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{{Short description|Cable wired to allow two similar devices to be directly connected}}
[[Image:Vergleich 2von2 Crossoverkabel.gif|thumb|Crossover cable suitable for use with 100BASE-T4 [[Fast Ethernet]]]]
[[Image:Crossover Adapter DSC01805.JPG|thumb|[[8P8C]] modular crossover adapter]]


An '''Ethernet crossover cable''' is a type of [[Ethernet cable]] used to connect computing devices together directly where they would normally be connected via a [[network switch]], [[network hub|hub]] or [[router]]. For example, one would use a crossover cable to directly connect two [[personal computer]]s via their [[Computer networking|network]] adapters.
An '''Ethernet crossover cable''' is a [[crossover cable]] for [[Ethernet]] used to connect computing devices together directly. It is most often used to connect two devices of the same type, e.g. two computers (via their [[network interface controller]]s) or two switches to each other. By contrast, ''straight through'' [[patch cable]]s are used to connect devices of ''different types'', such as a computer to a [[network switch]].


Intentionally crossed wiring in the crossover cable connects the transmit signals at one end to the receive signals at the other end.
''''''== Overview ==
The [[10BASE-T]] and [[Fast Ethernet|100BASE-TX]] [[Ethernet]] standards use one wire pair for transmission in each direction. The Tx+ line from each device connects to the tip conductor and the Tx- line is connected to the ring. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. When a [[Terminal (telecommunication)|terminal]] device is connected to a [[Network switch|switch]] or [[Network hub|hub]], this crossover is done internally in the switch or hub. A standard ''straight through'' cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.


Many [[network device]]s today support [[auto MDI-X]] (automatic crossover) capability, wherein a patch cable can be used in place of a crossover cable, or vice versa, and the receive and transmit signals are reconfigured automatically within the device to yield a working connection.
One terminal device may be connected directly to another without the use of a switch or hub, but in that case the crossover must be done externally in the cable. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This is a ''crossover cable''. A crossover cable must also be used to connect two internally crossed devices (e.g., two hubs) as the internal crossovers cancel each other out. This can also be accomplished by using a straight through cable in series with a modular crossover adapter.


== Motivation ==
Because the only difference between the [[TIA/EIA-568-A]] and T568B pin/pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one [[Electrical connector|connector]] following T568A and the other T568B. Such a cable will work for 10BASE-T or 100BASE-TX. [[Gigabit Ethernet|1000BASE-T4]] (Gigabit crossover) which uses all four pairs requires the other two pairs (1 and 4) to be swapped and also requires the solid/striped within each of those two pairs to be swapped.''''''
[[File:Crossover Adapter DSC01805.JPG|thumb|[[8P8C]] modular crossover adapter]]
The [[10BASE-T]] and [[100BASE-TX]] Ethernet standards use one wire pair for transmission in each direction. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. The 10BASE-T standard was devised to be used with existing [[twisted pair]] cable installations with straight-through connections.


When a [[Terminal (telecommunication)|terminal]] device (with an ''MDI'' port) is connected to a switch or hub, this crossover is done internally in the switch or hub (''MDI-X'' port). A standard ''straight-through'' cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.
== Crossover cable pinouts ==

{| class="wikitable"
One terminal may be connected directly to another without the use of a switch or hub, but in that case, the crossover must be done in the cabling. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This wiring scheme constitutes a ''crossover cable''. A crossover cable may also be used to connect two hubs or two switches on their [[Upstream (networking)|upstream]] ports.
|+ Two pairs crossed, two pairs uncrossed <br> 10baseT/100baseTX crossover ( shown as T568A )

! Pin || Connection 1 pair || Connection 2 pair || Connection 1 || Connection 2 || Pins on plug face (jack is reversed)
Because the only difference between the [[T568A]] and [[T568B]] pin and pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one [[modular connector]] following T568A and the other T568B (see [[TIA/EIA-568#Wiring|TIA/EIA-568 wiring]]). Such a cable will work for 10BASE-T or 100BASE-TX.

