Transmission Media
Various physical media can be used to transport a stream of bits from one device to another. Each has its own characteristics in terms of bandwidth, propagation delay, cost, and ease of installation and
maintenance. Media can be generally classified as guided (e.g. copper and fiber cable) and unguided (wireless) media. The main categories of transmission media used in data communications networks.
Some Bound Media are:-
A. Coaxial Cable
B. Twisted Pair cable
C. Optical Fiber Cable
Coaxial cable:
A coaxial cable has a central copper wire core, surrounded by an insulating (dielectric) material. Braided metal shielding surrounds the dielectric and helps to absorb unwanted external signals (noise), preventing it from interfering with the data signal travelling along the core. A plastic sheath protects the cable from damage. A terminating resistor is used at each end of the cable to prevent transmitted signals from being reflected back down the cable. The following diagram illustrates the basic construction of a coaxial cable.
Construction of coaxial cable
Coaxial cable has a fairly high degree of immunity to noise, and can be used over longer distances (up to 500 meters) than twisted pair cable. Coaxial cable has, in the past, been used to provide network
backbone cable segments. Coaxial cable has largely been replaced in computer networks by optical fiber
and twisted pair cable, with fiber used in the network backbone, and twisted pair used to connect
workstations to network hubs and switches. Thick net cable (also known as 10Base5) is a fairly thick cable (0.5 inches in diameter). The 10Base5 designation refers to the 10 Mbps maximum data rate, base band signaling and 500 meter maximum segment length . Thick net was the original transmission medium used in Ethernet networks, and supported up to 100 nodes per network segment. An Ethernet transceiver was connected to the cable using a vampire tap , so called because it clamps onto the cable, forcing a spike through the outer
shielding to make contact with the inner conductor, while two smaller sets of teeth bite into the outer
conductor. Transceivers could be connected to the network cable while the network was live. A separate drop cable with an attachment unit interface (AUI) connector at each end connected the transceiver to the network interface card in the workstation (or other network device). The drop cable was typically a shielded twisted pair cable, and could be up to 50 meters in length. The minimum cable length between
connections ( taps ) on a cable segment was 2.5 meters.
Thick net (10Base5) coaxial cable
Thin net cable (also known as 10Base2) is thinner than Thick net (approximately 0.25 inches in diameter)and as a consequence is cheaper and far more flexible. The 10Base2 designation refers to the 10 Mbps maximum data rate, base band signaling and 185 (nearly 200) meter maximum segment length . A T‐connector is used with two BNC connectors to connect the network segment directly to the network adapter card. The length of cable between stations must be at least 50 centimeters, and Thin net can
support up to 30 nodes per network segment.
Thin net (10Base2) coaxial cable
Coaxial cable has the following advantages and disadvantages:
Advantages
• Highly resistant to EMI (electromagnetic interference)
• Highly resistant to physical damage
Disadvantages
• Expensive
• Inflexible construction (difficult to
install)
• Unsupported by newer networking standards
Twisted pair cable:
Twisted pair copper cable is still widely used, due to its low cost and ease of installation. A twisted pair consists of two insulated copper cables, twisted together to reduce electrical interference between
adjacent pairs of wires. This type of cable is still used in the subscriber loop of the public telephone
system (the connection between a customer and the local telephone exchange), which can extend for
several kilometers without amplification. The subscriber loop is essentially an analogue transmission line, although twisted pair cables are also be used in computer networks to carry digital signals over short distances. The bandwidth of twisted pair cable depends on the diameter of wire used, and the length of the transmission line. The type of cable currently used in local area networks has four pairs of wires. Until recently, category 5 or category 5E cable has been used, but category 6 is now used for most new installations. The main difference between the various categories is in the data rate supported ‐ category 6 cable will support gigabit Ethernet. The main disadvantage of UTP cables in networks is that, due to the relatively high degree of attenuation and a susceptibility to electromagnetic interference, high speed digital signals can only be reliably transmitted over cable runs of 100 meters or less.
Twisted pair cable:
Shielded Twisted Pair (STP) cable was introduced in the 1980s by IBM as the recommended cable for
their Token Ring network technology. It is similar to unshielded twisted pair cable except that each pairs individually foil shielded, and the cable has a braided drain wire that is earthed at one end during installation. The popularity of STP has declined for the following reasons:
1) High cost of cable and connectors More difficult to install than UTP Ground loops can occur if incorrectly installed
2) There is still a cable length limitation of 100 meters
Shielded twisted pair cable
Advantages
• It is a thin, flexible cable that is easy to string between walls.
• More lines can be run through the same wiring ducts.
• UTP costs less per meter/foot than any other type of LAN cable.
• Electrical noise going into or coming from the cable can be prevented.
• Cross‐talk is minimized.
Disadvantages
• Twisted pair’s susceptibility to electromagnetic interference greatly depends on the pair twisting
schemes (usually patented by the manufacturers) staying intact during the installation. As a
result, twisted pair cables usually have stringent requirements for maximum pulling tension as
well as minimum bend radius. This relative fragility of twisted pair cables makes the installation
practices an important part of ensuring the cable’s performance.
