GUIDED TRANSMISSION MEDIA
The physical layer is to transport bits from one machine to another. Various physical media can be used for the actual transmission.
- • Magnetic Media
- • Twisted Pair
- • Coaxial Cable
- • Fiber Optics
Magnetic Media :
Magnetic tape or removable media (e.g., recordable DVDs), physically transport the tape or disks to the destination machine, and read them back in again. This method is more cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor.
Twisted Pairs :
Magnetic tape has poor delay characteristics. Transmission time is measured in minutes or hours, not milliseconds. A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form. Twisting is done because two parallel wires constitute a fine antenna. When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively. A signal is usually carried as the difference in voltage between the two wires in the pair. Provides better immunity to external noise. Used for transmitting either analog or digital information .
Twisted Pairs : Twisted-pair cabling comes in several varieties. The garden variety deployed in many office buildings is called Category 5 cabling, or ‘‘Cat 5’’. A category 5 twisted pair consists of two insulated wires gently twisted together. Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them together. Different LAN standards may use the twisted pairs differently. For example, 100-Mbps Ethernet uses two (out of the four) pairs, one pair for each direction.
Coaxial Cable :
• It has better shielding and greater bandwidth than unshielded twisted pairs, so it can span longer distances at higher speeds. • Two kinds of coaxial cable are widely used. 50-ohm cable is commonly used when it is intended for digital transmission from the start. 75-ohm cable is commonly used for analog transmission and cable television
Fiber Optics :
Fiber optics are used for long-haul transmission in network backbones, highspeed LANs, and high-speed Internet access such as FttH (Fiber to the Home). • An optical transmission system has three key components: the light source, the transmission medium, and the detector. • A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. • The transmission medium is an ultra-thin fiber of glass. The detector generates an electrical pulse when light falls on it. • Connects a light source to one end of an optical fiber and a detector to the other. There is a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.
