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.
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