14.2 Circuit switching, packet switching

##### Circuit switching

- a dedicated circuit is established before transmission starts //circuit is released after transmission ends
- data is transferred using the whole bandwidth
- all data is transferred over the same route
##### State, with a reason, where it would be appropriate to use circuit switching
- Circuit switching is used where a dedicated path needs to be sustained throughout the call / communication // where the whole bandwidth is required // where a real time communication is used. e.g. ==video conferencing // live streaming of a concert==
- A typical application is standard voice communications / video streaming / private data networks
##### Advantage
- Whole of bandwidth is available
- Dedicated communication channel increases the quality of transmission
- Data is transmitted with a fixed data rate
- No waiting time at switches
- Suitable for long continuous communication
- Fast method of data transfer
- Data /frames arrive in order and do not need to be reassembled
- Data can’t get lost
- Data all follows the same path / route
- Better for real-time
- Simple method of data transfer.

##### Disadvantage
- A dedicated connection makes it impossible to transmit other data even if the channel is free
- Not very flexible
- No alternative route in case of failure
- The time required to establish the physical link between the two stations can be too long
- The need to establish a dedicated path for each connection can have cost implications
- Dedicated channels require the whole bandwidth / bandwidth can’t be shared

##### packet switching
- A large message is divided up into a group of smaller chunks of the same size called packets
- The packet has a header and a payload
- The header contains a source IP address, destination IP address (and sequence number)
- Each packet is dispatched independently and may travel along different routes / paths
- The packets may arrive out of order and are reassembled into the original message at the destination
- If packets are missing / corrupted a re-transmission request is sent.

- data is split into packets
- each packet is given its own route
- the routing for a packet depends on the congestion
- packets may not arrive in the order sent

- Data are broken into equal sized packets
- Data packets have headers containing information such as the IP addresses of the sender and receiver
- Each packet of data is sent independently to the destination // Packets don’t necessarily follow the same route
- Each packet is sent via the most optimum path available
- Packets don’t necessarily arrive in the order they were sent // Packets are reconstructed at the destination in the correct order
- Missing / damaged packets are re-sent
##### where it would be appropriate to use packet switching
- Packet switching is most commonly used on data networks such as the internet to send large data files that don’t need to be live streamed
- Packet switching is used when it is necessary to be able to overcome failed/faulty lines by rerouting.
- Packet switching is used when it is necessary for the communication to be more secure.
- Packet switching is used for high volume data transmission.
- Packet switching is used when it isn’t necessary to use all the bandwidth.
- Specific examples e.g. ==email, text messages, documents, VOIP== etc.

##### the function of a router in packet switching

- The router reads the IP address of the destination from the packet header.
- A router uses a routing table to find information about e.g., available hops / netmask / gateway used / adjacent routers / the status of the routers along the route.
- The router determines the next hop / optimum route. // The router sends the packet on its next hop.
- The router manages the hop counter // The hop counter is reduced by 1 every time the packet passes a router.
##### benefits
- Accuracy – Ensures accurate delivery of the message
- Completeness – Missing packets can be easily detected and a re-send request sent so the message arrives complete
- Resilience – if a network changes the router can detect this and send the data another way to ensure it arrives
- Path also available to other users // Doesn’t use whole bandwidth //
- allows simultaneous use of channel by multiple users
- Better security as packets hashed and sent by different routes.
##### drawbacks
- Time delays to correct errors // Network problems may introduce errors in packets
- Requires complex protocols for delivery
- Unsuitable for real time transmission applications

##### differences between circuit switching and packet switching

- Circuit switching uses a dedicated channel to make communication, whereas packet switching forms data into packets to transmit over a digital network.
- The dedicated path for circuit switching must be established before the transfer of data can commence, which is not the case with packet switching (as it doesn’t require a dedicated path).
- Data in packet switching is split into packets, in circuit switching the message remains intact. 
- All of the transmission in circuit switching follows the same path whereas different packets in packet switching can take different routes.
- The message is received in the same order in which it is sent with circuit switching, but with packet switching, the packets can be received out of order (for assembly at the destination).
- Circuit switching is implemented at the physical layer while packet switching is implemented at the network layer.
- Circuit switching uses the whole bandwidth of the channel used, packet switching can share bandwidth.
- Circuit switching communication ends with an error but packet switching allows packets to be re-sent.
- Circuit switching is a simpler process than packet switching.