Module 2

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Module 2

• WAN Technologies

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WAN Technology

• A Wide Area Network (WAN) is used to interconnect
  Local Area Networks (LANs) that are separated by
  a large geographical distance.
• A Wide Area Network predominately operates at the
  OSI physical and data link layers.
• The WAN provides a data path between routers and
  the LANs that each router supports.

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• MAJOR CHARACTERISTICS OF WANS
• The network operates beyond the local LANs
  geographic scope. It uses the services of
  carriers such as Regional Bell Operating
  Companies (RBOCs).
• WANs use serial connections of various types to
  access bandwidth over wide-area geographies.
• By definition, the WAN connects devices separated
  by wide areas.

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• WAN DEVICES INCLUDE
• Routers that offer many services including
  internetworking and WAN interface ports.
• Switches that connect to WAN bandwidth for voice,
  data, and video communication.
• Modems that interface voice-grade services
  Include channel service units/digital service
  units (CSU/DSU) that interface T1/E1 services
  Terminal Adapters/Network Termination 1 (TA/NT1)
  that interface Integrated Services Digital
  Network (ISDN) services.
• Communication servers that concentrate dial-in
  and dial-out user communication.
• WANS use the OSI layered approach for
  encapsulation just like LAN's but are mainly
  focused on the physical and data link layers.

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WAN Physical Layer

• WAN physical layer protocols describe how to
  provide electrical, mechanical, operational, and
  functional connections for wide-area networking
  services.
• These services are most often obtained from WAN
  service providers such as Regional Bell Operating
  Companies (RBOCs), alternate carriers, and Post,
  Telephone, and Telegraph (PTT) agencies.

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WAN Data Link Protocols

• WAN data link protocols describe how frames are
  carried between systems on a single data path.
• They include protocols designed to operate over
  dedicated point-to-point, multipoint, and
  multi-access switched services such as Frame
  Relay.

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WAN Standards

• WAN standards typically describe both physical
  layer delivery methods and data link layer
  requirements including addressing and flow
  control encapsulation

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describes the interface between the data terminal
equipment (DTE) and the data circuit-terminating
equipment (DCE).
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DTE DCE

• DTE - data terminal equipment. Device at the user
  end of a user-network interface that serves as a
  data source, destination, or both. DTE connects
  to a data network through a DCE device (for
  example, a modem) and typically uses clocking
  signals generated by the DCE.
• DTE includes such devices as computers, routers,
  and multiplexers.
• DCE - Data communications equipment (EIA) or data
  circuit-terminating equipment (ITU-T). The
  devices and connections of a communications
  network that comprise the network end of the
  user-to-network interface. The DCE provides a
  physical connection to the network, forwards
  traffic, and provides a clocking signal used to
  synchronize data transmission between DCE and DTE
  devices. Ex Modems and interface cards

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The WAN data-link protocols describe how frames
are carried between systems on a single path
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The Data Link Layer WAN Protocols

• High-Level Data Link Control (HDLC)HDLC is an
  IEEE standard. It might not be compatible between
  different vendors because of the way each vendor
  has chosen to implement it.
• HDLC supports both point-to-point and multipoint
  configurations with minimal overhead
• Frame Relay - Frame Relay uses high-quality
  digital facilities.
• By using a simplified framing with no error
  correction mechanisms, Frame Relay can send Layer
  2 information much more rapidly than these other
  WAN protocols.
• Point-to-Point Protocol - Described by RFC 1661.
  PPP contains a protocol field to identify the
  network-layer protocol.
• Integrated Services Digital Network (ISDN) - ISDN
  is a set of digital services that transmits voice
  and data over existing phone lines.

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Wide Area Networking
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The WAN Cloud

• An overview of the WAN cloud organizes WAN
  provider services into
• Call setup serviceSets up and clears calls
  between telephone users.
• Also called signaling, call setup uses a separate
  telephone channel not used for other traffic.
• The most commonly used call setup is Signaling
  System number 7 (SS7). SS7 is an out-of-band
  signaling system for the exchange of call control
  information between network switching offices, in
  support of voice and nonvoice services

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Time-division multiplexing (TDM)

• Information from many sources has bandwidth
  allocation on a single media.
• Circuit switching uses signaling to determine the
  call route, which is a dedicated path between the
  sender and the receiver.
• By multiplexing traffic into fixed time slots,
  TDM avoids congested facilities and variable
  delays.
• Basic telephone service and ISDN services use TDM
  circuits.

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Time-division multiplexing (TDM)

• Transmits multiple signals simultaneously over a
  single transmission path. Each lower-speed signal
  is time sliced into one high-speed transmission.
• Example Three incoming 1,000 bps signals (A, B
  and C) can be interleaved into one 3,000 bps
  signal (AABBCCAABBCCAABBCC).
• The receiving end divides the single stream back
  into its original signals.

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• When your organization subscribes to an outside
  WAN provider for network connections, the
  provider assigns your organization the rules for
  connecting WAN calls.
• Your organization makes connections to
  destinations as point-to-point calls.

