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Data Communications
Chapter 1Introduction
2
A Communications ModelSource
generates data to be transmitted
Transmitter Converts data into transmittable signals
Transmission System Carries data
Receiver Converts received signal into data
Destination Takes incoming data
3
Simplified Communications Model - Diagram
4
Key Communications Tasks 1/3Transmission System Utilization
Efficient use of channel Multiplexing Congestion Control
Interfacing Connection to physical interface / media
5
Key Communications Tasks 2/3Signal Generation
Form & intensity Propagation Recovery of data
Synchronization Timing
Exchange Management Setup Data size Error contingencies
6
Key Communications Tasks 3/3Error detection and correction
Parity, ECCFlow ControlAddressing and routingRecovery
Recovery from fault in txMessage formatting
Binary codeSecurityNetwork Management
7
Simplified Data Communications Model
8
NetworkingPoint to point communication not usually
practical Devices are too far apart Large set of devices would need impractical
number of connections
Solution is a communications network Wide Area Networks Local Area Networks
9
Simplified Network Model
10
Wide Area NetworksLarge geographical areaCrossing public rights of wayRely in part on common carrier circuitsAlternative technologies
Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)
11
Circuit SwitchingDedicated communications path
established for the duration of the conversation
Physical connection is actually connectedConnection cannot be sharede.g. telephone network
12
Packet SwitchingData may be sent out of sequenceSmall chunks (packets) of data at a timePackets passed from node to node
between source and destinationUsed for terminal to computer and
computer to computer communicationsBasic rate of 64 kbps
13
Frame RelayPacket switching systems have large
overheads to compensate for errorsModern comms systems are more reliableErrors can be caught in end systemMost overhead for error control is stripped
outVariable packet lengthData rate up to 2 Mbps
14
Asynchronous Transfer ModeATM (Cell Relay)ATM is partly evolution of frame relay
Little overhead for error control Fixed packet (called cell) length
Partly evolution of circuit switching Multiple virtual channels Dynamic channel bandwidth of channel
Can offer constant data rate using packet switching technique
Anything from 10Mbps to Gbps
15
Integrated Services Digital NetworkISDNDesigned to replace public telecom
systemWide variety of servicesEntirely digital domainTwo Generations
Narrowband ISDNFrame Relay
Broadband ISDNATM
16
Local Area NetworksSmaller scope
Building or small campus
Usually owned by same organization as attached devices
Data rates much higherUsually broadcast systemsNow some switched systems and ATM are
being introduced
17
Protocols
Used for communications between entities in a system
Entities must speak the same language Entities
User applications e-mail facilities terminals
Systems Computer Terminal Remote sensor
18
Key Elements of a ProtocolSyntax
Data formats Signal levels
Semantics Control information Error handling
Timing Speed matching Sequencing
19
Protocol ArchitectureTask of communication broken up into
modulesFor example file transfer could use three
modules File transfer application Communication service module Network access module
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Simplified File Transfer Architecture
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A Three Layer ModelNetwork Access LayerTransport LayerApplication Layer
22
Network Access LayerExchange of data between the computer
and the networkSending computer provides address of
destinationMay invoke levels of service
Priority, through put
Software will be dependent on type of network used (LAN, packet switched etc.)
23
Transport LayerReliable data exchange
Data arrival Data order
Independent of network being usedIndependent of application
24
Application LayerSupport for different user applicationse.g. e-mail, file transfer
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Addressing RequirementsTwo levels of addressing requiredEach computer needs unique network
addressEach application on a (multi-tasking)
computer needs a unique address within the computer The service access point or SAP Port
26
Protocol Architectures and Networks
27
Protocols in Simplified Architecture
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Protocol Data Units (PDU) At each layer, protocols are used to communicate Control information is added to user data at each
layer Transport layer may fragment user data to make
it more manageable Each fragment has a transport header added
Destination SAP (DSAP) Sequence number Error detection code
This header combined with the application layer data make up a transport protocol data unit
29
Network PDUAdds network header
network address for destination computer (DHOST)
Facilities requestsPriorityThroughput
30
Operation of a Protocol Architecture
31
TCP/IP Protocol ArchitectureDeveloped by the US Defense Advanced
Research Project Agency (DARPA) for its packet switched network (ARPANET)
Used by the global InternetNo official model but a working one.
