Chapter 5: WAN Concepts

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CCNA Guide to Cisco Networking

• Chapter 5 WAN Concepts

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Objectives

• Describe WAN standards
• Explain the WAN connection methods
• Discuss WAN data link protocols
• Understand the WAN Physical layer
• Recognize the various WAN physical topologies
• Emerging WAN connection methods

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WAN Standardization

• Organizations that create standards
• International Organization for Standards (ISO)
• American National Standards Institute (ANSI)
• Electronic Industries Alliance (EIA)
• Internet Engineering Task Force (IETF)
• International Telecommunications
  Union-telecommunication (ITU-T)

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WAN Connection Methods

• Four connection methods
• Dedicated point-to-point
• Also known as a leased line (usually connects two
  office LANs)
• Point-to-multipoint
• TDM Time division multiplexing (Data, voice and
  video)
• Circuit-switched (defined Path but not
  pre-determined)
• Packet-switched (Frame relay)
• Standards organizations define several types of
  Data Link protocols used on WAN connections

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WAN Connection Methods (continued)
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WAN Connection Methods (continued)
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WAN Connection Methods (continued)
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WAN Data Link Protocols

• Three categories of Data Link protocols used for
  WAN
• Interface to IBM enterprise data centers
• Synchronous Data Link Control (SDLC)
• WAN connections using peer devices
• High-level Data Link Control (HDLC) - ANSI
• Point-to-Point Protocol (PPP)
• Switched or relay services
• X.25/Link Access Procedure Balanced (LAPB)
• Frame Relay

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WAN Data Link Protocols (continued)

• Three categories of Data Link protocols used for
  WAN (continued)
• Switched or relay services (continued)
• Integrated Services Digital Network (ISDN)/Link
  Access Procedure D-channel (LAPD)
• Asynchronous Transfer Mode (ATM)

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WAN Data Link Protocols (continued)

• SDLC
• IBM hosts system communication over WAN links
• Point-to-point
• Point-to-multipoint
• Connections between
• Remote devices
• Central mainframe
• Synchronous communication

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WAN Data Link Protocols

• HDLC
• Default protocol on WAN links
• Also known as Advanced Data Communications
  Control Procedure (ADCCP)
• Superset of the SDLC protocol
• Point-to-point
• Point-to-multipoint
• Supports full-duplex and half-duplex
• Synchronous and asynchronous
• Default HDLC does not support multiple protocols
• Cisco HDLC allows for multiple protocols

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WAN Data Link Protocols (continued)

• PPP
• SLIP (Serial Line Internet Protocol)
• Further extends default HDLC by providing a
  protocol field
• Runs over Dial-up or Leased lines
• Network Control Protocols (NPCs)
• Because It uses NCPs, it Can transfer IP, IPX,
  AppleTalk and other network layer protocols

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WAN Data Link Protocols (continued)

• PPP (continued)
• PPP provides the following connections
• Router-to-router
• Host-to-router
• Host-to-host
• Physical interfaces for PPP
• Asynchronous serial
• ISDN/synchronous serial
• High-speed Serial Interface (HSSI)

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WAN Data Link Protocols (continued)
LCP Link Protocol Control)
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WAN Data Link Protocols (continued)

• X.25/LAPB (Link Access Procedures Balanced)
• Packet-switching
• Is a CCITT (ITU-T) spec
• Widely implemented for international
  communications and in North America RBOCs
• Connections over virtual circuits (between 2 pts)
• Create to function over existing unreliable
  analog telecommunication lines
• Errors correction and flow control
• Older than OSI, does not directly translate
• Original throughput was 56Kbps
• CCITT revised the standard to support 2Mbps

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WAN Data Link Protocols (continued)
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WAN Data Link Protocols (continued)

• Frame Relay
• A Data Link protocol
• A service
• Improved upon X.25 standard
• Operates between 56 Kbps and 45 Mbps
• Higher layer protocols provide error checking and
  flow control
• Packet-switching
• Shared bandwidth with other frame relay
  subscribers

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WAN Data Link Protocols (continued)

• Frame Relay (continued)
• Defines the connection between
• Customer premise equipment (CPE)
• Service providers local access switching
  equipment
• Operates on almost any Physical layer interface
• Two types of virtual circuits
• Switched virtual circuits (SVC)
• Permanent virtual circuits (PVC)
• Uses statistical multiplexing to allocate
  bandwidth

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WAN Data Link Protocols (continued)

• ISDN/LAPD
• Circuit-switched technology
• Dedicated circuit for the period of transmission
• Two types of ISDN
• Basic rate interface (BRI)
• Primary rate interface (PRI)
• Bandwidth
• BRI 128 kbps
• PRI 1.544 mbps

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WAN Data Link Protocols (continued)

• ISDN/LAPD
• B-channels (bearer channels)
• 64-kbps
• D-channel (data channel)
• BRI 16 kbps
• PRI 64 kbps
• BRI is called 2BD
• Two B channels
• One D channel
• PRI is called 23BD
• 23 B channels
• One D channel

