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Issue of October 2002 
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Focus: ATM Vs. Gigabit Ethernet
The LAN technology choice

ATM and Gigabit Ethernet have both evolved into mature and robust LAN technologies. Each has unique advantages that make it suitable for particular industries. Have you made the right choice for yours? by Graeme K. Le Roux

In some respects, the competition between ATM and Gigabit Ethernet (GbE) has more than a passing resemblance to the battle between VHS and Beta technology. In those days, both VHS and Beta video cassette recorders were relatively expensive, of limited practical use, and both newcomers were aiming to replace an older technology (U-Matic format video tapes) which worked perfectly well for most of the market.

In the end, VHS won the consumer market because users were comfortable with a tape that looked like a miniature version of what they knew (U-Matic cassettes), and that VHS could accommodate four hours of recording on a single tape. Beta won the professional market because of its perceived superior picture quality and more functional equipment.

In the enterprise network marketplace today, ATM looks like playing the role of Beta and GbE that of VHS. Note that throughout this discussion we will use the term GbE as an umbrella term covering both 1Gb and 10GbE technologies.

Telco's only option
For a telco, ATM is the only option for building very large—national or global—network infrastructures that will carry a mixture of voice, data, and video services with constant Quality of Service (QoS) and predictable latency. On the other hand, non-telcos who look for a high-capacity network infrastructure, see GbE as just another form of the Ethernet they are using now.

Furthermore, the proponents of GbE point out that voice and video can also be transmitted over Ethernet. Just to complicate matters, both ATM and GbE seem to be evolving into more mature and robust products. So how do you choose between the two technologies?

Things to consider
The first thing to do is to figure out whether or not you need either of them. In most cases ATM is not an economically viable desktop technology and neither is GbE. Even if you have a few users with a need for extraordinary amounts of computing power, and who routinely handle enormous data sets, it is likely that you will find it more cost effective to install something like the newer pocket supercomputers.

Such pocket supercomputers can have either a GbE or an ATM interface, and you can have them in your data centre along with a SAN, and then give specific users high-end thin clients to access this machine via full-duplex 100BaseTx Ethernet links. CPU power is cheaper in large chunks so there is no reason to be hauling huge amounts of data across a network just to process it on a more expensive platform. Especially when doing so would also increase the cost of your network by forcing you to accommodate Gb data streams to users' desktops.

In general, an average user with a thin client on their desktop and a VoIP telephone handset will get by quite happily with a full duplex 10Base-T link to their desktop. Most PCs for business users have 10/100BaseTx Ethernet NICs built-in, so add a VoIP phone and you will still find that, at worst, full duplex 100BaseTx will do the job. The key in both cases is to build a backbone system that will support non-blocking full duplex connections and guarantee both stable latency and enough bandwidth for both the PC and the VoIP 'phone line'.

As far as desktop video conferencing is concerned, it basically places the same constraints on the network as VoIP, but requires more bandwidth. Once again 100BaseT full duplex Ethernet is usually more than sufficient.
Given that you can get by comfortably with 10/100BaseTx Ethernet to your users' desktops, the next question is what will you need as a backbone technology for the links between the local hubs and the hubs in your IT glass house? From this point there are several choices you can make using ATM, GbE or neither of them. Most of these choices center around how much cable, specifically fiber optic cable, you want to pull.

How much cable to pull?
A basic design rule for any network is that you use copper UTP to the desktop and fiber between your hubs. Short haul, multi-mode fiber is relatively inexpensive; single mode fiber of the sort used in campus applications is more expensive, but not unreasonably so.

Costs change drastically at the ports to which the fiber is attached. Fiber versions of 100 Mbps Ethernet are considerably cheaper than either GbE or ATM ports. Thus, you basically have to decide on whether you want to install lower cost 100 Mbps fiber ports, or less costly but faster GbE or ATM ports.

Using 'lesser' 100 Mbps fiber Ethernet means segmenting your network. The incidental effect of this segmentation is arguably greater reliability, but there is a limit. If your average user's desktop has a simple 10BaseT link to it, effectively you have a flat Ethernet, and you typically have 20 users per 10 Mbps segment. In this case a single 100 Mbps full duplex link can support up to 200 users without over subscription.

The reasoning for this is that each 10BaseT segment, which is half duplex, can only present 10 Mbps to the 10/100 switch controlling the full duplex backbone link. Each segment can be either transmitting or receiving at any one time and hence the 100 Mbps full duplex link can support up to 10 segments, or 200 users.

Obviously if you run this sort of network, the use of GbE or ATM is probably pointless, but what if you need to guarantee 10Base-T full duplex to each desktop? In this configuration a 100 Mbps full duplex link will support just 10 users without oversubscribing. In most cases you will oversubscribe, depending upon your traffic loads, by about 5-10 to one. In other words, about 50 to 100 users. Usually the number of users will be about 60; where VoIP is running, 40 users is probably as much as you want to try.

Add desktop video conferencing to the mix and oversubscribing becomes a very bad idea. Obviously, as the amount of users you can support per link falls, you are faced with the choice of upgrading your existing fibers to GbE or pulling more fiber.

In the average TCP/IP network, ATM with either OC-3 (155 Mbps) or OC-12 (622 Mbps) will not be an economical option, since the links are more expensive than GbE, as are the hubs and their switch engines.

Take into account the fact that you will have to pay a premium for your GbE switches, as you will need support for 802.1p, 802.1q, etc., to carry your VoIP and video conferencing traffic with the necessary QoS. Naturally, ATM gives you the QoS support you need automatically.

ATM users
By now, you are probably thinking that only telcos would use ATM, but this is not necessarily so. The examples we have discussed implicitly assume, at worst, a fairly even mix of data, VoIP and video conferencing. In most company networks, the bulk of the traffic is simple data which can cope with reasonably wide variations in latency. Even if you add VoIP, it is likely to constitute around a third of your traffic, albeit with much tighter constraints on variance in latency.

As for desktop video conferencing, it is just not something a lot of people do. In most cases, video conferencing is done between specifically configured conference rooms at different sites, and it is often done via ISDN rather than the company's TCP/IP network.

Video heavy traffic
But what if you had a network where video streams were the bulk of the traffic, voice was the major secondary application and data was only a relatively minor concern? For example, consider a video production facility with several studios, each with several digital video cameras, a shared post production facility, shared special effects shop which specialize in computer graphics, and an administration center with a central PABX, all grouped on one campus, or several in one city.

A thing to note is that a digital video camera produces a data stream which occupies a lot less bandwidth than a modern office PC can generate, and an audio stream occupies even less bandwidth. Video cameras produce streams of data, unlike the bursty traffic which a PC generates. And there are going to be a lot less users in this scenario than there are PCs in an average office building.

A typical TV studio has less than a dozen cameras, all of which will not be in use at once. One floor of an office block can have hundreds or more PCs, all of which will be in use at once. We could use GbE or ATM in the studio situation, but ATM is likely to be a more natural choice, particularly when one may have to send a video stream to a remote site via a telco's land line service. Chances are that the telco can sell the studio a wide area ATM service which can be directly interfaced with the studio's ATM network.

It is probably true that most networks are going to see GbE with its various enhancements as their most cost effective and straightforward choice. But in cases where QoS, streaming support, and the ability to directly interface with a carrier's wide area services are more of a priority than simple bandwidth, ATM is still arguably the best choice.

Graeme K. Le Roux is the director of Moresdawn (Australia), a company which specialises in network design and consultancy. He can be reached at

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