2What is data communication? Data communications deals with the transmission of signals in a reliable and efficient mannerUltimately, it’s about transmitting data (i.e., bits) across some physical transmission medium:Electricity - copper wire, twisted pair, undersea cableLight - infra-red through air, laser through fibre-optic cableElectromagnetic radiation - radio, microwave, satellite
3What is understood by the term ‘communication’? The term communication is defined as the act of disseminating information.It presupposes that:there is information to disseminatethe desire or requirement to disseminate existsthere is an agency to send/transmit informationthere is a means of encoding informationthere is a medium to carry the informationthere is a recipient to receive the informationthe recipient is capable of understanding the information received
4CommunicationIn a face-to-face conversation between two individuals following takes place:Conversion of brain waves into speech.Agreement of both individuals on which vocabulary to use.Agreement of both individuals on volume level at which both can be heard comfortably.Agreement of both individuals on the rate of talking at which each can understand the other’s speech.Agreement of both individuals on the rules used to decide when to speak and when to listen, i.e. how the flow of information is managed.Conversion of the audio signals into brain waves.Let us generalize the process just describedIn any communication between two entities the following properties are required:Modulation:Signal compatibilitySignal strengthData rateProtocolDemodulation
5Communication Model Source Transmitter Transmission System Receiver generates data to be transmittedTransmitterConverts data into transmittable signalsTransmission SystemCarries dataReceiverConverts received signal into dataDestinationTakes incoming data
7Key Communications Tasks Transmission System UtilizationInterfacingSignal GenerationSynchronizationExchange ManagementError detection and correctionAddressing and routingRecoveryMessage formattingSecurityNetwork Management
9Basic Elements of a Communication System In any communication between two entities the following 10 elements can be identified:A Sender.A Receiver.Addressing, to identify where the Receiver is.Protocol – a set of co-operation rules to achieve communication.Transmission code - an agreed “language” to be used.Transmission rate - the speed at which “what is being communicated” is being sent.Transmission synchronisation - how to recognise what is being communicated.Transmission medium.Error detection and correction.Transmission efficiency - how much overhead must be added to manage the transmission.
10Transmission Media Two wire -“telegraph wires” seen in old films. Simplest arrangement, with two wires, separated by air.Can pick up interference, and suffer “crosstalk”.Only reliable for low data rates.Twisted Pair - currently used for domestic phonesTwo insulated wires twisted together.Any interference affects both wires equally.May also have an additional protective screen of metallic foil – “shielded twisted pair”.Suitable for short distance medium speed links.Suffers from “skin effect”, leading to higher resistance at higher data rates.“Skin effect” – HF signals carried only on skin of wire, in effect reducing the area of the wire from a solid wire to a tube of the same diameter.
11Transmission Media Coaxial cable - commonly seen on TV aerial leads Single central wire, separated from woven outer conductor by plastic insulation.Not prone to interference.Can support medium to high data rates.Optical FibreSimilar to coaxial cable in appearance:Uses single strand of glass as core, with light shield around it.Immune to electrical interference , and difficult to eavesdropOften used in industrial or other electrically “noisy” environments.Capable of high data ratesMechanically weaker than electrical wires, and difficult to join.
12Transmission Media Microwaves -ultra high frequency radio waves Line of sight from sender to receiver.No need for wires, so good across rivers, or main roadsExtremely high data ratesSatellite microwaves:Mainly through space so long lines of sight.Little human interference, but affected by extreme solar activity.Terrestrial microwaves:Need repeater stations if lines of sight shortCurvature of earth, or mountains, or buildings
13Data Transmission Terminology Transmission may be simplex, half-duplex or duplex.Simplex – in one direction only.Half-duplex – in both directions, but only in one direction at any time.Full-duplex – in both directions simultaneously, if required.Transmission media may be guided or unguided.Guided – the medium is bounded and the transmission contained within it (e.g. fibre-optic or electrical cable)Unguided – the medium is unbounded (e.g. radio waves in the air, or in space).
