Introduction to WAN Technologies

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Introduction to WAN Technologies

Post  adnanbahrian on Fri Mar 13, 2009 10:14 am

Introduction to WAN Technologies




This chapter introduces the various protocols and technologies used in
wide-area network (WAN) environments. Topics summarized here include
point-to-point links, circuit switching, packet switching, virtual
circuits, dialup services, and WAN devices. Chapters in Part III, "WAN
Protocols," address specific technologies in more detail.




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.



Figure 3-1 WAN Technologies Operate at the Lowest Levels of 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 lines are usually leased
from a carrier and thus are often called leased lines. For a
point-to-point line, the carrier allocates pairs of wire and facility
hardware to your line only. These circuits are generally priced 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.



Figure 3-2 A Typical Point-to-Point Link Operates Through a WAN to a Remote Network







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.



Figure 3-3 A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call







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 the carrier's network, and many customers share the
carrier's network. The carrier can then create virtual 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. Figure
3-4 shows an example packet-switched circuit.



The virtual connections between customer sites are often referred to as a virtual circuit.



Figure 3-4 Packet Switching Transfers Packets Across a Carrier Network







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, and
circuit termination. The establishment phase involves creating the
virtual circuit between the source and destination devices. Data
transfer involves transmitting data between the devices over the
virtual circuit, and the circuit termination phase involves tearing
down the virtual circuit between the source and destination devices.
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 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 constant virtual circuit availability. PVCs are
generally configured by the service provider when an order 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, and ISDN terminal
adapters are discussed in the following sections. 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 reference model. Figure
3-5 illustrates two routers at remote ends of a WAN that are connected
by WAN switches.



Figure 3-5 Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches








Access Server





An access server
acts as a concentration point for dial-in and dial-out connections.
Figure 3-6 illustrates an access server concentrating dial-out
connections into a WAN.



Figure 3-6 An Access Server Concentrates Dial-Out Connections into a WAN








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.



Figure 3-7 A Modem Connection Through a WAN Handles Analog and Digital Signals








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.



Figure 3-8 The CSU/DSU Stands Between the Switch and the Terminal








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.
Figure 3-9 illustrates the placement of the terminal adapter in an ISDN
environment.



Figure 3-9 The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces







Review Questions






Q—What are some types of WAN circuits?



A—Point-to-point, packet-switched, and circuit-switched.



Q—What is DDR, and how is it different from dial backup?



A—DDR
is dial-on-demand routing. DDR dials up the remote site when traffic
needs to be transmitted. Dial backup uses the same type of services,
but for backup to a primary circuit. When the primary circuit fails,
the dial backup line is initiated until the primary circuit is
restored.



Q—What is a CSU/DSU used for?



A—A CSU/DSU interfaces a router with a digital line such as a T1.



Q—What is the difference between a modem and an ISDN terminal adapter?



A—A
modem converts digital signals into analog for transmission over a
telephone line. Because ISDN circuits are digital, the conversion from
digital to analog is not required.
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