GSM - Time Division Multiple Access

A TDMA frame is divided into 8 time slots (TS) or 8 time bursts.
One timeslot is allocated per "user", i.e. the Mobile Station emits once per TDMA frame (every 8th TS).
The Mobile Station emits 3 TS later than the BTS to avoid antenna diplexer equipment and to use the same Radio Frequency (RF) components for transmission and reception.
The TDMA frames of uplink is synchronized to the downlink by computing a "Timing Advance".
So the uplink TDMA frame follows the downlink TDMA frame with 3 time slots + Timing Advance.
The time of one TDMA frame is 4.615 ms. That means one time slots has 577 μs.
This time is shared by 156.25 bits.
The Gaussian Minimum Shift Keying (GMSK) is used for modulation of the RF energy.

Five different physical channels (or burst types) are defined:
Normal Burst ,
Frequency Correction Burst ,
Synchronization Burst ,
Access Burst and
Dummy Burst .

The definition of logical channels and their mapping to physical channels follows.

Although not present in each burst type, the different burst parts have different tasks:
The Tail Bits (TB) define the start and end of a burst. Their function is similar to the "flags" in a HDLC protocol.
The Training Sequence (Training) is a 26 fixed bit pattern defined by GSM as 8 different types. They are known by MS and BTS and are used for synchronization and bit error rate (BER) measurements to monitor the communication quality.
Stealing Flags (SF) are located aside the training sequence indicating whether the adjacent 57 bits in the "useful" burst parts contain speech / data information or spontaneous signaling.
The Encrypted Bits (info) are the "useful" bits of a burst. They are serving for speech or data transmission or signaling information.
The Guard Period (GP) between two consecutive bursts is necessary to switch on and off the transmitter power and to serve for timing advance.

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GSM - Physical Channels (Bursts)

A physical channel is defined by its position in the frequency time plan.

Normal Burst

This burst type is used for user and signaling information exchange in uplink and downlink direction.

Frequency Correction Burst

It is used for MS synchronization on the BTS frequency. This burst type is only existing in downlink direction. Due to the GMSK modulation 142 modulating bits of "0" produce a pure RF carrier sine wave information shifted to -67,7 kHz.

Synchronization Burst

The Synchronization Burst is used for MS synchronization on the BTS time. It does only exist in downlink direction. The following parameters are contained:
the Common extended training sequence to synchronize the MS onto the BTS,
the Base Station Identity Code (BSIC) to briefly describe the cell and
the TDMA frame number to be used with ciphering and information identification.

Access Burst

The Access Burst is used for MS access to the BTS air interface and to inform the network about the access reason. It makes the BSC calculate the distance between the MS and the BTS, i.e. max. 35 km in GSM 900. It exists only in uplink direction.

Dummy Burst

The Dummy Burst is used in downlink direction to modulate the transmitter if there is no useful information to be exchanged. It helps the MS in adjacent cells to monitor the RX level of the actual cell in order to prepare a handover.

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GSM - Logical Channels

A logical channel describes the kind of information to be transmitted.
GSM distinguishes between two groups of logical channels on the air interface.

Traffic Channels (TCH)

Example:
TCH/F9.6 means Full Rate Traffic Channel with 9600 bps data transmission rate.
There do also exist Half Rate TCHs (TCH/H) and Enhanced Full Rate TCHs (TCH/EFR).
Possible data rates are 2.4 kbps, 4.8 kbps and 9.6 kbps.

The Cell Broadcast Channel (CBCH) is also a Traffic Channel. It provides downlink broadcast information per cell, e.g. traffic announcements, cultural information or advertising.

Control Channels

This section consists of Broadcast Channels (BCH), Common Control Channels (CCH) and Dedicated Control Channels (DCCH).

The following table will give information on the mapping of logical channels on physical channels and their usage.

Logical Channel			Physical Channel	Direction	Function
T R A F F I C C H A N N E L		TCH/F TCH/2.4 TCH/4.8 TCH/9.6 Traffic Channel / Full Rate	Normal Burst (NB)	Uplink / Downlink	Contains speech information or data information
		TCH/EFR TCH/2.4 TCH/4.8 TCH/9.6 Traffic Channel / Enhanced Full Rate	Normal Burst (NB)	Uplink / Downlink
		TCH/H TCH/2.4 TCH/4.8 TCH/9.6 Traffic Channel / Half Rate	Normal Burst (NB)	Uplink / Downlink
		CBCH Cell Broadcast Channel	Normal Burst (NB)	(Uplink) / Downlink	Provides downlink broadcast information per cell
C O N T R O L C H A N N E L	B R O A D C A S T	FCCH Frequency Correction Channel	Frequency Correction Burst (FCB)	Downlink	After Switch-On, the MS scans the downlink frequency range to detect this known burst type and to align its receiver to this master frequency. Normally, the FCCH with highest field strength is chosen.
		SCH Synchronization Channel	Synchronization Burst (SB)	Downlink	Contains the number of the current TDMA frame and a brief network identification, BSIC.
		BCCH Broadcast Control Channel	Normal Burst (NB)	Downlink	Provides general information concerning the network, serving cell and adjacent cells (country, network operator, location area). Vital for MS to go to "idle mode".
	C O M M O N	RACH Random Access Channel	Access Burst (AB)	Uplink	The MS demands signaling by using a RACH if it replies to a paging information of if it wants to initiate a mobile originated call.
		AGCH Access Grant Channel	Normal Burst (NB)	Downlink	Allocates dedicated signaling capacity to proceed registration, call setup, etc.
		PCH Paging Channel	Normal Burst (NB)	Downlink	Information serves for calling a MS being in "idle mode" in a decent geographical area.
	D E D I C A T E D	SDCCH Stand Alone Dedicated Control Channel	Normal Burst (NB)	Uplink / Downlink	MS forwards signaling information to the GSM network concerning authentication, location update, dialing, etc. procedures. BTS uses it for downlink information. Also used for SMS information not related to a call.
		SACCH Slow Associated Control Channel	Normal Burst (NB)	Uplink / Downlink	Is used every 480ms during communication providing uplink measurement reports (RX level, RX quality) on active and adjacent cells and downlink commands for MS timing advance or power control. Will contain SMS if related to a call.
		FACCH Fast Associated Control Channel	Normal Burst (NB) + Stealing Flag	Uplink / Downlink	Is allocated in case of spontaneous signaling needs during active communication, e.g. during handover, service mode modification (data/speech) or connection release.