14Data Transmission Terminology In a direct link, (or data link), a transmission path:Propagates signals directly from transmitter (sender) to receiverWith no intermediate devices.except amplifiers (or repeaters) to increase signal strength.In guided transmission mediaA configuration is point-to-point if it provides a direct link between two devices, and those are the only two devices sharing the medium.A configuration is multipoint, if more than two devices share the same medium.
16Data Encoding Encoding means changing how data are represented. This can be for convenience:Morse code alphabet used in early radio transmissions.Encoding to hide the meaning of data is “encryption”.Computer data are represented in an encoded form for storage or transmission within and between computers.The most common codes used to store digital data are:ASCII (American Standards Committee for Information Interchange)EBCDIC (Extended Binary Coded Decimal Interchange Code)
17Data Encoding Data are transmitted using electromagnetic signals. Data exists in analogue or digital forms.Analogue or digital data can be encoded using either analogue or digital signals.For example digital data can be transmitted using analogue signals.The telephone network traditionally used analogue signals to represent voices.The telephone network was well-established when transmission of digital computer data became necessary.The latter allows normal computer communications using widely available telephone lines.This is achieved using Modems.
19Transmission direction Data Encodinganalogue signalsTransmission directionComputerModemPrinterdigital signalsModulation is the conversion of the digital signal into an analogue signal, and demodulation converts the analogue signal back into a digital signal. These processes are carried out by a Modem.Another device used is a Codec (coder-decoder).All transmissions occur within a range of frequencies called the Bandwidth.
20Signalling Technologies Baseband is the transmission of digital signals without modulation.In a baseband communication network, digital signals (0s and 1s) are put onto the medium as voltage pulses.The entire bandwidth is consumed by the signal.Broadband uses coaxial cable to provide data transfer by means of analogue signals.The bandwidth is divided in different frequency bands or channels.In a broadband communication network involving computers, digital signals are passed onto the medium through a modem and transmitted over one of the channels. So, several different communication networks can be implemented over the same medium.
21Signalling Technologies Analogue transmission is used to mean the transmission of analogue signals without regard to their content.Digital transmission, on the other hand, is used to mean the content of the signal.
25Transmission Synchronisation Synchronisation is essential for transmitter and receiver to understand each other.In serial transmission the following types of synchronisation are required:Bit synchronisation - how to detect each bit.Byte or character synchronisation - how to group the bits to make a character or byte.Block synchronisation - how to group the characters/bytes to make a block (a frame or a packet)Bit synchronisation depends on how the signal is encoded
26Transmission Synchronisation In serial transmission there are two standard ways of achieving character and block synchronisation:Asynchronous Transmission or Character SynchronisationThe time interval between characters is random.Each character is synchronised by the use of a start bit, and either one or two stop bits.The bit rate is constant on a per character basis
27Transmission Synchronisation Synchronous Transmission or Block SynchronisationEach block is synchronised by the use of a number of synchronisation characters that are transmitted firstThese are followed by a start of block character, which is followed by the data block, and transmission is finished with an end of block character.The bit rate is constant for the whole transmission of the block I.e. the time interval between characters is fixed.
29Data Transmission: Modes Computer based communications always use Digital Transmission,What is transmitted is digital data, using either an analogue or digital signal.Normally, the digital data are recovered and repeated at intermediate points to reduce the effects of noise.Irrespective of the type of communications facility being used, in most applications data are transmitted between computers in a bit-serial mode,more commonly known as serial transmission.
30Data Transmission: Modes Within a computer, data are transferred in a word-parallel mode, most commonly known as parallel transmission.In computer communications is necessary to perform a parallel-to-serial conversion, in the transmitter, serial-to-parallel conversion in the receiver.These conversions are done in the computer interface to the network
31Transmission efficiency Extra bits (start and stop bits) and characters (synchronisation and block delimiters) are needed to implement asynchronous and synchronous transmission.These add nothing to the content of the message, but must be included in what is sent.They reduce the overall information capacity of the transmissionThey reduce the overall efficiency of the transmission.
