[Beowulf] Jury rigged ethernet?

Robert G. Brown rgb at phy.duke.edu
Tue Jun 28 05:40:48 PDT 2005


One last remark.  Continuing with google (google is your friend, after
all) I encountered this:

 10BaseT and [100BaseTX] has the capability to run at 10M bits/sec.
 [100M bits/sec.] for transmission over Shielded Twisted Pair (STP)
 cables or Un-shielded Twisted Pair (UTP) (100 ohm) cable which forms
 the Ethernet architecture. The devices belonging to this subsystem
 require interconnection via RJ45-type connectors and twisted pair
 cables (28 AWG minimum), and must conform to IEEE802 specification.
 This interconnect is point-to-point, commonly in a star topology.
 Normally this runs out to 100 meters. Normally configured in a star
 configuration. 10BaseT at a minimum uses "Category 3". 100BaseTX at a
 minimum uses "Category 5" minimum. The [nominally] 100 ohm cable
 impedance should be between 85 and 115 ohms and have at least 2 twists
 per foot. The total number of devices on a single chain is 2.
 100Base-TX uses 2 pairs of Cat5 UTP or 2 pairs of 150 ohm shielded
 balanced cable as defined by ISO/IEC 11801.  With 100Base-TX one wire
 pair is used for transmission, the other pair for collision detection.
 100Base-T4 uses of 4 pairs of Cat. 3, 4, or 5 UTP in a star topology at
 a maximum distance of 100 meters per segment on RJ45 connectors 25Mbps
 per wire pair].

(Quote from:

  http://www.interfacebus.com/Design_Connector_Ethernet.html

)

So apparently the ethernet specification allows for the use of STP
explicity -- not really that surprising.  The question then becomes what
kind of STP is available in the space and whether or not it conforms
with ISO/IEC 11801 (or near enough).  And, perhaps, whether NIC and
switch designers have been conforming with this part of the spec, given
the paucity of STP-based cabling in anything but legacy situations.

So 100BT on STP is almost "guaranteed" to work and would obviously be
the cheapest alternative if it does.  gigabit also has an STP extension
-- I think its cabling specification is deliberately backwards
compatible with 100BT.  So this will almost certainly work as well if
all four pairs are used, at least over a 10 meter run (which is short).

To summarize, I think that the answer is: 

"Yes, you can run any of the 1 Gbps or slower ethernet segments over
STP, depending on precisely what kind of STP you have available, as
there are specifications on gauge, twists per meter, and impedance.
Because of impedance matching problems and reflections, one should avoid
mixing STP and UTP whereever possible, use care terminating cable runs
(regardless of the wire type used).  Short UTP patch cables are probably
OK on the ends of an STP backbone run."

   rgb
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