The polarity of each pair is not swapped, but the pairs crossed as a unit: the two wires within each pair are ''not'' crossed.<ref name="Spurgeon">{{cite book |title=Ethernet: the Definitive Guide |author=Charles E. Spurgeon |publisher=O'Reilly Media |year=2000 |page= [https://archive.org/details/ethernetdefiniti0000spur/page/247 247] |isbn=978-1-56592-660-8 |url= https://archive.org/details/ethernetdefiniti0000spur |url-access=registration }}</ref>

{{Ethernet crossover table}}

== Automatic crossover ==
{{main|Medium Dependent Interface#Auto MDI-X}}
Introduced in 1998, this made the distinction between uplink and normal ports and manual selector switches on older hubs and switches obsolete.<ref name="dove">{{cite web |title= 1000BASE-T Automatic Crossover Algorithm |author= Daniel Dove |date= February 1998 |work= Presentation to IEEE 802.3ab working group |url= http://www.ieee802.org/3/ab/public/feb98/ddmdix1.pdf |access-date= June 17, 2011 }}</ref> If one or both of two connected devices has the automatic MDI/MDI-X configuration feature, there is no need for crossover cables.

Although Auto MDI-X was specified as an optional feature in the [[Gigabit Ethernet|1000BASE-T standard]],<ref name=gigx>
Clause "40.4.4 Automatic MDI/MDI-X Configuration" in [[IEEE 802.3-2008]]:
({{cite web
| url = http://standards.ieee.org/getieee802/download/802.3-2008_section3.pdf
| archive-url = https://web.archive.org/web/20090711032121/http://standards.ieee.org/getieee802/download/802.3-2008_section3.pdf
| url-status = dead
| archive-date = July 11, 2009
| title = IEEE 802.3-2008, Part 3
| page = 192
| quote = Implementation of an automatic MDI/MDI-X configuration is optional for 1000BASE-T devices.
| date = 2010-06-22
| access-date = 2011-02-07
}})
</ref> in practice it is implemented widely on most interfaces.

Besides the eventually agreed upon ''Automatic MDI/MDI-X'', this feature may also be referred to by various vendor-specific terms including: ''Auto uplink and trade'', ''Universal Cable Recognition'' and ''Auto Sensing''.

==1000BASE-T and faster==
In a departure from both 10BASE-T and 100BASE-TX, [[1000BASE-T]] and faster use all four cable pairs for simultaneous transmission in both directions through the use of [[telephone hybrid]]-like signal handling. For this reason, there are no dedicated transmit and receive pairs. 1000BASE-T and faster require either a straight or one of the crossover variants only for the autonegotiation phase. The physical medium attachment (PMA) sublayer provides identification of each pair and usually continues to work even over cable where the pairs are unusually swapped or crossed.<ref>IEEE 802.3-2012 ''40.1.4 Signaling''</ref>

==Fiber==
For most [[optical fiber cable|optical fiber]] variants of Ethernet, fibers are used in pairs with one fiber for each direction. The transmitter on one end of the connection needs to be connected to the receiver on the other and vice versa. For this, fiber patch cables with [[duplex (telecommunications)|duplex]] connectors are normally configured as crossover as is the [[on-premises wiring]].{{cn|date=November 2018}} Thus, a simple connection with two patch cables at each end and a section of fixed cable in the middle has three crossovers in total, resulting in a working connection. Patch cable crossovers can usually be reconfigured very easily by swapping the [[optical fiber connector|connectors]] within a duplex bracket if required.

== Pinouts ==
In practice, it does not matter if non-crossover Ethernet cables are wired as T568A or T568B, just so long as both ends follow the same wiring format. Typical commercially available pre-wired cables can follow either format depending on the manufacturer. What this means is that one manufacturer's cables are wired one way and another's the other way, yet both are correct and will work. In either case, T568A or T568B, a normal (un-crossed) cable will have ''both'' ends wired identically according to the layout in either the ''Connection 1'' column or the ''Connection 2'' column.

