Data Communications & Networking

Lecture-14

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

Lecture overviewTypes of TDMSynchronous TDM/InterleavingAsynchronous TDM/Statistical TDMBit stuffing or Padding

Synchronous TDM/Interleaving

TDM can be visualized as two fast rotating switches, one on the multiplexing side and the other on the demultiplexing side.

The switches are synchronized and rotate at the same speed.

On the multiplexing side, as the switch opens in front of a connection, that connection has the opportunity to send a unit on to the path .

This process is called interleaving. On the demultiplexing side, as the switch opens in front of

a connection that connection has the opportunity to receive a unit from the path.

Synchronous TDM/Interleaving

Synchronization in TDMThe implementation of TDM is not as easy as

that of FDM.

If the multiplexer and the demultiplexer are out of synchronization, a bit belonging to one channel may be received by the wrong channel.

For this reason one or more synchronization bits are usually added to the beginning of each frame.

These bits are called framing bits.

Synchronization in TDM cont…These bits follow a pattern from frame to

frame, that allows the demultiplexer to synchronize with the incoming stream so that it can separate the time slots accurately.

In most cases this synchronization information consists of 1 bit per frame, alternating between 0 and 1.

Framing bits

Example Example 88We have four sources, each creating 250 characters per second.

If the interleaved unit is a character and 1 synchronizing bit is added to each frame,

find

(1)the data rate of each source,

(2) the duration of each character in each source,

(3) the frame rate,

(4) the duration of each frame,

(5) the number of bits in each frame, and

(6) the data rate of the link.

Solution

We can answer the questions as follows:

1. The data rate of each source is 250x8=2000 bps = 2Kbps.

2. The duration of a character is 1/250 s, or 4 ms.3. The link needs to send 250 frames per second.4. The duration of each frame is 1/250 s, or 4 ms. 5. Each frame is 4 x 8 + 1 = 33 bits.6. The data rate of the link is 250 x 33, or 8250

bps.

Different data rates in TDMIt is possible to multiplex data from devices of

different data rates.

If device B is two times faster than other devices than each device could use one slot while device B will use two slots.

The number of slots in each frame and the input lines to which they are assigned remain fixed throughout a given system but devices of different data rates may control different numbers of those slots.

What should we do for this technique to work?Each time slot length is fixed.

The requirement is that the different data rates must be integer multiples of each other.

For example we can accommodate a device five times faster than the others by allocating it five slots.

Bit stuffing/Padding/Pulse stuffingHowever we cannot accommodate a device

that is 5 n half times faster because we cannot introduce one half of a time slot to a frame.

When the speeds are not integer multiples of each other, they can be made to behave as if they were by a technique called bit stuffing/bit padding/pulse stuffing.

In bit padding,the multiplexer adds extra bits to a device’s source stream to force the speed relationships among the various devices in to integer multiples of each other.

Limitations of synchronous TDM /interleaving

Synchronous TDM does not guarantee that the capacity of the link is fully used.

It is more likely that only a portion of the time slots is in use at a given instant.

Because the time slots are preassigned and fixed whenever the connected device is not transmitting, the corresponding slot is empty and that much of the path is wasted.

Asynchronous TDM/ Statistical TDMAsynchronous TDM/ Statistical TDM is designed to

avoid the waste in synchronous TDM.The term asynchronous means flexible not fixed

here.In asynchronous system if we have n input lines,the

frame contains a fixed number of atleast n time slots.

In asynchronous system,if we have n lines, the frame contains no more than m slots with m less than n.

In this way asynchronous TDM supports the same number of lines as synchronous with a lower capicity link.

Or given the same link asynchronous TDM can support more devices than synchronous TDM.

Asynchronous TDM/ Statistical TDMThe number of slots m in asynchronous

TDM frame is based on a statistical analysis of the number of input lines that are likely to be transmitting at any given time.

Rather than being preassigned each slot is available to any of the attached input devices that has data to send.

The multiplexer scans the input lines accepts portions of data until a frame is filled.

Then it sends the frame across the link.

Asynchronous TDM/ Statistical TDMIf there are not enough data to fill all the

slots in a frame,the frame is only partially filled.

Thus full link capacity may not be used 100 percent of the time.

But the ability to allocate time slots dynamically,coupled with lower ratio of time slots to input lines greatly reduces the likely hood and degree of waste.

Synchronous and asynchronous TDM

Digital Signal(DS) ServiceTelephone companies implement TDM

through a hierarchy of digital signals called digital signal(DS) service.

DS Hierarchy

T LinesDS-0,DS-1 and so on are the names of services.To implement those services, the telephone

companies use T lines(T1-T4).These are lines with capacities precisely

attached to the data rates of the DS-1 to DS-4 services.

Service LineRate

(Mbps)Voice Channels

DS-1DS-1 T-1T-1 1.5441.544 2424

DS-2DS-2 T-2T-2 6.3126.312 9696

DS-3DS-3 T-3T-3 44.73644.736 672672

DS-4DS-4 T-4T-4 274.176274.176 40324032

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