GSM CHANNELS ECE 2526-MOBILE COMMUNICATIONS Tuesday, 19 February 2020
LINKS BETWEEN BTS, BSC & MSC Sector 1 Sector 2 E1 at 2048 Kbps Sector 3
GSM PHYSICAL CHANNELS • Physical channels in GSM are as a result of Frequency Division Multiplex Where the entire GSM band is divided into frequency channels denoted by their Absolute Radio Frequency Channel numbers (ARFCN). Each ARFCN is divided into 8 time slots using Time Division Multiple Access (TDMA). • A time slots may carry a variety of information, e.g. synchronization, paging,, traffic, etc which is organised at a higher level in the network hierarchy and referred to as logical channels.
NEED FOR SYNCHRONIZATION: CALL INITIATION PROCESS To understand the need for logical channels, let us re-examine the call- initiation process. Upon being switched on: 1. MS scans all ARFCNs and measures their quality. 2. MS selects an ARFCN with the best quality and tunes onto it. 3. MS looks for synchronisation information containing Base Station Information Code (BSIC) as well as the TDMA frame number (FN) in time-slot no. 1. 4. MS uses the synchronization information to align its transmissions with those of the BTS.
NEED FOR LOGICAL CHANNELS: REGISTRATION & AUTHENTICATION Registration and authentication follows the following steps: MS BTS 1. Request for a channel to establish a connection 2. Network acknowledges the request and allocates a channel. 3. MS receives and reads the information and connects to the channel and send back ACK Main Engineering Issues of concern: 1. Which time slot should requests be made? 2. Which time slot should network acknowledgement be made? 3. Which channel should MS acknowledgements be made? 4. Which time-slots should authentication information be transmitted?
NEED FOR LOGICAL CHANNELS : PAGING PROCESS MS MSC BTS 1. Paging message sent to all BTSs within the location area 2. MS acknowledges the request by sending a channel/service request 3. Network acknowledges the request 3. MS is assigned a traffic channel (Conversation starts) 4. MS measures the signal strength of adjacent cells and sends measurement reports 5. Release traffic channel request after calling party disconnects (conversation ends) Engineering Issues of concern: 1. Which time-slot should be used for paging MSs in a location area? 2. Which time-slot should carry acknowledgement requests? 3. Which time-slots should carry traffic( in this case voice)? 4. Which time-slot should carry the release traffic channel signal?
WHY LOGICAL CHANNELS? 1. The preceding simplified description of call-setup processes indicate that there are many functions need at the air interface to enable a subsriber to setup a call. 2. Each of these functions require a unique burst which is referred to as a logical channel. 3. Some logical channels are uplink, some are downlink while some are bidirectional. 4. Logical channels are allocated at higher layers in the OSI models.
TIME SLOTS & FRAMES IN GSM 1. A GSM timeslot has a duration of 3/5200 seconds (≈ 577 µs). 2. Eight timeslots form a TDMA frame (0.577x8 ≈ 4.62 ms in duration). 3 5200 Timeslot 0 Timeslot 1 Timeslot 2 Timeslot 3 Timeslot 4 Timeslot 5 Timeslot 6 Timeslot 7 Timeslot 0 3 × 8 5200 = 4.15 𝑛𝑡
GSM SM FR FRAME STRUCTURE / / 01 • GSM data frames and slots are organised in a logical manner so that the both MS and BTS understands when particular types of data are to be transmitted. • GSM frame structure enables the data to be organised in a logical fashion so that the system is able to handle the voice and signalling data correctly. • GSM data structure is split into slots, frames, multiframes, superframes and hyperframes to give the required structure and timing to the transmitted data.
GSM SM FR FRAME STRUCTURE / / 02 Hyper-frames Hyper-frame Super-frames Hyper-frames repeat every 3 hours 28 2048 Super-frames Super-frames last 6.12 seconds. They minutes 53.76 seconds. consist of 51 traffic multi-frames or 26 Hyper-frames support such functions control multi-frames. as frequency hopping and encryption. Superframe 51 Traffic Multi-frames or 21 Control Multi-frames Traffic Multi-frame Control Multi-frame Consisting of 26 frames and Comprises 51 bursts and taking 120 ms. occupies 235.4 ms. Control Multi-frame 24 bursts numbered 0 to 11 Traffic Multi-frame Is subdivided into logical and 13 to 24 are used for (52 frames) (26 frames) channels FCCH, BCCH, PACCH, traffic. One of the remaining SCCH, SDCCH which are time- bursts is then used to scheduled. accommodate the SACCH. The remaining is free. Frame (8 slots) Frame Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately Time Slot 4.615 ms (i.e.120/26 ms). A frame forms the basic unit Time Slot Fundamental unit of time is called a burst period for the definition of logical channels. and it lasts for approximately 0.577 ms
GSM FRAME STRUCTURE Finally, there are 156.25 Bits per Timeslot, each Bit being 3.69231 μs long. Within each Frame are 8 Timeslots at 576.92 μs per Timeslot (577 μs in round numbers). There are 26 Frames in each Multiframe, with each Frame being 4.61538 ms long (120 ms/26). GSM Multiframe is the basic unit, and is 120 ms long.
LOGICAL CHANNELS IN IN GSM LOGICAL CHANNELS COMMON CHANNELS DEDICATED CHANNELS TRAFFIC BROADCAST DEDICATED CONTROL COMMON CONTROL CHANNELS CHANNELS CHANNELS CHANNELS FACCH TCH/F TCH/H TCH/EFR SCH RACH AGCH FCCH PCH SDCCH BCCH SACCH
TD TDMA FR FRAMES SH SHOWING COMMON & DEDICATED FR FRAMES Slow Associated Control Channel (SACCH) SACCH is associated with the speech channel and is used to transmit measurement reports and also for power control and time alignment. Stand alone Dedicated Control Channel (SADCC) Used for call setup, authentication, location update, assignment of traffic channels and transmission of short messages
LOGICAL CHANNELS IS USED FOR SMS Broadcast Channels (BCH) 1. Synchronization channel 2. Frequency Correction Channel (FCCH) 3. Broadcast Control Channel (BCCH) Common Control Channels (CCH) 1. Paging Channel 2. Access Grant Channel 3. Random Access Channel Dedicated Control Channels (DCCH) 1. Standalone Control Channel (SDCCH 2. Slow-Associated Control Channel (SACCH) 3. Fast-Associated Control Channel (FACCH)
BROADCAST CHANNELS / 00 • Broadcast (Beacon) Channels (BCH) - are used to transfer system information such as timing references and synchronization information. • The broadcast channel provides system information, system configuration information (such a paging channel sleep groups), and lists of neighbouring radio channels to all mobile devices operating within its radio coverage area.
BROADCAST CHANNELS / 01 1. Frequency Correction Channel (FCCH) 1. FCCH burst consists of all 0’s which are transmitted as a pure sinewave. It acts as a flag and enables the MS to find a particular TRX. 2. Upon being switched on, the MS scan for this channel, since it has no information to which frequency to use. FCCH carrier enables a mobile to tune its frequency to that being broadcast on the FCCH channel by the particular TRX.
BROADCAST CHANNELS / 02 SYNCHRONISATION CHANNEL (SCH) SCH carries information for frame synchronization of the mobile station and identification of a BTS. It contain two encoded parameters: (a) Base transceiver station identity code (BSIC), (b) Reduced TDMA frame number (RFN) BSIC is required to enable the MS to establish that the transmissions are from a particular BTS. MS decodes the BSIC if the chosen BTS is GSM Base station within a cell.
BROADCAST CHANNELS / 03 BROADCAST CONTROL CHANNEL (BCCH) BCCH is used to broadcast control information to every MS within a cell. The information includes details of the control channel configuration used at the BTS, a list of the BCCH carrier frequencies used at the neighbouring BTSs and a number of parameters that are used by the MS when accessing the network. Specifically, BCCH contains the detailed Network and cell specific information such as : 1. Frequency used by Cell and its Neighbouring cells. 2. Frequency Hopping Sequence 3. Paging Groups 4. Location Area Identity (LAI) 5. Max output power allowed in the cell
COMMON CONTROL CHANNELS / 00 • Common Control Channels (CCCH) - are communication channels used to coordinate the control of mobile devices operating within its cell radio coverage area. • GSM control channels include the Random Access Channel (RACH), Paging Channel (PCH), and Access Grant Channel (AGCH).
COMMON CONTROL CHANNELS / 01 PAGING CHANNEL (PCH) 1. PCH s a downlink burst that is transmitted to all MS in a location area when a call or SMS arrives. 2. The BTS Broadcasts the paging burst to alert the mobile of an incoming call or Incoming SMS. Paging message includes the MS’s identity number TMSI 3. MS listens to the PCH. If it identifies its own mobile subscriber identity number on the PCH, it will respond using the Random Access Channel (RACH).
DEDICATED CONTROL CHANNELS • Dedicated Control Channels - are communication channels used to transfer signalling messages to specific devices.
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