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Chapter 5. WAN Internetworking
Technologies
In this chapter, we look at WAN internetworking technologies. When we examine
this subject, the first thing we notice is the clear separation between physical
transport (OSI-RM Layer 1) and data transmission (OSI-RM Layer 2) services. This
separation contrasts sharply with LAN technologies, where the services are handled
by both layers under the same protocol specification. This delineation might seem
odd to administrators who have most of their experience with LAN protocols, but it
is characteristic of WAN networking, and reflects the utilitarian nature of the Public
Switched Telephone Network (PSTN) that provide transport service.
PSTN was originally developed for voice transport, which required that each
transmission be distinguishable from every other, but that the continuity of each
separate transmission be maintained. To facilitate this kind of service, each voice
transmission was given its own dedicated channel. When the PSTN was later used
for transmitting data, this same model was followed. Each data path has its own
channel for transmitting signals across the PSTN, and for the duration of the "call,"
the available bandwidth of that channel can be used to transmit any signal. In order
for data to be transmitted over the PSTN channel, some kind of transmission
scheme (framing) needs to be employed to format the data so it can be interpreted
correctly when it reaches the other end of the channel. The PSTN is uninterested in
what is being carried over the channel. Its only function is to ensure that the data is
in a useable form when it reaches its destination.
As the transmission of data became a larger and larger percentage of the traffic
being carried over the PSTN, WAN packet-switching and cell-switching technologies
were developed. Fundamentally, these technologies utilize the dedicated physical
layer transport mechanisms to send data signals over the PSTN. They also provide
intelligent signaling mechanisms that enable the available transport bandwidth to
be used to support multiple, discrete connections between different endpoints.
These technologies afford a more dynamic, efficient, and cost-effective utilization of
the channel bandwidth and expand the functional usefulness of the PSTN beyond
that of a large channel-switching system.
To start, we look at the PSTN's history and basic structure. Then, we review three
transport technologies: AT&T/Belcore's Transport-1 (T1) digital carrier system,
SONET (Synchronous Optical Network), and SDH (Synchronous Digital Hierarchy),
which are ANSI and ITU-T international standards. We then take a brief look at the
major PSTN packet- and cell-switching technologies: ISDN, Frame Relay, and ATM.
We close the chapter by examining the popular "data-link" protocols: HDLC
(High-Level Data Link Control) and PPP (Point-to-Point Protocol). These two
protocols are used to provide data-link transmission services over dedicated and