Lecture 17

DATA COMMUNICATIONS & NETWORKING

LECTURE-17

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

LECTURE OVERVIEW

Blocking Time-Division Switching TSI switch TDM Bus Advantages & disadvantages of Space

Division Switching Advantages & disadvantages of Time Division

Switching Combination of Space Division and Time

Division Switching

BLOCKING

Blocking refers to times when one input cannot be connected to an output because there is no available path between them.

All the possible intermediate switches are occupied.

MULTIPLE PATHS

WHY BLOCKING OCCURS???

The reduction in the number of cross points results in blocking during periods of heavy traffic.

In a single-stage switch blocking does not occur.

Because every combination of input and output has its own cross point, there is always a path.

Cases where two inputs are trying to contact the same output do not count.

TIME DIVISION SWITCHING

Time-division switching uses time-division multiplexing to achieve switching.

Methods used in Time Division Switching

Time-slot interchange TDM bus

TIME SLOT INTERCHANGE A TSI consists of Random Access

Memory(RAM) with several memory locations.

The size of each location is the same as the size of a single time slot.

The number of locations is the same as the number of inputs.

In most cases the number of inputs are equal to the number of outputs.

The RAM fills up from the incoming data from time slots in the order received.

Slots are then sent out in an order based on the decisions of a control unit.

TIME SLOT INTERCHANGE

TIME-DIVISION MULTIPLEXING, WITHOUT AND WITH A TIME-SLOT INTERCHANGE

TDM BUS

The input and output lines are connected to a high speed bus through input and output gates(micro switches).

Each input gates is closed during one of the four time slots.

During the same time slot,only one output gate is also closed .

TDM BUS CONT…

This pair of gates allows a burst of data to be transferred from one specific input line to one specific output line using the bus.

The control unit opens and closes the gates according to switching needed.

A folded TDM bus can be made with duplex lines (input and output) and dual gates.

TDM BUS

ADVANTAGES & DISADVANTAGES OF SPACE DIVISION SWITCHING

The advantage of space-division switching is that it is instantaneous.

Its disadvantage is the number of cross points required to make space-division switching acceptable in terms of blocking.

ADVANTAGES & DISADVANTAGES OF

TIME DIVISION SWITCHING

The advantage of Time-division switching is that it needs no crosspoints.

Its disadvantage in the case of TSI,is that processing each connection creates delays.

Each time slot must be stored in the RAM,then retreived and passed on.

SPACE-AND TIME-DIVISION SWITCH COMBINATIONS

Combining the two results in switches that are optimized both physically(the number of cross points) and temporally(the amount of delay).

Multistage switches of this sort can be designed as time-space-time(TST),time-space-space-time(TSST),space-time-time-space(STTS) or other possible combinations.

TST SWITCH

HAVE A NICE DAY!!!