Computer Networks Overview

Importance of Computer Networks (4)

• Resource sharing

• Social impact

• easy access to knowledge and education

• economic impact

OSI Model Layers (7)

Application (APDU)
Presentation (PPDU)
Session (SPDU)
Transport (Segment)
Network (Packet)
Data Link (Frame)
Physical (Bit)

Define Computer Network

A collection of computers / nodes connected by communication links (wired / wireless)

Links can be point-to-point or broadcast

Define Broadcast

shared link where unique address of individual nodes is used in message headers, more scalable

Bus topology

Reliability issue as if bus breaks whole network goes down

Star topology

point-to-point link but more flexible as easier to add nodes

reliability issue as if central hub breaks whole network goes down

Ring topology

goes all the way around the ring

Tree topology

bus network of star networks

don’t need to go to the main bus if you want to send a message within a star network

Full mesh topology

Every node is connected

Partial mesh topology

nodes are connected to those they communicate with the most

Personal Area Network

Interprocessor Distance - 1m

Processors located in same square meter

local area network (interprocessor distance / processors located in same)

10m / room

100m / building

1km / campus

metropolitan area network (interprocessor distance / processors located in same)

10km / campus

wide area network (interprocessor distance / processors located in same)

100km / country

1000km / continent

the internet (interprocessor distance / processors located in same)

10,000km / planet

Define Pervasive Computing

Integration of computer capabilities and communication technologies into everyday objects and environments, making them interconnected and responsive to human needs without requiring conscious interaction

Mobile Ad-hoc NETwork (MANET)

node dynamically self-organizes without using any pre-established infrastructure

network of nodes where each node communicates wirelessly and acts as both a host and a router

topology not fixed - another router can be added to the network if another fails and easily take over

Radio Frequency Identification (RFID) network

Tags (stickers with not even a battery when passive) are placed on objects

Readers send signals that the tags reflect to communicate

Protocol / Service Hierarchies

Networks are organised as a series of layers, each has a set of protocols that offers services to the upper layers, shielding them from the details of how services are implemented

Define circuit switch

dedicated physical path established between sender and receiver for the entire duration of the exchange (exclusive resources)

Define packet switch

virtual circuit - behaves like a circuit switch but operates on a packet-switch network (pre-determined path) less robust if node fails then entire circuit breaks

Datagram - each packet is treated completely independently containing full destination address so can take different routes / be re-routed dynamically

OSI Model layer traversal

Source host - moves down the layers Application → Physical

Destination host - moves up the layers Physical → Application

Routers / Switches / Hubs

Routers 3 / Switches 2 / Hubs 1; the more layers a device operates on, the more information it can use to make forwarding decisions (smarter and more efficient)

Type of data: x (unit exchanged)

P-3300

P-33

IP-A

IP-B

method of transmission

Source port number: P-3300 (segment)

Destination port number: P-33 (segment) distinguishes between Host B’s two offered services

Source IP Address: IP-A (packet)

Destination IP Address: IP-B (packet)

Source MAC address: changes depending place in chain of nodes as transmission is between adjacent nodes (frame)

frame translated into bits → signal transmitted between routers until destination host

Define socket

a combination of a port-number and IP address - uniquely identifies a certain application on a certain host in the Internet

Kilo storage vs networks

storage 2^10 = 1024

networks 10^3 = 1000

TCP/IP protocols

narrow waist architecture

ubiquitous (everywhere at the same time) by keeping the IP layer everyone has to agree to minimal

which layers of the OSI model are host-to-host layers?

Transport, Session, Presentation, Application

Error control is the responsibility of what layers?

Data link and Transport

IP Packet routing step 1

check if destination is on local network (compare own IP address and subnet mask to destination address)
local → packet sent directly to destination device using layer 2 protocols
remote → packet sent to default gateway

IP Packet Routing step 2 (Router - default gateway)

default gateway almost always local router

examines destination IP address and decides which connected network is the best path to send this packet towards its final destination

IP packet routing step 3 (Routing table)

if the router doesn’t know the complete path to the destination it consults the routing table

list of rules that state ‘to get to network X, send the packet out interface Y or to next-hop router Z’

IP packet routing step 4 (Hop-by-Hop journey)

process in step 3 repeats at every router along the path until packet reaches a router on the same local network as destination node

at each hop the data link header (MAC address) is stripped and re-created for the next immediate link

Network Access / Host-to-Network layer (= OSI physical + data link)

Catch all layer that represents the underlying network infrastructure that an IP packet uses to travel from one device to the next

Model designed to be independent of the underlying hardware

Internet layer (=OSI network layer)

packet-switching network based on a connectionless layer

official packet format and protocol (IP); unreliable (no acknowledgement message)

packet routing is major issue here

Transport Layer (= OSI transport layer)

designed to allow peer entities on the source and destination hosts to exchange data

TCP (transmissionless control protocol) is a reliable connnection-oriented protocol, used for reliable application

UDP (User datagram protocol) is an unreliable connectionless protocol, used for applications which require prompt delivery

Application layer (=OSI application, session, presentation)

contains all higher-level protocols

prompt delivery → quickly (low-latency), in sequence, consistently (without large variations in delay, called jitter)