Packet switching is a method of data transmission where data is broken down into small units called packets. Each packet contains a header with addressing information (source and destination) and the data itself. These packets are then routed independently through the network, potentially taking different paths to reach the destination.

The key steps involved are:

• Segmentation: The sending device divides the data into packets.
• Packet Formation: Each packet is encapsulated with a header containing source and destination addresses, sequence numbers, and error detection codes.
• Routing: Packets are routed independently through the network based on their destination addresses. Routers examine the destination address in the packet header and forward the packet to the next hop.
• Reassembly: At the destination, the packets are reassembled in the correct order based on the sequence numbers in the headers.

Advantages of Packet Switching:

• Efficient Use of Bandwidth: Bandwidth is only used when packets are being transmitted.
• Robustness: If one path is congested or unavailable, packets can be rerouted.
• Flexibility: Supports various data types and variable data sizes.

Disadvantages of Packet Switching:

• Variable Delay: Packets may experience different delays due to varying network conditions.
• Overhead: The packet headers add overhead to the data transmission.
• Complexity: Requires more complex network infrastructure than circuit switching.

Comparison with Circuit Switching: Circuit switching establishes a dedicated, end-to-end connection before data transmission begins. Packet switching does not require a dedicated connection; packets are routed independently. Circuit switching guarantees bandwidth, while packet switching does not. Circuit switching has a fixed delay, while packet switching has variable delay. Packet switching is generally more efficient for bursty data traffic.

Call routing is the process by which a switching centre determines the optimal path for a call to reach its destination within a network of interconnected switching centres. The switching centre uses information contained in the dialed number (the destination number) to make this decision.

The routing process typically involves consulting a routing table. This table contains information about the network topology – the connections between switching centres and the associated costs (e.g., delay, congestion). The switching centre examines the destination number and uses the routing table to identify the sequence of switching centres that will lead to the destination.

Factors that might influence the routing decision include:

• Network Congestion: The switching centre may choose a path that avoids congested links.
• Link Capacity: The capacity of the links between switching centres will be considered.
• Delay: The switching centre may choose a path that minimizes the overall delay.
• Cost: In some networks, the cost of using certain links may be factored into the routing decision.
• Network Availability: The switching centre will avoid paths that are currently unavailable due to maintenance or failures.

Sophisticated routing algorithms are used to dynamically adapt to changes in the network topology and to ensure that calls are routed efficiently and reliably.

Switching centres (also known as exchanges) are crucial components of a circuit-switched network. They act as intermediate points that connect different circuits together. When a call is established, the switching centre identifies the destination and allocates a dedicated circuit between the caller and the callee. This circuit remains reserved for the duration of the call.

The call establishment process typically involves the following steps:

1. Dialing: The caller dials the destination number.
2. Signaling: The telephone system sends a signal (e.g., using DTMF tones) to the calling switching centre, indicating the destination number.
3. Circuit Allocation: The calling switching centre checks if a circuit is available to the destination. If available, it allocates a dedicated circuit.
4. Connection Establishment: The calling switching centre sends a connection request signal to the destination switching centre.
5. Call Connection: The destination switching centre confirms the connection and establishes the dedicated circuit between the caller and the callee.

Call termination involves the following:

1. Call Termination Signal: Either the caller or the callee initiates the termination by hanging up the phone or pressing a designated button.
2. Circuit Release: The switching centre sends a circuit release signal to the other switching centre, indicating that the circuit can be released.
3. Resource Release: The allocated circuit is released, and the resources are made available for other calls.

Key signals involved include: dialing tones (DTMF), connection request signals, and circuit release signals.