Newsgroups: comp.dcom.lans.ethernet

In comp.dcom.lans.ethernet, skilby@ucqais.cba.uc.edu (Steven M. Kilby) writes:

    1.  Standard states that an ethernet network may have 101 segments, (1
    backbone and 100 branch segments.)  Do segments off of a repeater, (the
    repeater taps directly into the backbone), count towards the 100 branch
    segment total?  A multiport transceiver?
In the original Ethernet, repeaters were only conceived of as 2-port devices
connecting 2 and only 2 coax segments (sometimes with an inter-repeater link
between the coax segments).  The maximum topology was a central coax segment
with at most 100 transceivers on it connecting to 100 repeaters which 
connect to 100 segments.  For that topology, 101 was the maximum number of
segments.

It was not until the repeater definition in IEEE 802.3 was being refined
(mid 1980's) that the repeater became an n-ported device.  Once a repeater
could connect more than 2 segments, the 101 segment limit was no longer 
valid.  

I can not find any statement of a 101 segment limit in my copy of Ethernet.
There certainly is no such limit in IEEE 802.3.  It was the effective limit
of the original Ethernet topology, but it is not important today.

    2.  How do you count the 1024 station limit.  (This is similar to number 
    one.  In a network with bridges, routers, repeaters, etc.... what devices 
    count toward the 1024?)

There is a definition in Section 13 of IEEE 802.3 (published as part of
802.3i 10BASE-T) of a "collision domain:"

  A single CSMA/CD network.  If two or more Media Access Control (MAC) 
  sublayers are within the same collision domain and both transmit at
  the same time, a collision will occur.  MAC sublayers separated by
  a repeater are within the same collision domain.  MAC sublayers separated
  by a bridge are within different collision domains.

All the Ethernet topology limits apply within a collision domain.  Once
you go through a bridge or router, you are in a new collsion domain and
all those counts start over.  This is because simultaneous transmissions
can go on at the same time on either side of the bridge.  No collision
happens.  The bridge just stores the packet for later forwarding (if they
need to be forwarded).

Any thing with a MAC (in other words anything that can initiate the
transmission of a packet counts as a station.  This includes routers
and bridges as well as other nodes.  It does not include repeaters if
they are just a repeater.  However, some repeaters includ a MAC for the
purpose of transmitting network management packets to the network.  A
repeater with a MAC included should be counted.

The limitation on the number of nodes is from the MAC and its ability to
perform properly even under exceedingly heavy loads.

Theoretically, since the the maximum backoff is to 1024 slot times, 1024
stations could try to transmit at the same time and network throughput
would still be stable.  There would be a lot of collisions, but a reasonable
amount of transmissions would succeed.  In reality, things aren't quite
that neat.  I would recommend staying under 500 nodes.

I have heard of larger networks (~3000 low utilization stations).  Such
a network can work just fine as long as you never have more than a few
hundred stations attempting to transmit at the same time.  The problem
is that if all the stations (or well over 1000) tried to transmit at
the same time, you would get a situation where virtually no traffic
gets through.

    3.  If a building has 2 LAN's, each with its own RG-8 backbone, and you want
    to connect them together.... What is the best way?  Can you stick a
    transceiver on both pieces and run RG-58 between them?  Why/Why not?

How long are the backbones?  If they total less than 500 meters and less
than 100 stations, you may be able to just join them with a barrel 
connector.  I assume you are asking the question because they are longer
(or have more stations) than that.  In that case, you may be able to
join them with a repeater.  It would depend on what else was attached
f the total network when you connected them together was larger than
the maximum allowed, you shouldn't do that.  In that case, you would
need to use a bridge.  You also might want to use a bridge (or a router)
if the two LANs were already very busy.

A transceiver connects a station or a repeater to a segment (a piece of
media).  It takes signals (and usually power) from a station and converts
them to the appropriate waveform for the media.
To connect two segments, you need additional functions (primarily retiming)
which are provided by a repeater.

I've got to leave now.  I'll try to answer some of the rest later.

Pat Thaler
Newsgroups: comp.dcom.lans.ethernet