Cat 5 Ethernet (LAN) Wiring

568A & 568B Wiring Schemes

There are two colour-code standards in common use: EIA/TIA 568A and EIA/TIA 568B. These standards derive from telecom usage and the pairs shown correspond to four phone lines, each with its own line pair.  This same wiring was adopted for LAN standard Ethernet RJ45 wiring as well. 

It is important to eliminate any confusion between 568, the standard, and 568, the wiring scheme.

If someone refers to 568A, are they talking about the standard, or the wiring scheme? The answer depends on the context.

If they were to say "The entire office fully complies with 568A", obviously, they would be talking about the standard.

If they were to say "The jacks and patch panels are all 568A", then they would be referring to the wiring scheme.

The definitions of the two are:

568A Standard

This standard was published in July of 1991. The purpose of EIA/TIA 568A, was to create a multiproduct, multivendor, standard for connectivity.

Prior to the adoption of this standard, many "proprietary" cabling systems existed. This was very bad for the consumer.

Among other things, the standard set the minimum requirements for category-5E cable and hardware.

The 568 "standard" is not to be confused with 568A or 568B wiring schemes, which are themselves, part of the "568A standard".

568A & 568B Wiring Schemes

When referring to a jack or a patch panel's wiring connection, it is to either the 568A, or 568B wiring scheme, which dictates the pin assignments to the pairs of cat 5E cable.

There is no difference, whatsoever, between the two wiring schemes, in connectivity or performance when connected from one modular device to another (jack to Patch panel, RJ-45 to RJ-45, etc.), so long as both of the two devices are wired for the same scheme (A or B).

The only time when one scheme has an advantage over the other, is when one end of a segment is connected to a modular device, and the other end to a punch block. In which case, the 568A has the advantage of having a more natural progression of pairs at the punch block side.

The 568 committee decided, with good intentions, to allow both wiring methods (568A & 568B) to exist within the 568A Standard. The reason was that at the time, a great deal of cabling plants had been installed to the B standard (formerly known as WECO or AT&T 258A). Even though they allowed both wiring methods, they stated in their standard that 568A wiring would be the preferred method for all new installations. Time, and popular opinion though, went in the other way and the most popular wiring method today is 568B.

Having both A & B methods is often the cause of errors and confusion. Patch panels and jacks were originally manufactured either A or B, but generally were not labeled as such. Most suppliers stocked only the B wired products. Fortunately almost all jacks and patch panels today show diagrams for both A and B. The only difference between the two is the interchanging of the 2nd and 3rd pairs (white/orange and white/green, respectively).

As to which method to choose? As stated earlier, there is no difference, whatsoever, between the two wiring schemes, in connectivity or performance when connected from one modular device to another. Therefore it does not matter at all, unless you are terminating one end onto a punch block, in which case, the A method has an advantage.

The charts below illustrate the difference between the A & B methods.

The important thing is to choose one method, and stick with it.

For those who are not familiar with telephony, tip (T) refers to the positive (+) side, and ring (R) refers to the negative side of the circuit. The white/blue pair (the first pair in the cable) consists of two wires that are twisted together. They are the white/blue (tip) and the blue/white (ring). The white/blue wire is predominately white with a blue stripe. The blue/white is the inverse, predominately blue with a white stripe.

About the Cable:

The cable is available in bulk at most computer or electrical stores and at many home centres. UTP (Unshielded Twisted Pair) Category 5 cable for is used for basic 10/100 functionality. CAT 5e is required for gigabit (1000BaseT) operation and CAT 6 gives you a measure of future proofing. Bulk cable comes in many types, there are 2 basic categories, solid and braided cable.

Braided cable tends to work better in "patch" applications for desktop use. It is more flexible and resiliant than solid cable and easier to work with, but really meant for shorter lengths. Solid cable is meant for longer runs in a fixed position. Plenum* rated cable should/must be used whenever the cable travels through an air circulation space. For example, above a false ceiling or below a raised floor.

* Plenum rated cable has a special insulation that has low smoke and low flame characteristics. Plenum cable is mandated to be installed in any "air handling" space. For example, most large office buildings use the ceiling to return air to the AC unit. This cablel is relatively expensive.

You're likely going to want braided type cable but it may be difficult or impossible to tell from the box.

Flat-untwisted telephone cable cannot be used for a network cable that runs any appreciable distance.  One must use a pair of twisted wires to connect a set of transmitter pins to their corresponding receiver pins.  One cannot use a wire from one pair and another wire from a different pair...

Internal Cable Structure and Colour Coding

Here is what the internals of the cable look like:

The cable features 8 colour coded wires, twisted into 4 pairs, each having a common colour theme. One wire in the pair being a solid coloured wire, sometimes with a thin white stripe, and the other being a primarily white wire with a thin coloured stripe (Sometimes cable doesn't have any colour on the striped cable, the only way to tell is to check which other wire it is twisted around).

