In Time-Division Multiplexing (TDM) and its variants like Time-Division Multiple Access (TDMA), a frame is a fundamental, repeating structure used to organize data transmission.
Understanding the TDM Frame
Based on the reference provided, a frame in TDM is defined as:
a cyclically repeated data block that consists of a fixed number of time slots, one for each logical TDM channel or TDMA transmitter.
Let's break down this definition to understand what a frame represents:
- Cyclically Repeated Data Block: This means the frame structure is not a one-time event but occurs continuously and sequentially. Data from multiple sources is packaged into these blocks, and these blocks are transmitted repeatedly in cycles.
- Fixed Number of Time Slots: Each frame is divided into a specific, unchanging number of subdivisions called time slots. The duration of the frame and the duration of each time slot are fixed.
- One for Each Logical TDM Channel or TDMA Transmitter: This is a crucial point. Within a single frame, each time slot is specifically allocated to a particular logical communication channel (in TDM) or a specific transmitting device (in TDMA). This allocation is fixed for each cycle of the frame.
Essentially, a TDM frame provides a scheduled "turn" within a repeating cycle for each user or channel to transmit or receive data over a shared medium.
Key Characteristics of a TDM Frame
- Fixed Structure: The number and sequence of time slots within a frame are predetermined and do not change.
- Repeating Cycle: Frames are transmitted one after another in a continuous loop.
- Resource Allocation: Each time slot is dedicated to a specific channel or user for the duration of that slot within the frame cycle.
- Synchronization: The transmitter and receiver must be synchronized to recognize the start and end of each frame and each time slot within the frame.
Illustrative Example
Consider a TDM system multiplexing data from three users (User A, User B, User C). A frame in this system would contain three time slots.
| Frame Cycle | Slot 1 | Slot 2 | Slot 3 |
|---|---|---|---|
| Frame 1 | Data from A | Data from B | Data from C |
| Frame 2 | Data from A | Data from B | Data from C |
| Frame 3 | Data from A | Data from B | Data from C |
| ... | ... | ... | ... |
In this simplified example, User A gets the first slot in every frame, User B gets the second, and User C gets the third. This structured, repeating pattern ensures that each user gets regular access to the shared transmission medium.
The frame structure is critical for the proper operation of TDM systems, ensuring that data streams from different sources are kept separate and can be demultiplexed correctly at the receiving end.
