What Is a WAN?
A WAN is a data communications network that covers a relatively broad geographic area and that often
uses transmission facilities provided by common carriers, such as telephone companies. WAN
technologies generally function at the lower three layers of the OSI reference model: the physical layer,
the data link layer, and the network layer. Figure 3-1 illustrates the relationship between the common
WAN technologies and the OSI model.
Point-to-Point Links
A point-to-point link provides a single, pre-established WAN communications path from the customer
premises through a carrier network, such as a telephone company, to a remote network. Point-to-point
linesareusuallyleasedfromacarrierandthusareoftencalledleasedlines.Forapoint-to-pointline,the
carrierallocatespairsofwireandfacilityhardwaretoyourlineonly.Thesecircuitsaregenerallypriced
based on bandwidth required and distance between the two connected points. Point-to-point links are
generally more expensive than shared services such as Frame Relay. Figure 3-2 illustrates a typical
point-to-point link through a WAN.
Circuit Switching
Switched circuits allow data connections that can be initiated when needed and terminated when
communication is complete. This works much like a normal telephone line works for voice
communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching.
When a router has data for a remote site, the switched circuit is initiated with the circuit number of the
remote network. In the case of ISDN circuits, the device actually places a call to the telephone number
of the remote ISDN circuit. When the
two networks are connected and authenticated, they can transfer data. When the data transmission is
complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.
Packet Switching
Packet switching is a WAN technology in which users share common carrier resources. Because this
allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally
much better than with point-to-point lines. In a packet switching setup, networks have connections into
thecarrier’snetwork,andmanycustomerssharethecarrier’snetwork.Thecarriercanthencreatevirtua
circuits between customers’ sites by which packets of data are delivered from one to the other through
the network. The section of the carrier’s network that is shared is often referred to as a cloud.
Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame
Relay, Switched Multimegabit Data Services (SMDS), and X.25.
WAN Virtual Circuits
A virtual circuit is a logical circuit created within a shared network 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. Communication over an SVC consists of three phases: circuit establishment, data transfer,
andcircuittermination.Theestablishmentphaseinvolvescreatingthevirtualcircuitbetweenthesource
and destination devices. Data transfer involves transmitting data between the devices over the virtual
circuit,andthecircuitterminationphaseinvolvestearingdownthevirtualcircuitbetweenthesourceand
destinationdevices.SVCsareusedinsituationsinwhichdatatransmissionbetweendevicesissporadic,
largely because SVCs increase bandwidth used due to the circuit establishment and termination phases,
but they decrease the cost associated with constant virtual circuit availability.
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 they increase costs due to
constantvirtualcircuitavailability.PVCsaregenerallyconfiguredbytheserviceproviderwhenanorder
is placed for service.
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 a call on a switched circuit when it needs
to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met,
such as a particular type of network traffic needing to be transmitted. When the connection is made,
traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop
the connection when the circuit has remained idle for a certain period.
Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used
to provide backup service for another type of circuit, such as point-to-point or packet switching. The
router is configured so that when a failure is detected on the primary circuit, the dial backup line is
initiated. The dial backup line then supports the WAN connection until the primary circuit is restored.
When this occurs, the dial backup connection is terminated.
WAN Devices
WANs use numerous types of devices that are specific to WAN environments. WAN switches, access
servers,modems,CSU/DSUs,andISDNterminaladaptersarediscussedinthefollowingsections.Other
devices found in WAN environments that are used in WAN implementations include routers, ATM
switches, and multiplexers.
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
referencemodel.Figure3-5illustratestworoutersatremoteendsofaWANthatareconnectedbyWAN
switches.
Access Server
Anaccessserveractsasaconcentrationpointfordial-inanddial-outconnections
Modem
A modem is a device that interprets digital and analog signals, enabling data to be transmitted over
voice-grade telephone lines. At the source, digital signals are converted to a form suitable for
transmission over analog communication facilities. At the destination, these analog signals are returned
to their digital form. Figure 3-7 illustrates a simple modem-to-modem connection through a WAN.
CSU/DSU
A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a
router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication
between these devices. Figure 3–8 illustrates the placement of the CSU/DSU in a WAN implementation.
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 on a router. A terminal adapter is essentially an ISDN modem,
although it is called a terminal adapter because it does not actually convert analog to digital signals.
Introduction to WAN Technologies