Usually refered to by colour/stripe such as: Orange (or Orange/White) for the solid coloured wire and White/Orange for the striped cable etc.

The twists are extremely important. They are there to counteract noise and interference. It is important to wire according to a standard to get proper performance from the cable.

The TIA/EIA-568-A specifies two wiring standards for a 8-position modular connector such as RJ45. These two wiring standards, T568A and T568B vary only in the arrangement of the coloured pairs.

The RJ45 connectors:

The RJ45 is a 8-position modular connector that looks like a large phone plug. There are a couple variations available. The primary variation you need to pay attention to is whether the connector is intended for braided or solid wire.

For braided/stranded wires, the connector has contacts that actually pierce the wire.

For solid wires, the connector has fingers which pierce the insulation and make contact with the wire by grasping it from both sides.

The connector is the weak point in an ethernet cable, choosing the wrong one will often cause grief later. If you just walk into a computer store, it's nearly impossible to tell what type of connector it is.

RJ45 Jack and Plug Pinout

These are the pin-out diagrams for straight through and crossover UTP Ethernet cables.

The TX (transmitter) pins are connected to corresponding RX (receiver) pins, with plus to plus and minus to minus.  A cross-over cable must be used to connect units with identical interfaces, such as two computers.  

When straight-through cables are used to connect Ethernet devices, one of the two units must, in effect, perform the cross-over function.  This is the reason that straight through cables work directly between hubs or switches and NIC cards.... the Hub or Switch is designed so that their RJ45 Jacks are pre-wired with the transmit and receive pairs already reversed. 

RJ45 receptacle wiring for both standards are shown below:

Note: Only pairs 2 and 3 are used for Standard Ethernet wiring.  Pairs 1 and 4 can be used for other purposes such as telephones or often, a second separate, complete Ethernet connection (adapters are made for this purpose).

Standard, Straight-Through Wiring
(both ends are the same):

Straight-Through Wiring Using The 586A Standard

RJ45 Pin #	Wire Colour (T568A)	Wire Diagram (T568A)	10Base-T Signal 100Base-TX Signal	1000Base-T Signal
1	White/Green		Transmit+	BI_DA+
2	Green		Transmit-	BI_DA-
3	White/Orange		Receive+	BI_DB+
4	Blue		Unused	BI_DC+
5	White/Blue		Unused	BI_DC-
6	Orange		Receive-	BI_DB-
7	White/Brown		Unused	BI_DD+
8	Brown		Unused	BI_DD-

Straight-Through Cable Pinout for T568A

The flat wiring diagram, above, shows the 568A colour code standard as the wiring for the PC side of the cable and the same 568A standard for the Hub, Switch or Router side of things (assuming that the Hubs, Switches or Routers are wired internally to perform the cross-over function).  The illustrations depict the wiring arrangement before insertion into an RJ45 connector prior to crimping.  

T568B

Although the preferred standard is T568A, T568B is more commonly used. This is often preferred in many installations because it has become the main standard through popular usage.

RJ45 Pin #	Wire Colour (T568B)	Wire Diagram (T568B)	10Base-T Signal 100Base-TX Signal	1000Base-T Signal
1	White/Orange		Transmit+	BI_DA+
2	Orange		Transmit-	BI_DA-
3	White/Green		Receive+	BI_DB+
4	Blue		Unused	BI_DC+
5	White/Blue		Unused	BI_DC-
6	Green		Receive-	BI_DB-
7	White/Brown		Unused	BI_DD+
8	Brown		Unused	BI_DD-

Straight-Through Cable Pinout for T568B

 

Cross-Over Wiring Using The 568A to 586B Standards

The flat wiring illustration, above, shows cross-over cable wiring using the 568A colour code standard as the wiring for the PC side of things and the 568B standard for wiring to the other PC.  Note that in both cases, all eight wires are shown but only four are actually needed.

Pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard.  Contrary to common tech-lore and what you may have read elsewhere, these pins and wires are not used or required to implement 100BASE-TX duplexing.  In fact, they can be used for other purposes such as a single line phones or even operating two separate Ethernet channels, provided care is taken to assure that these wire pairs are isolated from the other wires. 

