QoS is best for multimedia connections
An engineering axiom states that it's easier to add features early in the design rather than retrofit them later. Ethernet and TCP/IP were designed to handle data transmissions, which are more forgiving than video or voice connections, so enhancing TCP/IP to support multimedia transmissions has been challenging.
Currently, information travels across most Ethernet networks in a random fashion. In a series of 10 packets, packet No. 8 may arrive at the destination before packet No. 6. The computer at the receiving end reorders the packets so the information is presented correctly to an end user.
Packet arrival order is not important with most data applications, but it is vital with video and voice transmissions. If packets arrive out of sequence, a video transmission may jumble or a voice connection may become garbled.
Bandwidth contention is a related problem. On an Ethernet network, bandwidth is parceled out on the fly. Suppose a user begins sending a large file when no one else is using the network and the transmission starts out fine. If a neighbor then starts to access a database, the transmission could slow to a crawl. With a file transfer, the only impact is that the user has to wait a bit longer.
Video and voice applications cannot tolerate such fluctuations. If two users are conversing and the available bandwidth shrinks, a transmission will jar or possibly break completely. So these applications require bandwidth to be available throughout the session.
Quality of service (QoS) solves these problems by opening up a clear communication line between two end points so packets can move unencumbered. ATM was designed from the ground up to support this capability.
In recent years, vendors have worked to retrofit QoS for Ethernet. While they've made progress, the options are not as robust as those found with ATM. "ATM now offers users four classes of services with three widely deployed; that is not the case with TCP/IP," says Carl Engineer, director of marketing at Cisco Systems Inc.'s eWan Business Unit, in San Jose, Calif.
It's not clear whether IP proponents will be able to match the level of QoS functionality that ATM offers. "IP equipment vendors must overcome a series of significant technical challenges to match ATM QoS," notes Tim Hale, senior product marketing manager at 3Com Corp.'s Marlboro, Mass., office. "Can they be solved? I don't see how it will be done, but I've learned in this business never to say never." --P.K.