Towards Performance Measurement - Part 02 | Computer Networks | Prof. Neminath Hubbali, IITI

Circuit Switching and Packet Switching

Circuit switching reserves network resources before transmission, while packet switching forwards packets according to their destination addresses.

Circuit Switching

Before data is sent, the network establishes a path from the sender to the receiver. This is the circuit-establishment phase. The network reserves resources, such as bandwidth, on every link along that path.

The sender’s data then travels over the established circuit. Each transmission requires its own circuit setup. Because capacity is reserved, it remains unavailable to other users even when the sender has nothing to transmit.

Multiplexing in Circuit-Switched Networks

• Frequency Division Multiplexing (FDM): The shared link’s capacity is divided into frequency bands, with a portion assigned to each transmission. Multiple transmissions can use the link at the same time.
• Time Division Multiplexing (TDM): Users take turns using the full link capacity in assigned time slots. A user’s reserved slots cannot be used by others, even if that user has no data to send.
• To guarantee service for (n) users, a shared outgoing link must have enough capacity for the sum of their individual transmission rates. For example, three clients with 10 kbps links require a 30 kbps shared link to be served without contention.

Circuit-Switching Transmission-Time Example

A file of 640,000 bits is sent over a 1.536 Mbps link divided into 24 TDM slots:

• Rate per slot: (1.536\text{ Mbps} \div 24 = 64\text{ kbps})
• File transmission time: (640{,}000 \div 64{,}000 = 10) seconds
• Including the 0.5-second circuit setup time: 10.5 seconds total

Packet Switching

Packet switching avoids establishing a dedicated end-to-end circuit before transmission:

1. The sender divides its data into packets and attaches a destination address, such as an IP address, to each packet.
2. At each router, the router checks the packet’s destination address and consults its forwarding table.
3. The table identifies the outgoing interface for that destination, and the router forwards the packet accordingly.

Packets can pass through a sequence of routers without a dedicated circuit or its associated setup delay.

Sharing Capacity and Supporting More Users

Packet switching can use a shared link more efficiently when users transmit intermittently. With circuit switching, reserved bandwidth limits the number of users the network can guarantee service to, even if they often have nothing to send. With packet switching, users share the link as needed. This can allow more users to be supported, although simultaneous demand can cause congestion and packet loss.

For an example with a 100 Mbps link and users requiring 20 Mbps when active:

• Circuit switching supports (100 \div 20 = 5) users if each user’s full 20 Mbps is reserved. The subtitles later say four users, despite using these figures; this appears to be an arithmetic or transcription inconsistency.
• In the packet-switching example, 30 users each transmit only 10% of the time. The network may support more users by sharing capacity, though occasional packet loss is possible.
• Assuming users transmit independently, the probability that one specified user is transmitting while the other 29 are idle is: [ 0.1 \times (0.9)^{29} ]

Main Takeaway

Circuit switching provides reserved capacity along a pre-established path, but that capacity can sit idle. Packet switching forwards addressed packets hop by hop and shares link capacity among users, improving utilization when traffic is intermittent at the cost of possible congestion or packet loss.

Speakers or Sources Featured

• Professor Neminath Hubbali — lecturer and sole speaking source in the subtitles.