32Transmission efficiency Transmission efficiency = (useful data/total bits transmitted)*100For example for asynchronous transmission of 8-bit characters with 1 start and 1 stop bit, we have to send 10 bits for each character:Transmission efficiency = (8/10)*100 = 80%Effective Data Rate = (Transmission Efficiency/100)*Capacity
33Transmission CodesSymbolic data/information must be encoded in a format suitable for transmission.Normally, the codes used for transmission are similar to the codes used to store the information.The most common code is ASCIIASCII is a 7-bit code, permitting 128 different symbols to be encoded.The second most commonly used code is EBCDICEBCDIC is an 8-bit code enabling 256 different symbols to be encoded.
35Data Communication vs. Networking Communication: Two Nodes. Mostly EE issues.Networking: Two or more nodes. More issues, e.g., routing
36Distributed Systems vs. Networks Users are unaware of underlying structure.Mostly operating systems issues.Nodes are generally under one organization’s control.Networks:Users specify the location of resources.Nodes are autonomous.
37Networking Point to point communication not usually practical Devices are too far apartLarge set of devices would need impractical number of connectionsSolution is a communications network
38What are computer networks? Networking deals with the technology & architecture of the communications networks used to interconnect communicating devices.Computer network is a collection of autonomous computers interconnected by a single technology.The Internet is not a single network but a network of networks.
39Types of Networks Point to point vs. Broadcast Circuit switched vs. packet switchedLocal Area Networks (LAN)vs.Metropolitan Area Networks (MAN)Wide Area Networks (WAN)
40Communications Networks A Communications Network is a set of interconnected devices that provide data transmission facilities between user's end points.UserUser on Host BCommunications Network comprising communications links and connecting computersUser on Host A
42Objectives of Networking To share and exchange data between systems;To share expensive resources;To facilitate communication among humans and machines;
43Some terminology Host – a machine on the network; End system/end point – a machine “on the edge” of the network, rather than an “internal” (switching) node;Subnet – sub network, a subset of the whole network;Also used to refer to the internal “routing” part of a network.IMP – Interface Message Processor, hardware connecting host to network.
44Some terminologyPacket – we often break messages into many chunks, sent separately. The chunks are called packets.Size of packet and how it’s treated depends on network protocol in use.A packet might get split up further by another protocol.Some protocols (e.g. IP) use varying size packets; in others (e.g. ATM) they’re fixed. Small fixed-size packets are called cells
45Some terminologyinternetworking – act of connecting multiple networks together to form a larger network;Fun issues include how to route and address across multiple heterogeneous networks;internet – a network thus producedAlso the name of a common protocol for doing this (IP);Internet – “the” global internet;
46Network sizesComputer Networks can be classified by the area they cover:PAN – Personal Area Network: very smallLAN – Local Area Network: room/building/campusMAN – Metropolitan Area Network: city, regionWAN – Wide Area Network: country/continent.
47Interconnection of Networks Networks of low capacity may be connected together via a backbone (network of high capacity)LANs and WANs can be interconnected via T1 or T3 digital leased linesAccording to the protocols involved, networks interconnection is achieved using one or several of the following devices:Bridge: a computer or device that links two similar LANs based on the same protocol.Router: a communication computer that connects different types of networks using different protocols.B-router or Bridge/Router: a single device that combines both the functions of bridge and router.Gateway: a network device that connects two different systems, using direct and systematic translation between protocols.
48Broadcast vs. Point-to-point Broadcast Networks:A single communication channel shared by all machines on a network;Multicast: simultaneous transmission to a subset.Point-to-point networks:Many connections between individual pairs of machines;Transmission from A to C might go via B;Often multiple routes: a fundamental question is which to use?
49Local Area NetworksA Local Area Network (LAN) is a computer network intended to link computers and associated devices within a small geographical area.The linking distances are relatively short, with cable lengths rarely exceeding 5 kilometres.The linked computers may include large computers, word processors, or desktop computers.Associated devices include computer terminals, printers, plotters, scanners, etc.
