7 ms·

Electrically, there's not much reason to look down on Twisted Pair. It is an ingenuous way to achieve what coax does as an unbalanced transmission line with, we

by anyfoo 4y ago

Electrically, there's not much reason to look down on Twisted Pair. It is an ingenuous way to achieve what coax does as an unbalanced transmission line with, well, a twisted pair of wires that form a balanced (differential) transmission line. You might need to add some shielding in some situations, but that's just a piece of foil. Most importantly, the two wires that make up the pair really just are common single-ended wires, not elaborate coax or anything else. A single coax transmission line supporting 10Gbps Ethernet would likely be much more expensive than the little bundle of twisted pairs we typically use nowadays. In many ways, for its applications, twisted pair and RJ45 connections are better than coax wiring with BNC.

cesarb 4y ago
> Electrically, there's not much reason to look down on Twisted Pair. Yeah, but mechanically, the RJ45 plug with its finicky easily breakable plastic tab can be an annoyance. And it's easy to see that the pin ordering is not ideal, with the pair in the middle splitting another pair. AFAIK, there exists a more robust connector (the M12 connector), but it doesn't seem to be that common.
- anyfoo 4y ago
  True, the plastic tab can be annoying, but I guess the sheer abundance of patch cables nowadays means the cheapness of the connector, while retaining pretty good ease of use (better than many others), makes up for that drawback. Maybe something slightly more resilient could have been designed within the same parameters, had people known just how ubiquitous that connector would become. Maybe M12 is that, but it looks way more expensive at first glance. Possibly more laborious to connect/disconnect, too, with its screw-locking? Seems to be better for applications where a secured connection is more important (transportation is mentioned). And yeah, the 1000base-T pin ordering seems unusual. I'm curious about the history there, because even 10base-T (where I thought Ethernet for Twisted Pair begun) had this really weird pinout, which does not support my initial theory that it was because Ethernet kept progressively adding more differential pairs: https://www.arcelect.com/10baset.htm https://www.arcelect.com/10baset.htm It may well be because they added the original two pairs to a pinout that already carried something else, but the diagrams don't say what those other lines were for, so if anyone knows... According to those same diagrams, though, it seems to be more common to split up the pairs than not, which now makes me wonder if there is any benefit to that?
  - tmm 4y ago
    > but the diagrams don't say what those other lines were for, so if anyone knows... Telephones. Telephones are why. Those other two pairs were often used for voice communication. If you had four-pair station cabling, the pairs were provisioned on the modular jack from the inside out. So line one was the blue/blue-white pair on the inner pins, line two was the orange/orange-white pair on the next two pins, and so on. Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice. But why 4 pairs in the first place? Just about the time that Ethernet was transitioning from coax to twisted pair, the digital PBX was taking over from key systems (1A2) and reduced the number of wires required for a business telephone from 25 pairs (or more ... secretarial sets often had 100 or more pairs) per station down to 4 (for HORIZON[0]) and later two pairs (DIMENSION and eventually Merlin, Definity, etc.). So if you're wiring a new building, you can just run one CAT-3[1] cable to each desk and use the first two pairs for voice and the second two for data[2]. [0] OK, for the pedants out there, HORIZON wasn't ever very popular and really pre-dated Ethernet, but the telecom world moves kinda slow [1] Wasn't really CAT-3 until the early 90s [2] Not on the same jack, but by using pins 1, 2, 7, and 8 for data, you can plug the wrong cable in without risk of hurting the phone or your computer's network card
