Considering 10Base-T Half Duplex...
Is the maximum segment length of 100m a timing constraint (from CSMA/CD)? Is it derived from the slot time / minimum packet size? Or is it an electrical performance constraint (attenuation etc.) of the cable used?
Conversely, is there a stated minimum cable length (or minimum segment length) between end device and switch required, so as to ensure Collision Detection works during Half Duplex operation?
Regards.
The 100m constraint is a very old constraint. At the time, it was a design decision for 10BASE-T. IEEE could have used a larger distance (say 500m like 10BASE5 or 185m like 10BASE2), but then all the equipment that would implement 10BASE-T (cables, repeaters, NICs, bridges, etc) would have to be more sensitive and capable (translating to being more expensive). A basic tenet of the IEEE 802.3 Working Groups was that Ethernet had to be cheap. Cheap meant smaller distances. And to further lower costs, the cable type and characteristics that the 10BASE-T working group had to select had to be easy and streamlined to installa (lower training for installers, lower breakage during installations, more durable cabling).
After a compromise was made between usefulness (a 100m physical link is quite useful) and expected price per NIC/cable, the standard was written around that.
Another piece of baggage that 10BASE-T had to carry was the MAC sub-layer constraints of older Ethernet standards. (Ethernet frame format, minimum and maximum frame lengths, Inter-Frame Gap, etc) The actual timing constraints of 10BASE-T (for CSMA/CD to function) came from older standards.
After the Working Group had concluded on a maximum distance, all they then had to do was define the cabling's minimum characteristics.
To find the minimum length, as defined by the standard, I believe you must study the IEEE 802.1 standard itself. I have seen references to a 2.5m minimum distance between 10BASE-T nodes, but I am not sure if this restriction is written in the standard. I know for a fact that with newer standards' implementations I have used 0.5m patch cords without any issues, but only on Ethernet point-to-point links.
But I noticed a weird phrasing in your question: you are asking about cabling between an end-device and a switch, and whether Collision Detection will work. Unless you are using a hub or a bridge (a very old device type), you should not be even able to produce collisions in this connection.
So, to answer your question "what is the minimum cable length (or minimum segment length) between end device and switch, so as to ensure Collision Detection works during Half Duplex operation", the answer is:
It's an impossible question to answer. You cannot get a Collision on a device connected to a switch with a UTP cable. You need something else that creates a shared medium somewhere. So, no, there is no minimum length of cable that will allow collision detection to work on an end-device or a switch directly connected to each other with a UTP cable.
The 100 m limit for twisted pair - up to 90 m of stiff, solid-core plenum cable, up 10 m of flexible, stranded patch cable - is determined by the electrical performance (mainly attenuation, crosstalk).
The 5-4-3 rule tells us that a half-duplex collision domain may be longer than just two 10BASE-T links - up to five 10BASE-T links may be cascaded over up to four repeaters. You can do the math with known minimum frame size, velocity factors, repeater delays and detection delays (for 10BASE5/2) - see IEEE 802.3 13.4 for details.
Half-duplex Ethernet with CSMA/CD is long obsolete, of course. In theory you could exceed the 100 m with a better cable grade but there is no standard that tells you how far you could go, and you'd be on your own.
is there a stated minimum cable length?
There is no such thing for 10BASE-T. IEEE 802.3 Clause 14.1.1 specifically states operating over 0 m to at least 100 m of twisted pair without the use of a repeater.
There are minimum constraints for some other Ethernet variants, most notably 10BASE5, to ensure reliable collision detection on the shared wire, but there aren't any minimum lengths for TP (or fiber variants for up to 20 km).
