Internet Communication

what is circuit switching?

• Creates a direct link between two devices for the duration of the communication 

• When a caller dials a number, various switches in telephone exchanges set up a path between the caller and recipient 

• allows two people to hold a call without any delay in the delivery of speech 

• When circuit switching used in computers, bandwidth is wasted when no data is being sent 

• Circuit switched networks can only connect devices that operate at the same transfer rate 

• as it is an exclusive connection between the devices, data packets arrive in the same order that they were sent, so the message can be easily reconstructed 

• Switches are used to connect and disconnect the circuits, so electrical interference is produced 

• Therefore may produce corrupt or lost data 

what is packet switching?

• Communicating packets of data across a network on which other communications are happening simultaneously  

• Website data arrives as a series of packets 

• Packets are sent from sender to recipient along entirely separate routes from each other 

• Each packet takes the fastest route depending on which route is fastest/least congested at the specific time that the packet leaves the sender's computer 

• Packets are reassembled upon arrival and any missing packets can be requested 

what’s in a packet?

• most packets are between 500 and 1500 bytes 

• can be fixed or variable size 

• Contain header (containing destination and  source IP addresses, protocols and packet number in sequence)  payload containing body of data being sent 

• Time to live= number in header indicating when the data packet expires and is discarded (like after how many hops) 

• Can also have a trailer section with a checksum or cyclical redundancy check to detect transmission errors by using hash totals  

• if the before and after hash totals are different, the packet is refused with suspected data corruption and a new copy is requested 

who or what are routers?

• Connect at least two networks together (eg LAN and WAN, or a LAN and its ISP's network) 

• Traversing between one router and another = a hop 

• Router reads recipients IP address in each packet and forwards it to the recipient in the fastest and least congested way 

• They use routing tables to store and update the locations of other network devices 

• The decision process can be complicated so the routing algorithm can become a bottleneck 

• Dijkstras algorithm is used 

• When a router is connected to the Internet, its IP address must be registered with the Internet Registry as this IP address must be unique over the whole internet 

why are gateways?

• When the protocols differ between networks, a gateway is used instead of a router 

• Header data is stripped leaving only payload 

• New header data is added in format of new network and gateway sends packet on its way 

what are mac addresses?

• Each NIC has a unique "media access control" address which is assigned and hard-coded into the card by the manufacturer which uniquely identifies the device 

• 48 bits long, 12 hex digits 

what is pop3?

retrieves emails from a mail server that temporarily stores incoming mail 

what is a socket?

IP address+port number = socket, which specifies which device and application the packet should be sent to 

who adds mac addresses and port numbers to packets?

at each new router, the link layer adds mac addresses

the transport layer adds port numbers