Lecture 5

Terminal Handling & Polling

Terminal Handling

• A Terminal controller is a device that collects traffic from a set of terminals and directs them to a concentrator.

Terminal Handling• For many applications the cost of communication lines exceeds the cost of the

equipment connected by those lines. In an attempt to reduce communication costs, many networks provide a way for multiple terminals to share a single communication line. The conceptual model is that of fig, in which a terminal controller accepts input from a cluster of terminals, and funnels the output onto one line, as well as the reverse operation. In fig.(a), all the terminals are wired onto the same multidrop line, whereas in fig, each terminal has its own point-to-point line to the controller.

• Terminal controller can be divided into two general classes, multiplexers and concentrators. A multiplexer is a device that accepts input from a collection of lines in some static, predetermined sequence, and outputs the data onto a single output line in the same sequence. As each output time slot is dedicated to a specific input line, there is no need to transmit the input line numbers. The output line must have the same capacity as the sum of the input line capacities.

In TDM when a terminal has no traffic, an output time slot is wasted. The output slots are filled in strict rotation. If there are no data, dummy characters are used. It is not possible to skip a time slot, because the receiving end keeps track of which character came from which terminal by its position in the output stream. Initially, the multiplexer and the computer synchronize themselves. Both know that the order to be used. The data themselves carry no identification of their origin. If the multiplexers skipped a time slot when there were no data from a terminal, the receiver would get out of phase and interpret the origin of succeeding characters incorrectly.If each terminal has traffic only a small fraction of the time, TDM makes inefficient use of the output line capacity.

When the actual traffic is far below the potential traffic, most of the time slots on the output line are wasted. Consequently, it is often possible to use an output line with less capacity than the sum of the input lines. This arrangement is called concentration.

The general approach is to only transmit actual data and not dummy characters. However, this strategy introduces the problem of telling the receiver which character came from which input line. One solution to this problem is to send two output characters for each input character – the terminal number and the data.

Concentrators using this principle are often referred to as statistical multiplexers or ATDMS (Asynchronous Time Division Multiplexers), in contrast with the true (synchronous) multiplexers, or STDMS, although strictly speaking, a statistical multiplexer that had as much output capacity as input capacity would not be a concentrator.

Concentrator• In the evolution of modern telecommunications systems there was a

requirement to connect large numbers of low-speed access devices with large telephone company 'central office' switches over common paths. During the first generations of digital networks, analog signals were digitized on line cards attached to the central office switches. In an effort to reduce costs, it was decided to push this conversion closer to the customer premises by deploying small conversion devices in customer neighbourhoods. These devices would combine multiple digital signals on a single link to a larger telephone switch, which would provide service to the customer. These devices were initially called remote concentrators or simply remotes.

• In fibre-optic distribution systems which offer triple-play services (voice, television, internet) the digitization has arrived at the customer premises and signals are digitized at the source and combined using customer edge routers. This traffic enters the distribution network at an Optical Network Termination and is carried to the central office using Wavelength Division Multiplexing and Passive Optical Networking.

Concentrator • In telecommunication, the term concentrator has the

following meanings:• In data transmission, a functional unit that permits a

common path to handle more data sources than there are channels currently available within the path. A concentrator usually provides communication capability between many low-speed, usually asynchronous channels and one or more high-speed, usually synchronous channels. Usually different speeds, codes, and protocols can be accommodated on the low-speed side. The low-speed channels usually operate in contention and require buffering.

A device that connects a number of links with only one destination, the main function of this device is to make a kind of load balancing between two or more servers connected together, data distribution is done according to the server processing rate.A patch panel or other component in the cable plant where cable runs converge.ISP used concentrators to enable modem dialling; this kind of concentrator is sometimes called a modem concentrator or a remote access concentrator. The term "access concentrator" is also used to describe similar provider edge equipment used in computer networks that doesn't rely on modems anymore, e.g.

Polling • Polling works with topology.

• One device is designed as primary section and other devices are secondary station.

• Link control is done by primary device.

• All data exchange take place through primary device.

• Primary device decides, which device is allowed to use the channel at a given time.

• If primary device wants to receive data, it asks the secondaries if they have anything to send, this function is called polling.

• Select mode and poll mode are two functions of polling.

• In polling, primary device receive the data.

• In select mode, primary device sends data to secondary device. This figure shows the select method.

• Link is available if primary device is not sending or receiving any data.

• Before sending data, the primary creates and transmits a select (SEL) frame.

• SEL frame includes address of the intended secondary device. This figure shows the poll method.