GSM Time Division Multiple Access

Structure of the TDMA frame, time slots, and burst types in GSM.

Beyond Frequencies: The Art of Sharing Time

In our exploration of the GSM Frequency Plan, we learned how the radio spectrum is sliced into narrow frequency channels, much like a highway is divided into lanes. This technique, called Frequency Division Multiple Access (FDMA), was the first step in allowing multiple users to communicate without interfering with each other. However, with only about 124 usable frequency channels in the original GSM-900 band, FDMA alone would be wildly insufficient to serve the thousands of users in a typical urban cell.

To solve this capacity puzzle, GSM employs a second, equally ingenious layer of sharing that operates in a different dimension: time. This technique is called . The core idea of TDMA is to take each of the frequency lanes created by FDMA and have multiple users take turns using it. It is like having a single-lane road where cars are allowed to pass one after another in a highly organized, repeating cycle. This combination of FDMA and TDMA is the cornerstone of GSM's ability to achieve massive user capacity.

The TDMA Frame: Organizing Time into Repeating Cycles

To make time-sharing work, time itself must be meticulously structured. In GSM, the fundamental unit of this time structure is the TDMA frame.

A TDMA frame is a very short, repeating period of time during which a specific sequence of events occurs on a given radio frequency. Here are its precise characteristics:

Eight Time Slots: Each TDMA frame is divided into exactly eight smaller time intervals, known as . These are numbered from 0 to 7.
User Assignment: In the simplest case of a voice call, each of the eight time slots within a frame is assigned to a different user. This means that a single radio frequency channel can simultaneously support eight separate conversations. User 1 gets slot 0, user 2 gets slot 1, and so on. Each user's phone is active only during its assigned time slot and remains dormant for the other seven.
Duration of a Frame: The total duration of one TDMA frame is precisely $120/26 \approx 4.615$ milliseconds (ms).
Duration of a Time Slot: Since a frame contains eight slots, the duration of a single time slot is $4.615 \text{ ms} / 8 \approx 0.577$ milliseconds, or 577 microseconds (µs). This is the tiny window of time in which a user's phone must wake up, transmit or receive its information, and go back to sleep.

Inside a Time Slot: The Anatomy of a GSM Burst

A time slot is not just an empty period of time; it is the container for a very specific packet of information called a . A burst is the smallest unit of data transmitted in GSM. The standard GSM bit rate is $270.833 \text{ kbit/s}$ . Given the time slot duration of $577$ microseconds, we can calculate how many bits fit into a single burst:

\text{Bits per slot} = \text{Bit Rate} \times \text{Slot Duration} \approx 270,833 \text{ bits/s} \times 0.000577 \text{ s} \approx 156.25 \text{ bits}

This fractional result means that time in GSM is actually defined in terms of bit periods. The duration of one bit is exactly $1 / (270.833 \times 10^3) \approx 3.692$ microseconds. A time slot lasts for exactly 156.25 bit periods. A burst is a sequence of bits that is carefully structured to carry user data as well as essential control and synchronization information. There are several different types of bursts, each designed for a specific purpose.

Burst Types and Their Structures

Not all communication is about transmitting voice or user data. The network needs to handle synchronization, frequency correction, and initial access. To do this, GSM defines several types of bursts, each with a unique structure.

1. The Normal Burst (NB)

This is the workhorse of the GSM system, used to carry all voice and data traffic on dedicated traffic channels (TCH). Its structure is a masterpiece of efficiency, packing user data and control information into its $156.25$ bit periods.

Guard Period: 8.25 bit periods ( $\approx 30.46 \text{ µs}$ )

Encrypted Data (57 + 57 bits): These two blocks carry the actual payload of the burst, which can be digitized voice or user data. These bits are encrypted for security.
Training Sequence (26 bits): This is a fixed, predefined sequence of bits known to both the mobile phone and the base station. It is placed in the middle of the burst. Its critical purpose is to allow the receiver's to estimate the characteristics of the radio channel and compensate for distortions like multipath fading. By comparing the distorted training sequence it receives with the perfect one it has stored, the phone can figure out how the channel has altered the signal and "undo" that distortion for the actual data bits. There are eight different predefined training sequences, and a BTS will use one of them, which helps the phone to distinguish it from neighboring cells.
Tail Bits (3 + 3 bits): These are three zero bits at the beginning and end of the burst. They serve as a known starting and ending point, allowing the receiver's signal processing algorithms to initialize and settle.
Stealing Flag (1 + 1 bits): These are two important bits that indicate whether the data fields of this burst contain regular traffic or urgent control messages. If urgent signaling information needs to be sent during a call (like a handover command), the network can "steal" a Normal Burst from the voice call to send this message. The stealing flags inform the receiver to interpret the 57-bit data fields not as voice, but as a Fast Associated Control Channel (FACCH) message.
Guard Period (8.25 bit periods): This is not transmitted data, but a period of silence at the end of the time slot. Its purpose is to prevent bursts from different users from colliding. Since mobile phones can be at varying distances from the cell tower, their signals will arrive with slightly different delays. The guard period acts as a time buffer, ensuring that a late-arriving burst from a distant user does not overlap with the next burst in the following time slot from a closer user.

2. The Synchronization Burst (SB)

The SB is used to provide mobile stations with the timing information needed to synchronize with the network. It is transmitted on the Synchronization Channel (SCH).

Structure (148 bits transmitted): 3 (Tail) | 39 (Data) | 64 (Training Sequence) | 39 (Data) | 3 (Tail)

Guard Period: 8.25 bit periods

The most notable difference from a Normal Burst is the extended Training Sequence of 64 bits. A much longer and more complex training sequence is used here to allow for robust and accurate initial synchronization, which is more challenging than maintaining synchronization during an ongoing call. The 39-bit data fields carry crucial information, including the current TDMA frame number and the Base Station Identity Code (BSIC), which allows the phone to identify the cell it is listening to.

3. The Frequency Correction Burst (FB)

The FB, transmitted on the Frequency Correction Channel (FCCH), has the simplest structure of all. Its only purpose is to help a mobile station fine-tune its internal oscillator to the exact frequency of the base station.

Structure (148 bits transmitted): It consists of 142 fixed bits, which are all zeros, plus the standard tail bits and guard period. $142$ Zero bits

Transmitting a long sequence of zeros results in a pure, unmodulated sine wave at the carrier frequency. This creates a strong, clear frequency "tone" that a mobile station can easily lock onto to correct any drift in its own frequency reference.

4. The Access Burst (AB)

The AB is a special, shorter burst used by the mobile station only when it first contacts the network or initiates a call on the Random Access Channel (RACH). At this stage, the network does not yet know the exact distance to the mobile station and has not assigned it a timing advance value.

Structure (88 bits transmitted): 8 (Extended Tail) | 41 (Sync) | 36 (Data) | 3 (Tail)

Guard Period: 68.25 bit periods ( $\approx 252 \text{ µs}$ )

The Access Burst is much shorter than other bursts and features a very long Guard Period of 68.25 bit periods. This extended silence is necessary because, without knowledge of the phone's distance, the BTS must allow for the maximum possible round-trip delay of the signal within the cell. This long guard period ensures that the access burst does not collide with bursts in adjacent time slots, even if it comes from a user at the very edge of the cell's coverage area.

5. The Dummy Burst

This burst is transmitted by the BTS on a traffic channel when there is no user data to send. Its structure is similar to a Normal Burst but with mixed data bits. Its primary purpose is to fill an empty time slot with a valid signal. This allows phones in the vicinity to perform signal strength measurements on the channel, which is essential for making handover decisions.