What is Switching:
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the speech information.
The switch is referred to as a "gate" when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer i.e. a computer is a system of electronic switches which function as logical gates.
Before switching is a process of making a connection between end users. When a caller or the source want to call or communicate with the destination this involves switching of various trunks and systems before finally the communication between the source and the destination is made. So in short Switching is the vital component of a telephone exchange that is used to connect the the source and destination.
The main job of the exchange is to connect the two subscribers together. The unit responsible for this is known as a Switching system. Switching is organizing the time slots so that subscribers can communicate with each other. So switch is a system for handling the time slots.
Introduction to Digital Switching:
Digital exchanges establish the connections by rearranging the 8-bit PCMcode words of various telephone signals as dictated by the call request. Digital switch architecture is made up of two elements called T and S for time division switching (T) and space division switching (S) and can be made up of sequences of T and S. For example the AT&T NO 4 ESS is aTSSSST switch, No 3 EAX is an SSTSS and the classical Northern Telecom DMS 100 is TSTS folded. Many of these switches are still available. One thing in common with these switches is that they had multiple space (S) stage. This has now changed. Many of the new switches or enhanced versions of the switches just mentioned have very large capacities and are simply TST or STS switches.
Advantages of Digital Switching:
There are both economic and technical advantages of digital switching;
The economic advantages are discussed below.
There are notably fewer equivalent cross points for a given number of lines and trunks than in a space division switch.
"" A Digital switch is considerably smaller "" It has more common circuitry "" It is easier to achieve full availability with in economic constraints. "" The technical advantage include "" It is regenerative (it does not distort the signal, the output is cleaner then the input signal) "" It is noise resistant. "" It is computer based and thus incorporates all the advantages controlling, managing the hardware. "" The binary message format is compatible with digital computers. It is also compatible with signaling. "" A digital exchange is loss less. There is no insertion loss as a result of a switch inserted in a network.
Building Blocks of a Digital Switch:
The building blocks of a digital switch include:
- Time Switch
- Space Switch
In a most simplified way a time switch changes the time slot of the incoming traffic. As we know that a time slot in conventional PCMcontains 8 bits and that a basic frame is 125 microsecond in duration. Time slot interchanging involves moving the data contained in each time sot form the incoming bit stream to an outgoing bit stream but with different time slot arrangement in accordance with the destination of each time slot. What is done of course is to generate a new frame for transmission at the appropriate switch outlet.
Obviously to accomplish this at least one time slot must be stored in memory (write) and then called out of memory in a changed position (read). The operations must be controlled in some manner, and some of these control actions must be kept in memory together with the software managing such actions. Typical control functions are time slot "idle" or "busy".
Fundamental Blocks of a Time Switch:
Now we can identify three basic fundamental blocks of a time switch.
- 1. Memory for Speech (Buffer)
- 2. Memory for Control (Control Buffer)
- 3. Time Slot counter or processor
The time switch consists of a buffer memory also called time slot interchange memory for storing the time slot information that is speech to be switched.
Control memory has as many locations as there are time slots, contains the list i.e. the order of the cells from which unloading is to take place. It also contains the information where to switch a particular call.
There is a processor in the time switch handling and coordinating various operations of a time switch.
Implementation of a Time Switch:
For implementation of a Time Switch there are two choices:
- 1. Sequential write, random read
- 2. Random write, sequential read
Sequential write, random read: In sequential write and random read case the time slots are written into speech memory as they appear in the incoming bit stream. Then based on the discussion of the control memory the speech signals are read form speech memory and put on the output time slots. Random write, sequential read:
In this case the incoming time slots are written into the memory in order of appearance in the outgoing bit stream. This mean the incoming time slot are written into memory in the desired output order. The writing of incoming time slots into a speech memory can be controlled by a simple time slot counter and can be sequential.
Unlike the time switch the space switch operates without interchanging the time slots. The space switch can switch any 8-bit PCM word from the incoming highways to any outgoing highways without the change of time slot.
A space switch consists of a cross point matrix made up of logic gates (AND gate) that allow the switching of time slots in the spatial domain. The switch into pattern determined by the required network connectivity organizes these PCM time slot bit streams. The matrix consists of a number of input horizontals and a number of verticals with a logical gate at each cross point.
The array consists of a number of M inputs horizontals and N output verticals. For a given time slot the appropriate logic gate is enabled and the time slot passes from the input horizontal desired vertical output. The other horizontals, each serving a different serial stream of time slots, can have the same time slot switched into vertical enabling their gates. In the next time slot position a completely different path configuration could occur, again allowing time slots from the horizontals to be switched to the selected verticals. The selection of the course is a function of how the traffic is to be routed at that moment for calls in progress or being set up.
The exact sequence in which the AND gates are turned on and off is indicated in the control memory. The control addresses in the control memory designate the AND gate to be turned on for each time slot.
Difference between a Time and a Space Switch
When connections are made via a time switch the PCM words change their time slots between the input and the output.
When connections are made via space switch the PCM words change their highways between the input and the output, but retain their original time slot.