Kopetz's MARS system [#!Kopetz!#] prototype was also focused on process control applications, but used a TDMA protocol to provide real-time guarantees on Ethernet. described a multi-token-ring protocol that is designed The token ring in Totem [#!totem!#] provided ordered multicasting rather than real-time performance guarantees. Hermant [#!hermant!#] presented a variant of CSMA/CDR for real-time scheduling in distributed multi-access broadcast communication channels. This protocol was not meant for existing Ethernet hardware.
More recently several commercial products available in the market attempt to provide real-time performance guarantee over LANs. HP's 100VG-AnyLAN [#!ALBR95!#] uses advanced Demand Priority Access to provide users with guaranteed bandwidth and low latency, and is now the IEEE 802.12 standard for 100-Mbps networking. National Semiconductor's Isochronous Ethernet [#!XIAO93!#] includes a 10-Mbps P channel for normal Ethernet traffic, 96 64-Kbps B channels for real-time traffic, one 64-Kbps D channel for signaling, and one 96-Kbps M channel for maintenance. The 96 B channels can provide bandwidth guarantee to network applications because they are completely isolated from the CSMA/CD traffic. Isochronous Ethernet forms the IEEE 802.9 standard. 3COM's Priority Access Control Enabled (PACE) [#!3COM96!#] technology enhances multimedia (data, voice and video) applications by improving network bandwidth utilization, reducing latency, controlling jitter, and supporting multiple traffic priority levels. PACE technology uses star-wired switching configurations and enhancements to Ethernet that ensure efficient bandwidth utilization and bounded latency and jitter. Because the real-time priority mechanism is provided by the switch, there is no need to change the network hardware on the desktop machines. More recently, there are 802.1p and 802.1q efforts that support packet prioritization and virtual LANs. Peterson [#!peterson!#] summarized the current efforts in the Industrial Automation community to use Ethernet as the control network technology.
The difference between RETHER and all the above work is that RETHER provides bandwidth/delay guarantees to network packets, rather than just supports packet prioritization. In addition, all of the other schemes require changes to the existing infrastructure in the host network hardware and/or the wiring, while RETHER does not. RETHER 's ability to use commodity Ethernet hardware is crucial for industrial automation systems to ride with the technology momentum of the PC networking industry. Finally, RETHER is the only system that provides bandwidth guarantees for real-time connections that run cross multiple hops, an critical feature for system scalability.