Communication and Internet Technologies

//Circuit vs packet switching

Circuit switching reserves a complete path for the whole conversation — guaranteed bandwidth and no reordering, but the line is wasted during silence and setup takes time. Packet switching sends packets independently by any route — efficient and resilient, but packets may arrive out of order or late, so they must be reassembled.

//Why protocols are layered

Each layer only has to talk to the layer above and below it. That means one layer can be changed — for example swapping Wi-Fi for Ethernet — without rewriting the others. It also makes the whole system easier to understand, to build and to fix.

TCP/IP layers (top to bottom)

Application  HTTP, FTP, SMTP — what the user's program speaks
Transport    TCP/UDP — splits into packets, checks arrival
Internet     IP — addressing and routing
Link         the physical network and hardware

//The mail protocols

SMTP SENDS mail. POP3 downloads mail and normally deletes it from the server, so it lives on one device. IMAP keeps mail ON the server and synchronises, so the same mailbox looks the same on your phone and laptop.

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