Transcript Chapter 4 TRANSMISSION MODE

CHAPTER 4
TRANSMISSION MODE & TRANSMISSION
FUNCTION
TRANSMISSION DIRECTION
 The flow of data must be controlled in some way so
that the sending and receiving stations know when
they will receive data or when they should send data.
 Simplex Transmission
 Half-Duplex Transmission
 Full-Duplex Transmission
Simplex Transmission
 A simplex connection is a connection in which the
data flows in only one direction, from the transmitter
to the receiver. This type of connection is useful if the
data do not need to flow in both directions (for
example, from your computer to the printer or from
the mouse to your computer
HALF- DUPLEX
 A half-duplex connection (sometimes called an
alternating connection or semi-duplex) is a connection
in which the data flows in one direction or the other,
but not both at the same time. With this type of
connection, each end of the connection transmits in
turn. This type of connection makes it possible to have
bidirectional communications using the full capacity
of the line.
FULL- DUPLEX
 A full-duplex connection is a connection in which
the data flow in both directions simultaneously. Each
end of the line can thus transmit and receive at the
same time, which means that the bandwidth is divided
in two for each direction of data transmission if the
same transmission medium is used for both directions
of transmission.
Multiplexer & Demultiplexer
 Multiplexer : device that receive the input signals from
several devices, combines them into a single stream of
data, and then transmits the data over a single
communication line.
 Demultiplexer: recovers the input signals and presents
them as separate signals again.
Frequency Division Multiplexing
 Several signals are joined for transmission on one
communication line or channel
 The bandwidth at the entire communications line is
divided into narrower bandwidth, or channels so each
client on the line can use one frequency range to
transmit data
 * A guardband is a range of frequency that
prevents the current containing the data from
one transmission from interfering with
another transmission current on the same line.
Time Division Multiplexing
 TDM divides the transmission time slot into time
segment
 Multiplexer repeatedly transmits a fixed sequence of
time slots over a single transmission channels.
 Each client is assigned a fixed time slot in the rotation
during which it can transmit data, and the client is
given the full transmission capacity of the line for that
given time.
Time Division Multiplexing (cont.)
 A major difference between TDM and FDM is that,
instead of using frequency guardband to separate the
signal, TDM uses “ times” to separate the signal
 If a client has no data to transmit during its fixed time
slot, the transmission line remains idle during that
time, and no other client can use that time slot. For
most application, client do not transmit continuously.
In this case, TDM means some section of
transmission line always idle.
Statistical Time Division
Multiplexing
 Time slots are assigned to client on the network
dynamically.
 The difference is that the time slots is not fixed.
 TDM is driven by time, STDM is driven by data.
Wavelength Division Multiplexing
 A type multiplexing developed for both digital signal
and analog transmission on fibre optic cables.
 In WDM, each signal is assigned to a particular
wavelength on an optical fiber communication line.
 By using different frequency within optical
bandwidth, many different transmission can exist on
same individually cladded optical fibre
Frequency Division Multiple Access
 Wireless communication also make use of
multiplexing systems to allow multiple access with the
potential for multiple users to access any available
cellular frequency or channel.
 More efficient and cost effective than reserving a
unique frequency for each subscriber.
 The oldest multiple access technology and the most
commonly used for analog cellular communication.
Frequency Division Multiple
Access(Cont.)
 In other words, if cellular subscriber is using a
particular frequency for a call, no other subscriber can
use the same frequency to make different call until
first call is terminated, or until the first caller moves
into different cell. Others callers still can transmit on
other available channels, however.
Time Division Multiple Access
 In order to increase efficiency of digital cellular systems
used today, TDMA methodology was developed.
 TDMA operates in a manner similar to TDM, except that it
is was designed for cellular radio, rather than for guided
media.
 With TDMA, the radio signal is divided into time slots to
increase the amount of data that can be carried and allow
multiple users to access the same frequency
simultaneously. Although TDMA enables only three users
within a given a cell to use the same frequency, the added
capacity makes the system cost-efficient.
Code Division Multiple Access
 A more recent development, used in second generation (2G) and third
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generation (3G) wireless technology.
CDMA assigns each conversation a unique code.
Both the mobile telephone and the communication systems at the
base station use this code to identify signals that are part of the
conversation.
Used direct sequence spread spectrum technology, meaning that the
original signal is spread over several frequency simultaneously.
