Multiplexing of Signals

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When many nodes compete to access the network, some efficient techniques for utilizing the data link are very essential. The capacity or bandwidth of the network channel can be divided or allotted in different ways and the process of making the most effective use of the available channel capacity is called Multiplexing.

There are three basic multiplexing schemes,

• Frequency-division multiplexing (FDM): It is most popular and is used extensively in radio and TV transmission. Here the frequency spectrum is divided into several logical channels, giving each user exclusive possession of a particular frequency band.

• Time-division Multiplexing (TDM): It is also called synchronous TDM, which is commonly used for multiplexing digitized voice stream. The users take turns using the entire channel for short burst of time.

• Statistical TDM: This is also called asynchronous TDM, which simply improves on the efficiency of synchronous TDM.

The figure below depicts the multiplexing functions in general.

The multiplexer which has n separate inputs is connected to the de-multiplexer by a single data link. The multiplexer combines (multiplexes) data from these 'n' input lines and transmits them through the high capacity data link, which is being de-multiplexed at the other end, according to the channel and is delivered to the appropriate output lines.

Frequency-Division Multiplexing (FDM)

In frequency division multiplexing, the available bandwidth of a single physical medium is subdivided into several independent frequency channels. Independent message signals are translated into different frequency bands using modulation techniques, which are combined by a linear summing circuit in the multiplexer, to a composite signal. The resulting signal is then transmitted along the single channel by electromagnetic means.

To prevent the inter-channel cross-talk, some guard band is required between each successive channel, which is unused portion of the frequency spectrum.

At the receiving end the signal is applied to a bank of band-pass filters, which select out individual frequency channels. The band-pass filter outputs are then demodulated and distributed to different output channels.

FDM are commonly used in radio broadcasts and TV networks. Since, the frequency band used for voice transmission in a telephone network is 4000 Hz, for a particular cable of 48 KHz bandwidth, in the 60 to 108 KHz range, twelve separate 4 kHz sub-channels could be used for transmitting twelve different messages simultaneously. Each radio and TV station, in a certain broadcast area is allotted a specific broadcast frequency, so that independent channels can be sent simultaneously. For example, the AM radio uses 540 to 1600 KHz frequency bands while the FM radio uses 88 to 108 MHz frequency bands.

Time-Division Multiplexing (TDM)

In frequency division multiplexing, all signals operate at the same time with different frequencies, but in Time-division multiplexing all signals operate with same frequency at different times. This is a base-band transmission system, where an electronic commutator sequentially samples all data source and combines them to form a composite base-band signal, which travels through the media and is being de-multiplexed into appropriate independent message signals by the corresponding commutator at the receiving end.

As shown in the figure below, the composite signals have some dead space between the successive sampled pulses which is essential to prevent inter-channel cross talks. Along with the sampled pulses, one synchronizing pulse is sent in each cycle. The data pulses, along with the control information forms a frame. Each of these frames contain a cycle of time slots and in each frame, one or more slots are dedicated to each data source.

Synchronous TDM is called synchronous mainly because each time slot is pre-assigned to a fixed source. The time slots are transmitted irrespective of whether the sources have any data to send or not. Hence, for the sake of simplicity of implementation, channel capacity is wasted. Although fixed assignment is used, TDM devices can handle sources of different data rates. This is done by assigning less slots per cycle to the slower input devices than the faster devices.

Statistical Time-division Multiplexing

One drawback of synchronous TDM, is that many of the time slots in the frame are wasted. It is because, if a particular terminal has no data to transmit at particular instant of time, an empty time slot will be transmitted. An efficient alternative to this synchronous TDM is statistical TDM, also known as asynchronous TDM or Intelligent TDM. It dynamically allocates the time slots on demand to separate input channels, thus saving the channel capacity. As with Synchronous TDM, statistical multiplexers also have many I/O lines with a buffer associated to each of them. During the input, the multiplexer scans the input buffers, collecting data until the frame is filled and send the frame. At the receiving end, the de-multiplexer receives the frame and distributes the data to the appropriate buffers.

In case of statistical TDM, the data in each slot must have an address part, which identifies the source of data.

Switching and Broadcast techniques

The techniques of Communication between number of different devices on a network can be classified into two broad categories.

• Switched network methodology
• Broadcast Methodology

In the switched network methodologies, the network consists of a set of interconnected nodes, among which information is transmitted from source to destination in different routes which is controlled by the switching mechanism.

The three principal types of Switched networks are

1. Circuit Switching
2. Message Switching
3. Packet Switching

In case of broadcast network, each node is a transmitter-receiver pair and is attached to the transmission medium, which is shared by all nodes. A transmission by one station is received by all the nodes of the network.

Circuit Switching

Circuit switching is a very commonly used technique in telephony, where the caller sends a special message with the address of the callee (i.e. by dialing a number) to state its destination. A direct communication line is then established and the caller gets a confirmation (e.g. hearing a "hello" from the other side), he starts conversation. The line remains dedicated to the message exchange until one station initiates disconnection.

Thus the actual physical electrical path or circuit between the source and destination host must be established before the message is transmitted. This connection, once established, remains exclusive and continuous for the total duration of information exchange and the circuit becomes disconnected only when the source wants to do so.

However, for information transmission applications, the circuit switching method is very slow, relatively expensive and inefficient. First of all, the need to establish a dedicated connection before sending the message itself inserts a delay time, which might become significant for the total message transfer time. Moreover, the total channel remains idle and unavailable to the other users once a connection is made. On the other hand once a connection is established, guaranteed and orderly delivery of message is ensured.

Message Switching

In this switching method, a different strategy is used, where instead of establishing a dedicated physical line between the sender and the receiver, the message is sent to the nearest directly connected switching node. This node stores the message, checks for errors, selects the best available route and forwards the message to the next intermediate node. The line becomes free again for other messages, while the process is being continued in some other nodes. Due to the mode of action, this method is also known as store-and-forward technology where the message hops from node to node to its final destination. Each node stores the full message, checks for errors and forwards it.

In this switching technique, more devices can share the network bandwidth, as compared with circuit switching techniques. Temporary storage of message reduces traffic congestion to some extent. Higher priority can be given to urgent messages, so that the low priority messages are delayed while the urgent ones are forwarded faster. Through broadcast addresses one message can be sent to several users. Last of all, since the destination host need not be present when the message is sent, message switching techniques improve global communications.

However, since the message blocks may be quite large in size, considerable amount of storage space is required at each node to buffer the messages.

Packet Switching

Packet switching is the most commonly used technique for data communication in Local Area Networks. It works in the same principal of message switching techniques, but in this case the messages are divided into subsets of equal length called packets. The packets are being generated at the source host and are encapsulated in a bit frame, i.e. necessary headers and trailers are added to each packet which contains information like -

• Source host address.
• Destination address.
• Serial numbers
• Frame Check Sequence

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