An asynchronous TDM is also called as a Statistical TDM. ATDM has been designed to solve the problem of wastage of the multiplexed channels in synchronous TDM.
In ATDM, if there are n input lines, then each slot can contain m frames, where m is less than n, as shown in the figure below.
In ATDM, the multiplexer scans all the input lines and accepts the portions of data till the frame is filled. When the frame is filled, it is sent across the link. If the senders don't have enough data to fill the frame, it is transmitted as partially filled. This is depicted in the figure below.
Hence, in ATDM, all sending devices can send data in all slots. As we can look from the figure, the slots are variable. Therefore, the receiver cannot identify the characters by counting them.
An efficient addressing mechanism is required, which should supply a unique address to each time slot. These addresses are temporary and discarded by demultiplexer after reading them correctly.
The variation between Asynchronous TDM and Synchronous TDM is that few slots in Synchronous TDM are unused, but in Asynchronous TDM, the slots are entirely used. This leads to the shorter transmission time and practical uses of the volume of the medium.
It firmly assigns the time slots on demand to free the input channels. Hence the channel size can be stored. Similarly, as with synchronous TDM, statistical multiplexers can have many input/output (I/O) lines with a buffer related to each of them.
During the input, the multiplexer filters the input buffers and accumulates the record until the frame is filled. Later on the frame is transmitted. At the receiving end, the demultiplexer obtains the frame and circulates the data to the suitable buffer.
In Asynchronous TDM, each slot's data should have an address part, which recognizes the source of information. Therefore, the data arrives from and is circulated to I/O lines unpredictably. The address data is needed to guarantee suitable delivery. This generates extra overhead per slot.
One disadvantage of the TDM method is that many of the time slots in the frame are invalid. A specific terminal has no data to transmit instantly and will send an empty time slot.
Adding the address bits to each time slot puts overhead on the system and decreases the system's efficiency.