• In video applications that send information across multiple parallel signal wires, twisted pair
cabling can introduce signaling delays known as skew which results in subtle color defects and
ghosting due to the image components not aligning correctly when recombined in the display
device. The skew occurs because twisted pairs within the same cable often use a different
number of twists per meter so as to prevent common‐mode crosswalk between pairs with
identical numbers of twists. The skew can be compensated by varying the length of pairs in the
termination box, so as to introduce delay lines that take up the slack between shorter and
longer pairs, though the precise lengths required are difficult to calculate and vary depending on
the overall cable length.
Optical fiber:
Optical fibers are thin, solid strands of glass that transmit information as pulses of light. The fibers has a core of high‐purity glass, between 6μm and 50μm in diameter, down which the light pulses travel. The core is encased in a covering layer made of a different type of glass, usually about 125 μm in diameter, known as the cladding. An outer plastic covering, the primary buffer, provides some protection, and takes the overall diameter to about 250 μm. The structure of an optical fiber is shown below.
The basic construction of an optical fiber. The cladding has a slightly lower refractive index than the core (typical values are 1.47 and 1.5 respectively), so that as the pulses of light travel along the fiber they are reflected back into the core each time they meet the boundary between the core and the cladding. Optical fibers lose far less of its signal energy than copper cables, and can be used to transmit signals of a much higher frequency. More information can be carried over longer distances with fewer repeaters. The bandwidth achievable using optical fiber is almost unlimited, but current signaling technology limits the data rate to 1 Gbps due to time required to convert electronic digital signals to light pulses and vice versa. Digital data is converted to light pulses by either a light emitting diode (LED) or a laser diode. Although some light is lost at each end of the fiber, most is passed along the fiber to the receiver, where the light pulses are converted back into electronic signals by a photo‐detector. As the ray passes along the fiber it meets the boundary between the core and the cladding at some point. Because the refractive index of the cladding is lower than that of the core, the ray is reflected back into the core material, as long as the angle of incidence ( θ i ) is greater than the critical angle ( θ c). The critical angle depends on the refractive indices of the two materials. In the case of an optical fiber,the values are chosen so that almost all of the light is reflected back into the fiber, and there is virtually no loss through the walls of the fiber. This is called total internal reflection . The critical angle for a particular fiber can be calculated using Snell's Law. This states that:
n 1 sin θ 1 = n 2 sin θ 2
where θ 1 is the angle of incidence, θ 2 is the angle of refraction, and n 1 and n 2 are the refractive indices of the core and cladding respectively. The effect on a ray of light passing along the fiber is shown below.
Light transmission in an optical fiber In step‐index fibers,
the refractive Len of both the core and the carding has a constant value, so that the refractive index of the fiber steps from one value to the next.
A step‐index fiber
If the core diameter of the fiber is such that it allows light to enter at different angles and follow
multiple paths, it is said to be a multi‐mode fiber. The number of times the light is internally reflected
will vary according to the angle at which the light initially enters the fiber, which will in turn determine the path length of the light as it travels along the fiber. Over long distances, there will be a significant difference in path length between light rays that enter the fiber at different angles. They will, as a consequence, arrive at slightly different times, causing distortion of the transmitted signal ‐ an effect known as modal dispersion . For this reason, multi‐mode fibers are only used for short‐haul applications such as LAN backbone connections, where the distances involved are likely to be considerably less than
one kilometer.
A multi‐mode step‐index fiber
If the core diameter of the fiber is made small enough, the angle at which light can enter the fiber can
be limited such that most of the light travels down a single path, effectively eliminating modal
dispersion. This type of fiber is called a mono‐mode fiber, and is commonly used for long‐haul
applications such as long distance telecommunications. Distances of many kilometers are possible with mono‐mode fiber before a repeater is needed.
Multi‐mode and mono‐mode fibers
One way to improve the performance of multi‐mode fiber is to use a graded index fiber instead of a step index fiber. The refractive index of this type of fiber varies across the diameter of the core in such a way that light is made to follow a curved path along the fiber (see below). Light near the edges of the core travels faster than light at the center of the core, so although some rays follow a longer path than
others, they all tend to arrive at the same time, resulting in far less modal dispersion than would occur
in a step‐index multi‐mode fiber.
A graded index fiber
Light paths in a graded index fiber
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Advantages
• Better security ‐ very hard to tap a fiber without being noticed.
• Longer cable runs
• Greater bandwidth.
• Not affected by electromagnetic interference.
• Can connect between buildings with different earth potentials. These would could cause problems with a copper wired system.
• Not effected by near‐miss lightning strikes
• Lower cost for 2 to 3 km fiber runs. CAT5 twisted‐pair is limited to 100 meters.
• Carrier signals on different frequencies (colors) can be used to increase the capacity (frequency
division multiplexing).
• Single mode or mono mode fiber has a very thin inner glass layer.
• This is so thin that it behaves like a wave guide and the light can't follow different paths.
• This reduces the dispersion and makes longer fibers possible 30 km.
Disadvantages
• Attenuation is still a problem and this limits the maximum cable length
• Dispersion is still a problem and this also limits the maximum cable length
• Scattering occurs when there are imperfections in the fiber. This causes attenuation or energy
loss.
• Higher installation cost for small networks. For major backbones fiber works out cheaper per
megabit of bandwidth.
• Optical fibers can be fragile although they are reinforced with Kevlar fibers and an outer
protective plastic layer
• Optical fibers are difficult to connect to the transmitting light source and the receiving light
detector. A complex cutting and polishing operation is needed to make the fibre ends flat and
free from dirt or imperfections.
