grouped into a TDMA frame (120/26 ms, or approx ms; 120 frames in a multiframe that is 26ms in duration) Time slot = 4.62/ 8 ms (or approx ms) One physical channel is one time slot per TDMA frame.

Polytechnic University6 Conventional carriers The conventional carrier is a sine wave at a single frequency

Polytechnic University7 Slow frequency hopped carriers Each TDMA frame in a given channel is carried on a different carrier frequency The purpose is to reduce co-channel interference between signals in nearby cells Frequency hopping adds a new dimension of complexity to cellular reuse planning

Polytechnic University8 Frequency reuse

Polytechnic University9 The Cellular Concept Repeat tiling of seven-cell array Distance between like cells must be far enough to avoid interference Smaller cells lead to better frequency reuse More calls per unit area Transmitted power must be smaller to avoid interference Requires careful power management Requires larger number of base stations Hexagonally tiled cells Cannot reuse frequencies in six surrounding cells Minimum of seven frequency sets is required if N=7

Polytechnic University10 Spectrum efficiency Assume N =3 (depends on environment) Country side: N = 2 or 3; Metropolitan areas: N > 3 or higher Carriers: 124; each carrier has 8 channels: No. of physical channels = 124*8 = 992 Total frequency band: 25(uplink)+25(downlink)=50MHz Efficiency = 992/(3*50MHz) = 6.61 conversations/ cell/ MHz

Polytechnic University11 Traffic frames & control frames

Polytechnic University12 A traffic channel (TCH) is used to carry speech and data traffic. TCHs are defined using a 26-frame multiframe (a group of 26 TDMA frames) The length of a 26-frame traffic multiframe is 120 ms Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) (12 and 25 on alternate multiframes) and 1 is currently unused. Traffic channels

Polytechnic University13 Traffic channels categories

Polytechnic University14 GSM transmission rate A full-rate traffic channel has a bit rate of Within each time slot, some bits reserved for control; only 114 bits of bits are for voice data Contrast this to IS136 where transmission rate is 48.6kb/s and to the full-rate channel bit rate of 16.2kbps

Polytechnic University15 Speech coding new: Enhanced Full-Rate (EFR) coding original: Linear Prediction Coding with Regular Pulse Excitation (LPC-RPE) –coder: 13kbps –with channel coding, rate becomes 22.8kbps – for a full-rate channel

Polytechnic University16 Control channels Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange signaling information required to change to dedicated mode. Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information. The common channels are defined within a 51-frame multiframe, so that dedicated mobiles using the 26-frame multiframe TCH structure can still monitor control channels.

Polytechnic University17 GSM control channels categories

Polytechnic University18 Send message yes no Access Fails Access Conflict Access Succeeds begin Other transmissions in this slot? Base detects message ? Max attempts? Random time delay Another message with same 5-bit code? Slotted Aloha Scheme Reverse control channel access protocol terminal is assigned an SDCCH

Polytechnic University19 GSM signaling protocol architecture

Polytechnic University20 Messaging GSM specifies the communication protocols employed on ALL the Network Interfaces All of the signaling channels (except FCCH, SCH & RACH) transmit information in LAPDm format. PHY carries these messages in 184-bit segments Address Control LIDataFill 184 bits

Polytechnic University21 Functions Radio Resources Management (RRM) –Controls the setup, maintenance, and termination of channels, including handovers. Mobility Management (MM) –Manages the location updating and registration procedures, as well as security and authentication. Call Control Management (CCM) –Handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.

Polytechnic University22 Mobile Station Base Station RACH: “ Channel request ” AGCH: “ Immediate assignment ” SDCCH: “ Paging response ” FACCH: “ Connect ACK ” SDCCH: “ Assignment ACK ” Conversation SDCCH message exchange SDCCH message exchange (see Slide 34) Initial ProcedureInitial Procedure (see slide 33) Delivery of a call to a GSM mobile station MSC gets MS’s location I am here.

Polytechnic University23 SCH: “ Sync Channel Information ” BCCH: “ System Information ” PCH: “ Paging Request ” Initial procedure in delivery of a call to a GSM mobile station PCH: Paging Channel Purpose: To notify terminals of arriving calls. Mobile Station Base Station RACH: “ Channel request ”

Polytechnic University24 SDCCH: Standalone dedicated control channel Authentication request Authentication response CIPHERING MODE Ciphering Mode ACK setup Call Confirmed ALERTING CONNECT Assignment Command SDCCH message exchange in delivery of a call to a GSM mobile station Base Station Mobile Station

Polytechnic University25 How is the call terminated at MS? FACCH: “ Release complete ” FACCH: “ Release ” FACCH: “ Disconnect ” FACCH: “ Channel release ” Conversation Termination of the call (by MS) Mobile Station Base Station

Polytechnic University26 References Wireless Personal Communication Systems, David J. Goodman Overview of the Global System for Mobile Communications, John Scourias, University of Waterloo GSM Cellular Standards: A look at the world’s most common digital cellular system, Kevin Bolding, Electrical Engineering, Seattle Pacific University Wireless and Mobile Network Architectures, Yi Bin Lin, Imrich Chlamtac Mobile Radio Networking, Networking and Protocols, Bernhard H. Walke