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Semester 4 - Chapter 2 - WANs. Introducing the various protocols and technologies used in wide-area network (WAN) environments You will learn about the basics of WANs, including common WAN technologies, types of wide-area services, encapsulation formats, and link options.

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semester 4 chapter 2 wans
Semester 4 - Chapter 2 - WANs

Introducing the various protocols and technologies used in wide-area network (WAN) environments

You will learn about the basics of WANs, including

  • common WAN technologies,
  • types of wide-area services,
  • encapsulation formats, and link options.

In this chapter, you also will learn about

  • point-to-point links,
  • circuit switching,
  • packet switching,
  • virtual circuits,
  • dialup services, and
  • WAN devices.


WAN = Data CommunicationsNetwork operating beyond a LAN's geographic scope.

You must subscribe to an outside WANserviceprovider to use WAN carrier network services.

WANs use data links such as

Integrated Services Digital Network (ISDN) and


A WAN connects the locations of an organisation to:

each other

other organizations

external services (such as databases), and to

remote users



WANs generally carry a variety of traffictypes, such as

  • voice,
  • data, and
  • video.

WAN technologies function at the three lowest layers of the OSI reference model:

  • the physical layer,
  • the data link layer, and
  • the network layer.

Telephone and dataservices are the most commonly used WAN services.

An overview of the WAN cloud organises WAN provider services into three main types:

  • Call setup (also called signalling)
  • TimeDivisionMultiplexing (TDM)
  • Frame Relay


The most commonly used terms associated with the main parts of WAN services:

  • Customer premises equipment (CPE) -- Devices physically located on the subscriber's premises.
  • Demarcation (or demarc) -- The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building.
  • Localloop (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office.
  • COswitch -- A switching facility that provides the nearest point of presence for the provider's WAN service.
  • Tollnetwork -- The collective switches and facilities (called trunks) inside the WAN provider's cloud.


A key interface is between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE).

Typically, the DTE is the router, and the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility (eg a MODEM).

The WAN path between the DTEs is called the

  • link
  • circuit
  • channel
  • line

The DTE/DCE interface uses various protocols (such as HSSI and V.35) that establish the codes that the devices use to communicate with each other. This communication determines how call setup operates and how user traffic crosses the WAN.



A virtual circuit is a logical circuit created to ensure reliable communication between two network devices. There are two types:

  • switched virtual circuits (SVCs) and
  • permanent virtual circuits (PVCs).

SVCs are dynamically established on demand and terminated when transmission is complete. There are three phases:

  • circuitestablishment
  • datatransfer
  • circuittermination.

SVCs are used in situations where data transmission between devices is sporadic.

  • Disadvantage = increased bandwidth due to the circuit establishment and termination phases
  • Advantage = decreased costs compare with constant virtual-circuit availability.

A PVC is a permanently established virtual circuit that consists of one mode:

  • data transfer.

PVCs are used in situations where data transfer between devices is constant.

  • Advantage = PVCs decreased bandwidth use because there are no establishment and termination phases
  • Disadvantage = increased costs due to constant availability.


Routers are devices that implement the network service.

They provide interfaces for a wide range of links and subnetworks at a wide range of speeds.

Routers are active and intelligent network devices and can participate in managing the network by:

  • providing dynamic control over resources
  • supporting the tasks and goals for networks.

These goals are

  • Connectivity
  • reliable performance
  • managementcontrol
  • Flexibility

A WAN switch is a multiport networking device, which typically switches such traffic as

  • FrameRelay
  • X.25
  • Switched Multimegabit Data Service (SMDS).

This switching function operates at the data link layer of the OSI reference model.



A modem is a device that interprets digital and analogue signals by modulating and demodulating the signal

This enables data to be transmitted over voice-grade telephone lines.

At the source, digital signals are convertedto a form suitable for transmission over analogue communication facilities.

At the destination, these analogue signals are returnedto their digitalform.



A CSU/DSU is a digital-interface device

or sometimes two separate digital devices

A CSU/DSU adapts the physicalinterface on a DTE device to the interface of a DCE device in a switched-carrier network.



An ISDN TerminalAdapter (TA) is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces.



WANs use the OSI reference model layered approach to encapsulation, just as LANs do, but they are mainly focused on the physical and datalink layers.

