What is the difference between T568A and T568B?

Only pins 1, 2, 3 and 6 change: T568A puts the green pair on 1–2 and the orange pair on 3–6, T568B does the opposite. Pins 4, 5, 7 and 8 — the blue and brown pairs — are identical in both schemes.

Do I need a crossover cable?

Usually not. Gigabit and faster ports implement auto-MDI/MDI-X, which detects and corrects a straight-through connection between two like devices. A crossover (pins 1↔3, 2↔6) is only needed on older 10/100 hardware without that feature.

Which PoE pins carry power?

Alternative A superimposes power on the data pairs — pins 1–2 and 3–6 — and works at gigabit. Alternative B uses the spare pairs, pins 4–5 and 7–8, which only exist at 10/100. 802.3bt Type 3 and 4 power all four pairs at once.

What are MPO polarity types A, B and C?

They are three ways of getting position 1 on one end to reach position 2 on the other in a duplex link. Type A is a straight trunk with crossover patch cords, Type B is a reversed trunk with straight patch cords, and Type C flips adjacent fiber pairs.

How far does 10GBASE-T reach?

100 m over Cat 6A, but only 55 m over Cat 6 and 30 m over Cat 5e. Alien crosstalk, not attenuation, is what limits the shorter grades — which is why the reference table publishes the conservative figure.

Cabling Pinout & Fiber Polarity Reference

Verified against a real device

T568A and T568B side by side as tables and as color-coded strip diagrams, plus crossover and Cisco rollover wiring, PoE Mode A/B pin usage with the 802.3af/at/bt classes, MPO/MTP polarity A/B/C and the copper and fiber Ethernet standards.

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T568A and T568B

Pins 1, 2, 3 and 6 are the only ones that change between the two schemes

T568AGreen pair on 1–2, orange pair on 3–6

T568BOrange pair on 1–2, green pair on 3–6

Pin	T568A	T568B	Signal (10/100 · 1000BASE-T)	Differs
1	White / green	White / orange	TX+ / BI_DA+	differs
2	Green	Orange	TX- / BI_DA-	differs
3	White / orange	White / green	RX+ / BI_DB+	differs
4	Blue	Blue	unused / BI_DC+	—
5	White / blue	White / blue	unused / BI_DC-	—
6	Orange	Green	RX- / BI_DB-	differs
7	White / brown	White / brown	unused / BI_DD+	—
8	Brown	Brown	unused / BI_DD-	—

Pins that change:1236Pins 4, 5, 7 and 8 are identical, which is why a straight-through cable works with either scheme on both ends.

Crossover — pins 1↔3 and 2↔6

T568B on one end, T568A on the other. Each conductor keeps its color; only its position moves.

T568B pin	Color	T568A pin	Color	Signal
1	White / orange	3	White / orange	TX+ / BI_DA+
2	Orange	6	Orange	TX- / BI_DA-
3	White / green	1	White / green	RX+ / BI_DB+
6	Green	2	Green	RX- / BI_DB-

MDI / MDI-X

MDI ports (hosts, routers, uplinks) transmit on pins 1–2 and receive on 3–6; MDI-X ports (switch access ports) do the opposite. A straight-through cable connects the two. Auto-MDI/MDI-X detects and corrects a mismatch on gigabit and faster ports.

Cisco rollover (console)

A full reversal: pins 1↔8, 2↔7, 3↔6, 4↔5. No Ethernet traffic, ever.

RJ-45 pin	RJ-45 signal	DB-9	Direction
1	RTS	CTS (8)	From the PC
2	DTR	DSR (6)	From the PC
3	TxD	RxD (2)	From the device
4	GND	GND (5)	Ground
5	GND	GND (5)	Ground
6	RxD	TxD (3)	From the device
7	DSR	DTR (4)	From the device
8	CTS	RTS (7)	From the device

Never patch a rollover cable into a network port

It is a serial cable with a reversed pin order, not an Ethernet lead. On a console port it is harmless; between two switch ports it is a fault waiting to happen.