===Half crossed===
[[File:Ethernet MDI crossover.svg|thumb|upright=1.5|Crossover cable connecting two MDI ports]]
Certain equipment or installations, including those in which phone and/or power are mixed with data in the same cable, may require that the ''non-data'' pairs 1 and 4 (pins 4, 5, 7 and 8) remain un-crossed. This is the most common kind of crossover cable.

{| class="wikitable" style="text-align:center"
|+ Two pairs crossed, two pairs uncrossed 10BASE-T or 100BASE-TX crossover<ref>[https://web.archive.org/web/20111031162309/http://rj45pinout.net/?p=53 Crossover Cable for 10BASE-T and 100BASE-TX]</ref>
! rowspan="2" | Pin
! colspan="3" | Connection 1: T568A<br />[[File:RJ-45 TIA-568A Right.png|180x180px]]
! colspan="3" | Connection 2: T568B<br />[[File:RJ-45 TIA-568B Right.png|180px]]
! rowspan="2" | Pins on plug face
|-
| '''signal'''
| '''pair'''
| '''color'''
| '''signal'''
| '''pair'''
| '''color'''
|-
|-
| 1
| 1
| BI_DA+
| 3
| 3
|[[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
| BI_DB+
| 2
| 2
|[[Image:Wire white green stripe.svg|60px|Pair 3 Tip]]<br> white/green stripe
|[[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
| rowspan="8" | [[File:Rj45plug-8p8c.png|250px]]
|[[Image:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br> white/orange stripe
| rowspan="8" | [[Image:Rj45plug-8p8c.png|250px]]
|-
|-
| 2
| 2
| BI_DA-
| 3
| 3
|[[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
| BI_DB-
| 2
| 2
|[[Image:Wire green.svg|60px|Pair 3 Ring]]<br> green solid
|[[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
|[[Image:Wire orange.svg|60px|Pair 2 Ring]]<br> orange solid
|-
|-
| 3
| 3
| BI_DB+
| 2
| 2
|[[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
| BI_DA+
| 3
| 3
|[[Image:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br> white/orange stripe
|[[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
|[[Image:Wire white green stripe.svg|60px|Pair 3 Tip]]<br> white/green stripe
|-
|-
| 4
| 4
|
| 1
| 1
|[[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
|
| 1
| 1
|[[Image:Wire blue.svg|60px|Pair 1 Ring]]<br> blue solid
|[[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
|[[Image:Wire blue.svg|60px|Pair 1 Ring]]<br> blue solid
|-
|-
| 5
| 5
|
| 1
| 1
|[[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
|
| 1
| 1
|[[Image:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br> white/blue stripe
|[[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
|[[Image:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br> white/blue stripe
|-
|-
| 6
| 6
| BI_DB-
| 2
| 2
|[[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
| BI_DA-
| 3
| 3
|[[Image:Wire orange.svg|60px|Pair 2 Ring]]<br> orange solid
|[[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
|[[Image:Wire green.svg|60px|Pair 3 Ring]]<br> green solid
|-
|-
| 7
| 7
|
| 4
| 4
|[[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
|
| 4
| 4
|[[Image:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br> white/brown stripe
|[[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
|[[Image:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br> white/brown stripe
|-
|-
| 8
| 8
|
| 4
| 4
|[[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
|
| 4
| 4
|[[Image:Wire brown.svg|60px|Pair 4 Ring]]<br> brown solid
|[[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
|[[Image:Wire brown.svg|60px|Pair 4 Ring]]<br> brown solid
|}
|}


===Fully crossed===
Certain equipment or installations, including those in which phone and/or power are mixed with data in the same cable, may require that the "non-data" pairs 1 and 4 (pins 4, 5, 7 and 8) remain un-crossed.


