Transmission media is a pathway that carries the information from sender to receiver. We use different types of cables or waves to transmit data. Data is transmitted normally through electrical or electromagnetic signals. An electrical signal is in the form of current. An electromagnetic signal is series of electromagnetic energy pulses at various frequencies. These signals can be transmitted through copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different properties like bandwidth, delay, cost and ease of installation and maintenance.
Transmission media is also called Communication channel.
- Transmission media is broadly classified into two groups.
- Wired or Guided Media or Bound Transmission Media
- Wireless or Unguided Media or Unbound Transmission Media.
The data transmission capabilities of various Medias vary
differently upon:
- Bandwidth: It refers to the data carrying capacity of a channel or medium. Higher bandwidth communication channels support higher data rates;
- Radiation: It refers to the leakage of signal from the medium due to undesirable electrical characteristics of the medium;
- Noise Absorption: It refers to the susceptibility of the media to external electrical noise that can cause distortion of data signal;
- Attenuation: It refers to loss of energy as signal propagates outwards. The amount of energy lost depends on frequency.
- Radiations and physical characteristics of media contribute to attenuation.
Transmission Channel Parameters
Some parameters are required in description of transmission
channel. Some of the parameter is discussed below:
- Bandwidth: The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers.
- For example, if a composite signal contains frequencies between 2000 and 6000, its bandwidth is 6000 - 2000, or 4000. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. The bandwidth determines the channel capacity.
- Bit Rate: Most digital signals are non-periodic, and thus period and frequency are not appropriate characteristics. Bit rate is used to describe digital signals. The bit rate is the number of bits sent in 1s, expressed in bits per second (bps).
- Bit Length: The bit length is the distance one bit occupies on the transmission medium. Bit length=propagation speed x bit duration
- Data Rate Limits: one of the most important consideration in data communications is how fast we can send data, in bits per second over a channel. Data rate depends on three factors:
- a. The bandwidth available
- b. The level of the signals we use
- c. The quality of the channel (the level of noise)
- Noiseless Channel: Nyquist Bit Rate. For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate BitRate = 2 x bandwidth x log2L
Wired Transmission Media
Wired or Guided Media or Bound Transmission Media: Bound
transmission media are the cables that are tangible or have
physical existence and are limited by the physical geography.
Popular bound transmission media in use are twisted pair
cable, co-axial cable and fiber optical cable. Each of them has
its own characteristics like transmission speed, effect of
noise, physical appearance, cost etc.
Open Wire:
Open Wire is traditionally used to describe the electrical wire
strung along power poles. There is a single wire strung
between poles. No shielding or protection from noise
interference is used. We are going to extend the traditional
definition of Open Wire to include any data signal path without
shielding or protection from noise interference. This can
include multiconductor cables or single wires. This media is
susceptible to a large degree of noise and interference and
consequently not acceptable for data transmission except for
short distances under 20 ft.
Twisted Pair Cable:
A twisted pair consists of two conductors (normally copper),
each with its own plastic insulation, twisted together, as
shown in below figure. 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. Each pair would consist
of a wire used for the +ve data signal and a wire used for the -
ve data signal. Any noise that appears on 1 wire of the pair
would occur on the other wire. Because the wires are
opposite polarities, they are 180 degrees out of phase (180
degrees - phasor definition of opposite polarity). When the
noise appears on both wires, it cancels or nulls itself out at
the receiving end. Twisted Pair cables are most effectively
used in systems that use a balanced line method of
transmission.
Coaxial Cable:
Coaxial Cable consists of 2 conductors. The inner conductor
is held inside an insulator with the other conductor woven
around it providing a shield. An insulating protective coating
called a jacket covers the outer conductor.
The outer shield protects the inner conductor from outside
electrical signals. The distance between the outer conductor
(shield) and inner conductor plus the type of material used for
insulating the inner conductor determine the cable properties
or impedance. Typical impedances for coaxial cables are
75ohms for Cable TV, 50 ohms for Ethernet Thinnet and
Thicknet. The excellent control of the impedance
STP UTP77
characteristics of the cable allow higher data rates to be
transferred than Twisted Pair cable.
Wireless Transmission Media
In Wireless Transmission Media data signals flows through
the air.
Wireless or Unguided Media or Unbound
Transmission Media: Unbound transmission media are the
ways of transmitting data without using any cables. These
media are not bounded by physical geography. This type of
transmission is called Wireless communication. Nowadays
wireless communication is becoming popular. Wireless LANs
are being installed in office and college campuses. This
transmission uses Microwave, Radio wave, Infrared are some
of popular unbound transmission media.
Wireless Signal Propagation:
Wireless signals travel or propagated in three ways:
1. Ground-wave propagation: Ground Wave Propagation
follows the curvature of the Earth. Ground Waves have
carrier frequencies up to 2MHz. AM radio is an example
of Ground Wave Propagation.
2. Sky-wave propagation: Ionospheric Propagation
bounces off of the Earths Ionospheric Layer in the 82
upper atmosphere. It is sometimes called Double Hop
Propagation. It operates in the frequency range of 30 -
85MHz. Because it depends on the Earth's ionosphere,
it changes with weather and time of day. The signal
bounces off of the ionosphere and back to earth. Ham
radios operate in this range. Characteristics of Sky Propagation are as follows: Signal reflected from
ionized layer of atmosphere back down to earth; Signal
can travel a number of hops, back and forth between
ionosphere and earth„s surface; Reflection effect
caused by refraction.
3. Line-of-sight propagation: Line of Sight Propagation
transmits exactly in the line of sight. The receive station
must be in the view of the transmit station. It is
sometimes called Space Waves or Tropospheric
Propagation. It is limited by the curvature of the Earth
for ground based stations (100 km: horizon to horizon).
Reflected waves can cause problems.
Examples of Line
of Sight Propagation are: FM Radio, Microwave and
Satellite. Transmitting and receiving antennas must be
within line of sight.
LAN(Local area network)
Local Area Network A local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus. Depending on the needs of an organization and type of technology used, a LAN can be as simple as two desktops and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers. In addition to the size, LANs are distinguished from other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. LANs are designed to allow resources to be shared between personal computers or workstations. Early LANs had data rates in the 4 to 16 mega-bits-per-seconds (Mbps). Today, however, speeds are normally 100Mbps or 1000 Mbps. Wireless LANs (WLAN) are the newest evolution in LAN technology.
0 Comments