30-09-2016, 09:19 AM
(30-09-2016, 12:48 AM)Nick Wrote: There are two standards of GbE over copper, 1000BASE-T and 1000BASE-TX.
1000BASE-T uses all 4 pairs each running bidirectionally on CAT5 or better.
1000BASE-TX uses 2 pairs, one for TX, the other for RX, however it requires CAT6 or better.
1000BASE-TX was a commercial failure as CAT6 is expensive and most folk already have CAT5, also 1000BASE-T kit is cheap, so 1000BASE-TX never caught on. Pretty much every GbE installation is thus 1000BASE-T.
That's right - I said the same in my original post (#19). Today "Gb Ethernet" is synonymous with "BASE-T", but it wouldn't hurt to add "BASE-T" to my more recent posts for clarity.
(30-09-2016, 12:48 AM)Nick Wrote: PoE (Power over Ethernet) is extremely important in structured cabling environments - with all 4 pairs in 1000BASE-T being used for data, PoE is delivered by exploiting the fact that the pairs are differentially driven to reject common mode noise, so a DC level can be imposed and extracted with no effect on the data. All Ethernet terminations use transformers for galvanic isolation, however PoE terminations use center-tapped transformers to extract the imposed DC.
One slight difference between CAT5 and CAT6 is that CAT5 is 24 AWG and CAT6 is 23 AWG, so whilst PoE on CAT5 is tecnically limited to bundles of 100 cables due to heating (assuming everyone is drawing 30W or so), CAT6 bundles can be bigger...
The PoE point is a good one, and I remember a colleague trying that on me before I started flood-wiring my house. That was just about the only "concrete" reason I had to consider Cat 6 cable, but there were plenty of other reasons to stick with Cat 5, including cost (I already had the box!), ease of installation, etc. Ultimately, the bundle leaving the patch panel will only contain 40 or so cables when finished, and the runs are all quite short - it's only a 3-bed house. But when I win the lottery and move to that mansion in the country, it'll be Cat-8 all the way
(30-09-2016, 12:48 AM)Nick Wrote: Finally, 1000BASE-T devices use auto negotiation (auto MDI-X), so there is never a need to use different cables (crossover vs. straight through) for endpoints and uplinks, even when talking to older 100BASE -T devices which have distinct TX and RX pairs like 1000BASE-TX, thus reducing installation errors and simplifying switches (no longer any need for a distinct uplink port in low-end kit).
Yes, auto MDI-X made life so much easier.
Auto-negotiation was a bit problematic for a while. One example I remember is the Compaq/DEC Alphaservers that were used for audio playout: they would try to run at half-duplex while the switch would try to work at full-duplex. Manually setting the switch to full-duplex (100Mb/s) worked fine. Other switches had the same behaviour; others did seem to negotiate correctly - we didn't identify any patterns between brand or anything else IIRC. Happily, things have moved on quite a bit since then.