50Local Area NetworksLANs normally offer much higher data transmission rates than WANs.This difference is apparent in the network oriented protocols only.At application level, LANs provide the sharing of resources like programs, files, printers, plotters, scanners, etc.
51LAN TopologiesLAN topology is one of the issues that must be considered when selecting LAN technology.It defines the interconnection of stations to form the network.LAN topologies are classified as:Broadcast topologyStore-and-forward topology
52LAN Topologies Broadcast topology Store-and-forward topology This implies that all stations are connected to a common transmission medium.Store-and-forward topologyA complete message or packet is received into a buffer in the memory of an intermediate stationIt is then re-transmitted on the route to its destination.The stations in a store-and-forward topology network are connected by independent point-to-point transmission lines.
53LAN TopologiesThe topology of a LAN is important because it influences the following features of the network:expansion costthe incremental cost of adding another station to an existing network.reconfiguration capabilitiesthe ease of modifying the topology to deal with a failed node or component.reliabilityThe extent of dependency on a single component for network operation.
54LAN Topologies As well as: software complexity performance the complexity of the protocols required to achieve communications.performanceThe effectiveness of the LAN in terms of throughput, or delays in transmission.broadcast capabilitieshow difficult it is to broadcast in the LAN, i.e. to transmit a single message which is received by all other stations in the network.
55Bus Topology Single medium to which all hosts are connected The bus is a broadcast topology – anything placed on the medium by one host is available to all hosts attached to the bus.If the destination address is recognised by a node it copies the contents into that node.Network connection cable between NIC on host and the network medium
56Ring TopologyThe ring is a store-&-forward topology – a packet placed on the medium by one node is received by the next node along, and is then resent by that node, and so on.If the destination address matches that of the receiver, the packet is not resent.If the sender’s address matches that of the receiver, the packet has gone all the way round and will not be resent. It is a “lost” packet and is effectively removed from the ring.
57Star TopologyThe star is a store-&-forward topology – a packet placed on the medium by one node is received by central node which is forwarded to it’s destination by the central node.Central node
58Hub Topology The “hub” is derivative of the bus and ring topologies It has the appearance of the star topology, with a central hub in place of the central node.The hub is simply the bus or ring wiring “collapsed” into a central unit.Unlike the central node in the star topology, the hub does not perform any switching. The hub simply consists of a set of repeaters.Many modern networks are implemented using hubs for convenience.Care is needed when deciding what topology is being used in a real network.
59Hub Topology : Network with and without hub Bus network – long network “backbone” installed through building, with short network connection cables between host computers and backbone.The same bus network, implemented using a hubHub containing network “backbone” and stubs of connection cablesNetwork connecting cables extending through the building to the host devices
60Network topologies Tree Corresponding to an organisational hierarchy? Internal nodes may be bottlenecks.
61Network topologies Graph Complete graph (Mesh) Generalisation of a treeCycles allowedComplete graph (Mesh)Dedicated link from every node to every other nodeRapidly becomes prohibitively expensive
63Communications System A communications system is the combination of network hardware and communications system software that supports the communications between user-oriented processes running in remote computers.The communications system provides the services required by the applications to communicate. These services are outlined on the next slide.
64Communications System Communication System FunctionsNaming and Addressing of entities.Segmenting and reassembly of messagesBlocking of messagesConnection or session controlError controlCongestion and flow controlSynchronisationPriority
65Communication System Architecture The user-oriented layers:The application offers services to users through a set of rules or steps for accessing web-sites or sending s.Some applications operate on different types of user-interface. A means of converting alphabets and screen formats may be neededSome applications require a session of activity with a definite set-up and closedown of the session (e.g. logon and logoff)The transport layer provides an end-to-end virtual channel between the source and destination.
66Communication System Architecture The system-oriented layers:Implement the connections between nodes that make a machine part of a communications networkThe network layer is responsible for routing between nodesThe Data link and Physical layers provide the means of moving packages of data between pairs of nodes.
