Taking a Closer Look at ATM

ATM is emerging as an important worldwide standard for the transmission of information because of its ability to efficiently carry all traffic types, including the most time-sensitive, such as real-time voice and video. (For a more detailed description of ATM, please see Appendix A, "An ATM Primer.")

ATM is compatible with existing physical networks such as twisted pair, coax and fiber optics, because it isn't design-limited to a specific type of physical transport. This also means that ATM, unlike conventional LANs, has no inherent speed limit. In contrast, when Ethernet speed was increased from ten to 100 megabits per second, its architecture required a reduction in the length of Ethernet segments from 2,500 meters to 250 meters. Similarly, Token Ring has gating factors on its speed. But with ATM, there's nothing in the architecture that limits speed. An ATM network can operate as fast as a physical layer can be made to run.

Even more importantly, a homogenous ATM LAN can connect through an ATM WAN to another ATM LAN, providing end-to-end quality-of-service guarantees beyond that offered by any other transport system.

As noted earlier, ATM switching technology is so efficient that it already is being widely and rapidly adapted by the telecommunications industry, providing the WAN infrastructure that will allow ATM LANs to enable applications that would be either impossible or impractical with conventional LAN technology.

While 100-megabit Ethernet and other high speed networks can provide comparable bandwidth, only ATM can provide the QoS guarantees required for confidently deploying real-time telephony, video streaming, smooth videoconferencing, and other no-delay voice and video applications. The need for QoS is so vital to the industry that a number of initiatives are underway to provide QoS support for TCP/IP based networks, including the Resource Reservation Protocol (RSVP), a specification proposed by the Internet Engineering Task Force (IETF), allowing software developers to "reserve" bandwidth on the Internet to deliver real-time video and audio data streams. With the widespread adoption of IP as a protocol for the Internet and intranets, these QoS efforts are relevant, and Microsoft will support these other standards. But ATM is built from the ground up to support QoS.