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LAN/WAN Interconnectivity

Chapter 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 topologies

Describe major LAN transmission methods, including Ethernet, token ring, and FDDI

Explain 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 companies

Cable TV companies

Satellite 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 communicate

How a network device knows when to transmit and not transmit data

How physical network network devices are arranged and connected

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OSI Guidelines Specify…Methods to ensure that network transmissions are received correctly

How network devices maintain a consistent rate of data flow

How 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 signal

Sends signal along the transfer medium

Includes physical layout of network

Monitors for transmission errors

Determines voltage levels for data signal transmissions and to synchronize transmissions

Determines 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 information

Two important sublayersLogical 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 transmissions

Ensures data is sent and received in the same order

Sends acknowledgement when packet is received

Monitors for packet transmission errors and resends bad packets

Breaks 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 link

Determines which node transmits at any point in time

Disconnects when communication session is over

Translates node addresses

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

Performs data encryption

Performs data compression

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Application Layer FunctionsEnables sharing remote drivers and printers

Handles e-mail messages

Provides file transfer services

Provides file management services

Provides terminal emulation services

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Communicating Between Stacks

OSI model provides standards for:Communicating on a LAN

Communicating between LANs

Internetworking 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

Bus

Ring

Star

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Bus TopologyBuilt by running cable from one PC or file server to the next

Terminators signal the physical end to the segment

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Advantages of Bus TopologyWorks well for small networks

Relatively inexpensive to implement

Easy to add to it

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Disadvantages ofBus Topology

Management costs can be high

Potential 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 problem

Well-suited for transmitting signals over long distances on a LAN

Handles high-volume network traffic

Enables reliable communication

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Disadvantages ofRing Topology

Expensive

Requires more cable and network equipment at the start

Not used as widely as bus topologyFewer equipment options

Fewer options for expansion to high-speed communication

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Star TopologyOldest and most common network design

Multiple nodes attached to a central hub

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Advantages of Star TopologyGood option for modern networks

Low startup costs

Easy to manage

Offers opportunities for expansion

Most popular topology in use; wide variety of equipment available

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Disadvantages ofStar Topology

Hub is a single point of failure

Requires more cable than the bus

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Bus Networks in a Physical Star Layout

No exposed terminators

Capability for connecting multiple hubs to expand network in many directions

Expansion opportunities for implementing high-speed networking

Popular 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 network

Typically implemented in a bus or bus-star topology

Carrier sense

Collision

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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 technology

Employs physical star topology with logic of ring topology

Each 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)

Beaconing

Broadcast storms

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FDDIFiber-optic data transport method capable of a 100-Mbps transfer rate using a dual ring topology

Synchronous versus asynchronous communications

Nodes monitor network for error conditionsLong periods of no activity

Long 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 locations

Headend is the main focal point in the star

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Wireless WANsUse radio, microware, and satellite communications

Packet 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 data

SwitchingEnables multiple nodes to simultaneously transmit and receive data, or

Enables 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) model

Basic network topologies

Key LAN transmission methods

WAN communications options

WAN transmission methods