Date : 26th August 2009 (Wednesday)
Time : 11.00am-12.30pm
Place : DKG 3/9, UUM
Oh no, the class is getting difficult and difficult. I’m exposed to so many new knowledge that I never knew before. It is a bit difficult for me, indeed. Luckily, my lecturer does explain it in simple yet easy for me to understand.
Today, I learn about data transmission. One of the sub topics is wired transmission media. There are three types, as I will explain them one by one.
A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded and the outer conductor is typically a shield that might be braided or a foil.
Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Coaxial cables exist because we can't run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Coaxial cable consists of an insulated center conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement give quite good shielding against noise from outside cable, keeps the signal well inside the cable and keeps cable characteristics stable.
The primary advantage of coaxial cable compared to twisted pair is the braided metal shield is very good at blocking electromagnetic signals from entering the cable and producing noise. And has also been used for long-distance telephone transmission, as the cabling within a local area network, and as a connector between a computer terminal and a mainframe computer.
The twisting of wires reduces crosstalk because when electrical current flows through a wire, it creates a small, circular magnetic field around the wire. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out. They also cancel out any outside magnetic fields. Twisting the wires can enhance this cancellation effect.
Twisted pair wire is placed inside a thin metallic shielding, similar to aluminum foil, and is then enclosed in an outer plastic casing. The shielding provides further electrical isolation of the signal-carrying pair of wires. Shielded twisted pair wires are less susceptible to electrical interference caused by nearby equipment or wires and, in turn, are less likely to cause interference themselves. Because it is electrically "cleaner," shielded twisted pair wire can carry data at a faster speed than unshielded twisted pair wire can
The disadvantage of shielded twisted pair wire is that it is physically larger and more expensive than twisted pair wire, and it is more difficult to connect to a terminating block.
A fiber optic cable transmits a signal using a wave of light, while copper wires transmit a signal using an electric current. (Palais, 1998). A typical fiber optic wire consists of a plastic sheath surrounding a glass tube. Inside the tube is a tiny core in which the light waves travel. Separating the core and the tube is a cladding that prevents the light waves from escaping. One wire design has a core diameter of 125 microns and a sheath diameter of 2.5 millimeters.
The wire’s small size provides an advantage over copper wires. Fiber optic cables weigh less than copper wires, which makes installation easier and allows the wires to be placed in smaller areas. (Palais, 1998). Although the wires are made of glass, they are extremely flexible therefore easing the problems involved with installation.
Although the core of a fiber optic cable is very small, large amounts of information can be transported at one time. In copper wires, only one signal can be sent across the wire at a single time but in a fiber optic cable, many light waves can be sent at once. This allows the wires to be more cost effective. Silicon oxide is the material used to make fiber optic wires. This material is less expensive than the copper used in coaxial cables making fiber optics less expensive to make. Also, fewer fiber optic cables are needed for mass communication.
Fiber optic cables also have disadvantages. Because they use light to transmit information they are incompatible with the old copper wires. This creates a problem because telephone companies cannot use the wire system that is already present. They must build a new system using fiber optics. Fiber optic cables are also very difficult to connect. In order to connect the wires, the glass tube must be broken and then melted together with the new wire. With copper wires, you simply cut away the plastic coating and join the wires. This also creates a problem when repairing fiber optic wires. A copper wire is much easier to repair than a fiber optic wire. Special tools and highly trained professionals are needed to repair the damaged fiber wires.
In a fiber optic cable, many light waves of information are transmitted at one time. Sometimes so much information comes in that the cable becomes jammed and the information slows down. This cannot occur in copper wires because only one signal is generated at a time. Often, at points in the cable where traffic is heavy, bottlenecks occur. A bottleneck is when so much information tries to squeeze through but is lost.
The comparison between among twisted pair, coaxial cable and optical fiber for the bandwidth and the attenuation
The bandwidth is about 1 MHz.
The attenuation is about 0.7 dB/km @ 1 kHz.
The bandwidth is about 500 MHz.
The attenuation is about 7 dB/km @ 10 MHz.
The bandwidth is about 184 THz.
The attenuation is about 0.2 to 0.5 dB/km.
Low attenuation from 0 to 1 MHz.
The bandwidth in the low attenuation range is 1 MHz.
Low attenuation from 0 to 100 MHz.
The bandwidth in the low attenuation range is 100 MHz.
Low attenuation from 800 to 1700 nm.
The bandwidth in this is about 200 THz.