SWITCHING - Circuit, Packet, Message Switching

Switched communication networks are those in which data transferred from source to destination is routed between various intermediate nodes. Switching is the technique by which nodes control or switch data to transmit it between specific points on a network.

In large networks, there can be multiple paths from sender to receiver. The switching technique will decide the best route for data transmission. Switching technique is used to connect the systems for making one-to-one communication.

Various switching techniques are-

1. Circuit Switching

The Circuit Switching technique establishes a dedicated path or channel between the sender and receiver for data transmission, and once a dedicated path is established then it does not terminate it until and unless the connection between the two data transmission point terminates.

Circuit switching in a network operates in a similar way as the telephone works. A complete end-to-end path must exist before the communication takes place.

In case of circuit switching technique, when any user wants to send the data, a request signal is sent to the receiver then the receiver sends back the acknowledgment to ensure the availability of the dedicated path. After receiving the acknowledgment, dedicated path transfers the data.

Circuit switching is used in public telephone network. It is used for voice transmission. Fixed data can be transferred at a time in circuit switching technology.

Communication through circuit switching has 3 phases:

• Circuit establishment
• Data transfer
• Circuit Disconnect

Advantages of Circuit Switching:

• Establishment of a dedicated channel
• Improves data transmission rate
• Improves data loss
• Improves delay in the data flow

Disadvantages of Circuit Switching:

• Establishing a dedicated channel sometimes takes a very long duration of time.
• The amount of bandwidth required is more for establishing a dedicated channel.
• Even if a channel is free, it cannot be used to transmit any other data from any other source.

2. Packet Switching

The packet switching technique transmits data through the network by breaking it down into several data packets for more efficient transfer and it also utilizes multiple vacant resources, these network devices direct or route the data packets to the destination where the receiving device then collects all of them and reassembles to get the proper orientation of the message.

The packet switching is a switching technique in which the message is sent in one go, but it is divided into smaller pieces, and they are sent individually.

The message splits into smaller pieces known as packets and packets are given a unique number to identify their order at the receiving end. Every packet contains some information in its headers such as source address, destination address and sequence number.

Packets will travel across the network, taking the shortest path as possible. All the packets are reassembled at the receiving end in correct order.

If any packet is missing or corrupted, then the message will be sent by receiver to resend the message.

If the correct order of the packets is reached, then the acknowledgement message will be sent by the receiver.

There are two approaches to Packet Switching:

Datagram Packet switching:

• It is a packet switching technology in which packet is known as a datagram, is considered as an independent entity. Each packet contains the information about the destination and switch uses this information to forward the packet to the correct destination.
• The packets are reassembled at the receiving end in correct order.
• In Datagram Packet Switching technique, the path is not fixed.
• Intermediate nodes take the routing decisions to forward the packets.
• Datagram Packet Switching is also known as connectionless switching.

Virtual Circuit Switching

• Virtual Circuit Switching is also known as connection-oriented switching.
• In the case of Virtual circuit switching, a preplanned route is established before the messages are sent.
• Call request and call accept packets are used to establish the connection between sender and receiver.
• In this method the path is fixed for the duration of a logical connection.

Advantages of Packet Switching:

• Cost-effective: In packet switching technique, switching devices do not require massive secondary storage to store the packets, so cost is minimized to some extent. Therefore, we can say that the packet switching technique is a cost-effective technique.
• Reliable: If any node is busy, then the packets can be rerouted. This ensures that the Packet Switching technique provides reliable communication.
• Efficient: Packet Switching is an efficient technique. It does not require any established path prior to the transmission, and many users can use the same communication channel simultaneously, hence makes use of available bandwidth very efficiently.

Disadvantages of Packet Switching:

• Packet Switching technique cannot be implemented in those applications that require low delay and high-quality services.
• The protocols used in a packet switching technique are very complex and requires high implementation cost.
• If the network is overloaded or corrupted, then it requires retransmission of lost packets. It also lead to the loss of critical information if errors are nor recovered.

3. Message Switching

The Message Switching Technique was developed to act as an alternative to circuit switching, this was before packet switching was introduced.

Message Switching is a switching technique in which a message is transferred as a complete unit and routed through intermediate nodes at which it is stored and forwarded.

In Message Switching technique, there is no establishment of a dedicated path between the sender and receiver.

The destination address is appended to the message. Message Switching provides a dynamic routing as the message is routed through the intermediate nodes based on the information available in the message.

Message switches are programmed in such a way so that they can provide the most efficient routes. Each and every node stores the entire message and then forwards it to the next node. This type of network is known as store and forward network.

Message switching treats each message as an independent entity. These message switched data networks are also known as a hop-by-hop system.

Advantages of Message Switching

• Data channels are shared among the communicating devices that improve the efficiency of using available bandwidth.
• Traffic congestion can be reduced because the message is temporarily stored in the nodes.
• Message priority can be used to manage the network.
• The size of the message which is sent over the network can be varied. Therefore, it supports the data of unlimited size.

Disadvantages of Message Switching

• The message switches must be equipped with sufficient storage to enable them to store the messages until the message is forwarded.
• The Long delay can occur due to the storing and forwarding facility provided by the message switching technique.

Token Ring, Token Bus

Token Ring

Token ring (IEEE 802.5) is a communication protocol in a local area network (LAN) where all stations are connected in a ring topology and pass one or more tokens for channel acquisition. A token is a special frame of 3 bytes that circulates along the ring of stations. A station can send data frames only if it holds a token. The tokens are released on successful receipt of the data frame.

Token Passing Mechanism in Token Ring

If a station has a frame to transmit when it receives a token, it sends the frame and then passes the token to the next station; otherwise it simply passes the token to the next station. Passing the token means receiving the token from the preceding station and transmitting to the successor station. The data flow is unidirectional in the direction of the token passing. In order that tokens are not circulated infinitely, they are removed from the network once their purpose is completed. 

This is shown in the following diagram −

Token Bus

Token Bus (IEEE 802.4) is a standard for implementing token ring over virtual ring in LANs. The physical media has a bus or a tree topology and uses coaxial cables. A virtual ring is created with the nodes/stations and the token is passed from one node to the next in a sequence along this virtual ring. Each node knows the address of its preceding station and its succeeding station. A station can only transmit data when it has the token. The working principle of token bus is similar to Token Ring.

Token Passing Mechanism in Token Bus

A token is a small message that circulates among the stations of a computer network providing permission to the stations for transmission. If a station has data to transmit when it receives a token, it sends the data and then passes the token to the next station; otherwise, it simply passes the token to the next station. 

This is depicted in the following diagram −

Differences between Token Ring and Token Bus

Token Ring	Token Bus
The token is passed over the physical ring formed by the stations and the coaxial cable network.	The token is passed along the virtual ring of stations connected to a LAN.
The stations are connected by ring topology, or sometimes star topology.	The underlying topology that connects the stations is either bus or tree topology.
It is defined by IEEE 802.5 standard.	It is defined by IEEE 802.4 standard.
The maximum time for a token to reach a station can be calculated here.	It is not feasible to calculate the time for token transfer.