Network wiring notes - 8P8C / RJ45
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For other network related things, see:
Also: |
What you were probably looking for: Ethernet
(...for the pedants: mainly for 10Base-T, 100BaseTX, and 1000Base-T which are but three of a good deal more methods of the wider concept of Ethernet, but the others are much less relevant to consumer networking)
With pin positions are counted from left to right with the contacts pointing up, clip on the back, and the and pointing up (cable coming out the bottom):
Used by Used by Color (568B) Pin Color(568A) 10/100Mbit 1Gbit/10gBit Orange-white 1 Green-white ✓ ✓ Orange 2 Green ✓ ✓ Green-white 3 Orange-white ✓ ✓ Blue 4 Blue ✓ Blue-white 5 Blue-white ✓ Green 6 Orange ✓ ✓ Brown-white 7 Brown-white ✓ Brown 8 Brown ✓
cut for equal length (not shown),
insert into plug (left)
And preferably enough of the jacket there to clamp down on, ideally more than here (right)
Then use the crimping tool (not shown)
You typically want to make a straight cable, a.k.a. patch cable.
The other type is a crossover cable (568A-to-568B). There were useful in the 10/100 era when you wanted to connect a NIC to another NIC directly, skipping a pricy switch. These days, switches are cheap, plus any gigabit-capable NIC can do this crossover internally, making life simpler and these cables mostly unnecessary.
You care mainly about 568B, because the most typical straight cable is which is 568B-to-568B.
Also, yes, 568A-to-568A is functionally identical to 568B-to-568B but really only a way to confuse yourself. And no, 568A-to-568A is not to 568B-to-568B in terms of low-down details but if you're the type that cares, you already care.
When wiring plugs
- Cut the outer insulation (that holds the cables together)
- Untangle the wire ends, so that they won't pull back when crimped
- ideally, for ≥1gBit it is suggested that you keep the untwisted length as small as possible, to minimize near-end crosstalk[1]
- order them according to the colors you want
- cut them to equal length (if more than slightly different)
- ideally, the outer insulation should make it into the plug far enough so that the crimping digs into it - this provides a little more protection against pull the cable. which means the wires should stick out ~15mm from the sheath. And you want to do that at this stage, because ordering them will change length a little.
- insert into plug
- probably check the order again, they may have jumped around, and checking is faster than redoing
- check that all the wires go to the end (seeing shiny copper from the tip is a good sign)
- use a crimping tool, which shoves the sharp part of the pins into the wires
- (optional: put some solid glue in the socket, to spread force when accidentally pulling the cable)
Notes:
- technically, there are different plugs for braided and solid-copper wiring, that clamp/pierce the wire in different ways (see e.g. stranded solid contacts)
- most of the time you'll work with solid
- (in home use you can often get away with mixing this, in professional use too much of your network may depend on doing one thing properly)
- other things that may be using 8P8C includes
- ISDN
- RS-232 (varied pinouts)
- FireWire
- DMX
- ...and many others. It's a convenient plug.
More notes on ethernet wiring
On standards
The wiring used on 10Mbit, 100Mbit (specifically 10-BASE-T and 100-BASE-TX) ethernet over 8P8C (informally RJ45) plugs is defined by TIA/EIA-568-B, which define two plug wiring alternatives, 568A and 568B.
Notice the lack of dashes; 568-B is the standard that 568A and 568B are part of, 568-A a completely different standard (yes, that's stupidly confusing).
On coexistence
- 10Mbit and 100Mbit networking use only pair 2 and 3 (orange and green) in a 568-style cable
- Countries using analog phones with 4P or 6P plugs (in 8P sockets) can have analog phones and FE coexist, because they use only pair 1 (blue, in the center). Large setups can do this to make their wiring simpler
- It's also sometimes used for the (somewhat riskier) DIY variant of power over Ethernet
- on-the-cheap two-pair cables will work in most places, but only because the NICs fall back to 100MBit
- 1GBit and 10gBit ethernet uses all four pairs in a 568-style cable, so can't do the above trick,
- You want cables rated Cat5e or better for 1gBit, Cat6 or better for 10gBit
- so you can put in the wiring for whatever highest speed you expect to use in the next X years, and can do the actual speed upgrade later by replacing only the switch
On loopbacks
Loopbacks connect a port back to itself.
