of new network architectures are being proposed
for use in multimedia applications. At stake is
the next generation of LAN technology.
Although multimedia applications
today are few in number, users need to see how
the new LAN infrastructures can support
tomorrow’s needs for integrated voice, video,
image, and data traffic. Applications include
multimedia training, multimedia compound
documents, and teleconferencing.
This chapter reviews the
status of some technical approaches that are
promising for multimedia use. There is no clear
winner at this point because, simultaneously,
technology is evolving, user tests are ongoing,
and some standards are as yet incomplete.
Significant efforts are also being expended to
make maximum use of existing network standards
including fiber distributed data interface (FDDI),
802.3 (Ethernet), and 802.5 (Token Ring).
Several new technical
approaches lend themselves to multiuse
applications involving images, data, and full
motion video. The purpose of this chapter is to
review these approaches and some of the
trade-offs involved in deploying each. The
architectures surveyed include the following:
time division multiplexed LANs, Fibre Channel
LANs (ANSI Standards Committee X3T9.3), and
asynchronous transfer mode (ATM) LANs. The
chapter also outlines approaches to multimedia
that optimize the use of existing LANs.
Time division multiplexed (TDM)
LAN backbones are ideally suited for
transporting a wide variety of applications
information throughout an enterprise. The basic
concept is to allocate a fixed bandwidth for
each service — data, image, or video — and scale
that bandwidth to meet the application needs.
This backbone interconnects
subnetworks labeled A, B, and C. Each of these
subnetworks could run separate services such as
digitized video, image, and data. The networks
could be bridged together (as shown in Exhibit
1) in a central wiring closet. For more
demanding video or image applications, separate
servers might be employed for each subnetwork.
Several firms market backbone
products based on this concept. Luxcom, Inc.
(Fremont CA) and Fibermux Corp. (Chatsworth CA)
are the product leaders at present.
pros and cons of TDM technology may be
summarized as follows:
- • Hubs
are reasonably priced; TDM systems are
- • No
complex token-passing protocols.
- • Allows
for completely isolated subnetworks, with no
adverse interference effects between LANs.
- • Backbone
carries widest variety of information formats.
disadvantages of TDM technology are that it:
- • Is
not represented by any international LAN
- • Does
not allow flexible allocation of backbone
bandwidth to adjust to instantaneous needs.
A multiplexing alternative
that overcomes some of the shortcomings of TDM
is asynchronous transfer mode, a type of
asynchronous time division multiplexing. This
technology was originally developed for use in
wide area network connections under the umbrella
known as broadband integrated services digital
network (B-ISDN). It was developed with two
specific goals in mind: high throughput and
network transport independent of the network
These goals were developed to
allow efficient transport of voice, video, and
data information of varying formats and data
rates. These same objectives are being applied
to the LAN environment. Hence, ATM is actively
being considered for LAN backbones and for
direct workstation connections.