{| class="wikitable"
{| class="wikitable"
|+ All pairs crossed<ref>IEEE 802.3 ''40.8.2 Crossover function''</ref><br />''While this is the only crossover for 1G, it also works for 10M and 100M ethernet''
|+ <font color="red">'''Gigabit crossover'''</font><br>
! rowspan="2" style="text-align: center; font-weight:bold;" | Pin
All four pairs crossed <br> 10base-T/100base-TX/1000base-TX/T4 crossover (shown as T568B)
! colspan="2" style="text-align: center; font-weight:bold;" | TIA/EIA 568-A
! Pin || Connection 1 pair || Connection 2 pair || Connection 1 || Connection 2 || Pins on plug face (jack is reversed)
! rowspan="10" style="text-align: center; font-weight:bold;" |
! colspan="2" style="text-align: center; font-weight:bold;" | TIA/EIA 568-B
|-
|-
| style="text-align: center; font-weight:bold;" | Normal
| 1
| style="text-align: center; font-weight:bold;" | Crossover
| 2
| style="text-align: center; font-weight:bold;" | Normal
| 3
| style="text-align: center; font-weight:bold;" | Crossover
|[[Image:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br> white/orange stripe
|[[Image:Wire white green stripe.svg|60px|Pair 3 Tip]]<br> white/green stripe
| rowspan="8" | [[Image:Rj45plug-8p8c.png|250px]]
|-
|-
| style="text-align: center;" | 1
| 2
| style="text-align: center;" | [[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
| 2
| style="text-align: center;" | [[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
| 3
|[[Image:Wire orange.svg|60px|Pair 2 Ring]]<br> orange solid
| style="text-align: center;" | [[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
|[[Image:Wire green.svg|60px|Pair 3 Ring]]<br> green solid
| style="text-align: center;" | [[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
|-
|-
| style="text-align: center;" | 2
| 3
| style="text-align: center;" | [[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
| 3
| style="text-align: center;" | [[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
| 2
|[[Image:Wire white green stripe.svg|60px|Pair 3 Tip]]<br> white/green stripe
| style="text-align: center;" | [[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
|[[Image:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br> white/orange stripe
| style="text-align: center;" | [[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
|-
|-
| style="text-align: center;" | 3
| 4
| style="text-align: center;" | [[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
| 1
| style="text-align: center;" | [[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
| 4
|[[Image:Wire blue.svg|60px|Pair 1 Ring]]<br> blue solid
| style="text-align: center;" | [[File:Wire white green stripe.svg|60px|Pair 3 Tip]]<br />white/green stripe
|[[Image:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br> white/brown stripe
| style="text-align: center;" | [[File:Wire white orange stripe.svg|60px|Pair 2 Tip]]<br />white/orange stripe
|-
|-
| style="text-align: center;" | 4
| 5
| style="text-align: center;" | [[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
| 1
| style="text-align: center;" | [[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
| 4
|[[Image:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br> white/blue stripe
| style="text-align: center;" | [[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
|[[Image:Wire brown.svg|60px|Pair 4 Ring]]<br> brown solid
| style="text-align: center;" | [[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
|-
|-
| style="text-align: center;" | 5
| 6
| style="text-align: center;" | [[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
| 3
| style="text-align: center;" | [[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
| 2
|[[Image:Wire green.svg|60px|Pair 3 Ring]]<br> green solid
| style="text-align: center;" | [[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
|[[Image:Wire orange.svg|60px|Pair 2 Ring]]<br> orange solid
| style="text-align: center;" | [[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
|-
|-
| style="text-align: center;" | 6
| 7
| style="text-align: center;" | [[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
| 4
| style="text-align: center;" | [[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
| 1
|[[Image:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br> white/brown stripe
| style="text-align: center;" | [[File:Wire green.svg|60px|Pair 3 Ring]]<br />green solid
|[[Image:Wire blue.svg|60px|Pair 1 Ring]]<br> blue solid
| style="text-align: center;" | [[File:Wire orange.svg|60px|Pair 2 Ring]]<br />orange solid
|-
|-
| style="text-align: center;" | 7
| 8
| style="text-align: center;" | [[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
| 4
| style="text-align: center;" | [[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
| 1
|[[Image:Wire brown.svg|60px|Pair 4 Ring]]<br> brown solid
| style="text-align: center;" | [[File:Wire white brown stripe.svg|60px|Pair 4 Tip]]<br />white/brown stripe
|[[Image:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br> white/blue stripe
| style="text-align: center;" | [[File:Wire blue.svg|60px|Pair 1 Ring]]<br />blue solid
|}

