Data_Communications

Transmission medium - the physical path between transmitter and receiver in a data transmission system. 2 Categories o Guided transmission media - are those media with some form of conductor that provides a conduit in which electromagnetic signals arte contained. o Unguided transmission media - are wireless system. Unguided signals are emitted and then radiate through air or a vacuum. Metallic Transmission Lines - a transmission line is a metallic conductor system use to transfer electrical energy from one point to another using electrical current flow. Transverse Electromagnetic Waves 2 kinds of waves o Longitudinal – the displacement (amplitude) is in the direction of propagation. E.g. surface waves, sound waves. o Transverse –the direction or displacement is perpendicular to the direction of propagation. E.g. Electromagnetic waves Characteristics of Electromagnetic Waves 3 primary characteristics o Wave velocity – Waves travel at different speeds depending on the type of wave and the characteristics of the propagation medium. o Frequency – the oscillations of an electromagnetic wave are periodic and repetitive. o Wavelength – the distance of one cycle occurring in space. Transmission Lines Classifications 2. classifications o Balanced transmission lines - both conductors in a balanced line carry signal currents. o Unbalanced transmission lines – one wire is at ground potential, whereas the other wire is at signal potential. Metallic Transmission Line Types • Parallel-conductor transmission lines – comprised of two or more metallic conductors separated by a nonconductive insulating material called a dielectric. o Open-wire transmission lines –these two-wire parallel conductors, closely spaced and separated by air.. o Twin-lead – essentially the same as open-wire transmission line except that the spacers between the two conductors are replaced with a continuous solid dielectric ensuring the uniform spacing along the entire cable. o Twisted-pair transmission lines – is formed by twisting two insulated conductors around each other. .2 Basics types specified by EIA/TIA 568 Commercial Building Telecommunications Cabling Standard ▪ Unshielded twisted-pair (UTP) –an UTP cable consists of two copper wires where each wire is separately encapsulated in PVC (polyvinyl chloride) insulation. ▪ Shielded twisted-pair (STP) - is a parallel two-wire transmission line consisting of two copper conductors separated by a solid dielectric material. o Plenum cable – metal rectangular-shaped air ducts were traditionally placed in the plenum to control airflow in the building. • Coaxial (concentric) transmission lines – consists of center conductor surrounded by a dielectric material (insulation), then a concentric (uniform distance from the center) shielding, and finally a rubber environmental protection outer jacket. 2 types of coaxial cable connectors ▪ BNC connectors – are sometimes referred to bayonet mount, as they can be easily twisted on or off. ▪ N-type connectors – are threaded and must be screwed on and off. Metallic Transmission Line Equivalent Circuit – the characteristics of a transmission line are determined by its electrical properties like wire conductivity, insulator dielectric constant and its physical properties like wire diameter and conductor spacing. • Characteristics Impedance – for maximum power transfer from the source to load, a transmission line must be terminated in a purely resistive load equal to the characteristic impedance of the transmission line. Using Ohm’s law, the characteristic impedance is simply the ratio of the source voltage to the line current (Io), given by Zo = Eo / Io , where Zo is the characteristic impedance in ohms, Eo is source voltage in volts and Io is transmission line current in amps. ➢ Characteristic impedance of a two wire parallel transmission line with an air dielectric can be determined from its physical dimensions Zo = 276 log D/r where D is distance between the centers of the two conductors and R is the radius of the conductors. ➢ Characteristic impedance of a coaxial cable can also be determined from its physical dimensions: where, D is inside diameter Of the conductor and is relative dielectric constant of the insulating material. Wave Propagation on Metallic Transmission Line Types – EM waves travel at speed of light through vacuum and nearly the same through air, but they travel considerably slowly in metallic transmission lines, where the conductor is generally copper and the dielectric materials vary with cable type. ✓ Velocity factor– the ratio of the actual velocity of propagation of an electromagnetic wave through a given medium to the velocity of propagation through a vacuum. ✓ Dielectric constant – is simply the relative permittivity of a material. ✓ Coupling losses - it occurs whenever a connection is made to or from a transmission line or when two sections of transmission line are connected together. ✓ Corona – a luminous discharge that occurs between the two conductors of a transmission line.