WAN Networking Technologies

Circuit Switching

In a circuit-switched network, a dedicated communication path is
established between two stations through the nodes of the network. That
path is a connected sequence of physical links between nodes. On each
link, a logical channel is dedicated to the connection. Data generated
by the source station are transmitted along the dedicated path as
rapidly as possible. At each node, incoming data are routed or switched
to the appropriate outgoing channel without delay. The most common
example of circuit switching is the telephone network.

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 Service Digital Network(ISDN) is a good
example of circuit switching. When a router has data for 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. The picture illustrates an example of this type of circuit.

Packet Switching

A quite different approach is used in a packet-switched network. In
this case, it is not necessary to dedicate transmission capacity along
a path through the network. Rather data are sent out in a sequence of
small chunks, called packets. Each packet is passed through the network
from node to node along some path leading from source to destination.
At each node, the entire packet is received, stored briefly, and then
transmitted to the next node. Packet-switched networks are commonly
used for terminal-to-computer and computer-to-computer communications.

Packet switching is a WAN technology in which users share
common carrier resources. Because this allow 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 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.
The following two techniques are used in packet-switching technology:

• Variable-length packets
• Statistical multiplexing

Variable-length packets are used for more efficient and flexible
data transfers. These packets are switched between the various segments
in the network until the destination is reached.

Statistical Multiplexing technique control network access in a
packet-switched network. The advantage of this technique is that it
accommodates more flexibility and more efficient use of bandwidth. Most
of today’s popular LANs, such as Ethernet and Token Ring, are
packet-switched networks.

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:

1. Switched Virtual Circuits (SVCs) and
2. 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 Service

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.

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.

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.

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 an
essential an ISDN modem, although it is called a terminal adapter
because it does not actually convert analog to digital signals.