Introduction to WANs
‹A
WAN is a data communications network
that operates beyond the geographic
boundaries of a LAN:
» A WAN typically uses data links provided by
the WAN service provider
» WANs generally carry a variety of traffic
types, such as voice, data, and video
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WAN Link Options
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There are two classifications of WAN technologies:
» Circuit-Switched Connections
– Circuit switching can be used to establish a connection (for voice or
data) between two geographically separate points
– In the PoTs (an ordinary telephone system) this is achieved using the
dialed number
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However, this approach incurs delays as each switch has to process the
call request and set-up the internal connections. An alternative approach
is to use pre-built permanent circuits across the telephone system.
These are called leased lines.
As well as reducing delay these connections offer higher bandwidths
than is available with a switched circuit
– Examples of circuit-switched connections include:
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Public Switched Telephone Network (PSTN)
ISDN Basic Rate Interface (BRI)
ISDN Primary Rate Interface (PRI)
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WAN Link Options - continued
» Packet-Switched Connections
– Circuit-switched circuits are generally an expensive
way to move data
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Switched or permanent circuits can be very inefficiently
utilized for the transfer of computer data
– Packet-switched networks adopt an alternative
approach:
Allocate capacity to traffic only when it is needed
Share the available capacity among many users
‹ Here the data is moved in labeled cells, frames, or packets
through a packet-switched network
‹ Because the internal links between the switches are shared
among many users, the costs of packet switching are
lower than those of circuit switching.
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WAN Link Options - continued
‹ Two
fundamental differences between
packet and circuit-switched technologies:
» The inter-nodal links are shared between many
subscribers without any dedicated bandwidth
» Packets must be entirely received at one switch
before they can delivered to the next:
– This introduces delays (latency) and variability of
delay (jitter) which is not as significant in circuitswitched networks.
– Despite these problems, however, packet-switched
networks allow reasonable transport of voice and
video communications
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WAN Devices
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Connecting to a packet-switched network requires a local
loop to the nearest point of presence (POP).
» This local loop can be a dedicated leased line or a dialin service
Devices used to connect to a WAN comprises:
» Data Circuit-terminating Equipment or Data
Communications Equipment (DCE) which prepares the
data for transmission through the network
» Data Terminal/terminating Equipment (DTE) which
passes the data to the DCE
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WAN Standards
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WANs standards are mainly focused on the lowest two
layers of the OSI reference model
» The physical layer standards describes the physical delivery of the
bits into the network
» The data link layer defines addressing, flow control framing and
encapsulation
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Whilst a variety of technologies are used including X.25,
Frame Relay, and Asynchronous Transfer Mode (ATM),
they all use a framing mechanism similar to High-Level
Data Link Control (HDLC), an ISO standard.
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