Bob Baker

Communications

Bob Baker

September 1999

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Section 1

• Communications and networks

• Communications services

• Data equipment

• OSI reference model

• Encapsulation

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Communications and Networks

• Data Communications– Sender– Receiver– Channel (medium)– Message

• Telecommunications– At a distance

Sender

Receiver

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Communications Services

Network

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Services

• Sharing files (databases, applications)• Sending messages (e-mail, chat)• Collecting information (remote sensing)• Controlling devices (home automation)• Transferring funds (EFTPOS, E-commerce)• Transaction processing (ordering, booking)• Sharing information (WWW, libraries)• Sharing resources (printers, storage)

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medium

Data Equipment

Data Terminal (hosts)

Data Switching

Data Communication

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Data Terminal Equipment

• Workstations

• PCs

• Printers

• File Servers

• Application Servers

DTE

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Data Communications Equipment

• Modems

• ISDN adapters

• Satellite (and base stations)

• Microwave stations

• Network Interface Cards (NICs)

DCE

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Data Switching Equipment

• Hubs

• Bridges

• Switches

• Routers

• Gateways

DSE

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A campus network

http://www.comms.unsw.EDU.AU/net98/images/h_diagrm.jpg

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Open Systems Interconnection

• OSI Reference model– Layer 7 application– Layer 6 presentation– Layer 5 session– Layer 4 transport– Layer 3 network– Layer 2 data link– Layer 1 physical

Channel

Sender Receiver

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Services

– Layer 7 application

– Layer 6 presentation

– Layer 5 session

– Layer 4 transport

– Layer 3 network

– Layer 2 data link

– Layer 1 physical

– Identification, authentication

– Format conversion

– Set-up and coordinate conversation

– Ensures complete, error-free transfer

– Routing of data through network

– Error control and synchronisation

– Placing signals on the carrier

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Sample- Internet protocol suite

– Layer 7 application

– Layer 6 presentation

– Layer 5 session

– Layer 4 transport

– Layer 3 network

– Layer 2 data link

– Layer 1 physical

– E-mail, Web browser, Directory

– POP, SMTP, FTP, HTTP, DNS

– Sockets

– TCP

– IP

– PPP, Ethernet, Token ring

– 100baseT

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Encapsulation

Application

Presentation

Session

Transport

Network

Data Link

Physical

dataDevice 1 Device 2

dataH6 T6

dataH5 T5

dataH4 T4

(packet)H3 data T3

H2 data T2

H1 data T1

carrier frame

Application

Presentation

Session

Transport

Network

Data Link

Physical

frameframe frame

A typical frame DestinationAddress

SourceAddress

Data Padding CRCPreamble

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Section 2

• Layer 1 (Physical)– Media and Bandwidth– Speeds– Handling errors in signal transmission

• Dial up connections– Modulation/Demodulation

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Data format

1 0 0 0 0 1 001

(7E1) (7bits Even parity 1 stop bit)

B

Parity bitStop bit

Data bits

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Section 3

• Layer 2 (Data Link)– Local Area Network topologies

• Bus

• Ring

• Star

– Hardware addresses– Line access

• Polling

• Token passing

• CSMA/CD

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LAN topologies

• Three common topologies

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Hardware addresses

• At Data Link level

• Each Host (actually the interface) has a unique number

• Usually 48 bits and built in by manufacturer

• Example is the Ethernet Interface number

• Different to Internet Address

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Line access

• Polling– Each station is given a turn at accessing the line

• Token passing (as in Token Ring)– Token passed, only access the line when you have the

token

• CSMA/CD (as in Ethernet)– Carrier sense– Multiple access– Collision detection

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Media access and addressing

Data linkAA:F1 Data link Data link Data linkAB:F3 AA:C2 A1:53

A1:53 AB:F3 data

Each node only has to check the destination address of the frame

This happens at the Data link level

data

data

data

A1:53 AB:F3 data A1:53 AB:F3 data

data

data

data

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Servers

• Peer-to-peer– All nodes have equal status.