67Communication System Architecture The ISO Open Systems Interconnection (OSI) model has 7 layers:The top 3 layers are user or application oriented.The bottom 3 layers are system-oriented.The middle layer, transport, acts as a broker between the basic services provided by the network and the needs of the usersEach layer can be thought of as “talking” directly to its peer on another machine.A user of a web-browser holds a “conversation” with a remote web-siteOnly at the physical layer does direct communication take place, using signals.1. Application2. Presentation3. Session4. Transport5. Network6. Data Link7. Physical
68Communication System Architecture The TCP/IP model has 4 layers:The top layers is the application.The bottom 2 layers are system-oriented.The middle layer, transport, acts as a broker between the basic services provided by the network and the needs of the users.Although the model is simpler than OSI it recognises the same purpose and requirements.The transport level protocols are TCP and UDPThe network level protocol is usually IPThe data link and physical level protocols are specific to the network1. Application2. Transport3. Network4. Data Link / Physical
69Communication System Architecture 1. Application2. Transport3. Network4. Data Link / PhysicalApplication-level protocol based on messages used by the application. For example, the PIN and card details from a bank cardTransport level data packets exchanged using transport protocolNetwork level data packets exchanged using network protocolData Link frames exchanged using data link protocol.Frames transmitted over the physical medium using appropriate signalling techniquesCommunications Network
70Communication System Architecture Application-level protocol1. Application2. Transport3. Network4. Data Link / PhysicalTransport-level protocol acting end-to-end – e.g. TCPIPHost AHost BNetworkIntermediate nodes inside networkDCNode DNode C
71Circuit Switching vs. Packet Switching Fundamental question: how to move bits from one host to another, via ‘n’ others?Two key approaches (opposed):Circuit switchingEstablish fixed-bandwidth circuit & use it;Packet switchingSplit messages into packets, send separately;Trend is very much towards packet switching.
72Circuit SwitchingResources along a path are reserved for duration of communication.Buffers, link bandwidth, CPU time, etc.All nodes on path genuinely maintain connection state information;All data in a some communication is sent on the same circuit, through same nodes;Classic example: PSTN (Public Switched Telephone Network)
74Circuit Switching Each circuit has a fixed bandwidth for its lifetime. Channels typically split into n equal bandwidth circuits.Pro: Makes QoS (Quality of Service) guarantees easy to achieve;Con: Wasteful during “silent” periods.Data transmission tends to be bursty.
75Circuit Switched Multiplexing Multiplexing – combining information channels onto a common transmission medium.FDM (Frequency Division Multiplexing)Frequency spectrum of link is shared among circuits;Typically, each of n circuits gets 1/n;e.g. PSTN bandwidth divided in 4KHz bands;TDM (Time Division Multiplexing)Time divided into fixed size chunks;Each circuit gets a portion of the total time
76Packet Switching No prior reservation of resources; Each packet transmitted separately;Nodes don’t maintain connection state information:Each packet dealt with individually;Two packets might take different paths;Classic example: the Internet.
77Packet SwitchingCon: QoS harder to do, can only really make “best effort” promises;IPv6 addresses this somewhat – complex;Pros: more efficient use of bandwidth, no hard limit to number of comms.Ideally: “graceful degradation” curves;What happens when queues fill? Delays and, ultimately, packet loss.Store-and-forward (on routers):Read entire packet in, then send it out
79Delay & Loss in Packet Switching Processing delayTime to examine packet & decide where to send it; maybe also some error checking;Queuing delayDelay while packet is queued; depends on size of queue, ie traffic levels;Transmission delayTime taken for node to “push out” packet;Depends on size of packet & speed of outbound link.
80Delay & Loss in Packet Switching Propagation delayTime taken for packet to propagate across link to next node;Depends on speed of physical medium and distance to next node;Packet lossHappens when things get too busy, queues overflow, nodes can’t keep up;End-to-end delayTotal delay on transmission between two end points.