It's useful to check
- whether a patchbay was wired properly
- whether a wallplug or the cable to the nearest switch is broken
- whether a (long) cable is broken
- whether a switch/router port is broken
These days, it depends more on the test device you have.
- There are ten-buck just-continuity testers
- there are more moderate testers that do some communication and some nicer reporting.
- Classically you could do this without hardware - or rather with the loopback as the only hardware you needed
- ...just by seeing whether the link light lights up,
- but that no longer works when gigabit became standard. ...because Gigabit NICs have crosstalk detection (detects how much signal interferes onto other wires), and will often decide that a loopback cable/plug is actually an extreme amount of crosstalk - any may not show link at all. Which defeats the point entirely, and unless you know you can and have disabled crosstalk detection on the NIC and switch, you won't know what lack of link-light means.
If you still want to make one:
Connect: (If you're wiring a plug as a loopback, make sure you're not confused about which end is pin is pin 1)
Pin 1 to 3 Pin 2 to 6 (Pin 4 to 7 for a gBit loopback) (Pin 5 to 8 for a gBit loopback)
You could take an existing cord (e.g. one that has one the retainer clip broken), cut it in half, and connect:
Orange-white to Green-white Orange to Green (Blue to Brown-white for a gBit loopback) (Brown to Blue-white for a gBit loopback)
(This is the same for both 568A and 568B)
Other 8P8C wiring
Phone
FireWire
ISDN
DMX
RS-232
(varied pinouts)
More technical
On crossover cables
On cable standards (Cat5, etc.)
According to the specs:
- Cat5
- rarely seen - it has fallen out of favour and is barely sold
- ...so informally refers to Cat5a
- Cat5a
- Was the basic choice for a while, and you still see it a bunch
- designed to support 100 MHz signals
- Cat6
- designed to support 250MHz signals
- Cat6a
- Currently the basic choice for installs(verify)
- designed to support 500MHz signals
- Cat7
- designed to support 600MHz signals
- stricter about crosstalk (pairs individually insulated)
- Cat7a
- designed to support 1000MHz signals
- Cat8 (verify)
- designed to support 2000MHz signals
- only up to ~30m
The specs are only for electrical performance, not transfer speeds, but we associate that with speeds, because those rely for a large part on electrical performance, but also on cable length (and also on nearby interference, which is when you care about shielded versions, where that is optional for the category. In noisy situations the shieldedness may matter more than cat5/cat6 difference).
When installing many long cables in in large buildings you absolutely care about the guarantees under the worst case, and shielding.
while at home the fact that most cables are a tenth that length means that even a category lower might support a speed fine (for cables through walls you still want shielding).
So loosely:
- Cat5 should do 100MBit at 100m (but is rare now)
- Cat5e should carry 1gbit at 100m
- Cat6 should do 1gBit at 100m and 10gBit at 55m
- Cat6a should do 10gBit at 100m
- Cat7 should do 10gBit at 100m
- Cat7a should do 10gBit at 100m
- Cat8 should do 40gBit at 30m (...but at this point you might look at fiber too)
Notes:
- Any cabling that is not shielded will crosstalk
- with itself (hence pair shielding, see below)
- with others, meaning permissible distances are lower when many are bundled (relevant to company wiring)
- Cat5 and Cat6 are available unshielded
- The reason that Cat7 seems useless in that list seems to be that it was intended for the not-yet-designed step up from 10GBit, but the standards we actually got meant we wanted a Cat8(verify)
- Home NICs started doing 2.5GBASE-T, and 5GBASE-T.
- Moderate adoption for practical reasons (switch and other NICs has to also speak it)
- But theoretically a reason to go for at least Cat6 in home installations
- (and Cat6/Cat6a is so common that it's an easy choice over Cat5e)
- In theory anything that meets category specs will do the same speed
- The idea of cable quality is usually more about cheap cables that don't actually conform
On cable shielding
Cable shielding doesn't make it go faster, it lessens the influence of some things that might make it act up - mostly nearby cables and other sources of EMI.
