Communication channels refers to the medium used to convey
information from sender to receiver. Communication channels are either wireline
or wireless.for information to be of any value.
GUIDED AND UNGUIDED MEDIA
GUIDED media includes all wired media,also referred to as
conducted or bounded media or bounded media. The unguided media includes all
traditional wireless media also referred to as radiated or bounded or unguided.
PASS BAND
The actual range of frequencies supporting a given
communication is known as a pass band.
BANDWIDTH
Bandwidth shows the capacity of a channel to hold the range
of frequencies assigned to it . if the bandwidth is higher ,the more will be
data transmit rate or throughput. It is the difference between the highest and
the lowest frequencies of a band expressed in heartz.
SECURITY IN THE CONTEXT OF COMMUNICATION SYSTEM
It adresses the protection of data from
interception as it transverses the network. Particularly in the case of data
networking, it is also important that access to a remote system and the data
resident on it be limited to authorized users.
SWITCHING TECHNIQUES
1.MESSAGE SWITCHING
Message switching is the method of handling message
information over a channel through a switching node where the message
information is stored and forwarding to reach the destination. there is
no need for a connection to be established all the way from the source to
the destination.
2.CIRCUIT SWITCHING
CIRCUIT SWITCHING IS A switching method in which a dedicated
communication path in physical form between two stations within a network is
established,maintained and terminated for each communication session. It has
three phases : establishment, data transfer, and circuit disconnect. It
provides a fixed rate channels and both subscribers must operates at this rate.
It has two types of transmission :1.. data gram transmission have individually
addressed frames.2. data streams trans mission which have streams of data which
addressed checking only once.
3.PACKET SWITCHING
It’s a network technology that breaks up a message into
small packets of transmission.
In packed switched data networks ,error and flow control
procedures are applied on each link by the switching nodes. In the packet
switched data network all user data to be transmitted is first divided into one
or more message units, called packets by source DTE. These packets are
handled in the following two ways:
1 data gram
Data gram is a connection less services there is
no need of prior call setup. It is more flexible and provides alternate routes
to avoid the congested part of the network.
2..virtual circuit and permanent virtual circuit
It is a logical path setup prior to transmission and
therefore no routing decisions is to be made which ensures that packets are
forwarded quicker than datagram .it is less reliable because interruption
nin switching nodes may lose all circuits through the nodes.
PERMANENT VIRTUAL CIRCUITS
PERMANENT VIRTUAL CIRCUIT is used to establish long term
connection between DTE at the sending and the receiving end. In this user is
fixed and always connected in logical channel . the main features of PVC are
having a long term association, which is identical to the data transfer phase
of virtual call, and eliminating the need for repeating connection set up and
clearing.
WIRED TRANSMISSIONS
Bounded media or wired transmissions employs the physical
media which are tangible also known as conducted systems, wired media generally
employ a metallic or glass conductor which serves to conduct some form of
electromagnetic energy.
TWISTED PAIRS OR COPPER CONDUCTORS
Twisted pair is a pair of copper wires with diameters of
0.4-0.8 mm, twisted together and raped with plastic coating. Twisting increases
the electrical noise immunity and reduces the error rate transmission of
data .each conductor is separately insulated by some low smoke and fire
retardant substances.these are popular for telephone network. The energy flow
is in the guided media wires.
IT MAY BE DEFINED IN TWO CATAGORIES
1. UNSHIELD TWISTED
PAIR
IT is a copper media inherited from telephoney, which
is being used for higher data rates, and is rapidly becoming the de facto
standard for horizontal wiring specifies the connection between , and
including the outlet and the termination in communication closet horizontal
wiring limited to the maximum of 90m.
2. SHIELDED COPPER
OR STP
Shielded copper is metallic shield or screen sorrounds
the pairs , which may or may not be twisted. The pair can be individually
shielded. A single shield can surround a cable containing multiple pairs
or both techniques can be employed in tandem .shielded copper offers the
advantage of enhanced performance for reduced emissions and reduction of
electromagnetic interference. With the reduction of emissions , the strength of
the signal can be maintained through the confinement of the electromagnetic
field within the conductor i.e signal loss is reduced.
COAXIAL, CABLES –BASEBAND , BROAD BAND
Coaxial cable is a rebust shielded copper wired two
conductor in which a solid centre conductor runs concentrically inside a
solid outer circular conductor. This forms an electromagnetic shield around the
former and that improves signal strength and integrity. Two conductors are
separated by insulation. It comes under the category of a bounded media and is
still an effective medium to use in data communication. It includes
shield for improved performance and is therefore expensive. Example cable tv
and LAN .
Two types of coaxial cables
1. Baseband
It transmit a single signal at a time at very high speed
.the signal on baseband cable must be amplified at a specific distances.
It is used for LAN. In baseband there is transmission of only one signal at a
time over cable. It include frequencies that are equal to or vary near zero by
comparison with the higher frequencies. In analog signal information is first
converted into an electrical signal which is also known as base band electrical
signal. Example thick Ethernet or 10base5.
BROAD BAND
IT CAN TRANSMIT MANY signal using different
frequencies simultaneously . It includes or handles wide range of
frequencies. These frequencies may be divided into frequencies bins or
channels. Broadband ISDN service is a digital service in excess of 1.544 m b
per sec , It is the service of the future. The digital service can be in the
form of frame relay , SMDS, ATM.
BASIC RATE ACCESS as CCIT
IT AFFORDS AN ISDN user with simultaneous access
to two 64 kbps data channels utilizing the conventional twisted pair copper
telephone cable . it includes no of b channels carry data, voice and
other services. BRA is also sometimes called I.420 AS PER CCIT RECOMMENDED
TECHNOLOGY.
OPTIC FIBRE TRANSMISSION
microve transmission
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