Newsgroups: comp.dcom.lans.ethernet


    1) I find conflicting information (mostly from salespersons) about 
       how many repeaters I can have in my network. One source says
       2 another says 3 and yet another says 4. Which is correct?
       (I should say, how many can I have between to communicating
       systems on the same ethernet (i.e. not broken by a non-repeating
       apparatus such as a bridge or gateway?)

The source of confusion is due to early references that considered a
coax to fiber repeater to be a "half repeater".  This term is and always
has been a misnomer.  There is nothing "half" about a coax to fiber repeater.
It has the same functionality between the two transceivers that a coax
to coax repeater has.  Terms such as "local" and "remote" repeaters are
similarly misguided.  There are only repeaters.  The reason for differentiating
between coax-to-coax repeaters versus coax-to-fiber repeaters is that the
round trip delay of a 500 m coax _segment_ is slightly longer than the
delay of a 500 m FOIRL segment.  (To those confused by the fact that coax
prop delay is faster than fiber:  the mechanism for collision detection
used on coax works more slowly than that used on FOIRL.  This delay on
coax is large enough to make the total round trip delay longer for a 
coax segment.)

Until recently, considering fiber to coax repeaters to be half repeaters
and saying that you could have two repeaters end to end was virtually 
equivalent to the IEEE 802.3 rule which says you can have 4 repeaters
and 5 segments end to end with no more than 3 segments being coax. 
With the addition of 10BASE-T (and with fiber being used to the desk),
the definitions are no longer equivalent.  For instance, the 802.3
definition allows this:

  10BASE-T   -- R -- coax -- R -- coax -- R -- coax  -- R -- 10BASE-T
  or 10BASE-F                                                or 10BASE-F 

which has 4 repeaters, 5 segments, 3 of which are coax.  The old Ethernet
rule would count this as 3 or 4 repeaters (it isn't clear what they would
call a 10BASE-T to coax repeater or a fiber to coax repeater where the
fiber connects to a DTE; 3 assumes that the two end repeaters are counted
as half repeaters).  In either case, applying the old rule would say
it is an invalid configuration when in fact it is a perfectly good one.

For rules relating to configuration, I recommend you refer to Section 13
of IEEE 802.3i (the 10BASE-T standard).  This is the most current statement
of Ethernet/IEEE 802.3 configuration rules.  When 10BASE-F is released,
it will contain an expanded version of Section 13.

    2) Is a repeater effectively two transceivers? If so, then do
       the thin to fiber converters (one at each end of fiber) count
       as one repeater?

A repeater is more than two transceivers.  In order to restore the signal
it is decoding and reencoding it.  Non-standard devices such as transceiver
multiplexers and some media converters can have whatever characteristics
their manufacturer builds into them.  (Some other media converters are actually
small repeaters; the ones I know of in that category are thin to 10BASE-T.)
You can't count on apply standard rules to non-standard devices.  Ask the
manufacturer or consult the data sheet on how they should be used.

    3) Which is the correct way to wire a thinnet segment into a 
       multiport repeater?
  
       a)                                  b)
  
        .------------.                       .--------------.
        | mpr box    |                       | mpr box      |
        `---.--------'                       `----.---------'
            |                                     |
        |---T--------------|                      |
                                                  |
                                                  -     <-terminator
        (two terminators, t-connection        (one terminator, direct bnc
         to multiport repeater)                connection to multiport repeater)

Strictly speaking, a repeater does not include any terminators.  (Strictly
speaking means that a repeater is a very specifically defined device in
the IEEE 802.3 standard.)  However, a box may contain more than one device.
For instance, a repeater unit is the device that connects the transceivers.
The repeater set is the repeater unit plus the transceivers.  Some
manufacturers also include one T and one terminator per thinnet (10BASE2)
segment along with the repeater set in their box.  In some the terminator 
can be selected or not by configuration.  So, you need to look at the 
documentation for your repeater to see if it includes a terminator.

    4) Can a transceiver be plugged directly into the workstation, or
       is there some reason that you must use the tranceiver cable?
There does not need to be a transceiver cable (assuming the mechanical
design of the station and the transceiver allows direct connection).

    5) Are thinnet segments supposed to be grounded somewhere?
Grounding the thinnet segment is optional.  It may be grounded in one
place or no place.  It should not be allowed to come into contact with
multiple grounds.  Grounding the segment is a safety issue not an operatinal
issue.  When we looked into it while developing 10BASE2, we found that
codes in different jurisdictions had conflicting rules on this.  10BASE2
works fine either way.

The transceivers are AC coupled to the stations.  At low frequencies such
as 60 Hz there is a high impedance between the coax and the station.  At
the frequencies where the most energy of the data signal is, 3 to 30 MHz,
the impedance is less than 15 ohms.  To the data it looks like the coax
is grounded at each transceiver.

I am not a safety expert.  However, I believe if you are putting in a ground
for safety reasons, the preferred way to do it is with a direct connection
to a ground.  That is, for instance from a lug on a terminator or barrel
to a building ground.  If you go to the chassis of some station
and the station is unplugged, you lose your ground connection.  Yes,
I know that the same effect occurs if someone disconnects the terminator
or a T but at least then the segment doesn't work so it is likely to get fixed
soon.  The alternative is to not allow users to access the T-connectors
(eg put the cable and transceivers in the wall).  Connecting to the chassis
of a repeater in a wiring closet might be okay since it is less likely to
get unplugged.


Pat Thaler
Newsgroups: comp.dcom.lans.ethernet