(shielding per pair is more about lower crosstalk at transfer speeds(verify))
We informally might say UTP for ethernet cable, but that means 'unshielded twisted pair', and we did start adding shielding.
And introduced a number of distinct ways of doing so.
And fumbled the naming a little. There's...
- (an earlier three-letter system, but they're not quite consistent. Basically, anything with three letters (except UTP) is potentially not what you think)
- a four-or-five-letter system - looks like U/STP or U-STP - to indicate which one of those it is.
- a industry conventions that shortens it, but introduces a little ambiguity
Luckily, the most imformative one, the four-or-five-letter one, is pretty common.
Its parts:
- overall shielding type
- U - unshielded
- S - braided shield
- F - overall foil
- SF - braid and foil
- / or -
- pair shielding type (to reduce crosstalk, common on long cables, 10gBit cables(verify))
- U - unshielded
- F - foil around each pair
- TP (twisted pair)
For example,
- U/UTP means no shield at all
- F/UTP means foil around all, nothing per pair
- S/FTP means a braid around evreything, and foil per pair
- SF-UTP means Braid and foil overall, around unshielded pairs
Note that shielding is not part of the category (except that 7 and up require it(verify)), so this is effectively another choice while buying.
This in part because shielding is much more about the environment, than about the category specs.
Overall shielding: more obviously about environment, as any EMI can get in (and balanced differential signalling helps, but only so much)
Per-pair shielding: if you were worried only about crosstalk between pairs at high speeds, there are other ways to get that electrical performance (thicker conductors, more space and/or better control of placement may also get it that lower crosstalk.
Which is part of why you can find all of Cat5, Cat5e, Cat6, Cat6a in U/UTP.
Notes
- From the wall to your home PC you don't need to think about shielding (unless maybe you're doing 10gBit)
- If you're putting it in walls, bundled with other things, any overall shielding (e.g. F-UTP) is a decent idea, especially if price difference is low (and it often is)
- Shield, as a concept, needs to be grounded
- and in larger installations ideally only on one side
- at home this is rarely a problem, but in large installations you need your network people to think like electricians, and preferably be certified
- in industrial settings you also want someone who can think about interference
As to that confusing industry convention mentioned earlier,
there are a few names that looks similar but are different
- some are pretty clear, e.g. UTP is U/UTP
- others are fuzzier, e.g.
- STP might refer to U/FTP, F/UTP, S/UTP, SF/UTP
- SSTP might refer to S/FTP, or to SF/FTP
- SFTP might refer to SF/UTP, S/FTP, SF/FTP
- see e.g. https://en.wikipedia.org/wiki/Twisted_pair#Cable_shielding
Naming pendantics and telephony
When we say RJ45, we often mean something like "Ethernet wiring on an 8P8C plug."
RJ is the group of plugs that can be described by their positions and connectors, such as 8P8C in Ethernet, while RJ45 actually refers to a specific telephone wiring on the 8P-style plug (probably the most common one among several), while 8P8C refers to that plug itself and no specific wiring. Regardless, most people call the plug RJ45, regardless of wiring.
See also Common plugs and connectors#Modular_connector_.28.3FP.3FC.29.3B_Registered_Jack
Plugs may have fewer actually present conductors than they have positions, so 8P2C, 8P4C, 8P6C, 8P8C, 6P2C, 6P4C, 6P6C, 4P2C, 4P4C all exist.
When there are less connectors than positions, they are in the middle positions; RJ-style wiring is from the middle out.
For most of us, the is interesting only in that you can plug a phone with 6P plugs into a 8P (ethernet-plus-phone) socket and have the phone work - the clip aligns the plug in the middle.
If more more than the middle two wires are used in telephone wiring, they carry either power, or a second (RJ14) or even third (RJ25) telephone line on the same wire, but consumers rarely see this type of phone wiring.
There are exceptions to the 'always start in the middle', but they tend to be intentionally working around RJ-style wiring.
The most common use of 6P outside of the US is probably phone wiring according to RJ11, which often use just a single pair in the middle. In the US, 8P connectors with the RJ45 phone wiring is common.