A variety 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.

TDM 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.  The pros and cons of TDM technology may be summarized as follows:

•  Hubs are reasonably priced; TDM systems are straightforward.

•  No complex token-passing protocols.

•  Allows for completely isolated subnetworks, with no adverse interference effects between LANs.

•  Backbone carries widest variety of information formats.

Cons.  The disadvantages of TDM technology are that it:

•  Is not represented by any international LAN standard.

•  Does not allow flexible allocation of backbone bandwidth to adjust to instantaneous needs.

ATM LANs

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 application.

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.