WAN standards typically describe both physical-layer delivery methods and data link-layer requirements, including addressing, flowcontrol and encapsulation.

WAN standards are defined and managed by a number of recognized authorities, including the following agencies:

  • International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), formerly the Consultative Committee for International Telegraph and Telephone (CCITT)
  • International Organization for Standardization (ISO)
  • Internet Engineering Task Force (IETF)
  • Electronic Industries Association (EIA)
  • Telecommunications Industries Association (TIA)


The WAN data link layer defines how data is encapsulated for transmission to remote sites.

WAN data-link protocols describe how frames are carried between systems on a single data path.

Common data-link encapsulations associated with WAN lines are:

  • Frame Relay -- Simplified encapsulation with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly compared to the other WAN protocols.
  • Point-to-Point Protocol (PPP) -- Described by RFC 1661, PPP was developed by the IETF. PPP contains a protocol field to identify the network-layer protocol.
  • ISDN -- A set of digital services that transmits voice and data over existing phonelines.
  • Link Access Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. LAPB provides reliability and flowcontrol on a point-to-point basis.
  • Cisco/IETF -- Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Ciscorouters.
  • High-Level Data Link Control (HDLC) -- An ISO standard, HDLC 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.


The two most common point-to-point WAN encapsulations are HDLC and PPP. All the serial line encapsulations share a common frame format, which has the following fields: 

  • Flag - Indicates the beginning of the frame and is set to the hexadecimal pattern 7E. 
  • Address - A 1- or 2-byte field to address the end station in multidrop environments. 
  • Control - Indicates whether the frame is an information, a supervisory, or an unnumbered type frame. It also contains specific function codes. 
  • Data - The encapsulated data. 
  • FCS - The frame check sequence (FCS). 
  • Flag - The trailing 7E flag identifier.

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 need to configure the Layer 2 encapsulation type to use for each serial interface on a router. The choice of encapsulation protocol depends on the WAN technology and the communicating equipment. Encapsulation protocols that can be used with the WAN connection types covered in this chapter are PPP and HDLC.



The two most common point-to-point WAN encapsulations are


PPP can check for link quality during connection establishment.

In addition, there is support for authentication through

Password AuthenticationProtocol (PAP) and

Challenge Handshake Authentication Protocol (CHAP).

HDLC - a data link-layer protocol derived from the Synchronous Data Link Control (SDLC) encapsulation protocol.

HDLC is Cisco's default encapsulation for serial lines.



Dedicated lines, also called leased lines, provide full-time service.

Dedicated lines typically are used to carry




In data network design, dedicated lines generally provide core or backbone connectivity between major sites or campuses, as well as LAN-to-LAN connectivity.

Dedicated lines generally are considered reasonable design options for WANs.

The following are required for dedicated line connections.

a router port


actual circuit from the service provider.



Packet switching is a WAN switching method in which network devices share a permanent virtual circuit

The following are examples of packet-switched WAN technologies.

Frame relay

SMDS (Switched Multimegabit Data Service)




Frame Relay was designed to be used over high-speed, highqualitydigital facilities.

As a result, Frame Relay does not offer much error checking or reliability

Frame Relay expects upper-layer protocols to attend to these issues.

A data-link connection identifier (DLCI) identifies a PVC.

The DLCI number is a localidentifier between the DTE and the DCE

The DLCI identifies the logicalcircuit between the source and destination devices.



Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session.

ISDN is an example of a circuit-switched WAN technology.



Dial-on-demand routing (DDR) is a technique in which a router can dynamicallyinitiate and close circuit-switched sessions when transmitting end stations need them.

When the router receives traffic destined for a remote network, a circuit is established, and the traffic is transmitted normally.

DDR enables you to make a standardtelephone connection or an ISDN connection only when required by the volume of network traffic.

DDR can be used to provide backuploadsharing and interface backup.

Compared to LAN or campus-based networking, the traffic that uses DDR is typically lowvolume and periodic.



ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today.

ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels.

Two of the channels, called B (bearer) channels, operate at 64kbps and are used to carry voice or data traffic.

The third channel, the D (delta) channel, is a 16-kbps signalling channel used to carry instructions that tell the telephone network how to handle each of the B channels.

ISDN BRI often is referred to as 2B+D.