LAN/WAN Interconnectivity

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LAN/WAN Interconnectivity
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Learning Objectives

• Explain the OSI reference model, which sets
  standards for LAN and WAN communications
• Discuss communication between OSI stacks when two
  computers are linked through a network
• Apply the OSI model to realistic networking
  situations

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Learning Objectives

• Describe 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 Interconnectivity

• Intense competition between three sectors
• Telecommunications companies
• Cable TV companies
• Satellite communications companies

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OSI Reference Model

• Foundation that brings continuity to LAN and WAN
  communications
• Product of two standards organizations
• ISO
• ANSI
• Developed in 1974
• Set 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 Layers

• Bottom layers
• Support for physical connectivity, frame
  formation, encoding, and signal transmission
• Middle layers
• Establish and maintain a communication session
  between two network nodes
• Monitor for error conditions
• Uppermost layers
• Application/software support for encrypting data
  and assuring interpretation/presentation of data

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Physical Layer Functions

• Provides 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 Functions

• Constructs data frames
• Creates CRC information checks for errors
• Retransmits data if there is an error
• Initiates communications link makes sure it is
  not interrupted (ensures node-to-node physical
  reliability)
• Examines device addresses
• Acknowledges receipt of a frame

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Data Link Layer

• Data link frame contains fields consisting of
  address and control information
• Two important sublayers
• Logical link control (LLC)
• Media access control (MAC)
• Connectionless service versus connection-oriented
  service

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Network Layer Functions

• Determines network path for routing packets
• Helps reduce network congestion
• Establishes virtual circuits
• Routes packets to other networks, resequencing
  packet transmissions when needed
• Translates between protocols

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Transport Layer Functions

• Ensures 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 Functions

• Establishes 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 Functions

• Translates 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 Functions

• Enables 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 Networks

• Three main topologies
• Bus
• Ring
• Star

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Bus Topology

• Built 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 Topology

• Works 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 Topology

• Continuous path for data with no logical
  beginning or ending point, and thus no terminators

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Advantages of Ring Topology

• Easier 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 topology
• Fewer equipment options
• Fewer options for expansion to high-speed
  communication

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Star Topology

• Oldest and most common network design
• Multiple nodes attached to a central hub

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Advantages of Star Topology

• Good 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 Methods

• Ethernet
• IEEE 802.3 specifications
• Broadest options for expansion and high-speed
  networking
• Token ring
• IEEE 802.5 specifications
• FDDI (Fiber Distributed Data Interface)
• High-speed variation of token ring

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Ethernet

• Uses 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 Ring

• Developed 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 Terms

• Multistation access unit (MAU)
• Beaconing
• Broadcast storms

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FDDI

• Fiber-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 conditions
• Long 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 services
• Telecommunications companies
• Cable TV companies
• Satellite providers
• Newer sources of WAN connectivity
• Cable television networks
• Satellite TV companies
• Wireless WANs
• Wide use of star topology

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Telecommunications WANs

• Earliest source of WAN connectivity
• Regional telephone companies, also called
• Telcos
• Regional bell operating companies (RBOCs)
• Long-distance telecommunications companies
• Plain 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 WANs

• Also 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 WANs

• Use 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 Methods

• Use different switching techniques to create data
  paths (channels) for transmitting data
• Switching
• Enables 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 Techniques
Time 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 Summary

• Open Systems Interconnection (OSI) model
• Basic network topologies
• Key LAN transmission methods
• WAN communications options
• WAN transmission methods