Network Basics

Note on 15 Jul 2002 

Why Networks?

• Communication
• Sharing of Data
• Providing remotes services
• Sharing of Resources
• Failsafe
• Social Reason

A brief history of networks - the birth of the internet

• 1980's ATM matures
• 1982 - TCP/IP developed - the internet as we know it is born
• 1984 - DNS introduced - users no longer have to remember obscure IP addresses
• 1984 - Number of internet hosts reaches 1000
• 1984 - JANet introduced in the UK
• 1986 - NFSNET created - massive explosion of networks within US universities

Glossary

• Guilded Transmission - signal is sent down physical medium
• Unguilded Transmission - no physical medium
• POTS - Plain Old/Ordinary Telephone System
• Bandwidth - The amount of information that can be in transit at any one time
• Latency - The amount of time needed to send/receive information

A brief history of networks - prehistory

• Earliest established networks - runners
• A medical refinement - pigeon post
• Bell Telephone
• Marconi - Wireless Telegraph
• 1968 - ARPANET - 1st man WAN
• 1970's - X25 ( & associated standards) developed
• 1971 - email and basic FTP developed
• 1973 - UCL joins ARPANET making the first international network
• 1974 - Telnet developed

A brief history of networks - the rise of web & e-Commerce

• 1990 - The world, the first commercial ISP goes online
• 1991 - CERN releases the WWW, developed by Tim Berners-Lee
• 1993 - Mosaic brings graphes to the web - web increase at the rase of 341,634%
• 1995 - modems and NICs become standard on new PCs
• 1996 - Gigabit either & ATM technology takes off
• 1997 - Increased interest in cable modems
• 1997 - NorWeb demonstrates WAN through national grid
• 1997 - Modems exceed theoretical limit of 38k
• 1998 - 2 way satellite communication become commercial reality
• 1999 - xDSL standard ratified

A brief history of networks - the human element

• 2000 - Human rights issues
• 2000 - XML
• 2000 - .com bubble burst
• 2000 - increase interest in WAP, WML, etc
• 2001 - Broadband becoming commonplace in the home
• 2001 -Web services
• 2001 - September 11th
• 2002 - Increasing privacy issues
• 2002 - ICANN controversy increases, ICANN incharge all naming in internet and allow who to assign name in internet

Main types of network

InterProcessor	Processors located in same	Example
0.1M	Board	SMP
1m	System	SAN (Beowulf Cluster)
10m	Room	LAN
100m	Building	LAN
1km	Campus	LAN
10km	City	MAN
100km	Country	WAN
1,000km	Continent	WAN
10,000km	Planet	Internet
1 light hour	Solar System	Inter-Internet

SAN

• System Area Network (Not storage area network)
• Specialisation of LAN used for cluster computing
• Contains system level traffic only
• Often uses
• Channel bonded ether
• Hierarchical switching

Network hardware

• Generally 2 types
• Broadcast
• Common communication channel
• Messages sent to all machines
• Messages have address information
• Point-to-Point
• Many communication channels
• Maybe multiple routes to a given destination

LAN

• Local Area Network
• Restricted size - therefore known characteristics
• Low latency
• Medium speed (10-100Mb/s)
• Standard topologies
• Line/bus or hubbed (ethernet / IEEE802.3)
• Ring (token ring)

MAN

• Metropolitan Area Network
• High speed interconnects between (geographically) related LANs
• Uses DQDB (Distributed Queue Dual Bus) Standard/IEEE80206
• DQDB consists 2 unidirectional buses to which all the computers are connected

WAN

• Wide Area Network
• Collection of routers connected by high speed lines
• Topology is
• irregular
• redundant
• Router
• communication bridge between LAN and WAN
• Routers (normally)
• receive entire packet
• wait till suitable WAN connection is free
• send packet to WAN

The need for protocols

• Allows heterogenous platform to interact
• Allows flexibly in changing system
• Shields processes from what they don't need to know about
• Provides nice abstraction for building communications software

Interface & Services

• Each layer offers certain communication features/opportunities known as Services
• Services are provided by Service Provider (layer n) to the Service User (layer n+1) using the services of the next layer down (layer n-1)
• These services are described in terms of interfaces to Service Access Points

Protocol Stacks

                                    Host 1                                            Host 2

                                    Layer4  <--- Layer 4 protocol-->   Layer4

layer 3/4 interface          ^                                                        ^

                                       v                                                        v

                                    Layer3  <--- Layer 3 protocol-->   Layer3

layer 2/3 interface          ^                                                        ^

                                       v                                                        v

                                    Layer2  <--- Layer 2 protocol-->   Layer2

layer 1/2 interface          ^                                                        ^

                                       v                                                        v

                                    Layer1  <--- Layer 1 protocol-->   Layer1

                                       ^                                                        ^

                                       v                                                        v

                                   Physical Medium                             Physical Medium

Protocol Design Decisions

• Efficiency
• Identification of
• Hosts (sender & receiver) > mechanism for identifying sender and receiver some form of addressing needed in-order to specifying destination
• Message
• Error correction & control > both must agree on which one is being used. Receiving end must have something to telly the sender which receive & which not
• Message
• ordering > number the packets/pieces
• Size > mechanism to dis-assembling, transmitting and re-assembling
• Traffic control > multiplexing in physical layers for all connections has to be sent over at most a few physical circuits.