    - anyfoo 4y ago
      That makes perfect sense now, thanks. > so for signal integrity reasons those are moved to the outside and used for data I'm not sure about that bit, though. Would keeping the pair together not help with signal integrity?
    - garaetjjte 4y ago
      >Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice. But 10BASE-T uses pins 1, 2, 3, 6, thus green and orange pair.
  - 4y ago
- robocat 4y ago
  > finicky easily breakable plastic tab That is not really the fault of the RJ45 specifications. The choice is available between cheap breakable connectors or reliable well-designed connectors: it isn’t the fault of the specification that cheap is often chosen. > the pin ordering is not ideal A very minor nitpick. And designed that way for specific reasons. I like that it works well, was backwards compatible, and the connectors, wiring, and tools are cheap, available, and abundant. 1000Base-T is amazing technology (even if we are blasé about it!)
  - nomel 4y ago
    I've been using multi-gig [1] over short runs of cheap cat 5 cable just fine. Actually, I only have one span that links at 5G. The rest are short enough for 10G with cat-5e, with 100m achievable with Cat-6! Talk about incredible! 1. https://community.fs.com/blog/what-is-multigig-ethernet.html https://community.fs.com/blog/what-is-multigig-ethernet.html
  - anyfoo 4y ago
    > And designed that way for specific reasons. Do you know that reason? I was wondering in my other reply.
    - garaetjjte 4y ago
      It's some legacy from scheme used in RJ connectors used in telephony, where first pair was on connector center and it continued outwards with each pair (like this, where each digit is pair: 4 3 2 1 1 2 3 4). T568 only retains this scheme for two pairs, maybe they realized that splitting last pairs across entire 8 pin connector would be unwieldy. Nothing stops you from wiring connectors different way though, to the annoyance of anybody splicing that cable in the future :)
      - robocat 4y ago
        > Nothing stops you from wiring connectors different way though You can only swap the wires in a pair (eg swap solid orange and striped white/orange), or swap pairs (eg blue pair with orange pair). You must keep each twisted pair matched, so that the electrical signal travels correctly, otherwise you run into problems at length. For very short runs I would guess you could ignore pairs, even though it would increase crosstalk (between signals within a cable) and interference (aerial like transmission or reception, especially with different cables run close together). Great picture of different twists per centimetre comparing each pair within a cable (compare orange against blue in one cable), and also shows the different twists for different cats (less for cat5, more for cat6): https://store.chipkin.com/articles/differences-between-category-5-cat5e-and-cat6-patch-cables https://store.chipkin.com/articles/differences-between-categ... — if the twist lengths of all pairs were exactly the same within a cable, there would be a lot more crosstalk between pairs.
- Spooky23 4y ago
  Huh? Rigid coax and overtightened or damaged shielding is so much more annoying and more common.
- kjellsbells 4y ago
  M12 is quite common in "dirty" industrial environments and military apps. its vibration proof, resistant to corrosion, can meet low/no spark requirements, and you can manipulate the connectors with gloves on, which sounds silly but is not to be discounted when you are in a foxhole being shot at.
jbverschoor 4y ago
In the 90s, coax was way cheaper than utp.