Note: The cross over cable layout is suitable for 1000Base-T operation, if all 4 pairs are crossed as below:

RJ45 Pin # (END 1)	Wire Colour	Diagram End #1 T568B	RJ45 Pin # (END 2)	Wire Colour	Diagram End #2 T568A
1	White/Orange		1	White/Green
2	Orange		2	Green
3	White/Green		3	White/Orange
4	Blue		4	White/Brown
5	White/Blue		5	Brown
6	Green		6	Orange
7	White/Brown		7	Blue
8	Brown		8	White/Blue

Cross Over Cable Pinouts

How to wire Ethernet Cables:

The making of RJ45 Patch cables is complicated due to the connections to the pins in the RJ45 jack not being wire pair by wire pair, the orange pair of wires are not adjacent and the blue pair is upside-down.  Flattening out the cables in the correct order for insertion into the RJ45 jack before crimping is by far the most difficult part of the job of making twisted pair Ethernet patch cables. 

What You Need:

Required:

• CAT 5 Cable - bulk Category 5, 5e or 6 cable
• RJ45 Ends
• Crimper for RJ45
• Wire Cutters - to cut and strip the cable if necessary

Recommended:

• Wire Stripper
• Cable Tester

1. Strip off about 1 inch (25mm) of the cable sheath, no more.
2. Untwist the pairs - don't untwist them beyond what you have exposed, the more untwisted cable you have the worse the problems you can run into.
3. Align the coloured wires according to the diagrams above.
4. Trim all the wires to the same length, about 1/2" to 3/4" left exposed from the sheath.
5. Insert the wires into the RJ45 end - make sure each wire is fully inserted to the front of the RJ45 end and in the correct order. The sheath of the cable should extend into the RJ45 end by about 1/2" and will be held in place by the crimp.
6. Crimp the RJ45 end with the crimper tool
7. Verify the wires ended up the right order and that the wires extend to the front of the RJ45 end and make good contact with the metal contacts in the RJ45 end.
8. Cut the cable to length - make sure it is more than long enough for your needs. Remember, an end to end connection should not extend more than 100m (~328ft). Try to keep cables short, the longer the cable becomes the more it may affect performance, usually noticable as a gradual decrease in speed and increase in latency.
9. Repeat the above steps for the second RJ45 end.
10. If a cable tester is available, use it to verify the proper connectivity of the cable.

If your cable doesn't work, look closely at each end and see if you can find the problem. Usually a wire ended up in the wrong place or more commonly, one of the wires didn't extend to the front of the RJ45 connector and is making no, or poor contact. If you see a mistake or problem, cut the end off and start again with a new plug.

Category 5E Installation.

Run all cables in a "Star" configuration. They all emanate from, and are "homerun" to, one central location, known as the wiring hub. Visualize a wagon wheel, all of the spokes, start from on central point, known as the hub of the wheel.

Keep all cable runs to a maximum of 295 (90m) feet for each run.

Make gradual bends of the cable, where necessary. No sharper than a 1" (25mm) radius.

Use low to moderate force when pulling cable. Cable pulling lubricant makes an amazing difference for cable runs that may otherwise require great force to install.

Keep cat 5E cables as far away from potential sources of EMI (electrical cables, transformers, light fixtures, etc.) as possible.

Install proper cable supports, spaced no more than 5 feet apart.

Cross-connect cables (where necessary), using cat 5E rated punch blocks and components.

Always label every termination point. Use a unique number for each cable segment. The idea here, is to make moves, adds, changes, and troubleshooting as simple as possible.

Always test every installed segment with a cable tester. Continuity alone, is not an acceptable test. Even professionals have terminated an entire installation, before discovering that the two ends were wired for different methods. They then had to re-terminate all of the cables on one end to correct the situation. It is therefore recommended to terminate one segment at a time on both the jack and patch panel sides, and test for proper continuity.

Always install jacks in such a way as to prevent dust and other contaminants from settling on the contacts. The contacts (pins) of the jack should face up on flush mounted plates, or left, right, or down (never up) on surface mount boxes.

Always leave extra slack on the cables, neatly coiled up in the ceiling or nearest concealed place. It is recommended that you leave at least 5 feet at the work outlet side, and 10 feet at the patch panel (wiring hub) side.

Always use grommets to protect the cable where passing through metal studs or anything that can possibly cause damage to them.

Choose either 568A or 568B wiring standard, before you begin your project. Wire all jacks and patch panels for the same wiring scheme (A or B).

Always obey all local, and national, fire and building codes. Be sure to "firestop" all cables that penetrate a firewall. Use plenum rated cable where it is mandated.

Do Not:

Skin off more than 1" of jacket when terminating

Over tighten cable ties. Hook and Loop (Velcro) Cable Ties are recommend for commercial installations.

Splice or bridge category-5E cable at any point. There should never be multiple appearances of category 5E cable.

Use oil, or any other lubricant, not specifically designed for cable pulling. Oil, or other lubricants, can infiltrate the cable, causing damage to the insulation.

Tie cables to electrical conduits, or lay cables on electrical fixtures.

Install cable that is supported by the ceiling tiles (this is unsafe, and is a violation of the building codes).

