- Cabling is the medium for the transmission of data between hosts on the LANs.
- LANs can be connected together using a variety of cable types, such as unshielded twisted-pair, coax, or fiber.
- Each cable type has its own advantages and disadvantages .
- Three primary types of cable media can be used to connect systems to a network— coaxial cable, twisted-pair cable, and fiber-optic cable.
- Transmission rates that can be supported on each of these physical media are measured in millions of bits per second, or megabits per second (Mbps).
Coaxial Cable
- Coaxial, or coax, cable looks like the cable used to bring the cable TV signal to your television.
- One strand (a solid-core copper wire) runs down the middle of the cable.
- Around that strand is a layer of insulation, and covering that insulation is braided wire and metal foil, which shields against electromagnetic interference.
- A final layer of insulation covers the braided wire. Because of the layers of insulation, coaxial cable is more resistant to outside interference than other cabling, such as unshielded twisted-pair (UTP) cable.
- There are two types of coax cabling: thinnet and thicknet. The two differ in thickness and maximum cable distance that the signal can travel.
Thinnet Coaxial Cable
- This refers to RG-58 cabling, which is a flexible coaxial cable about a quarter-inch thick.
- Thinnet is used for short-distance communication and is flexible enough to facilitate routing between workstations.
- Thinnet connects directly to a workstation’s network adapter card using a Bayonet Neill–Concelman (BNC) connector and uses the network adapter card’s internal transceiver.
- The maximum length of thinnet is 185 meters.
Thicknet Coaxial Cable
- This coaxial cable, also known as RG-8, gets its name by being a thicker cable than thinnet.
- Thicknet cable is about a half-inch thick and can support data transfer over longer distances than thinnet.
- Thicknet has a maximum cable length of 500 meters and usually is used as a backbone to connect several smaller thinnet-based networks.
- Due to its thickness, this cable is harder to work with than thinnet cable.
- A transceiver often is connected directly to the thicknet cable using a connector known as a vampire tap.
- Connection from the transceiver to the network adapter card is made using a drop cable to connect to the attachment unit interface (AUI) port connector.
Twisted-Pair Cable
- Twisted-pair cabling gets its name from the four pairs of wires that are twisted to help reduce crosstalk or interference from outside electrical devices. (Crosstalk is interference from adjacent wires.)
- figure shows a twisted-pair cable. Just as there are two forms of coaxial cable, there are two forms of twisted-pair cabling—unshielded twisted-pair (UTP) and shielded twisted-pair (STP).
Unshielded Twisted-Pair Cable
- The typical twisted-pair cable for network use contains four pairs of wires.
- Each member of the pair of wires contained in the cable is twisted around the other.
- The twists in the wires help shield against electromagnetic interference.
- The maximum distance of UTP is 100 meters, with the exception of CAT 6, which has a maximum distance of 100 meters when used in 10/100/1000BaseT environments,.
- but has a maximum distance of 55 meters when used with 10GBaseT networks.
- UTP cable uses small plastic connectors designated as registered jack 45, or most often referred to as RJ-45.
- the network RJ-45 connector contains eight contacts, one for each wire in a UTP cable.
- UTP cable is easier to install than coaxial because you can pull it around corners more easily due to its flexibility and small size.
- Twisted-pair cable is more susceptible to interference than coaxial is, however, and should not be used in environments containing large electrical or electronic devices.
- UTP cabling has different categories. Each category of UTP cabling was designed for a specific type of communication or transfer rate.
- below Table summarizes the different UTP categories—the most popular today being CAT 6 , which can reach transfer rates of over 1 gigabit per second (Gbps).
- The difference between CAT 6 and CAT 6a is that with CAT 6 cabling, the maximum distance allowed for the cable length is reduced to 55 meters when connected to 10 Gbps networks, while CAT 6a can run at a maximum distance of 100 meters with 10 Gbps networks due to reduced crosstalk measures built into the cabling.
Wiring Standards
Straight-Through Cables
- UTP cabling usually uses only four wires when sending and receiving information on the network.
- The four wires of the eight that are used are wires 1, 2, 3, and 6.
- below Figure shows the meaning of the pins on a computer and the pins on a hub (or switch), which is what you typically will be connecting the computers to. When you configure the wire for the same pin at either end of the cable, this is known as a straight-through cable.
- wires 1 and 2 are used to transmit data (TX) from the computer, while wires 3 and 6 are used to receive information (RX) on the computer.
- the transmit pin on the computer is connected to the receive pin (RX) on the hub via wires 1 and 2.
- This is important because we want to make sure that data sent from the computer is received at the network hub. We want to make sure that data sent from the hub is received at the computer,
- so the TX pins on the hub are connected to the RX pins on the computer through wires 3 and 6. This will allow the computer to receive information from the hub.
Crossover Cables
- At some point, you may need to connect two computer systems directly together without the use of a hub, from network card to network card.
- You would not be able to use a straight-through cable for this because the transmit pin on one computer would be connected to the transmit pin on another computer.
- so we need to change the wiring of the cable to what is known as a crossover cable.
- In order to connect two systems directly together without the use of a hub, you will need to create a crossover cable by switching wires 1 and 2 with wires 3 and 6 at one end of the cable,
- . You will notice that the transmit pins on Computer A are connected to the receive pins on Computer B, thus allowing Computer A to send data to Computer B.
- The same applies for Computer B to send data to Computer A—pins A and B on Computer B are wired to pins 3 and 6 on Computer A so that Computer A can receive data from Computer B.
568A and 568B Standards
- Although only four of the wires are used to send and receive data in most environments today, some of the newer standards use all eight wires.
- Therefore, it is important to know the order of all eight wires in a UTP cable.
- There aretwo popular wiring standards today: 568A and 568B.
- If the goal is to create a straight-through cable, you would use the same wiring standard on each end (for example, 568B),
- but if you wanted to create a crossover cable, you would configure one end of the cable using the 568A wiring scheme, while the other end is set up using the 568B wiring scheme.
- below Table shows the wire order for the 568B standard of a straight-through cable at both ends.
Shielded Twisted-Pair Cable
Shielded twisted-pair (STP) cable is similar to UTP cabling, but it differs from UTP in that it uses a layer of insulation within the protective jacket, which helps maintain the quality of the signal.
Fiber-Optic Cable
- The third type of cabling is fiber-optic cabling.
- Fiber-optic cabling is unlike coax and twisted-pair, because both of those types have a copper wire that carries the electrical signal.
- Fiber-optic cables use optical fibers that carry digital data signals in the form of modulated pulses of light.
- An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding.
- There are two fibers per cable—one to transmit and one to receive. The core also can be an optical-quality clear plastic, and the cladding can be made up of gel that reflects signals back into the fiber to reduce signal loss.
There are two types of fiber-optic cables:
- Single-mode Fiber Uses a single ray of light, known as a mode, to carry the transmission over long distances.
- Multimode Fiber Uses multiple rays of light (modes) simultaneously, with each ray of light running at a different reflection angle to carry the transmission over short distances.
- the two major connector types: the straight-tip (ST) connector and the subscriber (SC) connector.
- The ST connector is based on the BNC-style connector, but has a fiber-optic cable instead of a copper cable.
- The SC connector is square and somewhat similar to an RJ-45 connector.
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