nbirkman@watserv1.waterloo.edu (Nancy Birkmann - DCS) (10/04/90)
IEEE 802.3 transceivers generate a signal known as Signal Quality Error Test (SQE). Ethernet V2.0 transceivers generate a similar signal known as Collision Presence Test or "heartbeat" and confirms that the collision detect circuitry in the transceiver is operating. I read a statement that the SQE signal is not allowed for transceivers that connect to 802.3 repeaters. Why? Also, many transceiver manufacturers (e.g. Allied Telesis, Cabletron, Interlan...) provide an option of either providing this signal or turning it off. Under what circumstances should the signal be turned off? That is about all the information I have. Most of the transceivers in our system to date have been Digital H4000s where these options were not available. Any information on this subject would be appreciated.
jstewart@ncs.dnd.ca (John Stewart) (10/04/90)
In article <1990Oct3.191251.26769@watserv1.waterloo.edu> nbirkman@watserv1.waterloo.edu (Nancy Birkmann - DCS) writes: > This is a recurring topic. I am sure that you will receive many answers, so I'll just address the points, and I'll leave the philosophical disucssions to others. :-) > >I read a statement that the SQE signal is not allowed for >transceivers that connect to 802.3 repeaters. >Why? Because SQE and Collision detect are two names for the same set of wires. The meaning of the wires is determined by the timeframe relative to the transmission of a packet; see the 802.3 documentation. Most repeaters do not buffer, so don't really know the wheres and whens of the beginning and end of packets. Remember that a repeater repeats what it sees on one segment to all others. If it sees the SQE as a collision, then it will jam all other segments to propagate that collision. Ditto for connecting a repeater up to a fanout box where the fanout box is connected via an SQE enabled tranceiver. > >Also, many transceiver manufacturers (e.g. Allied >Telesis, Cabletron, Interlan...) provide an option of >either providing this signal or turning it off. >Under what circumstances should the signal be turned off? I leave it off, unless the equipment really needs SQE. If it does, and one is not present, you will receive a message indicating such. You will probably receive many different opinions. >Most of the transceivers in our system to date have been >Digital H4000s where these options were not available. The H4000's are field retrofitable to enable/disable SQE/Heartbeat by cutting a capacitor lead inside them. Also, I think that the H4005 or the H4000-BA (with the lead cut from the factory) are the non-SQE'd tranceivers, while the standard H4000 is the one with the SQE. It's been a while since I have been around a lot of DEC stuff, so treat the above paragraph with due caution. John Stewart DREnet Coordinator jstewart@ncs.dnd.ca