Understanding Ethernet Patch Cords in Modern Networks

These ubiquitous cables have played a central role in the development of generic and structured cabling systems, and today are used for connecting virtually all networking components, without regard to a particular application or industry.

In many ways, patch cables are the Ether of the Ethernet.

As Ethernet systems provide increasingly flexible and cost-effective ways of transmitting voice, data, and multimedia over integrated networks, Ethernet patch cords are fast becoming a familiar part of our everyday experience.

We can see them in the work areas of commercial and educational buildings as they trail away from the backs of computers to wall plates and other computers. If we follow their trail, we can see them snake along the paths leading from wall plates to patch panels, and then sprout up again from patch panels to meet nearby hubs or switches.

But while the modular characteristics and abundance of patch cables seem to imply that their use is completely universal, they do have some important differences among them that can limit their interchangeability. Some of these differences stem from variations in the wiring configurations of their cable conductors and connector pins, and these are the differences that will be discussed here.

1. Ethernet Patch Cords and RJ-45 Connectors

Ethernet patch cords are flexible leads fitted at each end with an 8P8C (“8 position, 8 conductor”) connector plug for joining two corresponding 8P8C jacks together. Because they look like modular connectors originally used in telephone wiring systems, the 8P8C connectors used in Ethernet systems have adopted the common name RJ-45, an FCC designation for a Registered Jack with a similar 8P8C configuration.

In Ethernet networks, these RJ-45 plugs and jacks form a modular, gendered connector system that makes moving work areas and changing network components fast and easy. The male plugs and female jacks are held together by a spring-loaded tab—called a hook—that keeps them securely in place while in use, but allows them to be easily unplugged when changes are made to a network system or work area.

This modularization is accomplished through the eight conducting pins located on the top of RJ-45 plugs (shown in Figure 1), and just inside the tops of RJ-45 jacks. By connecting the ends of the conducting wires in a patch cable to individual pins in its two RJ-45 end-plugs, electronic data can be transferred via an 8-conductor Ethernet cable from one jack to another through its 8 connector pins.

Figure 1. 8P8C (“8 Position, 8 Conductor”) Connector Plug

In Ethernet Systems, 8P8C plugs and their corresponding jacks are commonly referred to as Rj-45 modular connectors. This RJ-45 plug shows the numbering convention for the pins and pin pairs from above, with the locking facing downward

2. Ethernet Patch Cords and Unshielded Twisted-Pair (UTP) Cabling

The patch cords used in most Ethernet systems are constructed using UTP (Unshielded Twisted-Pair) cable. UTP cable consists of eight insulated copper-core conductors grouped into four pairs, with each pair twisted together along the cable’s length.

The conductor pairs and individual conductors in UTP cables are represented by a color code that assigns a primary color—blue, orange, green, or brown—to each of the 4 twisted pairs. The insulation of a conductor within a pair is either a solid primary color, or white striped with that primary color. In this way, all conductors are identified as members of a specific twisted pair, and as individual members within that pair.

The conductor pairs are numbered 1 to 4, with Pair 1 corresponding to the blue pair, Pair 2 to the orange pair, Pair 3 to the green pair, and Pair 4 to the brown pair. The individual conductors in UTP cables can be solid copper-core wires with a well-defined thickness, or bundles of fine copper wire strands.

Figure 2. Unshielded Twisted Pair (UTP) Cable Cross Section

Pair 1 – Blue, Blue/White

Pair 2 – Orange, Orange/White

Pair 3 – Green, Green/White

Pair 4 – Brown, Brown/White

Even though the solid-conductor cables are less expensive and (much, much) easier to terminate, patch cords are almost always made from stranded cables. This is because the stranding of the conductors increases the cable’s flexibility and durability.

The extra flexibility of patch cords allows them to be readily switched among various wall outlets and patch panels, or easily routed through tight spaces between interconnected equipment. Their extra durability allows them to remain flexible and intact with repeated bending back and forth, giving them a longer lifetime.

This is especially important at the connector ends of the cable, where greater stresses are placed on the conductors through frequent handling, bending, and moving around at the cable-connector interface.

