Mumbai University > Information Technology > Sem4 > Computer Networks

Marks: 10M

Year: May 2014

Guided Media:

Guided media, which are those that provide a conduit from one device to another, include twisted pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light.

Twisted Pair:

• Although the bandwidth characteristics of magnetic tape are excellent, the delay characteristics are poor.
• A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together, as shown in fig. below
• One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two.
• In addition to the signal sent by the sender on one of the wires, interference (noise)and crosstalk may affect both wires and create unwanted signals.
• If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources(e,g., one is closer and the other is farther). This results in a difference at the receiver.
• By twisting the pairs, a balance is maintained. For example, suppose in one twist, one wire is closer to the noise source and the other is farther; in the next twist, the reverse is true.
• Twisting makes it probable that both wires are equally affected by external influences(noise or crosstalk). This means that the receiver, which calculates the difference between the two, receives no unwanted signals. The unwanted signals are mostly canceled out.
• The number of twists per unit of length(e.g., inch) has some effect on the quality of the cable

Applications of Twisted Pair Cables:

• In Telephone lines to carry voice and data channels
• In the local loop
• In the DSL line (ADSL)
• Local area networks such as 10 Base-T and 100 Base-T use the twisted pair cables
• In the ISDN(Integrated Services Digital Network)

Co-axial Cables:

• Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently.
• Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as shield against noise and as second conductor which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.

Applications

i.Analog telephone networks

ii. Digital telephone networks

iii. Cable T.V

iv. Traditional Ethernet LANS

v. Digital transmission

vi. Thick Ethernet

Fiber Optic Cable:

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.

Applications

i. Optical fiber transmission systems are widely used in the backbone of networks. Current optical fiber systems provide transmission rates from 45 Mbps to 9.6 Gbps using single wavelength transmission.

ii. They are used in the telephone systems and in LANs.