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Training session 23: Networking
Difficulty: Medium
Learn the principles of Networking
Creator: m101


In networking there are four main types of cables used, others such as DCE and DTE are used to connect routers and carry timing signals. Here are the standard cables and their general description:

Unshielded twisted-pair cable (UTP) is a four-pair wire medium - composed of pairs of wires - used in a variety of networks. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires are twisted around each other. This type of cable relies solely on the cancellation effect, produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable. When used as a networking medium, UTP cable has four pairs of either 22 or 24 gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms. This differentiates it from other types of twisted-pair wiring such as that used for telephone wiring. Because UTP has an external diameter of approximately .43 cm, its small size can be advantageous during installation. Since UTP can be used with most of the major networking architectures, it continues to grow in popularity. This type of cable is generally refered to as rj or rj45, however this just referes to the connection it uses to connect to the NIC.

Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires . Each pair of wires is wrapped in metallic foil. The 4 pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150 Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise, both within the cable (pair to pair coupling, or crosstalk) and from outside the cable (electromagnetic interference EMI and radio frequency interference RFI). Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). STP affords greater protection from all types of external interference, but is more expensive and difficult to install than UTP. This cable is used when low packet loss at a relatively cheap cost is required.

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements located in the center of the cable is a copper conductor. Surrounding it is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit, and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket. This cable generally uses a bus topology to provide connectivity.

Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. Compared to other networking media, it is more expensive, however, it is not susceptible to electromagnetic interference and is capable of higher data rates than any of the other types of networking media discussed here. Fiber-optic cable does not carry electrical impulses, as other forms of networking media that employ copper wire do. Instead, signals that represent bits are converted into beams of light. Even though light is an electromagnetic wave, light in fibers is not considered wireless because the electromagnetic waves are guided in the optical fiber. The term wireless is reserved for radiated, or unguided, electromagnetic waves. Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If viewed in cross section, you would see that each optical fiber is surrounded by layers of protective buffer material, usually a plastic such as Kevlar, and an outer jacket. The outer jacket provides protection for the entire cable. Usually made of plastic, it conforms to appropriate fire and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection for the fragile hair-thin glass fibers. Wherever buried fiber-optic cables are required by codes, a stainless steel wire is sometimes included for added strength. The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When the core glass is surrounded by a cladding layer of glass or plastic with a low index of refraction, light can be trapped in the fiber core. This process is called total internal reflection, and it allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends. More than one spectrum of light can be transmitted through the cable. Although this gives a higher bandwidth, it decreases the distance at which the cable can carry data.

The diferent types of devices to connect cables are as follows:

Repeaters regenerate, and retime signals, which then enables cables to extend farther to reach longer distances. They only deal with packets at the bit level. This device would generally use Coax or UTP to connect segments.

Multiport repeaters combine connectivity with the amplifying and re-timing properties of repeaters. It is typical to see 4, 8, 12, 24 and up to 48, ports on multiport repeaters. Multiport repeaters are often called hubs, instead of repeaters, when referring to the devices that serve as the center of a star topology network. Since the typical unmanaged hub only requires power and plugged-in RJ-45 jacks, they are great for setting up a network quickly. Like the repeaters on which they are based, they only deal with bits. Although UTP is generally their only media they use, some old hubs may use Coax. Generally these devices are passive, and do not strengthen the signal or retime it. If a hub is Active it provides connectivity over larger distances.

A bridge connects network segments and must make intelligent decisions about whether to pass signals on to the next segment. A bridge can improve network performance by eliminating unnecessary traffic and minimizing the chances of collisions. The bridge divides traffic into segments and filters traffic based on the station or MAC address. Bridges are not complicated devices. They analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. Bridges are only concerned with passing packets, or not passing packets, based on their destination MAC address. This device generally accepts UTP.

LAN switches are considered multi port bridges with no collision domain, because of microsegmentation. Data is exchanged at high speeds by switching the frame to its destination. By reading the destination MAC address, switches can achieve highspeed data transfers, much like a bridge does. The frame is sent to the port of the receiving station prior to the entire frame entering the switch. This leads to low latency levels and a high rate of speed for frame forwarding. This device generally uses UTP. When performing data transfer this device is much superior to the hub, but when latency/ping matter, this device gived less desirable results.

Hopefully you will have a greater knowledge of network devices and media, so when your lame friends try to convince you that their four port 10mbit switch is 1337, you can laugh at them...
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