Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!well!berger From: berger@well.sf.ca.us (Robert J. Berger) Newsgroups: comp.dcom.lans Subject: "real-time" over a lan: token ring vs ethernet vs ? Keywords: real-time, token ring, 802.5, ethernet Message-ID: <19300@well.sf.ca.us> Date: 29 Jul 90 20:33:04 GMT Distribution: comp Lines: 48 We are looking to make a special purpose dedicated lan for controlling up to 128 devices. These devices will run a real time os such as PSOS or VxWorks. There will be up to 16 master devices made up of unix workstations running Unix System V.4. The workstations will be initiating most traffic. We will not be using the lan for traditional computer traffic, but just for control and status exchanges. Most messages will be short and would easily fit into one datagram packet. They will probably be RPC's on top of UDP or TCP/IP. The main time critical response we need is to have a guaranteed worst case of the workstation sending a message to one or several of the slaves, where the message must get to the slave within 5 milliseconds. Most other traffic needs to get from master to slave within 16 milliseconds. There will be some backround traffic as well, but they are not generally time critical. It seems that ethernet would have problems guaranteeing the response time, particularly if the backround traffic got somewhat heavy and thus start having collisions. We are strongly considering 16mbs 802.5 token ring for our lan based on our reading (we have no actual experience with token ring). Theoretically token ring should be able to meet our worst case traffic estimates and give us the token often enough to meet the 5 millisecond response time, particularly if we take advantage of the alleged ability of 802.5 to support global priorities on the network. My questions for the net are: 1. Are we barking up the wrong tree here? Is it concievable to get the response we need at all? 2. Does token ring really offer the kind of response time that it shows on paper? 3. Does token ring global priorities really work and are they usable from UDP/ TCP/IP? 4. Has anyone done something like this before and have some hard numbers? (with ethernet, token ring, or something else?) Thanks, Bob PS. Please use the email address in the signature, not the return address of this posting! ------------------------- Bob Berger SONY Advanced Video Technology Center 677 River Oaks Parkway San Jose, CA 95134 408-944-4964 [uunet,mips]sonyusa!sfcsun!berger (soon: berger@sfc.sony.com)