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Chapter 9: WAN Design Considerations
·
X.25
-- Advantages
--It has guaranteed delivery and error-checking capability.
-- Disadvantages
--It works at slow speeds and is difficult to configure.
-- Cost
--It's expensive.
·
ISDN
-- Advantages
--It has a fast circuit setup time. It has great backup circuit
technology. It supports voice, video, and data.
-- Disadvantages
--It is difficult to configure and is distance-sensitive.
-- Cost
--It's expensive in most areas.
·
Leased lines
-- Advantages
--They are secure. They support HDLC encapsulation. They are not
shared.
-- Disadvantages
--Point-to-point circuits are not flexible.
-- Cost
--They are medium in cost compared to other alternatives.
·
ATM
-- Advantages
--It supports high speeds. It supports voice, video, and data. It has
an inherent Quality of Service.
-- Disadvantages
--It's difficult to configure.
-- Cost
--It's expensive.
·
DSL
-- Advantages
--It's fast. You can use existing phone lines for connectivity.
-- Disadvantages
--It isn't available everywhere. It is distance-sensitive.
-- Cost
--It has the lowest cost.
Many WAN options are available.
Frame Relay
is probably the most popular. It is considered a
packet-switched technology. The advantage of Frame Relay is the ability to connect multiple
sites to a central site with only one physical serial interface on the central site router by using
subinterfaces
. Subinterfaces provide a flexible solution for routing various protocols over
Frame Relay networks, as shown in Figure 9-2. A single physical interface can be logically
divided into multiple virtual subinterfaces. The subinterface may be defined as either a point-
to-point connection or a multipoint connection.
The
X.25
protocol suite maps to the lowest three layers of the OSI reference model, as shown
in Figure 9-3. X.25 was designed for use in nonreliable physical layers. It implements reliability
at Layer 2 with Link Access Procedure Balanced protocol. Today, with advancements being
made in software applications and telephone line connectivity, the upper layers can guarantee
delivery of packets using Frame Relay.
87200333.book Page 296 Wednesday, August 22, 2001 2:53 PM