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• Demarcation (or demarc)
• The point at which the CPE ends and the local
  loop portion of the service begins.
• Often occurs at the Point of Presence (POP) of a
  building.
• Local loop (or last-mile)
• Cabling (usually copper wiring) that extends from
  the demarc into the WAN service providers
  central office.

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• Central office (CO) switch
• A switching facility that provides the nearest
  point of presence for the providers WAN service.

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• Toll network
• The collective switches and facilities (called
  trunks) inside the WAN providers cloud.
• The callers traffic may cross a trunk to a
  primary center, then go to a sectional center,
  and then to a regional- or international-carrier
  center as the call goes the long distance to its
  destination.
• Switches operate in provider offices with toll
  charges based on tariffs or authorized rates.

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• A key interface in the customer site occurs
  between the data terminal equipment (DTE) and the
  data circuit-terminating equipment (DCE).
• Typically, DTE is the router.

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• DCE is the device used to convert the user data
  from the DTE into a form acceptable to the WAN
  services facility.
• In the graphic, the DCE is the attached modem,
  channel service unit/data service unit (CSU/DSU)
  or Terminal Adapter/Network Termination 1
  (TA/NT1).

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• Data communication over WANs interconnects DTEs
  so they can share resources over a wide area.
• The WAN path between the DTEs is called the link,
  circuit, channel, or line.
• The DCE primarily provides an interface for the
  DTE into the communication link in the WAN cloud.
  
• The DTE/DCE interface acts as a boundary where
  responsibility for the traffic passes between the
  WAN subscriber and the WAN provider.

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DSU/CSU

• A pair of communicating devices that connect an
  in-house line to an external digital circuit
  (T1). It is similar to a modem, but connects a
  digital circuit rather than an analog one.

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CSU

• Terminates the external line at the customer
  premises.
• Provides diagnostics and allows for remote
  testing.
• If the customer's communications devices are T1
  ready and have the proper interface, then the CSU
  is not required, only the DSU.

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DSU

• Does the actual transmission and receiving of the
  signal and provides buffering and flow control.
• DSU and CSU can be in the same unit.
• DSU may also be built into the multiplexor,
  (combines digital signals for high-speed lines).

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• Forms of WAN services with routers.
• The most common are
• Switched or relayed services.
• Frame Relay
• ISDN (Integrated Services Digital Network)
• ATM (Asynchronous Transfer Mode)
• X.25
• Peer Devices
• HDLC (High-level Data Link Control)
• PPP (Point-to-Point Protocol)
• DDR (Dial on Demand Routing)
• LAPB - point to point and X.25

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Both Frame Rely and X.25 use the concept of
virtual circuits
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X.25

• The first packet switched networks
• X.25 provides a connection-oriented technology
  for transmission over highly-error prone
  facilities.
• Error checking is performed at each node, which
  can slow overall throughput and and therefore
  would not make X.25 a choice for voice and video
• X.25 can be very cost effective because tariffs
  are based on the amount of data delivered rather
  than connection time or distance

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Asynchronous Transfer Mode (ATM)

• Lower latency at higher bandwidths
• Data rates beyond 155 Mbps
• Cell-based rather than frame-based
• Cell are a fixed length of 53 bytes
• Also uses PVCs
• Less efficient because of the small size

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Digital Subscriber Line (DSL)

• Uses existing telephone lines
• Uses multiple frequencies within the same
  physical medium to transmit data
• Bandwidth can vary
• Distance of the local loop is a factor must be
  less than 3.5 miles

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Cable Modem

• Two-way, high-speed data transmissions using the
  same coaxial lines that transmit cable
  television.
• Always on

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WAN Frame Encapsulation Formats
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• Layer 2 Encapsulation
• Each WAN connection type uses a Layer 2 protocol
  to encapsulate traffic while it is crossing the
  WAN link.
• To ensure that the correct encapsulation protocol
  is used, you will need to configure the Layer 2
  encapsulation type to use.
• The choice of encapsulation protocol depends on
  the WAN technology and the communicating
  equipment.

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• PPP
• Common for dialup single-user-to-LAN (dialup and
  ISDN) or LAN-to-LAN (router-to-router) access.
• PPP is standardized, so it supports vendor
  interoperability.
• It also supports the encapsulation of multiple
  upper-layer protocols including IP and IPX.

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• HDLC
• The Cisco default encapsulation type on
  point-to-point links.
• It is used typically when communicating with
  another Cisco device.
• If communicating with a non-Cisco device,
  synchronous PPP is a viable option.
• HDLC is normally proprietary between vendors.

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• LAPB (layer 2 of the X.25 protocol stack)
• For packet-switched networks, the LAPB protocol
  is used to encapsulate X.25 packets.
• It can also be used over point-to-point links, if
  the link is unreliable or there is an inherent
  delay associated with the link, such as in a
  satellite link.
• LAPB provides reliability and flow control on a
  point-to-point basis.