Power over Ethernet

Alternative A powers the data pairs, Alternative B the spare pairs — and 802.3bt uses both at once

Mode Apins 1–2 positive, 3–6 negative (data pairs)

Alternative	Positive	Negative	Pairs
Alternative A	1, 2	3, 6	Data pairs (works at 10/100 and gigabit)
Alternative B	4, 5	7, 8	Spare pairs (10/100 only)

Polarity is a convention here

Alternative A powers the data pairs through the center tap of the isolation transformers: pins 1–2 positive, pins 3–6 negative. It works at 10/100 and at gigabit, and it is the only alternative that works at 1000BASE-T on two pairs. The polarity shown is the common convention; an 802.3af/at PSE may apply either polarity, and only 802.3bt pins it down. 802.3af (Type 1, 44–57 V): 15.4 W at the PSE / 12.95 W at the PD, classes 0–3. 802.3at (Type 2, 50–57 V): 30 W / 25.5 W, class 4. 802.3bt Type 3 (50–57 V, 4-pair): 60 W / 51 W, classes 5–6. 802.3bt Type 4 (50–57 V, 4-pair): 90 W / 71.3 W, classes 7–8.

Class	Amendment	PSE	PD
0	802.3af	15.4 W	12.95 W
1	802.3af	4 W	3.84 W
2	802.3af	7 W	6.49 W
3	802.3af	15.4 W	12.95 W
4	802.3at	30 W	25.5 W
5	802.3bt Type 3	45 W	40 W
6	802.3bt Type 3	60 W	51 W
7	802.3bt Type 4	75 W	62 W
8	802.3bt Type 4	90 W	71.3 W

802.3afPoEType 12-pair

PSE output: 15.4 W
At the PD: 12.95 W
Voltage: 44–57 V DC
Per pair: 350 mA

IEEE 802.3af-2003 clauses 33.2 and 33.3

802.3atPoE+Type 22-pair

PSE output: 30 W
At the PD: 25.5 W
Voltage: 50–57 V DC
Per pair: 600 mA

IEEE 802.3at-2009 clause 33

802.3bt-type3PoE++ / 4PPoEType 34-pair

PSE output: 60 W
At the PD: 51 W
Voltage: 50–57 V DC
Per pair: 600 mA

IEEE 802.3bt-2018 Table 33-11

802.3bt-type4PoE++ / 4PPoE (high power)Type 44-pair

PSE output: 90 W
At the PD: 71.3 W
Voltage: 50–57 V DC
Per pair: 960 mA

IEEE 802.3bt-2018 Table 33-11

MPO / MTP polarity

TIA-568.3-D and TIA-604-5 (FOCIS 5) polarity methods A, B and C

Fiber count

Type A — straight

Fiber order is preserved: position 1 reaches position 1, position 2 reaches position 2, and so on through position 12. The connector key is up on one end and down on the other, so when the two ends are mated in an adapter the physical position order reverses — the trunk is straight, the mating is not. Method A therefore needs A-to-B (crossover) duplex patch cords at both ends.

Key orientation: Key up on one end, key down on the other
Duplex method: Method A — Type A straight trunk with A-to-B duplex patch cords at both ends

Type B — reversed

Fiber order is reversed end to end: position 1 reaches position 12, position 2 reaches position 11, and so on. Both connectors are key up, so the reversal comes from the cable itself and straight A-to-A duplex patch cords are used at both ends. This is the simplest method to install and the easiest to get wrong when a Type A cord is substituted.

Key orientation: Key up at both ends
Duplex method: Method B — Type B reversed trunk with A-to-A duplex patch cords at both ends

Type C — flipped pairs

Adjacent positions are flipped in pairs: 1↔2, 3↔4, 5↔6, 7↔8, 9↔10, 11↔12. Both connectors are key up and straight A-to-A duplex patch cords are used at both ends. Type C trunks are the least common of the three.

Key orientation: Key up at both ends
Duplex method: Method C — Type C pair-flipped trunk with A-to-A duplex patch cords at both ends

Position	Type A reaches	Type B reaches	Type C reaches
1	1	12	2
2	2	11	1
3	3	10	4
4	4	9	3
5	5	8	6
6	6	7	5
7	7	6	8
8	8	5	7
9	9	4	10
10	10	3	9
11	11	2	12
12	12	1	11

Read the table as: the fiber entering position n on the near connector leaves at the position shown on the far connector. Type A preserves the order, Type B reverses it, Type C flips adjacent pairs. The 8-fiber map is the same rule over the first eight positions. The highlighted row is position 1 — the transmit position a duplex link depends on.

