ween computers that communicate regularly, or network management software
can let administrators establish a connection that lasts only as long as it takes to transmit some data.
By contrast, so-called "connectionless" networks, like Ethernet and token ring, link individual workstations to the network but not to each other. This means that when you send data to someone, you launch it onto a shared network bus--along with all the other traffic everyone else has sent out there--with instructions for where the data is supposed to go. Rather than being analogous to a telephone call, data transport over traditional LANs is more like sending a delivery truck onto an expressway.
Why is ATM and its connection-oriented approach beneficial? First, it means you can dedicate a sustained level of bandwidth to the transmission; traffic from everyone else on the network doesn't horn in to delay or interrupt your communications. This is known as "quality of service," and within the ATM specification, you can negotiate different service levels. For example, under the se
rvice level that's called
available bit rate
(ABR), the ATM network tries to provide sufficient bandwidth, but there is no guarantee that there won't be disruptions.
Constant bit rate
(CBR) is a level of service that provides a continuous and specific amount of bandwidth throughout the session. You might specify ABR when you're sending large text files, while CBR would be necessary, for example, during a videoconferencing session.
Second, ATM gives you plenty of bandwidth. The technology provides from 25.6 Mbps to 1 Gbps, which can be scaled as needed for a particular application. Third, ATM promises to be a single network technology to handle all data types--text, audio, video, and graphics--for transmission over LANs and WANs.
Stumbling Blocks
ATM proponents have been making these promises for years, but so far only telecommunications service providers have found compelling reasons to deploy the technology on a large scale. Corporations have hesitated to com
mit to ATM because of high costs. For example, as recently as 18 months ago, a 155-Mbps ATM adapter card cost about $1400, or roughly the price of a midrange PC. The good news is that adapter cards with the same data rate cost about $850 today, while new 25-Mbps cards offer prices below $400 in some cases.
Other rays of hope for ATM have begun to shine. Just over a year ago, the ATM Forum released the LAN emulation (LANE) specification, which standardizes how legacy LANs, like Ethernet and token ring, can communicate with ATM networks. This gives managers the ability to gradually migrate select workgroups to ATM rather than having to initiate an abrupt changeover from one network technology to another.
ATM implementation also hasn't had smooth sailing because designers of competing technologies haven't been napping. For example, 100VG-AnyLAN offers 10-Mbps or 100-Mbps data rates with the advantages of a switched architecture and dedicated bandwidth. Adapter cards cost as little as $200. Fast Ethe
rnet, or 100Base-T, also offers 10 or 100 Mbps, although it uses a shared, rather than a switched, architecture, and it doesn't provide for dedicated bandwidth.
Switched Ethernet and switched token ring use the existing protocols but let administrators provide guaranteed bandwidth for multimedia and other heavy-duty applications thanks to more-sophisticated hubs. However, the switched versions of both technologies hit the same 10- and 16-Mbps bandwidth limits, respectively, as their predecessors.
What You'll Need
ATM backers counter by saying that competing technologies are just souped-up versions of legacy technologies. They may be good in the short haul, but long-term strategy argues for a new technology built from the start for high bandwidth, scalability, and flexibility in handling multiple types of data. Some corporations are heeding this message. As you make your move, or plan your move, to ATM, you'll need to understand some of the key components. The figure below provi
des some idea of things you have to consider when you start specifying ATM switches and adapter cards, as well as the management software that lets you set up, maintain, and disassemble your virtual connections.
illustration_link (35 Kbytes)
Alan Joch (
ajoch@bix.com
) is BYTE's senior editor/features.
Virtually Well Connected
