GSM infrastructure MSC, BSC, BTS, VLR, HLR, GSGN, GSSN Johan Montelius jm@sics.se

Compared to a fixed network Gateway MSC Transit exchang Local switches MSC BSC Subscriber stage Fixed phone MS

What’s new? Radio access network Encryption Voice coding Locate a mobile station Identify and authenticate Handle mobility during a call

A bird eye’s view GMSC Access Network MSC databases BTS BSC BSC BSC

”Typical” Mobile operator One to ten MSC per 1 M subscribers Ten to one hundred BSC per MSC Thousands of BTS per 1 M subscribers

Base Transceiver Station Controls the radio link encryption error control signal strength 1 - 6 duplex carriers, for example: two layers 120o sectors (6+8)*3 = 42 voice calls Cost ~ 100 – 200 k$

Base Station Controller communication with the mobile terminal local hand-over voice codecs carrier allocation paging to terminals talks to the MSC serves up to 40 BTSs could be co-located with MSC

Mobile Swicthing Center Mobility management identifies and authenticates locates switches between BSCs Handover to other MSCs 64 kbps regular voice channels

Radio Access network A carrier is divided into logical channels control channels CCH traffic channels TCH The first carrier holds 2 controll channels and 6 traffic channels. Additional carriers holds only traffic channels.

Control Channels Broadcast Common control Dedicated control Frequency correction: to synchronize (FCCH) Broadcast control: this is me, these are my neighbours (BCCH) Common control Paging: to notify a mobile station (PCH) Random access: for the mobile to request service (RACH) Access grant: reply with assigned dedicated control channel (AGCH) Dedicated control Standalone dedicated control: used during call set-up, SMS etc (SDCCH) Slow associated control: power, time etc (SACCH) Fast associated control: handover etc, uses the TCH (FACCH) Cell broadcast: cell broadcast

Traffic Channels Duplex Error control Full rate (TCH/F) Forward Error Correction (FEC) retransmission (ARQ) Full rate (TCH/F) 13 Kbps speech 9,6 or 14,4 Kpbs data Half rate (TCH/H) 6,5 Kbps speech 7,2 or 4,8 Kbps data

Logical channels Hyperframe: 2048 superframes 3h28m Superframe: 26 or 51 multiframes 6.12 s CCH CCH Multiframe: 26 or 51 TDMA frames CCH TCH 8 time-slot TDMA frame 4.143 ms

One TDMA frame Normal burst Frequency correction burst 148 b 0.577 ms tail guard 8.25 tail training Normal burst 3 57 b 26 b 57 b 3 user data user data stealing flags Frequency correction burst Synchronization burst Dummy burst Access burst

Keeping track of a mobile station VLR visiting location register Location area Location area update the VLR with new location area

Moving to a new MSC VLR which LA which MSC ? Location area update the VLR with new location area and new MSC

but also…. HLR home location register VLR update the home location register!

Home Location Register incomming call PSTN where is the mobile? GMSC HLR MSC How do we reach him? VLR

Mobile station numbers mobile subscriber ISDN number (MSISDN) i.e. the phone number PSTN GMSC HLR Mobile Station Roaming Number (MSRN). MSC VLR Temporary Mobile Subscriber Identity International Mobile Subscriber Identity

GPRS Access Network BTS BSC BTS MS BTS Internet Gateway GPRS Support Node GGSN Access Network Packet Control Unit BTS BSC UDP/TCP tunnel Serving GPRS Support Node SGSN BTS MS BTS