Time Division Multiplexing (TDM) methods

Conventional TDM systems usually employ either Bit-Interleaved or Byte-Interleaved multiplexing schemes.

Bit-Interleaved Multiplexing

In Bit-Interleaved TDM, a single data bit from an I/O port is output to the aggregate channel. This is followed by a data bit from another I/O port (channel), and so on, and so on, with the process repeating itself.A "time slice" is reserved on the aggregate channel for each individual I/O port. Since these "time slices" for each I/O port are known to both the transmitter and receiver, the only requirement is for the transmitter and receiver to be in-step; that is to say, being at the right place (I/O port) at the right time. This is accomplished through the use of a synchronization channel between the two multiplexers. The synchronization channel transports a fixed pattern that the receiver uses to acquire synchronization.

Total I/O bandwidth (expressed in Bits Per Second - BPS) cannot exceed that of the aggregate (minus the bandwidth requirements for the synchronization channel).

Bit-Interleaved TDM is simple and efficient and requires little or no buffering of I/O data. A single data bit from each I/O channel is sampled, then interleaved and output in a high speed data stream. But the main disadvantage of a bit interleaved transmission is that if we want to extract a lower order data from the stream, then we need to do the entire de-multiplexing process at the receiver. This is overcome by the Byte Interleaved mechanism explained later.

All the T-Carrier and E-Carrier multiplexers are using Bit Interleaved Multiplexing. Show below is the table containing the T-carrier and E-carrier hierarchies.

Byte-Interleaved Multiplexing

In Byte-Interleaved multiplexing, complete words (bytes) from the I/O channels are placed sequentially, one after another, onto the high speed aggregate channel. Again, a synchronization channel is used to synchronize the multiplexers at each end of the communications facility. Here individual user data can be picked by intermediate add-drop multiplexers without the demultiplexing process by using specific pointers as in SDH or SONET multiplexing.

Examples for the byte Interleaved multiplexing includes SDH (Synchronous Digital Hierarchy), SONET (Synchronous Optical Network) etc.