Ethernet standards

Reaches are the conservative figure where a standard allows a range by fiber grade

PHY	Speed	Medium	Reach	Encoding	Lanes	Standard
1000BASE-T	1 Gb/s	Balanced twisted pair, Cat 5 or better	100 m	4D-PAM5, 125 MBd over 4 pairs	4 pairs, full duplex	IEEE 802.3ab-1999
10GBASE-T	10 Gb/s	Balanced twisted pair, Cat 6A (Cat 6 to 55 m)	55 m	DSQ128 with Tomlinson precoding, 800 MBd per pair	4 pairs, full duplex	IEEE 802.3an-2006
10GBASE-SR	10 Gb/s	Multimode fiber, OM3 300 m / OM4 400 m	300 m	64b/66b, 850 nm VCSEL	1 duplex pair	IEEE 802.3ae-2002
10GBASE-LR	10 Gb/s	Single-mode fiber (OS2)	10 km	64b/66b, 1310 nm DFB	1 duplex pair	IEEE 802.3ae-2002
10GBASE-ER	10 Gb/s	Single-mode fiber (OS2)	40 km	64b/66b, 1550 nm EML	1 duplex pair	IEEE 802.3ae-2002
25GBASE-SR	25 Gb/s	Multimode fiber, OM4 100 m (OM3 70 m)	70 m	64b/66b with RS-FEC (528,514), 850 nm	1 duplex pair	IEEE 802.3by-2016
25GBASE-LR	25 Gb/s	Single-mode fiber (OS2)	10 km	64b/66b with RS-FEC, 1310 nm	1 duplex pair	IEEE 802.3cc-2017
40GBASE-SR4	40 Gb/s	Multimode fiber, OM3 100 m / OM4 150 m	100 m	4 × 10 Gbit lanes, 850 nm	8 fibers (MPO-12)	IEEE 802.3ba-2010
40GBASE-CR4	40 Gb/s	Twinax copper DAC	7 m	4 × 10 Gbit lanes over 4 shielded pairs	4 shielded pairs	IEEE 802.3ba-2010
100GBASE-SR4	100 Gb/s	Multimode fiber, OM4 100 m (OM3 70 m)	70 m	4 × 25 Gbit lanes with RS-FEC, 850 nm	8 fibers (MPO-12)	IEEE 802.3bm-2015
100GBASE-LR4	100 Gb/s	Single-mode fiber (OS2)	10 km	4 × 25 Gbit LAN-WDM, 1295–1310 nm	1 duplex pair	IEEE 802.3ba-2010
100GBASE-CR4	100 Gb/s	Twinax copper DAC	5 m	4 × 25 Gbit lanes with RS-FEC	4 shielded pairs	IEEE 802.3bj-2014
400GBASE-SR8	400 Gb/s	Multimode fiber, OM4 100 m (OM3 70 m)	70 m	8 × 50 Gbit lanes with RS-FEC, 850 nm	16 fibers (MPO-16 or 2 × MPO-12)	IEEE 802.3bs-2017
400GBASE-DR4	400 Gb/s	Single-mode fiber (OS2)	500 m	4 × 100 Gbit PAM4 lanes with RS-FEC, 1310 nm	8 fibers (MPO-12)	IEEE 802.3bs-2017

1000BASE-T — Cat 5 works at 100 m but Cat 5e is the practical minimum for a new installation.

10GBASE-T — Reach is 100 m on Cat 6A and 55 m on Cat 6 — and 30 m on Cat 5e. Alien crosstalk, not attenuation, is what shortens Cat 6.

10GBASE-SR — The 400 m OM4 figure assumes a compliant module and clean, low-loss connectors.

10GBASE-ER — Often paired with a dispersion-compensating module or an optical amplifier beyond 40 km.

25GBASE-SR — RS-FEC is mandatory; a port without it will not bring the link up.

40GBASE-CR4 — 7 m is the standard limit; passive assemblies are usually sold at 5 m or less to leave margin.

