TP Connectors

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Typical connection of a computer and an active component consist of three elements:

  1. A "patch" cable that connects the NIC in the computer to a wall socket.
  2. Structured cabling that connects wall sockets with corresponding patch panels.
  3. A "patch" cable that connects the panel and a corresponding active component (hub, switch, ...)

Note: Whenever hub is mentioned in the following text, it can also mean a switch or another similar component.

Structured cabling for TP Ethernet most often uses RJ-45 connectors. From the users point of view, these are exactly the same as the connectors on their NICs. However, the "hidden" portions for attachment to cables differ by manufacturer.

Today, S110 blocks or the Krone system are used, together with more or less perfect mechanical fixation of the cable. Keystone
Problems with fixation of cables have led to the development of many other attachment systems. For example, I consider the PanJack system by Panduit to be a very good one. PanJack

Structured cabling uses cables with four "twisted" wire pairs. Fixed cabling uses hard wires, patch cables are made of stranded wires. Typical cross-section is 24 AWG (wire, strand 7/32). A suitable connector needs to be selected for a given cable. Individual pairs are color-coded.

  1. Blue
  2. Orange
  3. Green
  4. Brown

One of the wires in the pair has solid color, the complementary one is either white or has stripes of the same color. Connector pinout is specified by a standard. However, two standards are used: T568A and T568B. They differ by the position of Green and Orange pair, which are used for 10BaseT/100BaseT communication. Both standards follow.

T568A   T568B

1 - White/Green
2 - Green
3 - White/Orange
4 - Blue
5 - White/Blue
6 - Orange
7 - White/Brown
8 - Brown
  1 - White/Orange
2 - Orange
3 - White/Green
4 - Blue
5 - White/Blue
6 - Green
7 - White/Brown
8 - Brown

front view of the socket   front view of the socket
T568A Patch Panel RJ45 T568B

front view of the plug   front view of the plug
T568A Plug Patch Kabel RJ45RJ45 Connector T568B Plug

10BaseT/100BaseT standard uses only two pairs for communication, 2 (orange) and 3 (green). Remaining pairs 1 (blue) and 4 (brown) are unused, and may be omitted if an extremely bendable and light cable is needed (e.g. to connect a notebook). Usually, a RJ-45 connector in a NIC is connected according to the MDI standard:

PinNameDirection
1TD+NIC -> hub
2TD-NIC -> hub
3RX+hub -> NIC
4N.C.
5N.C.
6RX-hub -> NIC
7N.C.
8N.C.

Patch cables that connect computers to hubs or LAN sockets are wired 1:1, that is, pins with the same numbers are connected together. However, when making a cable, high-frequency charasteristics must be taken into account by obeying correct signal pairing - 1/2, 3/6, 4/5, 7/8.

If only two computers are to be interconnected, no hubs or other active components are needed. A cross-cable is sufficient. Cross-cables are also used to connect two hubs or active components if they can't reverse their TD and RX pins. A switch or parallel "crossed" connector is usually used for uplinks. If a connector has TD and RX crossed, its pinout is called MDI-X. If two connectors wired in parallel are available, only one of them can be used, the other one must always remain unconnected.

Patch cables

1 - 1
2 - 2
3 - 3
4 - 4
5 - 5
6 - 6
7 - 7
8 - 8

1 - 1
2 - 2
3 - 3
4 N.C.
5 N.C.
6 - 6
7 N.C.
8 N.C.
Minimal version,
NOT RECOMMENDED,
used in special circumstances only
(whenever a cable needs to be as small and light as possible, e.g. for notebook connections)

Cross cables

1 - 3
2 - 6
3 - 1
4 - 8
5 - 7
6 - 2
7 - 5
8 - 4

1 - 3
2 - 6
3 - 1
4 - 4
5 - 5
6 - 2
7 - 7
8 - 8
Another version sometimes used, only TD and RX pairs are crossed, pairs 1 and 4 are connected 1:1

1 - 3
2 - 6
3 - 1
4 N.C.
5 N.C.
6 - 2
7 N.C.
8 N.C.
Minimal version, only TD and RX are crossed

Connectors in the pictures demonstrate T568B standard. For T568A, orange and green pairs have to be exchanged.

Marking of a cross cable with the letter X (from MDI-X or X-over) as shown is not a standard. Author prefers it since it allows to distinguish a crossed and a patch cable at the first glance. Otherwise, one needs to look at both connectors and check whether the orange or green pair is at the same location on both ends or not. If the leftmost (or rightmost) pair is green on both ends, it is a T568A patch cable. If it is orange, it is a T568B patch cable (used more often). If it is green on one end and orange on the other, it is a cross cable.





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