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HDLC

• HDLC is Ciscos default encapsulation for serial
  lines.
• This implementation is very streamlined.
• There is no windowing or flow control and only
  point-to-point connections are allowed (no
  multipoint).
• 2-byte proprietary type code is inserted after
  the control field, which means that HDLC framing
  is not interoperable with other vendors
  equipment.

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WAN Design Basics
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• When leased line connections are made
• a router port is required for each connection,
• along with a CSU/DSU and
• the actual circuit from the service provider.
• The cost of dedicated line solutions can become
  significant when deployed to connect many sites

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Dedicated connectivity, also referred to as
leased lines, provides full-time synchronous
connections. Dedicated, full-time connectivity
is provided by point-to-point serial links.
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Network Design

• Network designs tend to follow one of two general
  design strategies
• mesh
• hierarchical

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• Mesh structure
• Net topology is flat
• All routers perform essentially the same
  functions
• Usually no clear definition of where specific
  functions are performed.
• Expansion of the network tends to proceed in a
  haphazard, arbitrary manner.

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• Hierarchical structure the network is organized
  in layers that each have one or more specific
  functions.
• Benefits to using a hierarchical model include
  the following
• Scalability
• Ease of implementation
• Ease of troubleshooting
• Predictability
• Protocol support
• Manageability

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• The three-layer model consists of
• core
• distribution
• access layers

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• Core layer -
• Provides fast wide-area connections between
  geographically remote sites, tying a number of
  campus networks together in a corporate or
  enterprise WAN.
• Core links are usually point-to-point, and there
  are rarely any hosts in the core layer.
• Core services are typically leased from a telecom
  service provider (for example, T1/T3, Frame
  Relay, SMDS, and so on).

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• Distribution layer -
• Refers to the distribution of network services to
  multiple LANs within a campus network
  environment.
• This layer is where the campus backbone network
  is found, typically based on Fast Ethernet.
• This layer is implemented on sites that are large
  and is used to interconnect buildings.

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• Access layer -
• Usually a LAN or a group of LANs, typically
  Ethernet or Token Ring, that provide users with
  frontline access to network services.
• The access layer is where almost all hosts are
  attached to the network, including servers of all
  kinds and user workstations.

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• The three layers are bounded by Layer 3 devices
  or other devices that provide separation into
  broadcast domains.
• Note A three-layer model can usually meet the
  needs of most enterprise networks.
• However, not all environments require a full
  three-layer hierarchya one- or two-layer design
  may be adequate.
• Even in these cases, however, a hierarchical
  structure should be maintained.

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• The distribution layer would include the campus
  backbone with all its connecting routers.
• Because policy is typically implemented at this
  level, we can say that the distribution layer
  provides policy-based connectivity.
• Policy-based connectivity means that the layer 3
  routers are programmed to only allow traffic on
  the campus backbone that the network manager has
  determined acceptable.

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• The access layer connects users into LANs, and
  LANs into campus backbones or WAN links.
• This approach enables designers to distribute
  services across the CPUs of devices operating at
  this layer.
• The access layer allows logical segmentation of
  the network and the grouping of users based on a
  function.

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• The one-layer design is typically implemented
  where
• Only a few remote locations in the company
• access to applications are mainly done via the
  local LAN to the site file server.
• Each site is its own broadcast domain.

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• In a two-layer design, a WAN link is used to
  interconnect separate sites.
• VLANs may be implemented to create separate
  logical networks without requiring additional
  routers.
• Inside the site multiple LANs may be implemented
  with each LAN segment being its own broadcast
  domain.
• Router becomes a concentration point for WAN
  links.

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• Remote sites can access the WAN core network
  using WAN technologies other than dedicated
  links.
• Frame Relay or ISDN are two such alternatives.
• If a remote site is small and has low demand for
  access to services in the corporate network, ISDN
  would be a logical choice for this
  implementation.
• Perhaps another remote site cannot get access to
  dedicate WAN links from their service provider
  but has access to Frame Relay.
• In either case an entry point needs to be
  established for these types of WAN connections in
  to the WAN core.

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• One of the advantages of hierarchical WAN design
  is it provides a method for controlling data
  traffic patterns by putting Layer 3 routing
  points throughout the network.
• Since routers have the ability to determine paths
  from the source host to destination hosts based
  on Layer 3 addressing, data traffic will flow up
  the hierarchy only as far as it needs to to find
  the destination host.

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• If Host A were to establish a connection to Host
  B, the traffic from this connection would travel
  to Router 1 and be forwarded back down to Host B.
  
• Notice that this connection did not require any
  traffic be placed on the link between Router 1
  and Router 2, thus conserving the bandwidth on
  that link.

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• In a two-layer WAN hierarchy, the traffic
  patterns are still governed by host source and
  destination addresses and path determinations of
  the router.
• In this model again the traffic will only travel
  up the hierarchy as far as needed to get to the
  destination thus conserving bandwidth on other
  WAN links.

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Module 2

• WAN Technologies