In practice, it does not matter if your Ethernet cables are wired as T568A or T568B, just so long as both ends follow the same wiring format. It is just as valid to make a four-pair crossover using T568A, or a two pair crossover using T568B, as it is to wire them the way shown here. Typical commercially available "pre-wired" cables can follow either format depending on who made them. What this means is that you may discover that one manufacturer's cables are wired one way and another's the other way, yet both are "correct" and will work. In either case, T568A or T568B, a normal (un-crossed) cable will have '''both''' ends wired according to the layout in the first connections column.

== Other networking technologies ==
Other technologies use different pairs to transmit data, so crossover cables for them have different configurations to swap the transmit and receive pairs:
* Twisted pair [[Token ring]] uses T568B pairs 1 and 3 (the same as T568A pairs 1 and 2), so a crossover cable to connect two Token Ring interfaces must swap these pairs, connecting pins 4, 5, 3, and 6 to 3, 6, 4, and 5 respectively.
* A [[Digital Signal 1|T1]] cable uses T568B pairs 1 and 2, so to connect two T1 CSU/DSU devices back-to-back requires a crossover cable that swaps these pairs. Specifically, pins 1, 2, 4, and 5 are connected to 4, 5, 1, and 2 respectively.
* A 56K DDS cable uses T568B pairs 02 and 04, so a crossover cable for these devices swaps those pairs (pins 01, 02, 07, and 08 are connected to 07, 08, 01, and 02 respectively).

== Automatic crossover ==
''Automatic MDI/MDI-X Configuration'' is specified as an optional feature in the 1000BASE-T standard<ref>Clause 40.4.4 in [[IEEE 802.3-2005]]</ref>, meaning that straight-through cables will usually work between Gigabit capable interfaces. This feature eliminates the need for crossover cables, obsoletes the uplink/normal ports and manual selector switches found on many older hubs and switches, greatly reducing installation errors. Note that although Automatic MDI/MDI-X is generally implemented, a crossover cable would still be required in the occasional situation that neither of the connected devices has the feature implemented and enabled.

Even for legacy 10/100 devices, many NICs, switches and hubs automatically apply an internal crossover when necessary. Besides the eventually agreed upon ''Automatic MDI/MDI-X'', this feature may also be referred to by various vendor-specific terms including: ''Auto uplink and trade'', ''Universal Cable Recognition'' and ''Auto Sensing''.

== Networks created using crossover cables ==
A two-computer network, sometimes called a '''[[peer-to-peer|peer-to-peer network]]''', can be created using a crossover Ethernet cable. As in any other TCP/IP network, each computer needs to be assigned a unique [[Internet Protocol|IP]] address. In this network configuration, a [[default gateway]] is not used and can be unspecified.
{|class="wikitable"
|+Example crossover network configuration
|-
|-
| style="text-align: center;" | 8
!
| style="text-align: center;" | [[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
! Machine 1
| style="text-align: center;" | [[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
! Machine 2
| style="text-align: center;" | [[File:Wire brown.svg|60px|Pair 4 Ring]]<br />brown solid
|-
| style="text-align: center;" | [[File:Wire white blue stripe.svg|60px|Pair 1 Tip]]<br />white/blue stripe
! [[Internet Protocol|IP]]
| <tt>192.168.0.1</tt>
| <tt>192.168.0.2</tt>
|-
! [[Subnetwork|Subnet Mask]]
|colspan="2" align="center" | <tt>255.255.255.0</tt>
|}
|}

Information on how to deploy such network can be found [http://jamus.dannz.net.nz/technology/technology.htm#networking here]


== See also ==
== See also ==
* [[Networking cable]]
* [[Registered jack]] which expands on the introduction and evolution of these connectors.
* [[Null modem]]


== References ==
== References ==
{{reflist}}
{{reflist}}


{{Ethernet}}

{{DEFAULTSORT:Ethernet Crossover Cable}}
[[Category:Ethernet cables]]
[[Category:Ethernet cables]]
[[Category:Signal cables]]


[[fr:RJ45#Câblage croisé complet]]
[[de:Crosskabel]]
[[it:Cavo ethernet incrociato]]
[[pt:Crossover (cabo)]]

Latest revision as of 18:22, 13 September 2024

An Ethernet crossover cable is a crossover cable for Ethernet used to connect computing devices together directly. It is most often used to connect two devices of the same type, e.g. two computers (via their network interface controllers) or two switches to each other. By contrast, straight through patch cables are used to connect devices of different types, such as a computer to a network switch.

