Strategic Communications Choices

Computer and telecommunications vendors have done an excellent job of promoting their leading-edge technologies at network shows across the globe. Unfortunately, this information doesn't help the corporate manager faced with immediate strategic choices. On the contrary, the decision-making process grows more difficult as the number of options increases. Advances in transmission technologies, such as high-speed transmission over Unshielded Twisted Pair (UTP) media, and switching technologies, such as Asynchronous Transfer Mode (ATM) and Ethernet switching, have given rise to new competing approaches. To add to the confusion, technologies initially designed for one application are now proposed in contexts extending far beyond their initial scope, such as Frame Relay to support telephony and ATM to construct a LAN. This article describes how various LAN and WAN technologies compare and which one is best suited for what environment.

The Limits of Conventional Hubs
Let's examine LANs first. The rationale behind setting up shared-medium LANs--Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) where all systems exchange information over the same cabling system--stemmed from the fact that cables were expensive, as were installation and switching. Not surprisingly, the idea of using common cabling to connect the offices on a floor and the floors in a building without intermediary switching was certainly attractive. It turned out, however, that hooking up the connections was more complicated than it sounded.

First, deploying a simple linear cable created numerous operational difficulties for introducing new stations, locating faulty equipment, making physical interventions, or isolating part of the network. Because of these limitations, the physical bus topology was replaced by the physical star topology. Under the star topology, all LANs become physically organized around cascaded wiring hubs where administrators can perform technical intervention much more efficiently. In addition, for the LANs to take full advantage of the central position of the hub, developers decided to add increasingly powerful management functions.

If the topology remains a bus, LAN frames entering through a single port in a hub exit through all the ports in the hub. More precisely, the frame exits from one port, visits the stations cabled to that port, and returns to the hub so it can exit through the next port. This is the main characteristic of hubs: Frames visit all the attached stations. Both Ethernet and token-passing networks (FDDI and Token Ring) use probabilistic techniques to share access to the common medium, but FDDI and Token Ring employ a more deterministic scheme (i.e., token passing) where each station is guaranteed access to the medium. Ethernet provides no such guarantee. Therefore, under a heavy load from many stations, the performance of a Token Ring or an FDDI network will degrade far less than that of an Ethernet network.

Clearly, the conventional Ethernet hub approach has reached its limits. The propagation of frames sent to multiple recipients, called "multicast frames," over the LAN poses increasing difficulties, especially with the emergence of multimedia applications that depend on multicasting techniques. Also, sharing 10 megabits per second (Mbps) bandwidth between many stations cannot address the bit-rate requirements of a number of applications. What are the available options for providing higher-speed connections?

Segmentation Doesn't Scale
The simplest way to provide higher-speed connections is to reduce the number of stations per segment, increase the number of individual segments, and interconnect the segments using store-and-forward boxes (i.e., bridges or routers). This technique is called segmentation. Although it may be a stop-gap solution, segmentation doesn't scale well enough to turn it into a strategy.

A single bridge or router wouldn't be economical or sufficient in a medium-size organization. Several bridges or routers would be needed, and they would need to be interconnected by some form of high-speed point-to-point connections or by a backbone system. Unfortunately, those bridges or routers, which would have to completely store the frames before retransmitting them, would become so overloaded that they would become bottlenecks. They would also introduce additional delays in the end-to-end transit time.

A simple extrapolation of the segmenting approach would be to dedicate 10Mbps Ethernet segments to individual stations. This is a simple, practical way to connect certain demanding systems with special requirements, such as packet-based teleconferencing systems or multimedia servers, but like segmentation, it doesn't scale, so it's not a very good strategy.

FDDI: Too Little, Too Late
Another option would be to replace Ethernet or Token Ring networks with FDDI networks. FDDI uses a shared-medium technique similar to that of Token Ring but operates at 100Mbps instead of 16Mbps. A full-scale FDDI implementation is constructed of two physically separate rings that are dual-connected to all the FDDI adapters on the network. All traffic flows over the primary ring, but if the primary ring fails, the adapters switch to the secondary ring. The backbone of an FDDI network is normally made up of optical-fiber cable. You can achieve connectivity to the desktop with either optical fiber or copper twisted-pair.

FDDI has mainly been used for two purposes:

• To serve as a backbone interconnecting lower-speed LANs (e.g., 10Mbps Ethernet and 16Mbps Token Ring LANs)
• To provide direct, high-speed attachments for hosts and routers

Unfortunately, FDDI seems to be "too little, too late" for three reasons. First, FDDI in its current version doesn't support full isochronism, which is the ability to transport real-time data such as voice on a guaranteed basis. Thus, when it is heavily loaded, the quality of service FDDI provides may be limited. Second, FDDI remains a shared-medium technology unless you devote one ring per station. When the average traffic of each station is increased by an order of magnitude--driven by multimedia applications and the increasing power of individual stations--the limitations you've experienced with your current 16Mbps Token Ring LANs will reappear. As a result, each station will have access only to bandwidth equal to 100Mbps divided by the number of stations. Third, the cost of FDDI remains high, even though interfaces for twisted-pair cables are less expensive than those for their fiber counterparts.

FDDI currently has less than 1% penetration in the LAN interface market, but it will probably remain the backbone technology of choice for the next few years, especially for large campuses.

100BaseT's Popularity
A more recent option for high-speed communications is 100Mbps Ethernet. Two Versions exist: 100BaseT and 100VG-AnyLAN. Although both versions run on UTP, they are drastically different. 100BaseT Ethernet--a.k.a., Fast Ethernet--was developed by a group of companies called the Fast Ethernet Alliance. The idea behind Fast Ethernet is to keep the sharing mechanism used by conventional Ethernet, the Carrier Sense Multiple Access with Collision Detection (CSMA-CD), but to adapt the physical characteristics to enable it to operate 10 times faster (100Mbps). Stations connect to 100BaseT hubs in the same way that they connect to 10BaseT hubs.

One consequence of this approach is that the same collision problems you experience over conventional Ethernet affect the functioning of 100BaseT networks--each packet entering a 100BaseT hub visits all the stations sharing the 100Mbps bandwidth. (There are two modes of Fast Ethernet: 100BaseT4 and 100BaseTX, which run over different types of UTP cable.) During the spring of 1995, the penetration of 100BaseT technology was modest, but analysts have predicted that the number of installed ports will have reached 500,000 by the time this article is published.

The 100VG-AnyLAN technology has been proposed by another group of companies under the leadership of Hewlett-Packard and IBM. In fact, this technology has little to do with Ethernet. In contrast to the 100BaseT technology, 100VG-AnyLAN retains the Ethernet frame format but uses a deterministic protocol called the Demand Priority Access Method (DPAM) instead of CSMA-CD. DPAM provides guaranteed access to the media using an access method that is conceptually similar to token passing; in reality, however, it offers greater flexibility than CSMA-CD and can better handle prioritized traffic. Under 100VG-AnyLAN, stations are connected to a switching hub. These, in turn, may be connected to other switching hubs. 100VG-AnyLAN requires four unshielded twisted pairs in a Category 3 UTP cable. In the summer of 1995, about 60,000 ports had been installed, and analysts said that this figure should reach 200,000 by the time you read this.

How good is 100Mbps Ethernet for new applications which are bandwidth-intensive? High-speed networks that suffer from the same limitations as lower-speed networks may have problems scaling upward. Therefore, 100BaseT Ethernet technology is well-suited for multimedia applications in environments supporting one or a few stations per segment. 100VG-AnyLAN technology has the same limitations; however, it offers better guarantees because of the deterministic DPAM protocol that it employs.

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