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.

Network Topologies

A topology defines both the physical and logical structure of a network. 

Topologies come in a variety of configurations, including:

• Bus

• Star

• Ring

• Full or partial mesh

Ethernet supports two topology types – bus and star.

Ethernet Bus Topology

In a bus topology, all hosts share a single physical segment (the bus or the backbone) to communicate:

A frame sent by one host is received by all other hosts on the bus. However, a host will only process a frame if it matches the destination hardware address in the data-link header.

Bus topologies are inexpensive to implement, but are almost entirely deprecated in Ethernet. There are several disadvantages to the bus topology: 

• Both ends of the bus must be terminated, otherwise a signal will reflect back and cause interference, severely degrading performance.

• Adding or removing hosts to the bus can be difficult.

• The bus represents a single point of failure - a break in the bus will affect all hosts on the segment. Such faults are often very difficult to troubleshoot.

Ethernet Star Topology

In a star topology, each host has an individual point-to-point connection to a centralized hub or switch:

A hub provides no intelligent forwarding whatsoever, and will always forward every frame out every port, excluding the port originating the frame.

As with a bus topology, a host will only process a frame if it matches the destination hardware address in the data-link header. Otherwise, it will discard the frame.

A switch builds a hardware address table, allowing it to make intelligent forwarding decisions based on frame (data-link) headers. A frame can then be forwarded out only the appropriate destination port, instead of all ports.

Hubs and switches are covered in great detail in another guide.

Adding or removing hosts is very simple in a star topology. Also, a break in a cable will affect only that one host, and not the entire network.

There are two disadvantages to the star topology:

• The hub or switch represents a single point of failure.

• Equipment and cabling costs are generally higher than in a bus topology.

However, the star is still the dominant topology in modern Ethernet networks, due to its flexibility and scalability. Both twisted-pair and fiber cabling can be used in a star topology.

Ethernet technologies

Ethernet technologies :---

Ethernet is a family of technologies that provides data-link and physical specifications for controlling access to a shared network medium. It has emerged as the dominant technology used in LAN networking.

Ethernet was originally developed by Xerox in the 1970s, and operated at 2.94Mbps. The technology was standardized as Ethernet Version 1 by a consortium of three companies - DEC, Intel, and Xerox, collectively referred to as DIX - and further refined as Ethernet II in 1982.

In the mid 1980s, the IEEE published a formal standard for Ethernet, defined as the IEEE 802.3 standard. The original 802.3 Ethernet operated at 10Mbps and successfully supplanted competing LAN technologies, such as Token Ring.

Ethernet has several benefits over other LAN technologies:

• Simple to install and manage

• Inexpensive

• Flexible and scalable

• Easy to interoperate between vendors

Ethernet Cabling Types:---

Ethernet can be deployed over three types of cabling:

• Coaxial cabling – almost entirely deprecated in Ethernet networking

• Twisted-pair cabling

• Fiber optic cabling

Coaxial cable, often abbreviated as coax, consists of a single wire surrounded by insulation, a metallic shield, and a plastic sheath. The shield helps protect against electromagnetic interference (EMI), which can cause attenuation, a reduction of the strength and quality of a signal. EMI can be generated by a variety of sources, such as florescent light ballasts, microwaves, cell phones, and radio transmitters.

Coax is commonly used to deploy cable television to homes and businesses.

Two types of coax were used historically in Ethernet networks:

• Thinnet

• Thicknet

Thicknet has a wider diameter and more shielding, which supports greater distances. However, it is less flexible than the smaller thinnet, and thus more difficult to work with. A vampire tap is used to physically connect devices to thicknet, while a BNC connector is used for thinnet.

Twisted-pair cable consists of two or four pairs of copper wires in a plastic sheath. Wires in a pair twist around each other to reduce crosstalk, a form of EMI that occurs when the signal from one wire bleeds or interferes with a signal on another wire.

Twisted-pair is the most common Ethernet cable.

Twisted-pair cabling can be either shielded or unshielded. Shielded twisted pair is more resistant to external EMI; however, all forms of twisted-pair suffer from greater signal attenuation than coax cable.

There are several categories of twisted-pair cable, identified by the number of twists per inch of the copper pairs:

• Category 3 or Cat3 - three twists per inch.

• Cat5 - five twists per inch.

• Cat5e - five twists per inch; pairs are also twisted around each other.

• Cat6 – six twists per inch, with improved insulation.

An RJ45 connector is used to connect a device to a twisted-pair cable. The layout of the wires in the connector dictates the function of the cable.

While coax and twisted-pair cabling carry electronic signals, fiber optics uses light to transmit a signal. Ethernet supports two fiber specifications:

Ø Singlemode fiber – consists of a very small glass core, allowing only a single ray or mode of light to travel across it. This greatly reduces the attenuation and dispersion of the light signal, supporting high bandwidth over very long distances, often measured in kilometers.

Ø Multimode fiber – consists of a larger core, allowing multiple modes of light to traverse it. Multimode suffers from greater dispersion than single mode, resulting in shorter supported distances.

Singlemode fiber requires more precise electronics than multimode, and thus is significantly more expensive. Multimode fiber is often used for high-speed connectivity within a datacenter.

Encapsulation and Layered Communication

As data is passed from the user application down the virtual layers of the OSI model, each layer adds a header (and sometimes a trailer) containing protocol information specific to that layer. These headers are called Protocol Data Units (PDUs), and the process of adding these headers is called encapsulation. Note that in the TCP/IP protocol suite only the lower layers perform encapsulation, generally.

For example, a Transport layer protocol such as TCP will add a header containing flow control, port numbers, and sequencing. The Network layer header contains logical addressing information, and the Data-link header contains physical addressing and other hardware specific information. 

The PDU of each layer is identified with a different term:--

For example:--

Ø Transport Segments

Ø Network Packets

Ø Data-Link Frames

Ø Physical Bits

Each layer communicates with the corresponding layer on the receiving device. For example, on the sending device, source and destination hardware addressing is placed in a Data-link header. On the receiving device, that Data-link header is processed and stripped away (decapsulated) before being sent up to the Network and other upper layers.

Network devices are commonly identified by the OSI layer they operate at; or, more specifically, what header or PDU the device processes. 

For example, switches are generally identified as Layer-2 devices, as switches process information stored in the Data-Link header of a frame, such as Ethernet MAC  dresses. Similarly, routers are identified as Layer- 3 devices, as routers process logical addressing information in the Network header of a packet, such as IP addresses.

Encapsulation Illustrated

The following illustrates how basic encapsulation occurs with the TCP/IP stack, which typically performs encapsulation only at the lower layers: 

During encapsulation on the sending host:-- 

v Data from the user application is handed off to the Transport layer.

     v The Transport layer adds a header containing protocol-specific
     v information, and then hands the segment to the Network layer.
     v The Network layer adds a header containing source and destination
 v logical addressing, and then hands the packet to the Data-Link layer.
 v The Data-Link layer adds a header containing source and destination
 v physical addressing and other hardware-specific information.
 v The Data-Link frame is then handed off to the Physical layer to be
 v transmitted on the network medium as bits.

During De-capsulation on the receiving host, the reverse occurs:

v The frame is received from the physical medium.

v The Data-Link layer processes its header, strips it off, and then hands it off to the Network layer.

 vThe Network layer processes its header, strips it off, and then hands it off to the Transport layer.
 v The Transport layer processes its header, strips it off, and then hands
 v the data to the user application.