phil@amdcad.UUCP (Phil Ngai) (09/30/86)
Terry Slattery asks "<should> the interface monitor carrier during transmit?" I have reviewed the 802.3 spec and see no requirement to do so. (But I still find the language of the spec a bit confusing so I may have overlooked it.) The Ethernet 2 spec calls for a CarrierSenseTest process on page 53 which reports if carrier disappears while transmitting or if it never appears during an entire transmission. It does not seem to be used. Is Codenoll without fault? Ethernet assumes a certain maximum transceiver cable length and a certain minimum signal propagation velocity. The transceiver is supposed to send to the controller everything which appears on the cable. One may conclude that carrier should be present during transmission. The advantage of monitoring the presence of carrier is that you have more information with which to perform problem isolation. The Codenoll system alters the parameters which permit the monitoring of carrier. As such, calling it "Ethernet" or "Ethernet compatible" could be misleading and could cause network failures which are hard to diagnose such as your case. This is not to say the Codenoll product is bad or that you shouldn't use it. However, I will be cautious whenever someone proposes breaking one of the Ethernet configuration rules. If someone does break the rules and has a failure, then that would be one of the first places to look at.
leong@andrew.cmu.edu (John Leong) (10/03/86)
Re : "..... Ethernet assumes a certain maximum transceiver cable length and a certain minimum signal propagation velocity .... The Codenoll system alters the parameters which permit the monitoring of carrier. As such, calling it "Ethernet" or "Ethernet compatible" could be misleading and could cause network failures which are hard to diagnose such as your case. ...... " I am curious as to how the Remote LANBridge function in conjunction with the DEC remote repeater. Does the AMD chip inside the LANbridge, pointing to the direction of the long fibre link, programmed to carry out the CarrierSenseTest ? Furthermore, it is my contention that doing a CarrierSenseTest is probably a good thing but the timing is questionable bearing in mind that the 50 meter transciever cable distance is really a function of DC power attenuation between the controller an the transceiver. If the tranceiver is locally powered, that distance limitation would not have applied. The only relevant number will then be the 51.2 microseconds (512 bits) slot time.