81Frame RelayPacket switching systems have large overheads to compensate for errorsModern systems are more reliableErrors can be caught in the end systemMost overhead for error control is stripped out
82Asynchronous Transfer Mode ATMEvolution of frame relayLittle overhead for error controlFixed packet (called cell) lengthAnything from 10Mbps to GbpsConstant data rate using packet switching technique
83Virtual circuits vs. datagram networks We can, in fact, simulate circuit switching on packet switched networks:Virtual Circuits being the result;Otherwise, it’s a datagram network:Datagram: another word for packet;Choice has huge impact on routing;At IP level, Internet is a datagram network
84Virtual Circuit Networks Packets carry VC identifier;Hosts have table mapping VCIDs to outbound connections;Setting up involves both ends and every host in between;Every packet follows the same path;Requires complex state maintenance protocols.
85Datagram Networks Packets carry destination address; Host has (more complex) table to help it decide where to send next.Table at a given host can change over lifetime of a communication;Packets really can take different paths;No connection state information maintained (except maybe at ends);Almost all of the Internet.
86Connection-oriented vs. Connectionless Services Characterises end-to-end communication services available to end users.Connection-oriented:Application must establish connection to other end before sending any actual data;Each packet then sent via that connection.Allows delivery guarantees.Connectionless:Application just sends each packet individually;Thus, must know destination address every time you send a packet.No guarantee of delivery, generally.
87Caution: don’t get confused… Circuit-switched vs. packet switched:Concerns how packets are routed;Distinction made in “core” of network;Mainly at Network layer (see later).Connection-oriented vs. connectionless:Concerns how packets are sent/received;Distinction made at “edge” of network;Mainly at Transport layer (see later).
88Basic Types of Networks Yet another way to classify…
89Basic Types Peer-to-peer Server-based Does not require dedicated resource (dedicated server)Any host can share its resourcesTypically less expensive, easier to work withLess secure, support fewer users (10 or fewer), experience more problems with file system managementServer-basedConfiguration of nodes, certain of which are dedicated to providing resources (servers)Offer (better) user securityDedicated servers can be expensive, may require a full-time network administratorEnterprise network (which combines the two)Provide connectivity among all nodes in an organizationInclude (connect) both peer-to-peer and server-based networksMay consist of multiple protocol stacks and network architectures
90Client/Server Networks Client/Server is a networking model mainly applicable at the Application layer;Concerns the roles of end systems:Client – system requesting some service;Server – system providing some service.Ubiquitous example: HTTPClient is your web browserServer is (or whatever)
91Peer Networks Not all applications use Client/Server model; Often, all parties have equal status:In some sense they’re all clients and servers.Although sometimes have distinguished nodes providing certain services.
92Protocols Used for communications between entities in a system Must speak the same languageEntitiesUser applicationsfacilitiesterminalsSystemsComputerTerminalRemote sensor
93Key Elements of a Protocol SyntaxData formatsSignal levelsSemanticsControl informationError handlingTimingSpeed matchingSequencingProtocols define format, order of messages sent and received among network entities, and actions taken on message’s transmission, receipt
94In Summary, a protocol is .... An agreement about communication between two or more entitiesIt specifies– Format of messages– Meaning of messages– Rules for exchange– Procedures for handling problems
95Protocol Architecture Task of communication broken up into modulesFor example file transfer could use three modulesFile transfer applicationCommunication service moduleNetwork access module
97A Three Layer Model At the Top: In the Middle: At the Bottom: User Oriented layer-Application LayerIn the Middle:Transport LayerAt the Bottom:System Oriented Layer - Network Access Layer
98Network Access LayerExchange of data between the computer and the networkSending computer provides address of destinationMay invoke levels of serviceDependent on type of network used (LAN, packet switched etc.)