100GBASE-LR4 — WDM: four wavelengths on each fiber, so only one duplex pair is needed.

400GBASE-DR4 — 500 m, not 10 km. For longer single-mode runs use 400GBASE-FR4/LR4 on a duplex pair.

Cable types

What each assembly is wired for, and when you still need it

Assembly	Wiring	Used for
Straight-through (patch)	T568B on both ends (or T568A on both ends — never one of each)	Host to switch, switch to patch panel, router to switch: any MDI-to-MDI-X link.
Crossover	T568A on one end, T568B on the other (pins 1↔3, 2↔6)	Switch to switch, host to host, router to router — anything without MDI-X detection.
Rollover (Cisco console)	Reversed end to end: pins 1↔8, 2↔7, 3↔6, 4↔5	Cisco (and compatible) RJ-45 console port to a DB-9/DB-25 serial port or USB adapter.
T1 crossover	Pins 1↔4, 2↔5 (the two T1 pairs swapped)	Back-to-back T1 CSU/DSU units, or a loopback into a T1 interface for testing.
MDI / MDI-X and auto-detection	Not a cable — a port role	MDI (a host, router or uplink) transmits on pins 1, 2; MDI-X (a switch port) transmits on pins 3, 6. A straight-through cable connects MDI to MDI-X.

Breakouts

How many fibers each host port consumes

Host port	Becomes	Fibers	Connector
40GBASE-SR4	4 × 10GBASE-SR	8	MPO-12 (8 fibers used) → 4 × LC duplex
100GBASE-SR4	4 × 25GBASE-SR	8	MPO-12 (8 fibers used) → 4 × LC duplex
400GBASE-SR8	8 × 50GBASE-SR	16	MPO-16 APC, or 2 × MPO-12 APC → 8 × LC duplex
400GBASE-DR4	4 × 100GBASE-DR	8	MPO-12 APC → 4 × LC duplex

40GBASE-SR4 — The MPO-12 has four unused positions; a 12-fiber trunk still needs the full 12-fiber cable. (IEEE 802.3ba-2010 (40GBASE-SR4), IEEE 802.3ae-2002 (10GBASE-SR))

100GBASE-SR4 — The most common campus breakout. RS-FEC is mandatory on 25G lanes and must be enabled on the host port. (IEEE 802.3bm-2015 (100GBASE-SR4), IEEE 802.3by-2016 (25GBASE-SR))

400GBASE-SR8 — MPO-16 and MPO-12 ferrules are not interchangeable; the APC key and polish differ from the 12-fiber multimode parts. (IEEE 802.3bs-2017 (400GBASE-SR8), IEEE 802.3cd-2018 (50GBASE-SR))

400GBASE-DR4 — Parallel single-mode at 500 m per lane, so the breakout is 4 × 500 m — not 4 × 10 km. (IEEE 802.3bs-2017 (400GBASE-DR4), IEEE 802.3cd-2018 (100GBASE-DR))

Plain-text reference (copy or print)

# T568 pinout
| Pin | T568A | T568B | Signal (10/100 · 1000BASE-T) |
| --- | --- | --- | --- |
| 1 | White / green | White / orange | TX+ / BI_DA+ |
| 2 | Green | Orange | TX- / BI_DA- |
| 3 | White / orange | White / green | RX+ / BI_DB+ |
| 4 | Blue | Blue | unused / BI_DC+ |
| 5 | White / blue | White / blue | unused / BI_DC- |
| 6 | Orange | Green | RX- / BI_DB- |
| 7 | White / brown | White / brown | unused / BI_DD+ |
| 8 | Brown | Brown | unused / BI_DD- |

# Crossover (T568B end → T568A end)
| From pin | Color (T568B end) | To pin | Color (T568A end) | Signal |
| --- | --- | --- | --- | --- |
| 1 | White / orange | 3 | White / orange | TX+ / BI_DA+ |
| 2 | Orange | 6 | Orange | TX- / BI_DA- |
| 3 | White / green | 1 | White / green | RX+ / BI_DB+ |
| 6 | Green | 2 | Green | RX- / BI_DB- |