Application layer Host to host or transport layer Internet layer Network access layer Physical layer
32
Physical LayerPhysical interface between data
transmission device (e.g. computer) and transmission medium or network
Characteristics of transmission mediumSignal levelsData ratesetc.
33
Network Access LayerExchange of data between end system
and networkDestination address provisionInvoking services like priority
In some models the Network Access layer and the Physical layer are combined into a single layer
34
Internet Layer (IP)Systems may be attached to different
networksRouting functions across multiple
networksImplemented in end systems and routers
35
Transport Layer (TCP)Reliable delivery of dataOrdering of delivery
36
Application LayerSupport for user applicationse.g. http, SMPT
37
TCP/IP Protocol Architecture Model
38
OSI ModelOpen Systems InterconnectionDeveloped by the International
Organization for Standardization (ISO)Seven layersA theoretical system delivered too late!TCP/IP is the de facto standard
39
OSI LayersApplicationPresentationSessionTransportNetworkData LinkPhysical
40
Application LayerThe application layer represents the level
at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.
41
Presentation layerThe Presentation layer translates data
from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network.
42
Session LayerThe Session layer allows two applications
on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits. The services provided are dialogue discipline – full & half duplex communication; Grouping – grouping of data and Recovery – use of check points in the data transmission.
43
Transport Layer The Transport layer handles error recognition and
recovery. It also repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.
A connection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplications.
The transport layer is concerned with optimising the network and providing Quality of Service parameters to session entities. These may specify acceptable error rates, maximum delay priority and security.
44
Network Layer The Network layer addresses messages and
translates logical addresses and names into physical addresses.
It determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.
It relieves higher layers from knowing about underlying data transmission and switching technologies used to interconnect systems. This layer may use packet switched and circuit switched networks.
The packet level of the X.25 standard is a network layer standard.
45
Data Link LayerThe Data Link layer packages raw bits
from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer. Examples of standards at this layer are HDLC, LAPB,LLC and LAPD.
46
Physical LayerThe Physical layer transmits bits from one
computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.
47
OSI v TCP/IP
48
StandardsRequired to allow for interoperability
between equipmentAdvantages
Ensures a large market for equipment and software
Allows products from different vendors to communicate
Disadvantages Freeze technology May be multiple standards for the same thing
49
Standards OrganizationsInternet SocietyISOITU-T (formally CCITT)ATM forum
50
Internet SocietyInternet Architecture Board
Defines overall architecture of internet
Internet Engineering Task Force (IETF) Protocol engineering & development
Internet Engineering Steering Group Technical management of IETF & Internet
standards process
51
Internet RFC Publication Process
Internet Draft: Up to 6 monthsProposed Standard 6 monthsDraft Standard 4 months
RFC’s are the Proposed Standards
52
International Organisation for standardisation (ISO)Standardisation steps:
Proposal: Proposal to technical committee Preparatory:Working draft of technical solution Committee:Concensus obtained for submission
of Draft International Standard (DIS) Enquiry:5 month period of enquiry resulting in
a Final DIS Approval: 2 month for yes/no approval Publication
53
ITU-T (formally CCITT)International Telecommunications Union
Telecommunications Standardisation Section
United Nations specialised agencyPrimary objective is to standardise
techniques & operations to enable end to end compatibility of international telecommunication connections, regardless of the countries of origin of the source or destination.
54
ATM ForumInternational non profit body with 600
member companiesAccelerated development of standards wrt
ATMStandards based on majority as opposed
to concensusEffort feeds into the ITU-T standardisation
effort
55
Further Reading
Stallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1
Web site for Stallings book www.shore.net/~ws/DCC6e.html
Web sites for IETF, IEEE, ITU-T, ISO Internet Requests for Comment (RFCs) Tanenbaum, Computer Networks, chapter 1
TCP/IP model Usenet News groups
comp.dcom.* comp.protocols.tcp-ip