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LAN/WAN InterconnectivityChapter 2

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Learning ObjectivesExplain the OSI reference model, which sets standards for LAN and WAN communicationsDiscuss communication between OSI stacks when two computers are linked through a networkApply the OSI model to realistic networking situations

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Learning ObjectivesDescribe the types of networks as represented through LAN topologiesDescribe major LAN transmission methods, including Ethernet, token ring, and FDDIExplain basic WAN network communications topologies and transmission methods, including telecommunications, cable TV, and satellite technologies

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LAN/WAN InterconnectivityIntense competition between three sectors:

Telecommunications companiesCable TV companiesSatellite communications companies

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OSI Reference ModelFoundation that brings continuity to LAN and WAN communicationsProduct of two standards organizations:

ISOANSI

Developed in 1974Set of communication guidelines for hardware and software design

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OSI Guidelines Specify…How network devices contact each other; how devices using different protocols communicateHow a network device knows when to transmit and not transmit dataHow physical network network devices are arranged and connected

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OSI Guidelines Specify…Methods to ensure that network transmissions are received correctlyHow network devices maintain a consistent rate of data flowHow electronic data is represented on network media

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OSI Layers

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OSI LayersBottom layers

Support for physical connectivity, frame formation, encoding, and signal transmission

Middle layersEstablish and maintain a communication session between two network nodesMonitor for error conditions

Uppermost layersApplication/software support for encrypting data and assuring interpretation/presentation of data

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Physical Layer FunctionsProvides transfer medium (eg, cable)Translates data into a transmission signalSends signal along the transfer mediumIncludes physical layout of networkMonitors for transmission errorsDetermines voltage levels for data signal transmissions and to synchronize transmissionsDetermines signal type (eg, digital or analog)

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Analog Signals

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Digital Signals

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Data Link Layer FunctionsConstructs data framesCreates CRC information; checks for errorsRetransmits data if there is an errorInitiates communications link; makes sure it is not interrupted (ensures node-to-node physical reliability)Examines device addressesAcknowledges receipt of a frame

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Data Link LayerData link frame contains fields consisting of address and control informationTwo important sublayers

Logical link control (LLC)Media access control (MAC)

Connectionless service versus connection-oriented service

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Network Layer FunctionsDetermines network path for routing packetsHelps reduce network congestionEstablishes virtual circuitsRoutes packets to other networks, resequencing packet transmissions when neededTranslates between protocols

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Transport Layer FunctionsEnsures reliability of packet transmissionsEnsures data is sent and received in the same orderSends acknowledgement when packet is receivedMonitors for packet transmission errors and resends bad packetsBreaks large data units into smaller ones and reconstructs them at the receiving end for networks using different protocols

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Session Layer FunctionsEstablishes and maintains communications linkDetermines which node transmits at any point in timeDisconnects when communication session is overTranslates node addresses

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Presentation Layer FunctionsTranslates data to a format the receiving node understands (eg, from EBCDIC to ASCII)Performs data encryptionPerforms data compression

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Application Layer FunctionsEnables sharing remote drivers and printersHandles e-mail messagesProvides file transfer servicesProvides file management servicesProvides terminal emulation services

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Communicating Between Stacks

OSI model provides standards for:Communicating on a LANCommunicating between LANsInternetworking between LANs and WANs and between WANs and WANs

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Peer Protocols

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Primitives

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Layered Communications

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Applying the OSI Model

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Types of NetworksThree main topologies

BusRingStar

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Bus TopologyBuilt by running cable from one PC or file server to the nextTerminators signal the physical end to the segment

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Advantages of Bus TopologyWorks well for small networksRelatively inexpensive to implementEasy to add to it

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Disadvantages ofBus Topology

Management costs can be highPotential for congestion with network traffic

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Ring TopologyContinuous path for data with no logical beginning or ending point, and thus no terminators

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Advantages of Ring TopologyEasier to manage; easier to locate a defective node or cable problemWell-suited for transmitting signals over long distances on a LANHandles high-volume network trafficEnables reliable communication

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Disadvantages ofRing Topology

ExpensiveRequires more cable and network equipment at the startNot used as widely as bus topology

Fewer equipment optionsFewer options for expansion to high-speed communication

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Star TopologyOldest and most common network designMultiple nodes attached to a central hub

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Advantages of Star TopologyGood option for modern networksLow startup costsEasy to manageOffers opportunities for expansionMost popular topology in use; wide variety of equipment available

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Disadvantages ofStar Topology

Hub is a single point of failureRequires more cable than the bus

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Bus Networks in a Physical Star Layout

No exposed terminatorsCapability for connecting multiple hubs to expand network in many directionsExpansion opportunities for implementing high-speed networkingPopular design; wide range of equipment available

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LAN Transmission MethodsEthernet

IEEE 802.3 specificationsBroadest options for expansion and high-speed networking

Token ringIEEE 802.5 specifications

FDDI (Fiber Distributed Data Interface)High-speed variation of token ring

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EthernetUses CSMA/CD access method for data transmission on a networkTypically implemented in a bus or bus-star topologyCarrier senseCollision

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Ethernet Communications

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Ethernet II

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Ethernet Standards

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Token RingDeveloped by IBM in the 1970s; remains a primary LAN technologyEmploys physical star topology with logic of ring topologyEach node connects to a central hub, but the frame travels from node to node as though there were no starting or ending point

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Token Ring Frame

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Token Ring TermsMultistation access unit (MAU)BeaconingBroadcast storms

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FDDIFiber-optic data transport method capable of a 100-Mbps transfer rate using a dual ring topologySynchronous versus asynchronous communicationsNodes monitor network for error conditions

Long periods of no activityLong periods where the token is not present

Class A and Class B nodes

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WAN Network Communications

Typical providers of WAN network servicesTelecommunications companiesCable TV companiesSatellite providers

Newer sources of WAN connectivityCable television networksSatellite TV companiesWireless WANs

Wide use of star topology

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Telecommunications WANsEarliest source of WAN connectivityRegional telephone companies, also called:

TelcosRegional bell operating companies (RBOCs)

Long-distance telecommunications companiesPlain old telephone service (POTS) or public switched telephone network (PSTN)

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General Topology Linking LATA and IXC Lines

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Connecting LANs througha T-Carrier Line

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T-Carrier Services and Data Rates

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Cable TV WANsAlso called cablecos or multiple system operators (MSOs)Use a distributed architecture that consists of several star-shaped centralized locationsHeadend is the main focal point in the star

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Wireless WANsUse radio, microware, and satellite communicationsPacket radio communications

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Topology of a Radio Wave WAN Joining Two LANs

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WAN Transmission MethodsUse different switching techniques to create data paths (channels) for transmitting dataSwitching

Enables multiple nodes to simultaneously transmit and receive data, orEnables data to be transmitted over different routes to achieve maximum efficiency in terms of speed and cost

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Switching TechniquesTime division multiple access (TDMA)

Divides channels into distinct time slots

Frequency division multiple access (FDMA)

Divides channels into frequencies

Statistical multiple access Dynamically allocates bandwidth based on application need

Circuit switching Uses a dedicated physical circuit

Message switching Uses store-and-forward method of data transmission

Packet switching Combines circuit and message switching

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Chapter SummaryOpen Systems Interconnection (OSI) modelBasic network topologiesKey LAN transmission methodsWAN communications optionsWAN transmission methods