# Cisco rollover (console)
| RJ-45 pin | Signal | DB-9 |
| --- | --- | --- |
| 1 | RTS | CTS (8) |
| 2 | DTR | DSR (6) |
| 3 | TxD | RxD (2) |
| 4 | GND | GND (5) |
| 5 | GND | GND (5) |
| 6 | RxD | TxD (3) |
| 7 | DSR | DTR (4) |
| 8 | CTS | RTS (7) |

# PoE alternatives
| Alternative | Positive pins | Negative pins | Pairs used |
| --- | --- | --- | --- |
| Alternative A | 1, 2 | 3, 6 | Data pairs (works at 10/100 and gigabit) |
| Alternative B | 4, 5 | 7, 8 | Spare pairs (10/100 only) |

# MPO polarity, 12 fibers
| Position (1–12) | Type A → | Type B → | Type C → |
| --- | --- | --- | --- |
| 1 | 1 | 12 | 2 |
| 2 | 2 | 11 | 1 |
| 3 | 3 | 10 | 4 |
| 4 | 4 | 9 | 3 |
| 5 | 5 | 8 | 6 |
| 6 | 6 | 7 | 5 |
| 7 | 7 | 6 | 8 |
| 8 | 8 | 5 | 7 |
| 9 | 9 | 4 | 10 |
| 10 | 10 | 3 | 9 |
| 11 | 11 | 2 | 12 |
| 12 | 12 | 1 | 11 |

# Ethernet standards
| PHY | Speed (Mb/s) | Medium | Reach (m) | Encoding | Lanes | Standard |
| --- | --- | --- | --- | --- | --- | --- |
| 1000BASE-T | 1000 | Balanced twisted pair, Cat 5 or better | 100 | 4D-PAM5, 125 MBd over 4 pairs | 4 pairs, full duplex | IEEE 802.3ab-1999 |
| 10GBASE-T | 10000 | Balanced twisted pair, Cat 6A (Cat 6 to 55 m) | 55 | DSQ128 with Tomlinson precoding, 800 MBd per pair | 4 pairs, full duplex | IEEE 802.3an-2006 |
| 10GBASE-SR | 10000 | Multimode fiber, OM3 300 m / OM4 400 m | 300 | 64b/66b, 850 nm VCSEL | 1 duplex pair | IEEE 802.3ae-2002 |
| 10GBASE-LR | 10000 | Single-mode fiber (OS2) | 10000 | 64b/66b, 1310 nm DFB | 1 duplex pair | IEEE 802.3ae-2002 |
| 10GBASE-ER | 10000 | Single-mode fiber (OS2) | 40000 | 64b/66b, 1550 nm EML | 1 duplex pair | IEEE 802.3ae-2002 |
| 25GBASE-SR | 25000 | Multimode fiber, OM4 100 m (OM3 70 m) | 70 | 64b/66b with RS-FEC (528,514), 850 nm | 1 duplex pair | IEEE 802.3by-2016 |
| 25GBASE-LR | 25000 | Single-mode fiber (OS2) | 10000 | 64b/66b with RS-FEC, 1310 nm | 1 duplex pair | IEEE 802.3cc-2017 |
| 40GBASE-SR4 | 40000 | Multimode fiber, OM3 100 m / OM4 150 m | 100 | 4 × 10 Gbit lanes, 850 nm | 8 fibers (MPO-12) | IEEE 802.3ba-2010 |
| 40GBASE-CR4 | 40000 | Twinax copper DAC | 7 | 4 × 10 Gbit lanes over 4 shielded pairs | 4 shielded pairs | IEEE 802.3ba-2010 |
| 100GBASE-SR4 | 100000 | Multimode fiber, OM4 100 m (OM3 70 m) | 70 | 4 × 25 Gbit lanes with RS-FEC, 850 nm | 8 fibers (MPO-12) | IEEE 802.3bm-2015 |
| 100GBASE-LR4 | 100000 | Single-mode fiber (OS2) | 10000 | 4 × 25 Gbit LAN-WDM, 1295–1310 nm | 1 duplex pair | IEEE 802.3ba-2010 |
| 100GBASE-CR4 | 100000 | Twinax copper DAC | 5 | 4 × 25 Gbit lanes with RS-FEC | 4 shielded pairs | IEEE 802.3bj-2014 |
| 400GBASE-SR8 | 400000 | Multimode fiber, OM4 100 m (OM3 70 m) | 70 | 8 × 50 Gbit lanes with RS-FEC, 850 nm | 16 fibers (MPO-16 or 2 × MPO-12) | IEEE 802.3bs-2017 |
| 400GBASE-DR4 | 400000 | Single-mode fiber (OS2) | 500 | 4 × 100 Gbit PAM4 lanes with RS-FEC, 1310 nm | 8 fibers (MPO-12) | IEEE 802.3bs-2017 |