Intentionally crossed wiring in the crossover cable connects the transmit signals at one end to the receive signals at the other end.

Many network devices today support auto MDI-X (automatic crossover) capability, wherein a patch cable can be used in place of a crossover cable, or vice versa, and the receive and transmit signals are reconfigured automatically within the device to yield a working connection.

Motivation

[edit]
8P8C modular crossover adapter

The 10BASE-T and 100BASE-TX Ethernet standards use one wire pair for transmission in each direction. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. The 10BASE-T standard was devised to be used with existing twisted pair cable installations with straight-through connections.

When a terminal device (with an MDI port) is connected to a switch or hub, this crossover is done internally in the switch or hub (MDI-X port). A standard straight-through cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.

One terminal may be connected directly to another without the use of a switch or hub, but in that case, the crossover must be done in the cabling. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This wiring scheme constitutes a crossover cable. A crossover cable may also be used to connect two hubs or two switches on their upstream ports.

Because the only difference between the T568A and T568B pin and pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one modular connector following T568A and the other T568B (see TIA/EIA-568 wiring). Such a cable will work for 10BASE-T or 100BASE-TX.

The polarity of each pair is not swapped, but the pairs crossed as a unit: the two wires within each pair are not crossed.[1]

Cable requirement for Ethernet link
To
From
 MDI MDI-X Auto MDI-X
MDI crossover straight any
MDI-X straight crossover any
Auto MDI-X any any any

Automatic crossover

[edit]
Main article: Medium Dependent Interface § Auto MDI-X

Introduced in 1998, this made the distinction between uplink and normal ports and manual selector switches on older hubs and switches obsolete.[2] If one or both of two connected devices has the automatic MDI/MDI-X configuration feature, there is no need for crossover cables.

Although Auto MDI-X was specified as an optional feature in the 1000BASE-T standard,[3] in practice it is implemented widely on most interfaces.

Besides the eventually agreed upon Automatic MDI/MDI-X, this feature may also be referred to by various vendor-specific terms including: Auto uplink and trade, Universal Cable Recognition and Auto Sensing.

1000BASE-T and faster

[edit]

In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T and faster use all four cable pairs for simultaneous transmission in both directions through the use of telephone hybrid-like signal handling. For this reason, there are no dedicated transmit and receive pairs. 1000BASE-T and faster require either a straight or one of the crossover variants only for the autonegotiation phase. The physical medium attachment (PMA) sublayer provides identification of each pair and usually continues to work even over cable where the pairs are unusually swapped or crossed.[4]

Fiber

[edit]

For most optical fiber variants of Ethernet, fibers are used in pairs with one fiber for each direction. The transmitter on one end of the connection needs to be connected to the receiver on the other and vice versa. For this, fiber patch cables with duplex connectors are normally configured as crossover as is the on-premises wiring.[citation needed] Thus, a simple connection with two patch cables at each end and a section of fixed cable in the middle has three crossovers in total, resulting in a working connection. Patch cable crossovers can usually be reconfigured very easily by swapping the connectors within a duplex bracket if required.

Pinouts

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In practice, it does not matter if non-crossover Ethernet cables are wired as T568A or T568B, just so long as both ends follow the same wiring format. Typical commercially available pre-wired cables can follow either format depending on the manufacturer. What this means is that one manufacturer's cables are wired one way and another's the other way, yet both are correct and will work. In either case, T568A or T568B, a normal (un-crossed) cable will have both ends wired identically according to the layout in either the Connection 1 column or the Connection 2 column.

