NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/bit/listserv/biglan/68

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Date:         Fri, 21 Aug 1992 08:53:30 EDT
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Subject:      BIG-LAN Digest, Volume 4, Number 55, Friday, August 21, 1992
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BIG-LAN DIGEST            Friday, 21 August 1992       Volume 4 : Issue 55

Today's Topics:

                                Crosstalk
                       More on Repeater Count Rules
                         Daisy Chaining 10BT hubs
                     Ethernet card to IBM RISC-6000?
                         Those pesky "hub" rules

Moderated by John Wobus, Syracuse University

Relevant addresses:
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Date:         Tue, 18 Aug 92 10:28:48 EST
From:         Jack Lubowsky <LUBOWSKY@SNYBKSAC>
Subject:      Crosstalk

A contractor has just wired our building with 10Base-T to the floors
from fiber risers. The 10Base-T cable (4 pair of #24) has been wired
through carrels in the labs alongside unshielded ac power cables. The
power cables are typical lab use lines (ie: not running heavy machinery)
and the runs can be as long as 20 feet.

Does anyone know whether there will be crosstalk problems sufficient to
reduce performance. The decision is, do we...
      1: leave things alone, it will probably be ok,
      2: move the twisted pair cable and power cable apart and secure
             with wire ties anywhere we can, (Could be labor intensive)
      3: require the contractor to pull back the twisted pair and rerun
             to minimize proximity to power cable?

Any experiences or advice would be apreciated....Thanks

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Date: Tue, 18 Aug 92 16:03:34 CST
From: Andrew Birner <[e-mail]>
Subject:      More on Repeater Count Rules

In BIG-LAN 4.54, William Magill <[e-mail]> writes:

>The real issue in ANY CSMA/CD Carrier Sense, Multiple Access/Collision
>Detection - network ie. Ethernet IEEE 802.3 - is the ability for the two
>FURTHEST stations on a segment to "simultaneously" detect a collision,
>back-off and to retransmit. This dimension is measured in TIME, not feet.
>The FURTHEST separated stations are NOT necessarily the two stations which
>you are interested in. But they represent the LONGEST POSSIBLE path in TIME
>through that physical segment.

While this is true, it is only half the story of the the repeater rule.  The
other half has to do with inter-frame gap shrinkage:  Repeaters make frames
longer, so if you have too many repeaters, they use up all the space between
the frames.  Seriously.

A repeater doesn't start repeating until it knows it has something to repeat.
Since frames are generated asynchronously, the repeater has to see some
number of bits before it can sync up to the signal and determine that a frame
is coming in; this is the function of the frame preamble--to give the
receiver (in this case a repeater) something to synchronize to.  Detecting
and synchronizing to the signal consumes some number of preamble bits,
however; the exact number consumed for any given frame is indeterminate, but
must be less than some number (12, I think).  So, receiving the frame
essentially makes it "shorter", since preamble bits are included in the frame
size.

However, the specs require a minimum preamble length.  So, the repeater needs
to regenerate any preamble bits it consumed, on the assumption that the
transmitter generated the minimum allowed; but, the receiver doesn't know how
many it consumed!  The repeater does, however, have an upper bound (namely,
the maximum permitted by the standard) on the number of bits it consumed; so,
it adds that number of bits to the preamble of the outgoing frame.

If, as is usually the case, the repeater actually consumes less than the
maximum number of preamble bits, the outgoing frame will be longer than the
incoming frame.  Thus, the end of the frame will be displaced backwards in
time, relative to the front of the frame; this brings it closer to the front
of any following frame on the wire!  If two successive frames start out
spaced at the minimum time/distance, they may be indistinguishable to a
receiver after some number of repeaters!

Note that this preamble restriction is a function of the broadcast nature of
"coax" segments.  Inter-Repeater Link (IRL) segments are full-duplex, point-
to-point links; they are implemented such that the receiver does not need to
consume preamble bits to detect the start of a frame.  (In some synchronous
implementations, code violations are used, as I recall).  Thus the different
rules for coax versus IRL segments in the repeater-count rules.

