Ambo University Woliso Campus Technology and Informatics School
Ambo University Woliso Campus , Technology and Informatics School Department of Computer Science Data Communication and Computer Networks 1
CHAPTER THREE 2
Data Transmission Media Transmission media refers to the physical media through which communication signals (data and information are transmitted). Transmission media can be divided into two broad categories: guided/wired media and unguided/wireless media. Guided/wired media uses a cabling system that guides the data signals along a specific path. Unguided/wireless media: transmits the signals through the air by electromagnetic waves. 3
Guided Media A signal travelling along the guided media is directed and contained by the physical limits of the medium. The data signals are bound by the cabling system. Cabling refers to transmission medium that consists of cables. There are three major groups of cables: 1. Twisted-pair cable 2. Coaxial cable Fiber-optic cable Twisted-pair and Coaxial cable accepts and transports signal in the form of electric current. Fiber-optic cable as a light 3. • • 4
Transmission Impairments With a communication system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments can degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa. The most significant impairments are 5 Noise Attenuation
Noise: is any undesirable influence that may degrade or distort a signal. Many different types of noise may affect transmission. A common source of noise is EMI (electromagnetic interference), or waves that are generated from electrical devices or cables carrying electricity. Motors, power lines, televisions, copiers, fluorescent lights, manufacturing machinery, and other sources of electrical activity (including a severe thunderstorm) can cause EMI. 6
Noise Another form of noise that hinders data transmission is cross talk. Cross talk occurs when a signal travelling on one wire or cable infringes on the signal travelling over an adjacent wire or cable. When cross talk occurs between two cables, it’s called alien cross talk. 7
Attenuation is the loss of a signal’s strength as it travels away from its source. To compensate for attenuation, both analog and digital signals are boosted. However, the technology used to boost an analog signal is different from that used to boost a digital signal. Analog signals pass through an amplifier, an electronic device that increases the voltage, or strength, of the signals. 8
Attenuation When an analog signal is amplified, the noise that it has accumulated is also amplified. After multiple amplifications, an analog signal may become difficult to interpret. When digital signals are repeated, they are actually retransmitted in their original form, without the noise they might have accumulated previously. This process is known as regeneration. A device that regenerates a digital signal is called a repeater. 9
Twisted-Pair Cable A twisted pair cable consists of two conductors (copper), each with its own plastic insulation, twisted together. Ø There are two types of twisted pair cables: - Unshielded twisted-pair cables (UTP cables) Shielded twisted-pair cables (STP cables) Ø The total number of pairs in a cable varies. Ø Twisting of the cables cancels out electrical noise from adjacent pairs. 10
Ø Unshielded Twisted-Pair (UTP) Cable UTP is the most popular type of twisted-pair cable. It is the most popular LAN cabling. The maximum cable length segment is 100 meters. UTP specifications govern how many twists are permitted per foot of cable. The number of twists allowed depends on the purpose to which the cable will be put. 11
Ø There are 7 standard categories of UTP: • Category 1 - refers to traditional UTP telephone cable that carry voice but not data transmissions. • Category 2 - certifies UTP cable for data transmissions up to 4 megabits per second (Mbps). It consists of four twisted pairs. • Category 3 - certifies UTP cable for data transmissions up to 16 Mbps. It consists of four twisted pairs of copper wire with three twists per foot. • Category 4 - certifies UTP cable for data transmissions up to 20 Mbps. It consists of four twisted pairs of copper wire. • Category 5 - certifies UTP cable for data transmissions up to 100 Mbps. It consists of four twisted pairs of copper wire. • Category 5 e - Improved version of cat 5 category. The bandwidth is 1000 MBPS. • Category 6 Similar to CAT 5 wire, but contains a physical separator between 12
UTP Cable UTP is particularly susceptible to crosstalk. Ø Greater number of twists per foot of cable, the more effective the protection against crosstalk. • Crosstalk occurs when signals from one line bleed into another line. 13
Ø UTP advantages It is familiar technology. It is relatively inexpensive and easy to install. Most LAN systems are readily capable of running over UTP. Ø UTP disadvantages UTP is potentially more sensitive to external electromagnetic interference, attenuation than other media. 14 crosstalk, and
Shielded Twisted-Pair (STP) Cable STP cable uses a wounded copper-braid jacket that is more protective and of a higher quality. STP advantage • Provides better performance than UTP in environments with: - high noise levels - high levels of unwanted electrical signals. STP disadvantage • It is more labor-intensive than UTP to install. • Not all LAN systems work readily over STP. 15
Twisted-Pair Cable Components • Connection hardware -Twisted-pair cable uses RJ 45 (Registered Jack 45) telephone connectors to connect to a computer or Network Interface Card (NIC). 16
NIC (Network Interface Card) (network interface card)—The device (pronounced nick) inside a computer that connects a computer to the network media. NIC-Network Adapters NIC 17
Coaxial Cable It consists of a core of copper wire surrounded by insulation, a braided metal shielding, and an outer cover. 18
Coaxial Cable The core of a coaxial cable carries the electronic signals that make up the data. Surrounding the core is a dielectric insulating layer that separates it from the braided shielding. The braided shielding acts as a ground and protects the core from electrical noise and crosstalk. A non-conducting outer shield—usually made of rubber or plastic—surrounds the entire cable. Coaxial cable is more resistant to interference and attenuation than twisted-pair cabling. 19
Types of Coaxial Cable Ø There are two types of coaxial cable: Thin (thinnet) cable Thick (thicknet) cable Ø Thinnet Cable: is a flexible coaxial cable about 0. 64 centimeters thick. Thinnet coaxial cable can carry a signal for a distance of up to approximately 185 meters before the signal starts to suffer from attenuation. Ø Thicknet Cable: is a relatively rigid coaxial cable about 1. 27 centimeters in diameter. Thicknet cable can carry a signal for 500 meters. 20
Ø Coaxial-Cable Connection Hardware The BNC cable connector: is used to connect the end of the cable to a device, such as a TV set. . 21
Coaxial Cable The BNC T connector: This connector joins the network interface card (NIC) in the computer to the network cable. 22
The BNC barrel connector: This connector is used to join two lengths of thinnet cable to make one longer length. ØThe BNC terminator: it closes each end of the bus cable to absorb stray signals. Otherwise, the signal will bounce and all network activity will stop. 23
Ø Coaxial Cable Advantages It is less susceptible to interference than twisted-pair cable. It can transmit data for greater distances. It can transmit voice, video, and data. Offer a familiar technology with reasonable data security. Ø Coaxial Cable Disadvantages Due to its high metallic content, coaxial cable is usually more expensive than twisted pair cables. 24
Fiber-Optic Cable It contains glass (or in some cases, plastic) fibers rather than copper wire. Signals are transmitted across these fibers in the form of light pulses rather than electrical pulses. Optical fiber strands are thin filaments of glass consisting of an inner core and an outer cladding. The diameter of the core varies with the type of optical fiber. Single-mode optical fiber has a core diameter of approximately 8. 5 µm. Multimode optical fiber has a core diameter of 62. 5 µm. 25
Fibber optic (outdoor) 26
Optical Fiber Advantages The bandwidth of optical fiber is greater than that of twisted-pair or coaxial cable. Optical signals through glass encounter less loss than electrical signals through copper. It has lower attenuation and higher bandwidth than copper. Resist to electromagnetic interference signal is sent as light and not as electricity. Ø Optical fiber disadvantage Conversions of light to electricity, more expensive electronics are required. Requires specialized installation procedures. 27
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