Newsgroups: comp.dcom.lans.ethernet


    |> be 1)coax with no workstations, 2)twisted pair or 3)fiber.

    The spec only lists Fiber as a vendor independent IRL. (Inter-
    Repeater Link)

It is easy to understand how this confusion arises.  The original text
of IEEE 802.3 was not entirely clear on the definition of a link 
segment.  It could be read to include 10BASE2 and 10BASE5 segments
attached to two repeaters and no workstations.  However, the delay
of a coax segment in a worstcase collision scenario is not effected
by the presence of other stations on the segment.  In making additions
to the standard, we have tried to be more clear.

For those who wonder why:  The time to detect collision is longer on 
a coax segment since it depends on a rise in the DC level of the cable.
The link segments, 10BASE-T and FOIRL, are full duplex at the PMD level.
Detection of collision has about the same delay as detection of data on 
link segments.  Therefore, the contribution to round trip delay of a 
500 m 10BASE5 segment is about 10 Bit Times (BT) longer than that of
a 500 m FOIRL segment.

The most recent information on configurations in IEEE 802.3 is in 
IEEE 802.3i, the supplement which added 10BASE-T, in Section 13
Systems Considerations for Multisegment 10 Mb/s Baseband Networks.

A link segment is defined in IEEE 802.3i as: The point-to-point full 
duplex medium connection between two and only two Medium Dependent
Interfaces (MDIs).  (An MDI is the connector on the transceiver to the
media.)

This definition includes 10BASE-T segments (regardless of whether they
are repeater to repeater or repeater to station) and FOIRL segments.
It excludes 10BASE2 and 10BASE5 coaxial segments.

One of the goals in developing the configuration rules for IEEE 802.3
was to create a simple set of rules (they currently are less than
half a page of text) which would always work.  All configurations
that are within the rules will work.  Some configurations which are
outside the rules would also work but were not included inorder to
keep things simple.  For instance, the delay of a maximum length 10BASE2
coax segment is less than the delay of a 10BASE5 coax segment, but we
did not put in exception cases allowing that delay savings to offset
using more delay elsewhere such as a longer than 500 m FOIRL segment.

With the development of 10BASE-T and increasing numbers of star configured
twisted-pair and fiber ethernets, it has become desirable to allow
5 repeaters in certain situations.  (Some vendors support
such configurations though they are outside the current IEEE 802.3 
standard.)  The development of 10BASE-F which will allow fiber optic
segments of up to 2 km and which will allow higher numbers of repeaters
in some cases has increased the need for a more comprehensive (but
unfortunately more complicated) set of rules.

10BASE-F is currently beginning sponsor ballot in IEEE.  If accepted
in its current form, it will add an additional model for calculating
valid network configurations.  

"13.1 Overview.  This section provides informantion on building 10 Mb/s
multi-segment baseband networks within a single collision domain.  The 
proper operation of a CSMA/CD network requires network size to be limited
to control round trip propagation delay to meet the requirements of 
4.2.3.2.3 and 4.4.2.1 and the number of repeaters between any two DTEs to
be limited in order to limit the shrinkage of the inter-packet gap a
it travels through the network.
  This section provides two network models.  Transmission System Model
1 is a set of configurations that have been validated under conservative 
rules and have been qualified as meeting the two requirements set forth 
above.  Transmission System Model 2 is a set of calculation aids that
allow a configuration to be qualified against the two requirements.
This set of calculation aids allows those configuring a network to
test a proposed cconfiguration against a simple set of criteria that
allows it to be qualified.  The Model 2 Transmission System Model
validates an additional broad set of topologies which are fully functional
and do not fit within the simpler but more restrictive rules of Model 1."

So, if the simple rules fill your needs, they will still be there.  If
you need segments longer than 500 m, more than 4 repeaters, etc. and
are willing to do some calculations, you can check configurations
with Model 2.  To validate a path, you perform two calculations: 
one for round trip delay which involves adding a number (from a table)
for each segment and adding the length of the segment * delay per meter
(also from the table); and one for interpacket gap shrinkage which 
involves adding a number for each segment.

Pat Thaler
Chair of IEEE 802.3

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