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New technologies entering the market give Ethernet determinism when needed. These technologies typically fall into one of two camps: One uses Ethernet switches; another implements timing functions into protocols on top of Ethernet.

The Ethernet switch solution is being driven by the convergence of video and voice-over networks, two technologies with very sensitive timing requirements. An Ethernet switch (in effect a multiple-port bridge with a very fast backplane) lies at the center of the network and connects directly to each device. It provides each device with its own private network, which reduces signal collisions to almost zero.

This can be improved further if full-duplex Ethernet is used between the device and the switch. Full-duplex Ethernet technology uses separate channels for transmissions to and from the switch, so there are never any collisions.

The switch can also arbitrate who gets to talk to whom, and when, thus allowing prioritization of Ethernet packets. Applications and messages are assigned priority and time-sensitivity parameters, and the switch manages these according to specified rules. This prioritization is referred to as quality of service, and two standards for this in Ethernet environments (IEEE 802.1Q and IEEE 802.1p) are now in draft form and will soon be ratified.

Upper-level timing proponents use a protocol on top of Ethernet that coordinates interdevice timing and time stamps all packets. The devices then correct for any network or processor-induced delay.

This solution was demonstrated at the ISA EXPO/98 Conference in Houston by Hewlett-Packard (HP) and described in detail by John Eidson and Wesley Cole in the June 1998 InTech.

By using this technique, Eidson and Cole were able to demonstrate timing accuracy in the 200-nanosecond range, far beyond anything required in normal process control applications. HP will release to equipment manufacturers this timing technology in its embedded Ethernet controller in third quarter 1999.

Networking analyst Richard Caro says, "The fact that we can solve the determinism issue is good enough for most people. They will now use Ethernet knowing if problems arise they can be dealt with." --Eric Byres

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Figures and Graphics

• Shared Ethernet hub.

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