Types of Switching:
- Circuit switching was designed for voice communication.
- In a telephone conversation e.g., Once a circuit is established, it remains connected for the duration of the session.
- It creates a direct physical connection between two devices such as phones or computers.
- In Circuit Switching a dedicated link is established across the sender and the receiver which is maintained for the entire duration of the conversation.
- In circuit switching the routing, the decision is made when the path is set up across the given network.
- After the link has been set in between the sender and the receiver then the information is forwarded continuously over the provided link.
- Communication via circuit switching involves three phases: Circuit Establishment, Data Transfer, and Circuit Disconnect.
- Connection path must be established before data transmission begins. Nodes must have switching capacity and channel capacity to establish the connection
- Space Division Switching: Separates the path in the circuit from each other spatially.
- Time Division Switching: Uses time-division multiplexing to achieve switching.
- Guaranteed bandwidth
- A reliable communication channel between hosts
- Low per-packet overhead: No IP (and TCP/UDP) header on each packet
Disadvantages of Circuit Switching
- Less suited to data and other non-voice transmissions.
- A circuit switched link creates the equivalent of a single cable between two devices and thereby assumes a single data rate for both devices. This assumption limits the flexibility and usefulness of a circuit switched connection.
- Once a circuit has been established, that circuit is the path taken by all parts of the transmission, whether or not it remains the most efficient or available.
- Circuit switching sees all transmissions as equal. Any request is granted to whatever link is available. But often with data transmission, we want to be able to prioritise.
- To overcome the disadvantages of circuit switch. Packet switching concept came into the picture.
- In a packet switched network, data are transmitted in discrete units of potentially variable length blocks called packets.
- Each packet contains not only data but also a header with control information (such as priority codes and source and destination address).
- A packet contains three major fields: The header, the message, and redundancy check bits.
- The packets are sent over the network node to node.
- At each node, the packet is stored briefly, then routed according to the information in its header.
- There are two popular approaches to packet switching.
- Virtual circuit
- Each packet is treated independently from all others.
- It is also called connection-less packet switching.
- Each packet treated independently.
- Packets can take any practical route in the network.
- Packets may arrive out of order. Packets may go missing.
- Routers in the internet are packet switches that operate in datagram mode.
Virtual Circuit Approach:
- Sender divides the data into packets
- Store-and-forward transmission: Multiple packets may arrive at once
- A single route is chosen between sender and receiver at the beginning of the session.
- Network can provide sequencing and error control.
- When the data are sent, all packets of the transmission travel one after another along that route.
- We can implement it into two formats:
- Switched Virtual Circuit (SVC)
- Permanent Virtual Circuit (PVC)
Switched Virtual Circuit:
- This SVC format is comparable conceptually to dial-up lines in circuit switching.
- Here, a virtual circuit is created whenever it is needed and exists only for the duration of the specific exchange.
- The PVC format is comparable to leased lines in circuit switching.
- In this method, the same virtual circuit is provided between two users on a continuous basis.
- The circuit is dedicated to specific users. No one else can use it and because it is always in place, it can be used without connection establishment and connection termination.
- It is also known as store and forward.
- In this mechanism, a node receives a message, stores it, until the appropriate route is free, and then send it along.
- Store and forward is considered a switching technique because there is no direct link between the sender and receiver of a transmission.
- A message is delivered to the node along one path, then rerouted along with another to its destination.
- In message switching, the messages are stored and relayed from secondary storage (disk), while in packet switching the packets are stored and forwarded from primary storage (RAM).
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