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Wide Area Network Protocols

This page introduces various protocols and technologies used in Wide Area Network (WAN) environments.  Sections include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices.

To begin with, a WAN is a data communications network that covers a relatively broad geographic area and often uses transmission facilities provided by common carriers, such as telephone companies.  WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

Point-to-Point Links

A point-to-point link provides a single, preestablished WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network.  A point-to-point link is also known as a leased line because its established path is permanent and fixed for each remote network reached through the carrier facilities.

Circuit Switching

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.  Circuit switching accommodates two types of transmissions: datagram transmissions and data-stream transmissions.  Used extensively in telephone company networks, circuit switching operates much like a normal telephone call.  Integrated Services Digital Network (ISDN) is an example of a circuit-switched WAN technology.

Packet Switching

Packet switching is a WAN switching method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network.  Statistical multiplexing is used to enable devices to share these circuits. Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25 are examples of packet-switched WAN technologies.

WAN Virtual Circuits

A virtual circuit is a logical circuit created to ensure reliable communication between two network devices.  Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete.  SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but decrease the cost associated 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 in which data transfer between devices is constant.  PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but increase costs due to constant virtual circuit availability.

WAN Dialup Services

Dialup services offer cost-effective methods for connectivity across WANs.  Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup.

DDR is a technique whereby a router can dynamically initiate and close a circuit-switched session as transmitting end station demand.  A router is configured to consider certain traffic interesting (such as traffic from a particular protocol) and other traffic uninteresting.  DDR can be used to replace point-to-point links and switched multiaccess WAN services.

Dial backup is a service that activates a backup serial line under certain conditions.  The secondary serial line can act as a backup link that is used when the primary link fails or as a source of additional bandwidth when the load on the primary link reaches a certain threshold.

WAN Devices

WAN Switch

A WAN switch is a multiport internetworking device used in carrier networks.  These devices typically switch such traffic as Frame Relay, X.25, and SMDS and operate at the data link layer of the OSI reference model.

Access Server

An access server acts as a concentration point for dial-in and dial-out connections.


A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines.


A CSU/DSU (channel service unit/digital service unit) is a digital-interface device that adapts the physical interface on a DTE (Data Terminal Equipment) device to the interface of a DCE (Data Circuit-Terminating) device in a switched-carrier network.  The CSU/DSU also provides signal timing for communication between these devices.

ISDN Terminal Adapter

An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232.  A terminal adapter is essentially an ISDN modem.

High-Speed Serial Interface

The High-Speed Serial Interface (HSSI) is a DTE/DCE interface developed by Cisco Systems to address the need for high-speed communication over WAN links.  HSSI defines both the electrical and the physical DTE/DCE interfaces. It therefore corresponds to the physical layer of the OSI reference model.  The flexibility of the HSSI clock and data-signaling protocol makes user (or vendor) bandwidth allocation possible.  The DCE controls the clock by changing its speed or by deleting clock pulses.  In this way, the DCE can allocate bandwidth between applications.  HSSI assumes a peer-to-peer intelligence in the DCE and DTE.  The control protocol is simplified, with just two control signals required ("DTE available" and "DCE available").  Both signals must be asserted before the data circuit is valid.  The DCE and DTE are expected to be able to manage the networks behind their interfaces.  HSSI technical characteristics are summarized as follows:

Characteristic Value

Maximum signaling rate

52 Mbps

Maximum cable length

50 feet

Number of connector pins




Electrical technology

Differential ECL

Typical power consumption

610 mW



Cable type

Shielded twisted-pair wire

WAN Connection Types:

Leased Lines

Typically referred to as a point-to-point connection or dedicated connection.  Is is a pre-established WAN communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting date.  It uses synchronous serial lines up to 45Mbps.

Circuit Switching

Sets up like a phone call.  No data can transfer before the end-to-end connection is established.  Uses dial-up modems and ISDN.  It is used for low-bandwidth data transfers.

Packet Switching

WAN switching method that allows you to share bandwidth with other companies to save money.  As long as you are not constantly transmitting data and are instead using bursty data data transfers, packet switching can save you a lot of money.  However, if you have a constant data transfers, you will need to use a leased line.  Frame Relay and X.25 are packet switching technologies.  Speeds can vary from 56Kbps to 2.048Mbps.

WAN Protocols:

Frame Relay

A packet switching technology that emerged in the early 1990's.  Frame Relay is a Date Link and Physical Layer specification that provides high performance.  Frame Relay assumes that the facilities used are less error prone than when X.25 was being implemented and that they use less overhead.  Frame Relay is more cost-effective than point-to-point links and can run at speeds of 64Kbps to 45Mbps.  Frame Relay provides features for dynamic-bandwidth allocation and congestion control.


ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs(Public Data Networks).  X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol.  Frame Relay has to some degree superseded X.25.


Integrated Services Digital Network is a set of digital services that transmit voice and data over existing phone lines.  ISDN can offer a cost-effective solution for remote users who need a higher-speed connection than an analog dial-up link offers.  ISDN is also a good choice as a backup link for other types of links such as Frame Relay or a T-1 connection.


Link Access Procedure Balanced was created to be used as a connection-oriented protocol at the Data Link layer for use with X.25.  It can also be used as a simple Data Link transport.  LAPB has a tremendous amount of overhead because of its strict timeout and windowing techniques.  You can use LAPB instead of the lower-overhead HDLC if your links are very error prone.  However, that typically is not a problem anymore.


High-Level Data Link Control was derived from Synchronous Data Link Control (SDLC), which was created by IBM as a Data Link connection protocol.  HDLC is a connection-oriented protocol at the Data Link layer, but it has very little overhead compared to LAPB.  HDLC was not intended to encapsulate multiple Network layer protocols across the same link.  The HDLC header carries no identification of the type of protocol being carried inside the HDLC encapsulation.  Because of this, each vendor that uses HDLC has their own way of identifying the Network layer protocol, which means that each vendor's HDLC is proprietarty for their equipment.


Synchronous Data Link Control.  SNA data link layer communications protocol.  SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous protocols, including HDLC and LAPB.


Point-to-Point protocol is an industry standard protocol.  Because many versions of HDLC are proprietary, PPP can be used to create point-to-point links between different vendor's equipment.  It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol.  It allows authentication and multilink connections and can be run over asynchronous and synchronous links.

Click the Links to see protocol specific information.