Although CDMA does have some capacity limitations, no maximum
number of calls per single, wide bandwidth channel is established.
The number of calls that a channel can accommodate is based on the
total bandwidth of the channel, the required geographical coverage
and the quality of sound.
Modulation
• modulation is the process of varying one or more
properties of a high-frequency periodic waveform,
called the carrier signal, with a modulating
signal which typically contains information to be
transmitted
• In telecommunications, modulation is the process of
conveying a message signal, for example a digital bit
stream or an analog audio signal, inside another signal
that can be physically transmitted
Analog Modulation
• Analog data is transmitted on any communications
medium by means of an electrical signal called carrier
signal.
• The change made to the signal is called signal
modulation, or modulation.
• Three types of modulation are
– Amplitude Modulation (AM)
– Frequency Modulation (FM)
– Phase Modulation (PM)
Amplitude Modulation (AM)
 refers to a change in the height of the carrier wave for
analog signals.
 When data is sent to the carrier signal, the height of
the signal is changed to indicate a 0 bit or a 1 bit.
 The highest peaks of the signal represent a 1 bit, and
the lowest peak represents a 0 bit.
Frequency Modulation (FM)
 Refers to the number of waved used to represent a
single cycle.
 The number of waves changes for a 0 bit or a 1 bit; this
change in frequency is indicated by a difference in the
tone of the signal.
 A higher tone indicates more waves per unit of time
and a lower tone indicate fewer waves per unit of time.
Phase Modulation
 A phase shift occurs to indicate a change in the type of
bits being transmitted.
 To indicate a 1 bit, the wave moves in a specific
direction. When a 0 bit is detected, the direction of the
wave changes 180O
Digital Modulation
 Digital data must be converted to an analog form to be
transmitted on an analog transmission medium
Using Analog Signal to Transmit Digital
Data
• When amplitude modulations converts digital signals
into a form that can be transmitted on an analog line,
“Amplitude-shift keying (ASK)”
• When digital signals modulate using the frequency
modulation technique, “Frequency-shift keying
(FSK)”
• Digital data that changes using phase modulation is
called “Phase-shift keying (PSK)”.
• Another type of digital phase modulation is called
“Differential phase-shift keying (DPSK)”.
Using Digital Signals to Transmit
Analog Data
• The most common form today – Pulse Code
modulation.
– PAM is performed on the data to get a sampled signal.
– Then the signal is quantized (assigning integer values
within a range to the samples).
– The quantization assigns values of 0 to 255, depending
on the amplitude of the signal at the instant that it is
sampled.
– The values then are converted to their binary equivalent,
and the digital signal thus is encoded.
Using Digital Signals to Transmit Digital
Data
 To transmit the 1s and 0s generated by the computer,
even in a digital form, the bit must be changed into a
digital signal.
 Three types of conversion for digital signals
 Unipolar
 Polar
 Bipolar
Synchronous transmission
 The term synchronous is used to describe a continuous and consistent
timed transfer of data blocks.
 These types of connections are used when large amounts of data must
be transferred very quickly from one location to the other. The speed of
the synchronous connection is attained by transferring data in large
blocks instead of individual characters.
 The data blocks are grouped and spaced in regular intervals and are
preceded by special characters called syn or synchronous idle
characters. See the following illustration.
 Figure 1. Synchronous transmission
Synchronous transmission (Cont.)
 After the syn characters are received by the remote device, they are decoded
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and used to synchronize the connection.
After the connection is correctly synchronized, data transmission may begin.
An analogy of this type of connection would be the transmission of a large text
document. Before the document is transferred across the synchronous line, it is
first broken into blocks of sentences or paragraphs. The blocks are then sent
over the communication link to the remote site. With other transmission
modes, the text is organized into long strings of letters (or characters) that
make up the words within the sentences and paragraphs.
These characters are sent over the communication link one at a time and
reassembled at the remote location.
The timing needed for synchronous connections is obtained from the devices
located on the communication link. All devices on the synchronous link must
be set to the same clocking.
Synchronous transmission (Cont.)
 The timing needed for synchronous connections is obtained from the
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devices located on the communication link. All devices on the
synchronous link must be set to the same clocking.
The following is a list of characteristics specific to synchronous
communication:
There are no gaps between characters being transmitted.
Timing is supplied by modems or other devices at each end of the
connection.
Special syn characters precede the data being transmitted.
The syn characters are used between blocks of data for timing
purposes.