v  The most common type of cable used in data communications is copper.

v  There are different types of copper cable but they all share the feature of transmitting data as an electrical signal. This gives rise to one of the weaknesses of this type of media, which is that sending an electronic signal causes heat that is absorbed by the media as the signal travels thus causing signal loss, i.e. attenuation.

v  The faster the data transfer rate over copper cable the greater the possibility of signal loss and interference.

v  Guided media are manufactured so that signals will be confined to a narrow path and will behave predictably. Commonly used guided media include twisted-pair wiring, similar to common telephone wiring; coaxial cable, similar to that used for cable TV; and optical fiber cable.

v  The purpose of the physical layer is to transport a raw bit stream from one machine to another.

v  Various physical media can be used for the actual transmission. Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and maintenance.

v  Waves are guided along a solid medium such as a transmission line.

v  We will look at all of these in the following sections.

v  A plastic coating on each wire prevents the copper in one wire from touching the copper in another.

v  The twist helps reduce interference by preventing electrical signals on the wire radiating energy (causing interference) and by preventing signals on other wires interfering with the pair.

v  The historical foundation of the public switched telephone network (PSTN) lies in twisted-pair, and even today, most people who have access to networks access them through a local loop built on twisted-pair.

v  Twisted pair cable most used in  modern Ethernet networks

v  Although twisted-pair has contributed a great deal to the evolution of communications, advanced applications on the horizon require larger amounts of bandwidth than twisted-pair can deliver, so the future of twisted-pair is diminishing.

v  The total usable frequency spectrum of telephony twisted-pair copper cable is about 1MHz (i.e., 1 million cycles per second).

v  Newer standards for broadband DSL, also based on twisted-pair, use up to 2.2MHz of spectrum. Loosely translated into bits per second (bps)—a measurement of the amount of data being transported, or capacity of the channel—twisted-pair cable offers about 2Mbps to 3Mbps over 1MHz of spectrum.

v  But there’s an inverse relationship between distance and the data rate that can be realized. The longer the distance, the greater the impact of errors and impairments, which diminish the data rate. In order to achieve higher data rates, two techniques are commonly used:

          The distance of the loop can be shortened, and

          Advanced modulation schemes can be applied, which means we can encode more bits per cycle.

v  New developments continue to allow more efficient use of twisted-pair and enable the higher data rates that are needed for Internet access and Web surfing, but each of these new solutions specifies a shorter distance over which the twisted-pair is used, and more sophisticated modulation and error control techniques are used as well.

v  Another characteristic of twisted-pair is that it requires short distances between repeaters. Again, this means that more components need to be maintained and there are more points where trouble can arise, which lead to higher costs in terms of long-term operation.

v  Twisted-pair is also highly susceptible to interference and distortion, including electromagnetic interference (EMI), radio frequency interference (RFI), and the effects of moisture and corrosion. Therefore, the age and health of twisted-pair cable are important factors.

v  The greatest use of twisted-pair in the future is likely to be in enterprise premises, for desktop wiring. Eventually, enterprise premises will migrate to fiber and forms of wireless, but in the near future, they will continue to use twisted-pair internally.

There are two types of twisted-pair:

v  UTP (Unshielded twisted pair)

v  STP (Shielded twisted pair)

1)   UTP (Unshielded twisted pair)

 

v  A coaxial cable consists of four concentric cylinders: an inner conductor, surrounded by an insulating cylinder, surrounded by an outer conductor, surrounded by a final protective cover. This combination is called a coax (following figure).

v  Coaxial cables are superior to twisted pairs both in terms of bandwidth and communication distance, and can provide bandwidth to distance ratios in order of 10s of MHz per kilometer. Like twisted pairs, multiple coaxes are usually housed within one cable, which may also contain twisted pairs. Coaxial cables are extensively used in LANs and long distance telephone trunk lines.

v  A second type of copper wire is coaxial cable, similar to that used for TV aerials. The coaxial cable provides better protection from interference by providing a metal shield as illustrated in above figure.

v  The metal shield forms a flexible cylinder around the inner wire providing a barrier to electromagnetic radiation, both incoming and outgoing. The cable can run parallel to other cables and can be bent round corners.

v  Highly insensitive to EMI

v  Coaxial cable supports high bandwidths

v  Heavier types of coax are sturdy and can withstand harsh environments

v  They can transmit signals up to 10 kilometers.

v  It can be easily installed.

v  Represents a mature technology that is well understood and consistently applied among vendors

v  Although fairly insensitive to EMI, coax remains vulnerable to EMI in harsh conditions such as factories.

v  Disadvantage of coaxial cable is that they support only bus topology, they do not support star topology, star topology is most used in LAN’s

v  Coax can be bulky.

v  Optical fibers use light to transmit data. A thin glass fiber is encased in a plastic jacket which allows the fiber to bend without breaking. A transmitter at one end uses a light emitting diode (LED) or laser to send pulses of light down the fiber which are detected at the other end by a light sensitive transistor.

v  Optical fibers carry cable TV to each street with the houses fed by coaxial cable (a). Optical fibers also carry the Plain Old Telephone Service (POTS) to the nearest exchange, with the local loop to the house consisting of twisted pairs (b).

 

v  An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material. The core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light from escaping from the core. Because optical fiber uses a light signal instead of electrons, it does not suffer from the various noise problems associated with electromagnetic signals. The signal is usually generated by a laser or Light Emitting Diode (LED). Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer. Like other cables, hundreds of optical fibers are usually housed within one cable. They are being increasingly used by telecommunication carriers for long distance 2 Crosstalk is the unwanted coupling effect between two or more signal paths, which causes signal distortion.

v  Current trends promise that they will replace twisted pair residential loops in the near future.

v  Fiber optic cable transmits light signals rather than electrical signals. It is enormously more efficient than the other network transmission media. As soon as it comes down in price (both in terms of the cable and installation cost), fiber optic will be the choice for network cabling.

v  A light pulse can be used to signal a ‘1’ bit; the absence of a pulse signals a ‘0’ bit. Visible light has a frequency of about 108 MHz, so the bandwidth of an optical transmission system is potentially enormous. An optical transmission system has three components: the transmission medium, the light source and the detector. The transmission medium is an ultra-thin fiber of glass or fused silica. The light source is either a LED (Light Emit Diode) or a laser diode, both of which emits light pulses when a electrical current is applied. The detector is a photo diode, which generates an electrical pulse when light falls on it.

Advantages of Optical fibers.

v  Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair

v  It covers distance up to 2 Kms without a repeater.

v  It also has the capability to carry information at vastly greater speeds.

v  They use light which neither causes electrical interference nor are they susceptible to electrical interference

v  They are manufactured to reflect the light inwards, so a fiber can carry a pulse of light further than a copper wire can carry a signal

v  Light can encode more information that electrical signals, so they carry more information than a wire

v  Light can carry a signal over a single fiber, unlike electricity which requires a pair of wires

Disadvantages of Optical fibers.

v  Optical fiber cables are most expensive among all the cables.

v  Installation and maintenance of coaxial cable is high.

v  Glass fiber is more easily broken than wire