Andrew Birner
Zenith Electronics Corporation

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Date:        Wed, 19 Aug 92 09:13:00 EDT
From:        NCUT000 <NCUT@SLUMUS>
Subject:      Daisy Chaining 10BT hubs

While it's true that "Daisy Chaining" counts as a repeater hop when you
connect from the up/down ports that is NOT the only answer to the
question - 3Com makes a 12 port hub that can be interconnected to
another of the same type via a backplane scsi 2 connector.  Using this
backplane connector you can stack 4 hubs on "top" of each other and the
entire stack still only counts as one repeater hop.  We have just
received a few of these to put into our labs.  Having not actually
installed them yet I can't comment on my satisfaction or
dis-satisfaction, but I suspect they will work just fine.  We decided
to try these for this feature and a number of other quite well thought
out features that 3Com has built in.   These are the 10BT series
concentrator.

- - Sorry if that sounded like a sales pitch.  'Just thought you should
know.

Nick Cutry                         voice: [phone]
Network Manager                    Bitnet: NCUT@SLUMUS or CUTRY@STLAWU
Computer Services                  Compu$erve: 76424,144 or 73257,2537
St. Lawrence University

"...so they are protecting all of us PC users from having our computers
made obsolete."
              - Peter Norton on IBM's intro. of the PC/XT (march '83)

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Date: Thu, 20 Aug 1992 16:48 -0300
From: Marcelo Spohn <[e-mail]>
Subject:      Ethernet card to IBM RISC-6000?

        Please,

        Anyone can tell me if there is other supplier than IBM
of Ethernet cards for IBM RISC-6000 workstations?

        Thanks in advance!

MARCELO SPOHN
Federal University of Rio Grande do Sul - UFRGS
Institut of Informatic
Data Communication Group
Porto Alegre - RS - BRAZIL

-------------------------------------------------------

Date: Fri, 21 Aug 92 08:47:28 -0400
From: "John M. Wobus" <[e-mail]>
Subject:      Those pesky "hub" rules

Richard Letts writes:
>> From: "John M. Wobus" <[e-mail]>
>>
>>                                        Here are the rules: there can be
>> no more than 4 repeaters and 3 coax segments between any two stations
>> on an Ethernet.  By "coax segment", I mean thickwire or thinwire.
>> The other segments can be fiber or twisted pair.
>>
>Nope, these aren't the rules:
> -     You can have 3 data segments and 2 link segments between the
>    stations. (including the segments the stations are attached to)
> -    Stations can only be connected to the 'data' segments
> -    The segments can be of any type.

Actually, what Richard Letts said and what I said are very close: 4
repeaters is enough to connect 5 segments, i.e., 3+2.  The kinds
of segments defined by the standards include two that are point to
point (10BASE-T and FOIRL) and two that can have lots of stations
connected to them (10BASE-5/thickwire and 10BASE-2/thinwire).  So the
only differences are the choice of terminology, the fact that my
statement of the rules allowed more link segments as long as the
total is not more than 5 and that Richard's definition of data segments
was that there are stations on them rather than that they are coax.

The relevant chapter of the standard (Section 13, System Considerations
for Multisegment 10 Mb/s Baseband Networks) has been rewritten more
than once to "clarify" it.  The latest standardized version is in IEEE
Std 802.3i-1990, the 802.3 supplement that includes 10BASE-T.  The
relevant sentence is item (4) on page 16:

   (4) When a network path consists of four repeater sets and five
       segments, up to three of the segments may be coax and the
       remainder must be link segments.

William H. Magill writes:
>> From: "John M. Wobus" <[e-mail]>
>>
>> The point is: add a hublet only if you aren't already 4 repeaters away
>> from some station on the ethernet.
>>
>The real issue in ANY CSMA/CD Carrier Sense, Multiple Access/Collision
>Detection - network ie. Ethernet IEEE 802.3 - is the ability for the two
>FURTHEST stations on a segment to "simultaneously" detect a collision,
>back-off and to retransmit. This dimension is measured in TIME, not feet.

William Magill is right.  I was quoting the standard which actually
promotes an "easy way" of maintaining the necessary time constraints:
by counting lengths and repeaters rather than caclulating a time
budget.

John Wobus
Syracuse University

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End of BIG-LAN Digest
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