Computer Networks

Computer Networks · Study · FE Electrical and Computer · FE → PE Prep

OSI layer order (top to bottom)

\text{App} \to \text{Pres} \to \text{Sess} \to \text{Trans} \to \text{Net} \to \text{Link} \to \text{Phys}

The seven OSI layers, layer 7 down to layer 1. Mnemonic: All People Seem To Need Data Processing.

TCP/IP layer order

\text{Application} \to \text{Transport} \to \text{Internet} \to \text{Network Interface}

The four TCP/IP layers, top to bottom; the practical model used on the internet.

OSI to TCP/IP mapping

\{7,6,5\}\!\to\!\text{App},\;4\!\to\!\text{Trans},\;3\!\to\!\text{Internet},\;\{2,1\}\!\to\!\text{Net.\ Interface}

PDU by layer

\text{message} \to \text{segment} \to \text{packet} \to \text{frame} \to \text{bits}

Protocol Data Unit at Application, Transport, Network, Data Link, and Physical respectively. Packet = datagram.

Encapsulation rule

\text{PDU}_{n} = \text{Header}_{n} \,\|\, \text{PDU}_{n+1}

Device-to-layer map

\text{hub}\!=\!L1,\quad \text{switch/bridge}\!=\!L2,\quad \text{router}\!=\!L3

Physical, Data Link, and Network layers respectively; the layer a device reads to make forwarding decisions.

Full-mesh link count (subarea B)

L = \dfrac{n(n-1)}{2}

Topology metric (FE subarea B, Network topologies): number of direct links in a full mesh of n nodes; quantifies the cost of full redundancy.

TCP vs UDP

\text{TCP: connection-oriented, reliable};\;\; \text{UDP: connectionless, best-effort}

TCP detects loss/duplication/reordering and corrects; UDP has less overhead but no delivery guarantee.

A. Routing and switching