NETWORK MEDIA AND CONNECTORS
Networks are built using a topology of bus, star, or ring, but how the systems will be connected in the topology that you choose. 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.
There are three primary types of cable media that can be used to connect systems to a network:
- Coaxial cable
- Twisted-pair cable
- 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).
1. 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 another cabling, such as unshielded twisted-pair (UTP) cable. Figure 1 shows a coaxial cable with a copper core and layers of insulation.
|Figure 1: Coaxial 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.
This refers to RG-58 cabling, which is a flexible coaxial cable about ¼-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 British naval connector (BNC) and uses the network adapter card’s internal transceiver. The maximum length of a thinnet is 185 meters. Figure 1.1 displays the thinnet coaxial cabling and the BNC connector on the end.
This coaxial cable, also known as RG-8, gets its name by being a thicker cable than thinnet. Thicknet cable is about ½-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 the thickness of ½ inch, 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 adapter unit interface (AUI) port connector.
2. TWISTED-PAIR CABLE:
Twisted-pair cabling gets its name by having four pairs of wires that are twisted to help reduce crosstalk or interference from outside electrical devices. (Crosstalk is interference from adjacent wires.) Figure 2 shows a twisted-pair cable.
There are two forms of twisted-pair cabling unshielded twisted-pair (UTP) and shielded twisted-pair (STP).
|Figure 2: Twisted-Pair Cable|
I) UNSHIELDED TWISTED-PAIR (UTP) CABLE:
Unshielded twisted-pair (UTP) cables are familiar to you if you have worked with telephone cables. 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.
UTP cable uses small plastic connectors designated as registered jack 45 or most often referred to as RJ-45. RJ-45 is similar to the phone connectors, except that instead of four wires, as found in the home system, the network RJ-45 connects to contains eight contacts, one for each wire in a UTP cable. The bottom cable in Figure 2.1 is an RJ-45 connector.
It can be easy to confuse the RJ-45 connector with the RJ-11 connector. The RJ-11 connector is a telephone connector and is shown in Figure 2.1 (the cable on the top). In an RJ-11 connector, there are four contacts; hence there are four wires found in the telephone cable. With RJ-45 and RJ-11, you will need a special crimping tool when creating the cables to make contact between the pins in the connector and the wires inside the cable.
UTP cabling has different flavors, known as grades or categories. Each category of UTP cabling was designed for a specific type of communication or transfer rate.
i) STRAIGHT-THROUGH CABLES:
CAT 5 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. The 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.
|Figure: Pinout Diagram for a Straight-Through Cable|
You will see in the figure that 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. You will also notice that 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 that is sent from the computer is received at the network hub.
We also want to make sure that data sent from the hub is received at the computer, so you will notice that the transmit pins (TX) on the hub are connected to the receive pins (RX) on the computer through wires 3 and 6. This will allow the computer to receive information from the hub. The last thing to note about Figure 1-18 is that pin 1 on the computer is connected to pin 1 on the hub by the same wire, thus the term straight-through. You will notice that all pins are matched straight through to the other side in Figure.
ii) CROSSOVER CABLES:
At some point, you may need to connect two computer systems directly together without the use of a hub, from the network card to the network card. To do this, you would not be able to use a straight-through cable because the transmit pin on one computer would be connected to the transmit pin on another computer, as shown in Figure a. How could a computer pick up the data if it was not sent to the receive pins? This will not work, so we will 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, as shown in Figure b. 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 to Computer B to send 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.
A rollover cable is a popular cable type in the networking world and is used to connect to a Cisco device such as a router or a switch. Also known as a console cable, this cable connects from the computer’s serial port to the console port of the router or switch. Once the network administrator connects to the console port, he or she is then able to configure the router or switch.
II) SHIELDED TWISTED-PAIR (STP) CABLE:
Shielded twisted-pair (STP) cable is very 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. Figure 1-22 shows the size of STP cabling as compared to UTP.
3. 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 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. The figure shows fibers in a fiber-optic cable.
There are two types of fiber-optic cables: single-mode fiber (SMF) and multimode fiber (MMF).
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 fiber-optic cable supports up to 1000 stations and can carry the signal up to and beyond 2 kilometers. Fiber-optic cables are also highly secure from outside interference, such as radio transmitters, arc welders, fluorescent lights, and other sources of electrical noise. On the other hand, fiber-optic cable is by far the most expensive of these cabling methods, and a small network is unlikely to need these features. Depending on local labor rates and building codes, installing fiber-optic cable can cost as much as $500 per network node.