Half crossed

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Crossover cable connecting two MDI ports

Certain equipment or installations, including those in which phone and/or power are mixed with data in the same cable, may require that the non-data pairs 1 and 4 (pins 4, 5, 7 and 8) remain un-crossed. This is the most common kind of crossover cable.

Two pairs crossed, two pairs uncrossed 10BASE-T or 100BASE-TX crossover[5]
Pin Connection 1: T568A
 Connection 2: T568B
 Pins on plug face
signal pair color signal pair color
1 BI_DA+ 3 Pair 3 Tip
white/green stripe 		BI_DB+ 2 Pair 2 Tip
white/orange stripe
2 BI_DA- 3 Pair 3 Ring
green solid 		BI_DB- 2 Pair 2 Ring
orange solid
3 BI_DB+ 2 Pair 2 Tip
white/orange stripe 		BI_DA+ 3 Pair 3 Tip
white/green stripe
4 1 Pair 1 Ring
blue solid 		1 Pair 1 Ring
blue solid
5 1 Pair 1 Tip
white/blue stripe 		1 Pair 1 Tip
white/blue stripe
6 BI_DB- 2 Pair 2 Ring
orange solid 		BI_DA- 3 Pair 3 Ring
green solid
7 4 Pair 4 Tip
white/brown stripe 		4 Pair 4 Tip
white/brown stripe
8 4 Pair 4 Ring
brown solid 		4 Pair 4 Ring
brown solid

Fully crossed

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All pairs crossed[6]
While this is the only crossover for 1G, it also works for 10M and 100M ethernet
Pin TIA/EIA 568-A TIA/EIA 568-B
Normal Crossover Normal Crossover
1 Pair 3 Tip
white/green stripe 		Pair 2 Tip
white/orange stripe 		Pair 2 Tip
white/orange stripe 		Pair 3 Tip
white/green stripe
2 Pair 3 Ring
green solid 		Pair 2 Ring
orange solid 		Pair 2 Ring
orange solid 		Pair 3 Ring
green solid
3 Pair 2 Tip
white/orange stripe 		Pair 3 Tip
white/green stripe 		Pair 3 Tip
white/green stripe 		Pair 2 Tip
white/orange stripe
4 Pair 1 Ring
blue solid 		Pair 4 Tip
white/brown stripe 		Pair 1 Ring
blue solid 		Pair 4 Tip
white/brown stripe
5 Pair 1 Tip
white/blue stripe 		Pair 4 Ring
brown solid 		Pair 1 Tip
white/blue stripe 		Pair 4 Ring
brown solid
6 Pair 2 Ring
orange solid 		Pair 3 Ring
green solid 		Pair 3 Ring
green solid 		Pair 2 Ring
orange solid
7 Pair 4 Tip
white/brown stripe 		Pair 1 Ring
blue solid 		Pair 4 Tip
white/brown stripe 		Pair 1 Ring
blue solid
8 Pair 4 Ring
brown solid 		Pair 1 Tip
white/blue stripe 		Pair 4 Ring
brown solid 		Pair 1 Tip
white/blue stripe

See also

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References

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  1. ^ Charles E. Spurgeon (2000). Ethernet: the Definitive Guide. O'Reilly Media. p. 247. ISBN 978-1-56592-660-8.
  2. ^ Daniel Dove (February 1998). "1000BASE-T Automatic Crossover Algorithm" (PDF). Presentation to IEEE 802.3ab working group. Retrieved June 17, 2011.
  3. ^ Clause "40.4.4 Automatic MDI/MDI-X Configuration" in IEEE 802.3-2008: ("IEEE 802.3-2008, Part 3" (PDF). 2010-06-22. p. 192. Archived from the original (PDF) on July 11, 2009. Retrieved 2011-02-07. Implementation of an automatic MDI/MDI-X configuration is optional for 1000BASE-T devices.)
  4. ^ IEEE 802.3-2012 40.1.4 Signaling
  5. ^ Crossover Cable for 10BASE-T and 100BASE-TX
  6. ^ IEEE 802.3 40.8.2 Crossover function
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