• Client/server– Server provides services for clients– Web servers, print servers, mail servers etc

• Master/slave– Master processor controls operation of many slave

processors.– Eg slave robots.

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Client / Server

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Section 4

• Connecting and segmenting LANS– Physical level [Hubs]– Data Link level [Bridge] / [LAN Switch] /

[Switching Hub]– Network level [Routers] / [Multilayer Switch]– Application level [Gateways]

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HUB / BRIDGE / ROUTER / GATEWAY

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Segmenting LANS• Why

– Reduce wiring– Reduce and restrict traffic– Extend distance, maintain speed– Use same protocols

• How– Hub (layer 1)– Bridge (layer 2)– LAN Switch (layer 2)

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Hubs

Hub passes packets at physical layer without processing

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Hubs/Bridge

Bridge checks packets at data link layer

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Bridged rings

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Switched rings

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Bridge Tables

345

127

H

265H

215

614299

B/S1

2

3

387

H122

345 437

132

portMAC345215299127614132

1331

Bridge table

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Wide Area Networks

• Point to Point

• Uses PPP• Usually an ISDN or dedicated line

B/S

H345

127

265B/S

H122

345 437

132387

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Section 5

• Layer 3 (Network level)

• Internet Protocol– Internet addresses– TCP/IP

• Routers and Gateways

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Sample hybrid topology

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Hybrid Network

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Internet connectivity

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Hub/Switch/Router

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Bridging and Routing Internets

• Bridge same network

• Route across different networks

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Domains

• WWW.edfac.usyd.edu.au

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Internet (IP) Addresses

• Addresses 32 bit binary written as “dotted quad”• 11000000101010000000000100000000 = 192.168.1.0

• Class C addresses (network part in blue)– 192.0.0.0 to – 223.0.0.0

• Private Intranet addresses– 192.168.0.0 to– 192.168.255.0

– Allows 254 hosts on each network

• Network numbers assigned by ISP or interNIC

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Internet Protocol

• IP Operates at the Network Layer (3)

• Packages up data into packets

• Each packet has a source IP address and a destination IP address

• Packet passed to Data Link layer where machine address is determined

• Packaged into a frame at Physical level

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Address Resolution

• Address Resolution Protocol• Hardware address from IP address

– Each Interface has two addresses• Hardware address and Internet address

– ARP Matches Machine address to IP address– Table kept in ARP cache on each machine

Internet address192.168.1.1 AA:12:34:21

Machine address

192.168.1.4 12:11:7A:12192.168.1.n AA:12:23:A6

192.168.1.2

This is a router

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Routers

• Operate at Network Layer

• Connect networks

• Have an interface for each network they connect– Each of these interfaces has:

• An IP address

• A machine address

• Forward IP packets based on routing table and algorithms

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Routers (continued)

• Generally Hardware & Software

• Can filter packets

• Can convert protocols between different networks– Eg can link an Ethernet to a token ring

• Can be managed, secured and set up to “sniff” packets passing through.

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Routers (concluded)

PhysicalData Link Data Link

PhysicalData LinkPhysical

Network Layer

A router with three interfaces

Net 1

Net 2

Net 3

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Gateways

• Operate up to Layer 7

• Many of the functions of Routers

• Include security, authentication, access

• Firewalls, Cache servers, Proxy servers

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An Intranet

• With two subnets

192.168.1.4

192.168.1.1

192.168.1.2

H

192.168.1.0

H192.168.1.37

192.168.1.36 192.168.1.40

192.168.1.41192.168.1.33

192.168.1.32

R192.168.1.8

192.168.1.44

Internet

192.168.1.65

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Section 6

• Transport Layer

• Transmission Control Protocol– Handshaking– Sequencing– Error checking

• Effectively creates a connection between hosts

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What TCP does

• Handshaking– Does the destination exist– Establish port connection– Establish appropriate size of packet

• Create and send packets– Break up message– Create and number packet– Pass to IP and wait for acknowledgement

• Terminate connection

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TCP processes

Sender Receiver

SynAckAck

PacAck

FinAck

AckFin

PacAck

PacAck

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