Bit Stuffing

It is possible to connect devices of different data rates to synchronous TDM
For Example, device A uses one time slot, while the faster device B uses two slots
The Time slot length is FIXED
Therefore data rates should be integer multiples of each other
For example, we can accommodate a device that is 5 times faster than the other device by giving it five slots to one for each of the other devices
We cannot accommodate a device which is five and a half times faster using this method because we cannot introduce half a time slot into a frame
When the speeds are not integer multiples of each other, they can be made to behave as if they were
This is done by a technique known as BIT STUFFING
In bit stuffing, MUX adds extra bits to a device
For Example, if we have one device with a bit rate of 2.75 times that of other devices, we can add enough bits to raise this rate to 3 times that of others
The extra bits are then discarded by the Demultiplexer

Asynchronous TDM

Synchronous TDM does not guarantee full utilization of the timeslots
Because the time slots are fixed and pre assigned, whenever a connected device is not transmitting, the corresponding slot is empty and much of the channel capacity is wasted
For Example, imagine that we have multiplexed the o/p of 20 identical computers onto a single line
Using synchronous TDM, the speed of that line must be at least 20 times the speed of each i/p line
But what if only 10 computers are in use at a time?
Half of the capacity of the line is wasted
Asynchronous TDM or Statistical TDM is designed to avoid this type of waste
Asynchronous means flexible or Not fixed
In an asynchronous system, if we have ‘n’ input lines, the frame contains no more than ‘m’ slots, where m is less than n

In this way asynchronous TDM supports the same number of I/p lines as synchronous TDM with a lower capacity link
A slot is available to any device that wants to send data
MUX scans I/p lines, accepts data until a frame is filled and then sends the frame across the link

Two major advantages:
- Ability to allocate time slots dynamically
- Lower ration of time slots to I/p lines

Above two factors greatly reduce the likelihood of a waste
Advantages of Asynchronous TDM

Fig. shows a system with 5 I/p lines sharing a link using Asynchronous TDM
Frame size is 3 slots per frame
Fig shows how MUX handles 3 levels of traffic
In the first case, only 3 of the 5 computers have data to send
In the second case, 4 lines are sending data
In the third case, all devices are sending data
In each case, MUX scans the devices in order from 1 to 5 filling time slots as it encounters data to be sent

In the first case, the 3 active i/p lines correspond to the 3 slots in each frame
For the first 4 frames, the I/p is symmetrically distributed among all the devices.
By the 5 frame however, devices 3 and 5 have completed their transmission but device 1 still has two characters to go

The MUX picks up the A from device 1, scans down the line without finding another transmission and returns to device 1 to pick up the last A
There being no data to fill the final slot, the MUX then fills the 5th frame with only 2 slots filled
Compare with Synchronous TX: 6 frames of 5 slots each would be required=30 slots, 14 slots used only
In second case, there is one more I/p line than there are slots in each frame
This time MUX scans from 1 to 5 and fills up a frame before each of the lines are checked
The first frame contains data from device 1, 3,and 4

MUX continues the scan and puts first portion of 5th device into the first slot of next frame and so on
When the number of active senders does not equal the number of slots in a frame, the time slots are not filled symmetrically
Device 1 occupies the first slot in the first frame , 2 slot in second frame and so on
In the third case, frames are filled as shown above
All 5 I/p lines are active
In this case device 1 occupies the 1 slot in the first frame, the 3rd slot in the second frame and so on

Case 2 & 3 above show a major weakness of Asynchronous TDM
How does the DEMUX know which slot belongs to which output line?
As opposed to Synchronous TDM, in this case, data from a given device might be in the first slot of one frame and in the third of the next
Therefore, each time slot must carry an address telling the DEMUX how to direct data
This address is for local use only attached by the MUX and detached by the DEMUX
In the figure above address is specified by a digit
Adding address bits to each time slot increase the overhead of an Asynchronous system and limits its efficiency
Addresses usually consist of only a small number of bits
Need for Addressing makes Asynchronous TDM inefficient for bit or byte interleaving
Imagine bit interleaving with each bit carrying an address
One bit of data plus 3 bits of address
Asynchronous TDM is efficient only when the size of the time slot is kept relatively large

Opposite of Multiplexing
Takes data from one high speed line and breaks it into portions that can be sent over several lower speed lines simultaneously

An organization wants to send data, voice and video each of which requires a different data rate
To send voice it needs 64Kbps,
To send data, it needs 128 Kbps link
To send video it may need 1.544 Mbps link
It can lease a 1.544 Mbps line from a common carrier and only use it fully for sometime
Or it can lease several separate channels of lower data rates
Voice can be sent over any of these channels
Data & Video can be broken into smaller portions using Inverse Multiplexing and TX

Multiplexing has long been used as an essential tool in the Telephone industry
A country’s telephone system may include various carriers that offer local and long-distance service
These various carriers form a Telephone Network I.e. PTCL

Each subscriber is connected to the telephone network as a service line

Section 8.4,8.5 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan