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LAN/WAN Interconnectivity
Chapter 2
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
continued…
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
LAN/WAN InterconnectivityIntense competition between three sectors:
Telecommunications companies
Cable TV companies
Satellite communications companies
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
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
OSI Layers
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
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)
Analog Signals
Digital Signals
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
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
Network Layer FunctionsDetermines network path for routing packetsHelps reduce network congestionEstablishes virtual circuitsRoutes packets to other networks, resequencing packet transmissions when neededTranslates between protocols
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
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
Presentation Layer FunctionsTranslates data to a format the receiving node understands (eg, from EBCDIC to ASCII)
Performs data encryption
Performs data compression
Application Layer FunctionsEnables sharing remote drivers and printers
Handles e-mail messages
Provides file transfer services
Provides file management services
Provides terminal emulation services
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
Peer Protocols
Primitives
Layered Communications
Applying the OSI Model
Types of NetworksThree main topologies
Bus
Ring
Star
Bus TopologyBuilt by running cable from one PC or file server to the next
Terminators signal the physical end to the segment
Advantages of Bus TopologyWorks well for small networks
Relatively inexpensive to implement
Easy to add to it
Disadvantages ofBus Topology
Management costs can be high
Potential for congestion with network traffic
Ring TopologyContinuous path for data with no logical beginning or ending point, and thus no terminators
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
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
Star TopologyOldest and most common network design
Multiple nodes attached to a central hub
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
Disadvantages ofStar Topology
Hub is a single point of failure
Requires more cable than the bus
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
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
EthernetUses CSMA/CD access method for data transmission on a network
Typically implemented in a bus or bus-star topology
Carrier sense
Collision
Ethernet Communications
Ethernet II
Ethernet Standards
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
Token Ring Frame
Token Ring TermsMultistation access unit (MAU)
Beaconing
Broadcast storms
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
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
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)
General Topology Linking LATA and IXC Lines
Connecting LANs througha T-Carrier Line
T-Carrier Services and Data Rates
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
Wireless WANsUse radio, microware, and satellite communications
Packet radio communications
Topology of a Radio Wave WAN Joining Two LANs
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
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
Chapter SummaryOpen Systems Interconnection (OSI) model
Basic network topologies
Key LAN transmission methods
WAN communications options
WAN transmission methods