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WAN Data Link Protocols (continued)

• ATM
• Used within and between LAN connections
• Based on Frame Relay technology
• Cell-switched technology
• Circuit-oriented in the sense that ATM cells
  follow the same path for the duration of the
  connection
• Cell are fixed length of 53 bytes

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WAN Data Link Protocols (continued)

• ATM (continued)
• Provides high-speed data transmission
• No error correction like X.25
• Bandwidth up to 10 gbps

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WAN Physical Layer
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WAN Connections

• Provide connections between two LANs
• Connections between
• Channels service unit/ data service unit CSU/DSU
• Router
• Routers now include internal CSU/DSU
• Data terminal equipment (DTE)
• Data-circuit terminating equipment (DCE)

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WAN Connections (continued)

• Demarcation Point or demarc
• Point of presence (POP)
• Local loop or last mile
• Central office switch (CO)
• Toll network

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WAN Physical Standards

• Physical WAN serial interface standards
• EIA/TIA-232
• EIA/TIA-449
• EIA-530
• High-Speed Serial Interface (HSSI)
• V.24
• V.35
• X.25

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WAN Physical Standards (continued)

• Physical WAN serial interface standards
  (continued)
• X.21
• G.703

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WAN Topologies

• WAN topologies
• Peer
• Star
• Partial mesh
• Mesh

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WAN Topologies (continued)
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WAN Topologies (continued)

• Peer topology
• Daisy-chained
• Simplest WAN topology
• Least expensive
• Easy to configure
• No redundancy

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WAN Topologies (continued)

• Star topology
• Most implemented design
• Also know as the hub-and-spoke topology
• Simple hierarchical design
• One central router acting as the hub
• Several edge routers connect to hub

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WAN Topologies (continued)

• Full mesh
• Most expensive topology
• Most fault-tolerant design
• Each router has a connection to every other
  router
• Partial mesh
• Compromise between star and full mesh
• Connection are made according to need and traffic
  flow

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Emerging WAN Connection Methods

• Broadband access
• Two new WAN connection methods
• Digital subscriber lines (DSL)
• Cable access with cable modem
• DSL
• Two types of DSL
• Asynchronous DSL
• Symmetric DSL

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Emerging WAN Connection Methods (continued)

• DSL (continued)
• Modulates voice and data over existing copper
  phone lines
• Download speeds vary
• 256 kbps to faster than T1 (1.544 mbps)
• Upload speeds are typically slower than download
  speeds
• Speed and cost are attractive
• Distance limitations to CO

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Emerging WAN Connection Methods (continued)

• Cable access and cable modems
• Uses existing cable TV infrastructure
• Shared bandwidth
• Distance limitation not an issue like DSL
• Need VPN!

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Summary

• WANs connect LANs in geographically separate
  areas
• WAN connections typically function at the
  Physical and Data Link layers of the OSI
  reference model, and are made over serial
  connections
• WAN connections operate at a lower speed than LAN
  connections, and can be made as point-to-point,
  point-to-multipoint, and switched WAN connections
• You can use several different data link protocols
  for WAN connections

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Summary (continued)

• The different types of WAN connections are (1)
  interfaces to IBM mainframes provided by SDLC
  (2) WAN connections using peer devices with HDLC
  and PPP and (3) switched or relayed services
  including X.25, Frame Relay, ATM, and ISDN
• X.25 is the oldest of the switched or relayed
  services and provides the least efficient service
  because of its excessive error checking
• Frame Relay is an enhancement over X.25 because
  it is faster and does not provide redundant error
  checking

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Summary (continued)

• ATM is similar to Frame Relay, except that it
  uses fixed-length cells instead of
  variable-length packets
• In addition, the ATM protocol can be used on LANs
  as well as WANs
• ISDN is a leased digital line that can support
  X.25 and Frame Relay connections, among others
• ISDN comes in PRI and BRI levels

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Summary (continued)

• BRI only provides 128-Kbps throughput over two
  B-channels, and 16 Kbps over one D-channel that
  is used for controlling the connection
• PRI provides 23 B-channels of 64 Kbps each and
  one 64-Kbps D-channel for controlling the
  connection
• This allows PRI to offer 1.544-Mbps throughput
• The Physical layer WAN connections concern the
  interface between the DTE and DCE

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Summary (continued)

• The DTE is the endpoint of the users network,
  which connects to the WAN interface
• This is typically a router, computer, or terminal
  of some type
• The WAN service provider usually provides the
  DCE, which is often a CSU/DSU, modem, or terminal
  adapter
• The DCE is then connected to a demarc, which is a
  communications facility owned by the WAN service
  provider
• The local loop is the connection (usually copper
  cable) that links the demarcation to the WAN
  service providers CO switch, which is actually
  part of the toll network or PSTN

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Summary (continued)

• The physical WAN topologies are
• Peer, which is simply customer facilities
  connected in a daisy-chained fashion
• Star, which involves connecting remote customer
  facilities to a central facility
• Full mesh, which connects every location to every
  other location
• Partial mesh, which provides redundancy only
  where necessary