Never install cables "taught" in the ceiling, or elsewhere. A good installation should have the cables loose, but never sagging.

Do Not Mix 568A and 568B wiring on the same installation.

Do Not Use staples on category-5E cable that crimp the cable tightly. The common T-18 and T-25 cable staples are not recommended for category 5E cable.

The T-59 insulated staple gun is ideal for fastening cat5 & 6 and fiber optic cabling as it does not put any excess pressure on the cable.

Power over Ethernet (PoE):

Power over Ethernet has been implemented in many variations before IEEE standardized 802.3af. 802.3af specifies the ability to supply an endpoint with 48V DC at up 350mA or 16.8W. The endpoint must be capable of receiving power on either the data pairs [Mode A] (often called phantom power) or the unused pairs [Mode B] in 100Base-TX.

PoE can be used with any ethernet configuration, including 10Base-T, 100Base-TX and 1000Base-T. Power is only supplied when a valid PoE endpoint is detected by using a low voltage probe to look for the PoE signature on the endpoint.

PoE power is typically supplied in one of two ways, either the host ethernet switch provides the power, or a "midspan" device is plugged in between the switch and endpoints which supplies the power. No special cabling is required.

RJ45 Pin #	Wire Colour (T568A)	Wire Diagram (T568A)	10Base-T Signal 100Base-TX Signal	PoE
1	White/Green		Transmit+	Mode A +
2	Green		Transmit-	Mode A +
3	White/Orange		Receive+	Mode A -
4	Blue		Unused	Mode B +
5	White/Blue		Unused	Mode B +
6	Orange		Receive-	Mode A -
7	White/Brown		Unused	Mode B -
8	Brown		Unused	Mode B -

Power over Ethernet Power Delivery

Protocol Details:

	Freq (MHz)	Symbol Encoding	Signal Rate (Mbaud)	Symbol Rate	Data En- coding	Data Bits per Symbol	Pairs per Chan- nel	Pairs Used	Minimum Cable Category
10BaseT	10	Manchester	10	10	None	1	1	2	3
100BaseT4	12.5	Multi-level, 2T/Hz	25	25	8B6T	8/6	3	4	3
100BaseTX	31.25	MLT-3	125	125	4B5B	4/5	1	2	5
100BaseT2	12.5	PAM5x5 (2D-PAM5)	25	12.5	None	4 (2x2)	2	2	3
1000BaseT	31.25	4D-PAM5	125	31.25	None	8 (4x2)	4	4	5*

*Designed to work on MOST category 5 cable, category 5e specifications ensure 1000Base-T operation

Cable Category Details:

Cable Category	Rated Frequency Bandwidth (MHz)	Common Uses
1	None
2	1	Telephone Wiring
3	16	Telephone Wiring, 10Base-T
4	20	Token-Ring, 10Base-T
5	100	100Base-TX, 10Base-T
5e	100	1000Base-T, 100Base-TX, 10Base-T
6	250	1000Base-T, 100Base-TX, 10Base-T

Increasing category levels are backward compatible.
Manufacturers will often test and certify their cable well beyond the standards.

Category 5, 5 E, 6 and 7 Performance Specification Chart

Parameter	Category 5 and Class D with additional requirements TSB95 and FDAM 2	Category 5E ('568-A-5)	Category 6 Class E (Performance at 250 MHz shown in parentheses)	Proposed Category 7 Class F (Performance at 600 MHz shown in parentheses)
Specified frequency range	1-100 MHz	1-100 MHz	1-250 MHz	1-600 MHz
Attenuation	24 dB	24 dB	21.7 dB (36 dB)	20.8 dB (54.1 dB)
NEXT	27.1 dB	30.1 dB	39.9 dB (33.1 dB)	62.1 dB (51 dB)
Power-sum NEXT	N/A*	27.1 dB	37.1 dB (30.2 dB)	59.1 dB (48 dB)
ACR	3.1 dB	6.1 dB	18.2 dB (-2.9 dB)	41.3 dB (-3.1 dB)**
Power-sum ACR	N/A	3.1 dB	15.4 dB (-5.8 dB)	38.3 dB (-6.1 dB)**
ELFEXT	17 dB (new requirement)	17.4 dB	23.2 dB (15.3 dB)	ffs***
Power-sum ELFEXT	14.4 dB (new requirement)	14.4 dB	20.2 dB (12.3 dB)	ffs***
Return loss	8 dB* (new requirement)	10 dB	12 dB (8 dB)	14.1 dB (8.7 dB)
Propagation delay	548 nsec	548 nsec	548 nsec (546 nsec)	504 nsec (501 nsec)
Delay skew	50 nsec	50 nsec	50 nsec	20 nsec