Interface & Services

• Interface Data Unit (IDU) = Interface Control Info (ICI) + Service Data Unit (SDU)
• SDU is information passed across network to peer entity
• ICI = Needed for lower layer to do job (no of  byte in SDU)
• SDU = Header + SDU (Protocol Data Unit n-PDU)
• At N layer, SDU fragment into smaller pieces known of (PDU) > packet PDU header = control info, provide sequence no and counts, 

Connection and connectionless

• Connection
• Both parties must be available at same time (synchronous)
• Data is ordered
• Connectionless
• Parties can attend to message when they want (asynchronous)
• Data may be unordered

OSI (Open Systems Interconnection) Reference Model

Application <> Presentation <> Session <> Transport <> Network <> Data Link <> Physical

        ^                        ^                        ^                ^                ^                    ^                    ^

        v                        v                        v                v                v                    v                    v

Application <> Presentation <> Session <> Transport <> Network <> Data Link <> Physical

  (APDU)            (PPDU)            (SPDU)        (TPDU)      Packet            Frame               Bit

• Open systems interconnection reference model (aka 7 layers model) is an ISO standard for worldwide communication Networks that defines a networking framework for implementing protocols in seven layers.
• Layering the communications process means breaking down the communication process into Smaller and Easier to handle interdependent categories. The convention and rules used in such communications are collectively known as Layer protocol. 
• Designed to provide a standard for different platforms to communicate
• Good theory - in practie a bit unwiedly
• Model widely used - protocols aren't

Network & Transport Layer

• Network Layer
• Concerned with subnet control - routing & avoiding congestion
• Often not implemented
• Transport Layer
• Mainly concerned with efficiently providing information to network layer
• Handles setting up, and deletion of connections through the network layer.
• Also handles naming of connections (eg. ports)

Physical & Data Link Layers

• Physical Layer
• Concerned with transfer of raw bits
• Defines what a bit is and how long it lasts
• Data Link Layer
• Concerned with giving transmission that is free from undetected errors
• Also deals with flow regulation
• Must know what to do about damaged, lost & duplicate frames

Session & Presentation layer

• Session layer
• Provides added value services on top of transport layer, in particular
• Dialogue control
• Token management
• Synchronisation
• Presentation layer
• Responsible for providing certain commonly used functions (eg. data conversion)
• Rarely used

Application Layer

• Contains commonly used high level protocols eg.
• SMTP > Simple Mail Tranport Protocol
• NNTP > Network News Transfer Protocol
• HTTP > Hyper text Transfer Protocol
• FTP > File Transfer Protocol
• Telnet
• Define an abstract network virtual terminal that editors and programs can be written to deal with
• A piece of s/w written to map the function of network virtual terminal onto the real terminal eg. when cursor move to up left corner of screen, this software must issue proper sequence command to real terminal to get its cursor there too.

TCP/IP Reference Model

• Application (Appliaction Layer) <> Transport (Transport Layer) <> Internet (Network Layer) <> Host to Network (Physical & Data Link Layer)
• Application Layer, eg TELNET, FTP, SMTP, DNS
• Transport Layer, eg. TCP, UDP
• Network Layer, eg. IP
• Physical & Data Link Layer, eg. ARPANET, NSFNET, Packet radio, LAN 

Transport & Application Layer

• Transport Layer
• Provides 2 protocols
• TCP (Transmission Control Protocol)
• Main protocol
• Provides reliable, connection-oriented protocol
• UDP (Universal Datagram Protocol)
• Provides unrealible, connectionless protocol
• Used where TCP is not needed/inappropriate
• Application Layer
• Effectively the same as OSI

TCP/IP reference model

• Protocol stack for the internet
• Not a good reference model, but has good protocols
• Very widely implemented

Host-to-network & Internet Layer

• Host-to-network - not defined. Throw packets into network & have them travel independently to the destination
• Internet Protocol (IP)
• Provides packet switching, connectionless network
• Responsible for
• routing
• congestion control
• Provides no support/guarantees about
• sequence > upper layer take care of re-ordering packet
• path taken > packet flow freely along paths

Internet Packet exchange

• IPX format in byte
• 2 (Checksum) + 2 (Packet length) + 1 (Transport control) + 1 (packet type) + 12 (Destination Address) + 12 (Sender Address) + Data
• Checksum > Seldom used, since underlying data link layer also provides a checksum
• Packet length > how long entire packet is (header + data)
• Transport control > count how many networks the packets has traversed
• Packet Type > mark various control packets
• Destination Address > 32 bits network numbers + 48 bits machine number
• Source Address > 16 bits local address