by anyfoo 4y ago

cesarb 4y ago

> Electrically, there's not much reason to look down on Twisted Pair. Yeah, but mechanically, the RJ45 plug with its finicky easily breakable plastic tab can be an annoyance. And it's easy to see that the pin ordering is not ideal, with the pair in the middle splitting another pair. AFAIK, there exists a more robust connector (the M12 connector), but it doesn't seem to be that common.

anyfoo 4y ago
True, the plastic tab can be annoying, but I guess the sheer abundance of patch cables nowadays means the cheapness of the connector, while retaining pretty good ease of use (better than many others), makes up for that drawback. Maybe something slightly more resilient could have been designed within the same parameters, had people known just how ubiquitous that connector would become. Maybe M12 is that, but it looks way more expensive at first glance. Possibly more laborious to connect/disconnect, too, with its screw-locking? Seems to be better for applications where a secured connection is more important (transportation is mentioned). And yeah, the 1000base-T pin ordering seems unusual. I'm curious about the history there, because even 10base-T (where I thought Ethernet for Twisted Pair begun) had this really weird pinout, which does not support my initial theory that it was because Ethernet kept progressively adding more differential pairs: https://www.arcelect.com/10baset.htm https://www.arcelect.com/10baset.htm It may well be because they added the original two pairs to a pinout that already carried something else, but the diagrams don't say what those other lines were for, so if anyone knows... According to those same diagrams, though, it seems to be more common to split up the pairs than not, which now makes me wonder if there is any benefit to that?
- tmm 4y ago
  > but the diagrams don't say what those other lines were for, so if anyone knows... Telephones. Telephones are why. Those other two pairs were often used for voice communication. If you had four-pair station cabling, the pairs were provisioned on the modular jack from the inside out. So line one was the blue/blue-white pair on the inner pins, line two was the orange/orange-white pair on the next two pins, and so on. Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice. But why 4 pairs in the first place? Just about the time that Ethernet was transitioning from coax to twisted pair, the digital PBX was taking over from key systems (1A2) and reduced the number of wires required for a business telephone from 25 pairs (or more ... secretarial sets often had 100 or more pairs) per station down to 4 (for HORIZON[0]) and later two pairs (DIMENSION and eventually Merlin, Definity, etc.). So if you're wiring a new building, you can just run one CAT-3[1] cable to each desk and use the first two pairs for voice and the second two for data[2]. [0] OK, for the pedants out there, HORIZON wasn't ever very popular and really pre-dated Ethernet, but the telecom world moves kinda slow [1] Wasn't really CAT-3 until the early 90s [2] Not on the same jack, but by using pins 1, 2, 7, and 8 for data, you can plug the wrong cable in without risk of hurting the phone or your computer's network card
  - anyfoo 4y ago
    That makes perfect sense now, thanks. > so for signal integrity reasons those are moved to the outside and used for data I'm not sure about that bit, though. Would keeping the pair together not help with signal integrity?
  - garaetjjte 4y ago
    >Ethernet comes along and lots of places where you'd want a network connection already had a phone jack with two pairs unused, so for signal integrity reasons those are moved to the outside and used for data, leaving the inner two pairs where they were to be used for voice. But 10BASE-T uses pins 1, 2, 3, 6, thus green and orange pair.
- 4y ago
robocat 4y ago
> finicky easily breakable plastic tab That is not really the fault of the RJ45 specifications. The choice is available between cheap breakable connectors or reliable well-designed connectors: it isn’t the fault of the specification that cheap is often chosen. > the pin ordering is not ideal A very minor nitpick. And designed that way for specific reasons. I like that it works well, was backwards compatible, and the connectors, wiring, and tools are cheap, available, and abundant. 1000Base-T is amazing technology (even if we are blasé about it!)
- nomel 4y ago
  I've been using multi-gig [1] over short runs of cheap cat 5 cable just fine. Actually, I only have one span that links at 5G. The rest are short enough for 10G with cat-5e, with 100m achievable with Cat-6! Talk about incredible! 1. https://community.fs.com/blog/what-is-multigig-ethernet.html https://community.fs.com/blog/what-is-multigig-ethernet.html
- anyfoo 4y ago
  > And designed that way for specific reasons. Do you know that reason? I was wondering in my other reply.
  - garaetjjte 4y ago
    It's some legacy from scheme used in RJ connectors used in telephony, where first pair was on connector center and it continued outwards with each pair (like this, where each digit is pair: 4 3 2 1 1 2 3 4). T568 only retains this scheme for two pairs, maybe they realized that splitting last pairs across entire 8 pin connector would be unwieldy. Nothing stops you from wiring connectors different way though, to the annoyance of anybody splicing that cable in the future :)
    - robocat 4y ago
      > Nothing stops you from wiring connectors different way though You can only swap the wires in a pair (eg swap solid orange and striped white/orange), or swap pairs (eg blue pair with orange pair). You must keep each twisted pair matched, so that the electrical signal travels correctly, otherwise you run into problems at length. For very short runs I would guess you could ignore pairs, even though it would increase crosstalk (between signals within a cable) and interference (aerial like transmission or reception, especially with different cables run close together). Great picture of different twists per centimetre comparing each pair within a cable (compare orange against blue in one cable), and also shows the different twists for different cats (less for cat5, more for cat6): https://store.chipkin.com/articles/differences-between-category-5-cat5e-and-cat6-patch-cables https://store.chipkin.com/articles/differences-between-categ... — if the twist lengths of all pairs were exactly the same within a cable, there would be a lot more crosstalk between pairs.
Spooky23 4y ago
Huh? Rigid coax and overtightened or damaged shielding is so much more annoying and more common.
kjellsbells 4y ago
M12 is quite common in "dirty" industrial environments and military apps. its vibration proof, resistant to corrosion, can meet low/no spark requirements, and you can manipulate the connectors with gloves on, which sounds silly but is not to be discounted when you are in a foxhole being shot at.

jbverschoor 4y ago

In the 90s, coax was way cheaper than utp.