99Transport Layer Reliable data exchange Independent of network being usedIndependent of application
100Application LayerSupport for different user applicationse.g. , file transfer
101Addressing Requirements Two levels of addressing requiredEach computer needs unique network addressEach application on a (multi-tasking) computer needs a unique address within the computerThe service access point or SAPThe port on TCP/IP stacks
102Addressing Different levels of entity use different addresses. MAC address: Identifies the NIC and set by manufacturer.Used by Physical and Data Link layerIP address: Identifies a computer in a network.Used by the Network layerSocket: Identifies a process (running program).Used by the Transport layerApplication level addresses vary:One example is the Uniform Resource Locator (URL) used by WWW applications
103IP Addresses IP = Internet Protocol Each IP address is 32 bits longAn IP address has a network part and host partThe former identifies a specific network and the latter a specific computer, or host, on that network.IP addresses may be in one of five network classes:Class A: Used for a small number of networks, each with many hosts.Class B: Used for a larger number of networks, each with a medium number of hostsClass C: Used for a large number of networks, each with only a few hostsClasses D and E are for special purposes.
104IP Addressing ExampleAll hosts on a network have the same network prefix
105User Oriented Names and DNS Human users prefer names to numbers.The communications system translates these names into IP addresses, and vice versa.The translation is done using the Domain Name System (DNS) application.This is a “directory” service.It uses multiple levels of server to resolve queries as close to the point of issue as possible.All servers cache query results to reduce need for repeat queries in the near future.
106Name Resolution in DNS Each computer has a name resolver routine ‘gethostbyname’ in UNIXEach resolver knows the name of a local DNS serverResolver sends a DNS request to the serverDNS server either gives the answer, forwards the request to another server, or gives a referralReferral = Next server to whom request should be sent
107How the DNS works Intermediate Name Server Root Name Server Requesting HostRoot Name ServerLocal Name ServerIntermediate Name ServerAuthoritative Name ServerA “requesting host” issues a query to its local name server to translate from a host name to an IP address. This request is routed to a server which can supply the IP address.The root server may not know the IP address, but may “know the address of someone who does”. This can result in queries being forwarded to an “authoritative name server, via an intermediate name server, if necessary.All name servers employ caches which store the results of recent queries. This can speed up later requests for the same IP address.Name embedded in queryIP address, or error message, embedded in reply
110Protocol Data Units (PDU) At each layer, protocols are used to communicateControl information is added to user data at each layerTransport layer may fragment user dataEach fragment has a transport header addedDestination SAPSequence numberError detection codeThis gives a transport protocol data unit
114Standards Required to allow for interoperability between equipment AdvantagesEnsures a large market for equipment and softwareAllows products from different vendors to communicateDisadvantagesFreeze technologyMay be multiple standards for the same thing
115Standardized Protocol Architectures Required for devices to communicateVendors have more marketable productsCustomers can insist on standards based equipmentTwo standards:OSI Reference modelNever lived up to early promisesTCP/IP protocol suiteMost widely used
116OSI Open Systems Interconnection Developed by the International Organization for Standardization (ISO)Seven layersA theoretical system delivered too late!TCP/IP is the de facto standard
117OSI - The Model A layer model Each layer performs a subset of the required communication functionsEach layer relies on the next lower layer to perform more primitive functionsEach layer provides services to the next higher layerChanges in one layer should not require changes in other layers
120TCP/IP Protocol Architecture Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)Used by the global InternetNot official model but a working one.Application layerHost to host or transport layerInternet layerNetwork access layerPhysical layer
121TCP/IP Protocol Architecture: Physical Layer Physical interface between data transmission device (e.g. computer) and transmission medium or networkCharacteristics of transmission mediumSignal levelsData ratesetc.
122TCP/IP Protocol Architecture: Network Access Layer Exchange of data between end system and networkDestination address provisionInvoking services like priority
123TCP/IP Protocol Architecture: Internet Layer (IP) Systems may be attached to different networksRouting functions across multiple networksImplemented in end systems and routers
124TCP/IP Protocol Architecture: Transport Layer (TCP) Reliable delivery of dataOrdering of delivery
125TCP/IP Protocol Architecture: Application Layer Support for user applicationse.g. http