AI.CLIVE@MCC.COM (Clive Dawson) (09/10/86)
Here's a bit of info to add to the general folklore: We moved to a new building last week, and after putting everything back together, we found that TELNET connections from our Sun workstations to our DEC-20 (they're on separate Ethernets, gateway is a Sun) were suffering from horribly poor throughput. Echo delays ranged from 1 to 10 seconds, with no load to speak of on any of the systems or the nets. After being stumped for a couple of days, we finally realized that the only thing that had changed was that the Sun gateway and the DEC-20 were now sharing the same DELNI, whereas before they had been on two different DELNIs. We made a quick switch and put the Sun onto its own transceiver two meters away from the DELNI's transceiver. The results were immediate and dramatic. Echoing delays disappeared completely and users were once again happy. Now the obvious question: Does anybody have an explanation? Is there something fundamentally different about intra-DELNI communication that could cause such enormous performance problems? Thanks, Clive -------
leong@andrew.cmu.edu (John Leong) (09/12/86)
Clive, I don't really know what specifically is your problem with the DELNI. However, one thing you may care to check is this : The DELNI (and for that matter, almost all multiport tranceiver in the market) does not handle "heart beat" correctly. For all practical purposes, heart beat is a kind of acknowledgment from the tranceiver to the station. Within a very short period of time after the tranceiver finished sending the packet into the coax cable, it issue a heart beat signal on the collision pairs of the tranceiver cable - essentially saying "all's well". In the case of the DELNI, this signal is (mistakenly) fanned out to ALL attached stations. The station doing the transmit will know it is a heart beat. However, the other attached stations will treat it as an asynchonous collision notification. Most station will ignore it but I don't know about the DEC-20. However, definitely, do not attached a repeater to a DELNI. Repeater will not toss away heart beat. It will act on it and generate collision reinforcement (jam) on the other side!!! The result is excessive collisions. Very bad news to the overall band width availability. John Leong .