Switching techniques

Switching Techniques for Data Transmission

Presented by:

Amit Kumar(100101ECR019)

Sl. No- 41

Differences between Voice and Data Traffic

Telephone networks are basically designed to carry voice traffic & there are some significant differences in the nature of voice & data traffic. Voice traffic is generally continuous, whereas data traffic is bursty in nature.

Voice traffic Data traffic

Continuous Bursty

Low bandwidth for long duration

High bandwidth for short duration

Typical line utilisation 85-95%

Typical line utilisation 5-15%

Half duplex Half or full duplex

Real time Non real time or near real time

Loss acceptable Loss unacceptable

Error tolerable Error unacceptable

Switching Techniques for Data Transmission

The recognition of the diverse characteristics of voice and data traffic has led to the development of a switching technique other than the one used for voice transmission.

This technique is better suited for transmitting data traffic.

Two switching techniques are prevalent for data transmission :

1. Circuit Switching

2. Store & forward (S&F) switching

Circuit Switching

In circuit switching, an electrical path is established between the source and destination before any data transfer takes place.

The electrical path may be realised by physical wires or coaxial cables or radio or satellite links.

It remains dedicated to the communicating pair for the entire duration of the transmission irrespective of whether data is actually transferred or not.

The connection is released only when specifically signalled so by either of the communicating entities.

It is entirely analogous to the telephonic switching.

Circuit Switched Network

There are three phases involved in circuit switched data transfer:

1. Connection establishment

2. Data transmission

3. Connection release

The time taken to data transfer in a circuit switched connection is given by

= + +

Where

= time for path setup or connection establishment

= time for data transmission

= time for path release

Now,

= (N-1)

Where

N = number of switching nodes in the path

= average route selection time in each node

= M/R

Where

M = message length in bits

R = data rates in bps

= N

Where

= time taken by a node to make housekeeping entries

Therefore, time taken to data transfer in a circuit switched connection is given by

= (N-1) + M/R + N

If is the one way propagation time

then,

= (N-1) + M/R + N + 4

Circuit Switching Applications

Public Switch Telephone Network (PSTN)

Private Branch Exchanges (PBX)

Private Wide Area Networks (often used to interconnect PBXs in a single organization)

Data Switch

Advantages and Disadvantages of Circuit Switching

Advantages: • The communication channel (once established) is dedicated.

• Developed for voice traffic (public telephone network) but can also

applied to data traffic.

Disadvantages: • Channel capacity is dedicated for the whole duration of a connection, If

no data, capacity is wasted• Long initial delay: circuit establishment takes time

• More expensive than any other switching techniques, because a dedicated path is required for each connection.

• Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.

Store and Forward Switching

In store and forward switching, the switching nodes have the ability to store user messages and forward the same towards the destination when the links available.

Each node is equipped with a processor and some buffer storage.

No end-to-end link is set up prior to data transmission.

It is analogous to the postal or telegraph system.

Store and Forward Switching

S & F switching is of two types:

1. Message switching

2. Packet switching

Message Switching

In message switching, a message is transmitted in its entirety without a break from one node to another.

The node processor receives the full user message and store the same.

Check the message for data transmission errors and perform error recovery if required.

Determine the destination address from the user message.

Choose an appropriate link based on routing algorithm.

Forward the message to the next node on the chosen link.

Disadvantages of Message Switching

For long message there is a chance of overflow.

If an error occurs during transmission, the entire message may not be retransmitted.

If a high priority short message arrives while a long message is in transmission, it will have to wait until the transmission of the long message ends.

Packet Switching

In packet switching, messages are split into a number of packets, and they are transmitted in S & F fashion.

Messages are split at the source host and reassembled at the destination host.

Each packet transmission is independent of the others.

Each packet needs to carry the complete information such as destination id, source id, message id, and packet id.

Packet Format

Ex: indicates that this is the second packet of the third message originating from the host 4 and destined to the host 6.

Packet Switching

Advantages and Disadvantages of Packet Switching

Advantages:

• Packet switching is cost effective, because switching devices do not need massive amount of secondary storage.

• Packet switching offers improved delay characteristics, because there are no long messages in the queue (maximum packet size is fixed).

• Packet can be rerouted if there is any problem, such as, busy or disabled links.

Disadvantages:

• Protocols for packet switching are typically more complex.

• It can add some initial costs in implementation.

• If packet is lost, sender needs to retransmit the data.

• Packets of same message may arrive out of sequence at the destination node.

Conclusion

Thus, we can conclude that switching technique can be used for transmission of data traffic. Though there is a dedicated path in circuit switching due to long initial delay we are going for S & F switching instead of circuit switching. In S & F switching instead of using message switching we are using packet switching because of improved delay characteristics and less buffer storage.

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