# Breakouts
| Host | Lanes | Becomes | Fibers | Connector | Standards |
| --- | --- | --- | --- | --- | --- |
| 40GBASE-SR4 | 4 | 4 × 10GBASE-SR | 8 | MPO-12 (8 fibers used) → 4 × LC duplex | IEEE 802.3ba-2010 (40GBASE-SR4), IEEE 802.3ae-2002 (10GBASE-SR) |
| 100GBASE-SR4 | 4 | 4 × 25GBASE-SR | 8 | MPO-12 (8 fibers used) → 4 × LC duplex | IEEE 802.3bm-2015 (100GBASE-SR4), IEEE 802.3by-2016 (25GBASE-SR) |
| 400GBASE-SR8 | 8 | 8 × 50GBASE-SR | 16 | MPO-16 APC, or 2 × MPO-12 APC → 8 × LC duplex | IEEE 802.3bs-2017 (400GBASE-SR8), IEEE 802.3cd-2018 (50GBASE-SR) |
| 400GBASE-DR4 | 4 | 4 × 100GBASE-DR | 8 | MPO-12 APC → 4 × LC duplex | IEEE 802.3bs-2017 (400GBASE-DR4), IEEE 802.3cd-2018 (100GBASE-DR) |

Frequently asked

What is the difference between T568A and T568B?: Only pins 1, 2, 3 and 6 change: T568A puts the green pair on 1–2 and the orange pair on 3–6, T568B does the opposite. Pins 4, 5, 7 and 8 — the blue and brown pairs — are identical in both schemes.
Do I need a crossover cable?: Usually not. Gigabit and faster ports implement auto-MDI/MDI-X, which detects and corrects a straight-through connection between two like devices. A crossover (pins 1↔3, 2↔6) is only needed on older 10/100 hardware without that feature.
Which PoE pins carry power?: Alternative A superimposes power on the data pairs — pins 1–2 and 3–6 — and works at gigabit. Alternative B uses the spare pairs, pins 4–5 and 7–8, which only exist at 10/100. 802.3bt Type 3 and 4 power all four pairs at once.
What are MPO polarity types A, B and C?: They are three ways of getting position 1 on one end to reach position 2 on the other in a duplex link. Type A is a straight trunk with crossover patch cords, Type B is a reversed trunk with straight patch cords, and Type C flips adjacent fiber pairs.
How far does 10GBASE-T reach?: 100 m over Cat 6A, but only 55 m over Cat 6 and 30 m over Cat 5e. Alien crosstalk, not attenuation, is what limits the shorter grades — which is why the reference table publishes the conservative figure.

Position	Type A reaches	Type B reaches	Type C reaches
1	1	12	2
2	2	11	1
3	3	10	4
4	4	9	3
5	5	8	6
6	6	7	5
7	7	6	8
8	8	5	7
9	9	4	10
10	10	3	9
11	11	2	12
12	12	1	11

Position	Type A reaches	Type B reaches	Type C reaches
1	1	12	2
2	2	11	1
3	3	10	4
4	4	9	3
5	5	8	6
6	6	7	5
7	7	6	8
8	8	5	7
9	9	4	10
10	10	3	9
11	11	2	12
12	12	1	11

Frequently asked

Next

Position	Type A reaches	Type B reaches	Type C reaches
1	1	12	2
2	2	11	1
3	3	10	4
4	4	9	3
5	5	8	6
6	6	7	5
7	7	6	8
8	8	5	7
9	9	4	10
10	10	3	9
11	11	2	12
12	12	1	11