Isochronous Ethernet is not the only method of handling isochronous data over networks. Here's the current field.
WAN Links
When it comes to digital services, ISDN is popular for multimedia applications, because it was designed from the ground up to support isochronous applications. With modern compression algorithms and multiple B channels, ISDN can efficiently support audio and video. However, it requires service orders, telephone company installation, and widespread availability of ISDN services to be fully useful.
T1 circuits are leased lines for point-to-point communications. These dedicated circuits are limited because only predetermined people can be included in the multimedia session
s. Like ISDN, it takes service orders to the telephone carriers to execute changes to add or delete sites, and customers may sometimes have to wait weeks for changes to be made.
Workgroup Lineup
Pace from 3Com sends a mix multimedia and traditional data traffic over Ethernet and awards the highest priority to isochronous traffic. Regular data traffic is slowed because of its lower priority, but time-sensitive information has a better shot at getting through. However, this leads to potential problems with data collisions, particularly if several isochronous applications are attempting to run simultaneously over the same Ethernet segment.
Western Digital's ISOnet provides a single, stand-alone isochronous mode that handles isochronous traffic similarly to isochronous Ethernet. There is, however, no provision for a data channel running on the same Ethernet with ISOnet.
Like isochronous Ethernet, SDV (switched digital video) from AT&T uses ISDN's Q.931 signaling proto
col and a connection-oriented service. Unlike isochronous Ethernet, SDV runs strictly at the application level and requires an infrastructure, such as ISDN, for the equivalent of isochronous Ethernet's C channel. SDV is only a higher level (4 and up) protocol.
Proposed by Hewlett-Packard and AT&T, 100VG-AnyLAN supports Ethernet packets at 100 Mbps rather than the traditional Ethernet bandwidth of 10 Mbps. A protocol called Quartet Signaling uses four cable pairs to transmit data giving 100VG-AnyLAN the same 100-Mbps bandwidth as FDDI (Fiber Distributed Data Interface) or other fast Ethernet solutions. What 100VG-AnyLAN doesn't offer is a stand-alone circuit-switched channel for multimedia services. The channels compete for the same 100-Mbps bandwidth as does data traffic. However, some compromises have been made to favor isochronous applications.
100VG-AnyLAN's Demand Priority arbitrates packet transmission requests. This tends to avoid packet collisions by allowing multimedia traffic to get thr
ough in a more timely fashion. In addition, a hub can arbitrate requests by acknowledging higher-priority multimedia packets before lower-priority requests.
Migration to 100VG-AnyLAN is much like a move to isochronous Ethernet. It is possible to connect 100VG-AnyLAN devices to existing Ethernet networks and upgrade in stages. Corporations can upgrade individual stations or hubs on an as-needed basis.
FDDI is designed for data transport and, like Ethernet, doesn't efficiently handle isochronous traffic. FDDI is half-duplex. The only major advantage to running multimedia applications over FDDI is enhanced bandwidth (100 Mbps).
ATM, the standard for 25- or 155-Mbps and faster LANs, is perhaps isochronous Ethernet's biggest nemesis. ATM provides the same services that isochronous Ethernet does but at significantly higher bandwidths. ATM promises to bring voice, video, and data together into one big interoperable high-speed mix.
Once fully deployed, ATM will support wide-area access to
data, audio, voice, and video. However, the slow rate of ATM deployment plays into the hands of isochronous Ethernet's proponents. ATM equipment is expensive, and it's not always interoperable. Today's ATM connections are largely limited to local installations. Unlike isochronous Ethernet, ATM has no widely spread infrastructure.
Both ATM and isochronous Ethernet are ISDN-based protocols. ATM is based on broadband ISDN, and isochronous Ethernet is based on narrowband ISDN Basic Rate Interface and Primary Rate Interface service. In the end, vendors may be squaring off to fight the battle of the ISDNs.
THE USUAL SUSPECTS
Digital services: ISDN and T1
LAN services: Pace, Isonet, SDV, 100VG-AnyLAN, FDDI, and ATM
Networking Alternatives
