5. DATA TRANSMISSION MEDIA
1. Twisted pair wire or unshielded twisted pair(UTP) used in local
telephone communication and short distance up to 1 km for digital data
transmission
• Normally used to connect terminals to the main computer
• Data transmission speed of up to 9600 bits/sec , if the distance is not
more than 100 meters
• Inexpensive medium
• Easy to install and use
• Easily pick up noise signals when length extends 100 meters
2. Coaxial cable: capable of transmitting digital signals at rates of 10 mbps
• They are extensively used in long distance telephone lines and as cables
for cable TV
• Higher noise immunity
8. 5. Optical fibers
Electrical signal Optical fiber Electrical
signal
Electric to light Light to electrical
wave converter wave converter
amplifier
Light waves
9. MODEMS
• The process of modulation and demodulation is carried out by a special
device called a modem
• Modulation: conversion of digital data to analog
• Demodulation: conversion of analog to digital
Analog signals on Digital
Digital
telephone lines signals
signals
0 0
Modulator Demodulator 1 1
Terminal 0 1 1 0 Computer
Demodulator Modulator 0 0
0 0 0 1
0 1
A modem at A modem at
terminal end computer end
11. DATA TRANSMISSION SERVICES
• Dial‐up line
• Leased line
• Integrated services digital network
• Value added network
12. TRANSMISSION MODES
1. ASYNCHRONOUS TRANSMISSION: data is transmitted character by
character at regular intervals i.e A sender can send a character at any
time and the receiver will accept it.
• In order to enable the receiver to recognize a character when it arrives a
transmitter frames each character: Preceding a character a start bit and
following a character one or two stop bits
• The time between transmitting any two characters can be zero or of any
length.
• Best suited to key board type terminals, printers
• Advantage is this does not require any local storage at the terminal
,because transmission takes place character by character. So it is cheaper
to implement
• Since the transmission line is idle during the time intervals between
transmitting characters , the idle time of transmitting line may be a matter
of concern, if the line cost is high
13. Each character framed
Irregular time intervals
by start bit and stop bits
between two characters
character character character
14. 2. SYNCHRONOUS TRANSMISSION
• It involves blocking a group of characters .
• Each block is framed by header and trailer information
• Header contains synchronization information and information to identify
the sender and receiver
• Following the header is a block of characters which contains actual
message to be transmitted
• The message characters in the block are terminated by a trailer
• The trailer contains an end of message followed by a check character to
aid detection of any transmission error
• Its advantage is its efficiency.
• No need of individual start and stop bits and gives higher data rates
• Its disadvantage is need of local storage at the two ends of line and need
of synchronized clock at both ends
15. indefinite time interval
between two blocks of
data
char char trailer char char
T
Header containing
Trailer containing
synchronizing and other
end of character
information
16. SWITCHING TECHNIQUES
CIRCUIT SWITCHING:
ADVANTAGES:
1. Once the circuit is established data is transmitted with no delay
2. It is suitable for low speed communication between a terminal and a computers
the method is simple and requires no special facilities
3. Because of a dedicated path ,it is suitable for long continuous transmission
4. Full capacity of the circuit is available for exclusive use by the connected pair of
nodes, the transmission time required to send a message can be known
DISADVANTAGES
1. Since the physical link is dedicated one ,the channel capacity is dedicated to one
source for the entire duration of a connection. So network resources are not
properly used
2. Is very uneconomical since communication between computers occur in short
fast bursts with long silent periods
3. Before actual data transfer the time required to establish a physical link between
the two stations is of the order 10 secs or more, depending on the distance. For
many computer applications this time elapse is too large and unsuitable
18. D
Broadcast method
5
A 1 4 B
3
2 1 2 3 -------- N
C
message
Store and forward Broadcast channel
method
19. Packet switching
ADVANTAGES
1. Packets are of small and fixed size. Hence the storage requirement for
buffering the packets at intermediate nodes is minimal
2. Since the routing is done on packet basis, the intermediate nodes do not
have to wait for the entire message. Hence the transmission is very fast
3. The method is fast enough for interactive/real time applications
4. Is used in wide area networks
DISADVANTAGES
1. Due to the need to buffer each packet at every at intermediate node ,
and to reassemble the packet at the destination node ,the overhead
incurred per packet is large
2. There is no guarantee of how long it takes a message to go from its
source to its destination node, because the time taken for each packet
depends on the route chosen for that packet ,along with the volume of
data being transferred along that route
21. NETWORKING TOPOLOGIES
• The term topology refers to the way in which the nodes of a network are
linked together
• It determines the data paths which may be used between any pair of
nodes in the network
The four major topologies are:
1. STAR NETWORK:
Multiple computers are connected to the
Host
host computer computer
The computers in the network are not linked
directly to each other and can communicate only via host
computer
The routing function is performed by the host
computer , which centrally controls communication
between any two other computers by establishing a logical path between
them
22. 2.RING NETWORK
• Each computer has communicating subordinates, but there is no master
computer for controlling other computers
• A node receives data from one of its two adjacent nodes
• The only decision a node has to take is whether
the data is for its own use. If it is addressed
to it, it utilizes it else passes it on to the
next node
• More reliable than star network as communication
is not dependent on a single host
23. 3.MESH NETWORK
• It has a separate physical link for connecting
each node to any other node
• Each computer has a point‐to‐point link with
all other computers
24. BUS NETWORK
• A single transmission media is shared by all nodes
• When a computer wants to send a message to another computer
it appends destination address to the message and checks
whether the communication line is free or not
• As soon as the line gets free it
broadcasts the message on the line
• As the message travels the line
each computer checks whether it is
addressed to it
• The message is picked up by
Single communication line shared by all
the addressee computer which sends an
nodes
acknowledgement to the source
computer and frees the line.
• Also known as multidrop or multipoint
or broadcasting network
• It is appropriate for use in LAN where a high speed communication channel is used
and when satellite communication is used (one satellite channel may be shared by
many computers at a time)
25. HYBRID NETWORK
• The exact configuration of the network depends on the needs and overall
organization structure of the company involved
mesh
ring
star
27. Network linking devices
• Hubs: not a fast connection because it broadcasts the packets it receives
to all nodes attached to its ports.
• Bridges : it is used to connect two LANS or two segments of the same LAN.
• It looks at the information in each packet header and forwards data that is
travelling from one LAN to another
• Switches: it is a device that learns which machine is connected to its port
by using the PC, printer or other devices
• It reduces the amount of broadcast traffic and is currently the most
popular network linking device
• Modern switches can even function as routers , allowing multiple LANs to
be interconnected by linking each LAN ‘s switches together. This is called
an UPLINK
• Routers: it is a complicated device that stores the routing information for
networks
• It determines the best routes for the packet
• These are located as gateways, the places where two or more networks
connect
• It include security options, filters data
