Twisted-Pair Cabling

Twisted-Pair Cabling Overview:--

A typical twisted-pair cable consists of four pairs of copper wires, for a total of eight wires. Each side of the cable is terminated using an RJ45 connector, which has eight pins. When the connector is crimped onto the cable, these pins make contact with each wire.

The wires themselves are assigned a color to distinguish them. The color is dictated by the cabling standard - TIA/EIA-568B is the current standard:

Each wire is assigned a specific purpose. For example, both Ethernet and Fast Ethernet use two wires to transmit, and two wires to receive data, while the other four pins remain unused.

For communication to occur, transmit pins must connect to the receive pins of the remote host. This does not occur in a straight-through configuration:

The pins must be crossed-over for communication to be successful. The crossover can be controlled either by the cable, or an intermediary device, such as a hub or switch. 

Twisted-Pair Cabling – Cable and Interface Types

The layout or pinout of the wires in the RJ45 connector dictates the function of the cable. There are three common types of twisted-pair cable:

• Straight-through cable

• Crossover cable

• Rollover cable

The network interface type determines when to use each cable:

• Medium Dependent Interface (MDI)

• Medium Dependent Interface with Crossover (MDIX)

Host interfaces are generally MDI, while hub or switch interfaces are typically MDIX.

Twisted-Pair Cabling – Straight-Through Cable 

A straight-through cable is used in the following circumstances:

Essentially, a straight-through cable is used to connect any device to a hub or switch, except for another hub or switch. The hub or switch provides the crossover (or MDIX) function to connect transmit pins to receive pins.

The pinout on each end of a straight-through cable must be identical.

The TIA/EIA-568B standard for a straight-through cable is as follows:

Twisted-Pair Cabling – Crossover Cable

A crossover cable is used in the following circumstances: Remember that a hub or a switch will provide the crossover function.

However, when connecting a host directly to another host (MDI to MDI), the crossover function must be provided by a crossover cable.

A crossover cable is often required to uplink a hub to another hub, or to plink a switch to another switch. This is because the crossover is performed twice, once on each hub or switch (MDIX to MDIX), negating the crossover.

Modern devices can now automatically detect whether the crossover function is required, negating the need for a crossover cable. This functionality is referred to as Auto-MDIX, and is now standard with Gigabit Ethernet, which uses all eight wires to both transmit and receive. Auto-MDIX requires that auto negotiation be enabled.

To create a crossover cable, the transmit pins must be swapped with the receive pins on one end of the cable:

• Pins 1 and 3

• Pins 2 and 6

Twisted-Pair – Rollover Cable

A rollover cable is used to connect a workstation or laptop into a Cisco

device’s console or auxiliary port, for management purposes. A rollover

cable is often referred to as a console cable, and its sheathing is usually flat

and light-blue in color.

To create a rollover cable, the pins are completely reversed on one end of the

cable:

Rollover cables can be used to configure Cisco routers, switches, and firewalls.

Power over Ethernet (PoE)

Power over Ethernet (PoE) allows both data and power to be sent across

the same twisted-pair cable, eliminating the need to provide separate power

connections. This is especially useful in areas where installing separate

power might be expensive or difficult.

PoE can be used to power many devices, including:

Ø Voice over IP (VoIP) phones

Ø Security cameras

Ø Wireless access points

Ø Thin clients

PoE was originally formalized as 802.3af, which can provide roughly 13W of power to a device. 802.3at further enhanced PoE, supporting 25W or more power to a device.

Ethernet, Fast Ethernet, and Gigabit Ethernet all support PoE. Power can be sent across either the unused pairs in a cable, or the data transmission pairs, which is referred to as phantom power. Gigabit Ethernet requires the phantom power method, as it uses all eight wires in a twisted-pair cable.

The device that provides power is referred to as the Power Source Equipment (PSE). PoE can be supplied using an external power injector, though each powered device requires a separate power injector.

More commonly, an 802.3af-compliant network switch is used to provide

power to many devices simultaneously. The power supplies in the switch

must be large enough to support both the switch itself, and the devices it is

powering.