3. Twisted Conductor Pairs – What’s All the Twisting About?

The twisted conductor pairs in UTP cables form a balanced circuit. This is because the voltages of each member in a given pair have the same amplitude (the same voltage magnitude), but their voltages are opposite in phase (one voltage is positive, and the other is negative). The uniform twisting of each of these balanced pairs reduces electromagnetic interference (EMI) and radio frequency interference (RFI) originating from other conducting pairs inside the cable, or from equipment in the cable’s environment.

Figure 3. Category 5E Unshielded Twisted Pair (UTP) Cable

The conductor pairs inside a twisted-pair cable influence one another through a type of EMI called crosstalk. Crosstalk occurs when the electromagnetic field generated by one pair is large enough (the pair’s signal is strong enough) to cross over to the location of a neighboring pair. This ‘talking’ is a kind of co-mingling of the two fields through a constant exchange of energy between them, with some parts of each signal imparted to the other during each energy exchange. For the smaller signal, the result of all these exchanges will be the canceling out of its finer details (always the good parts), or an increase in the level of noise surrounding it. External sources of EMI and RFI interfere with signals in a similar way, further distorting or degrading them as they travel along cables located near ‘noisy’ office and communication equipment.

Twisting individual conductors together to form conducting pairs reduces the size of the current-produced electromagnetic fields surrounding them, limiting their influence on signals traveling inside other conductors by limiting the field interactions between the conducting pairs. Since the pairs are also balanced—with the two wires in each pair carrying signals of equal and opposite polarity—their twisting ensures equal exposure to interference generated by the fields of other pairs or external sources.

Subtracting the magnitudes of the opposite-polarity signals gives the total magnitude of the transmitted signal as it arrives at the opposite end of the cable, and automatically cancels out the same-magnitude, same-polarity interference signals. In this way, the twisting and balancing of the conductors in UTP cables can effectively remove the noise components within transmitted signals without removing or disturbing the signals themselves.

The greater the number of conductor twists, the better a cable’s immunity to EMI and RFI. This immunity gets even better when the number of twists per unit length (the twist rate) is varied among the four pairs. For example, manufacturers of higher-grade cables employ variations in the twist rates of individual conductor pairs, using a different twist rate for each of the four pairs in order to minimize the crosstalk between them. But adding any more variety than this to the twists will bring cable performance down instead of up, since signals traveling along the individual conductors within a pair will no longer be influenced equally by the interfering fields.

This results in interference components for the two signals that are different in magnitude, and an incomplete cancellation of these components when the signal voltages are later subtracted. So each of the pairs has to stick with the same twist rate along its entire length, or any added immunity provided by applying different twist rates to each of the pairs will be lost.

Wrapping each conductor pair with a foil shielding further reduces the crosstalk among pairs, and wrapping all four of the twisted-pairs in a foil or braided metallic shield reduces a cables susceptibility to EMI and RFI in noisy cable environments. STP (Shielded Twisted Pair) cables employ both types of shielding, giving them the highest immunity to all interference types. FTP (Foil Twisted Pair) and ScTP (Screened Twisted Pair) cables employ only the outer foil or braided-conductor shielding, giving them enhanced immunity against external EMI and RFI, but no more protection against crosstalk than an equally-constructed UTP cable.

4. Ethernet Applications

10Base-T and 100Base-T are the IEEE (Institute of Electrical and Electronics Engineers) standards defining the electrical and physical characteristics of twisted-pair cabling for use in 10 Mbps (Megabits per second) and 100 Mbps Ethernet connections. The “T” stands for Twisted pair, and these two Ethernet connections use wire pairs 2 and 3 to transmit and receive information, corresponding to the orange and green twisted pair conductors shown in Figure 2, and pins (1,2) and (3,6) in the RJ-45 connector plugs and jacks shown in Figures 1 and 4.

One of these balanced conductor pairs is used for sending information, and the other is used for receiving information. The two pairs are usually labeled (TX+, TX-) for the transmitting pair, and (RX+, RX-) for the receiving pair. For 10Base-T and 100Base-T connections, the other two cable pairs—corresponding to the blue and brown cable pairs and the four remaining connector pins (4,5) and (7,8) on RJ-45 connectors—are not used. With the introduction of Gigabit Ethernet (or 1000Base-T), all four conductor pairs are used to transmit and receive information simultaneously.

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