1. Concept and Model of Communications
Concept and Model of Communications
General Communications: face-to-face conversation, write a letter, etc.
Electronic Communications: telephone, wireless phone, TV, radar, etc.
Our Focus  Computer Communication
General Communication Model
S(t) T(t) Transmission Tr(t) Sd(t)
Source Transmitter Receiver Destination
System
Microphone Transformer Line/Cable Transformer Speaker
Telephone Encoder Fiber/Air Decoder Earphone
Computer Compress Satellite Uncompress Computer
Scanner Modulator Network Demodulator Printer
Basic Communication Criteria: Performance, Reliability, Security

2. Simplified Communications Model (2)
Source
generates data to be transmitted
Transmitter
converts data into transmittable signals
Transmission System
carries data
Receiver
converts received signal into data
Destination
takes incoming data from the receiver

3. Components of a data communications system
l.Message The message is the information (data) to be communicated.
Popular forms of information include text, numbers, pictures, audio,
and video.
2. Sender The sender is the device that sends the data message.
It can be a computer, workstation, telephone handset, video camera.
3. Receiver The receiver is the device that receives the message.
It can be a computer, workstation, telephone handset, television.
4. Transmission medium The transmission medium is the physical path
by which a message travels from sender to receiver.
Some examples of transmission media include twisted-pair wire,
coaxial cable, fiber-optic cable, and radio waves.
5. Protocol A protocol is a set of rules that govern data communications.
It represents an agreement between the communicating devices.
Without a protocol, two devices may be connected but not
communicating, just as a person speaking French cannot be understood
by a person who speaks only Japanese

4. Transmission Media
Transmission Media
A transmission medium: - a connection between a sender and a receiver
- a signal can pass but with attenuation/distortion
- a special system with a transmission bandwidth
Guided (Wired) Media Unguided (Wireless) Media
(lines) (air, vacuum, water, etc.)
- Twisted pair (0~10MHz) - LF (30~300KHz, Navigation)
- Coaxial cable (100K~500MHz) - MF/HF (300~3000KHz, AM/SW radio)
- Optical fiber (180~370THz) - VHF (30~300MHz, TV & FM radio)
- UHF (0.3~3GHz, TV, mobile phone)
- SHF (3~30GHz, satellite, microwave)
- EHF (30~300GHz, experimental com)
- Infrared (no frequency allocation)

5. What is Data Communications?
Exchange of digital information between two
digital devices is data communication

6. Data Transmission
Data transmission is the transfer of data from
point-to-point often represented as an
electromagnetic signal over a physical point-to-
point or point-to-multipoint communication
channel
A communication channel refers to the
medium used to convey information from a
sender (or transmitter) to a receiver, and it can
use fully or partially the medium.
Examples of channels: copper wires, optical
fibbers or wireless communication channels.

7. Requirements of Data Communications
 At least Two Devices ready to communicate
 A Transmission Medium
 A set of Rules & Procedure for proper
communication (Protocol)
 Standard Data Representation
 Transmission of bits either Serial or Parallel

8. case of Asynchronous Transmission
Bit synchronisation using Start/stop bits in
In Synchronous Transmission the agreed
pattern of Flag
Signal encoding rules viz. NRZ or RZ
And other higher layer protocol

9. Data Representations
A group of bits are used to represent a
character/number/special symbol/Control
Characters
5-bit code can represent 32 symbols (25=32)
7-bit code can represent 128 symbols (27=128)
8-bit code can represent 256 symbols (28=256)

10. Code Set
A code set is the set of codes representing the
symbols
Very common code sets are :
– ASCII : this is ANSI’s 7-bit American
Standard Code for Information Interchange
ASCII code(7-bit) is often used with an 8th bit
known as parity bit used for detecting errors
during Data Transmission

11. Parity bit is added to the Most Significant bit
(MSB)
– EBCDIC : this is IBM’s 8-bit Extended
Binary Coded Decimal Interchange Code

12. ASCII Code
ASCII is defined in ANSI X3.4
– Corresponding CCITT recommendation is
IA5 (International Alphabet No.5)
– ISO specification is ISO 646
Total 128 codes
– 96 codes are graphic symbols (in row. 2~7 in code
chart).
 94 codes are printable

13.  And 2 codes viz. SPACE & DEL characters are non printable
– 32 codes control symbols (row. 0 & 1 in code chart)
 All are non printable

16. Parallel Transmission and Serial Transmission
Parallel Transmission and Serial Transmission
…011000110111010111…
Segment the 0/1 ?
stream into Sender Receiver
N bits groups
N N N N
… 01…00 01…10 11…10 10…11 …
Parallel Transmission Serial Transmission
0 0 0
1 1 1
1 1 0110001 1
0 Sender 0 0 Receiver
Sender Receiver
0 0 0
0 0 0
1 1 1
P/S converter S/P converter
7 (N) bits are sent together 7 (N) bits are sent one after another
7 (N) lines are needed Only 1 line is needed

17. Parallel Transmission
 Parallel transmission allows transfers of multiple data bits at the same
time over separate media
 In general, parallel transmission is used with a wired medium that uses
multiple, independent wires
 Furthermore, the signals on all wires are synchronized so that a bit
travels across each of the wires at precisely the same time
 Engineers use the term parallel to characterize the wiring
17

18. Parallel Transmission
The figure omits two important details:
(1) In addition to the parallel wires that each carry data, a parallel
interface usually contains other wires that allow the sender
and receiver to coordinate
(2) To make installation and troubleshooting easy, the wires for a
parallel transmission system are placed in a single physical
cable
A parallel mode of transmission has two chief advantages:
(1) High speed: it can send N bits at the same time
 a parallel interface can operate N times faster than an equivalent serial
interface
(2) Match to underlying hardware: Internally, computer and
communication hardware uses parallel circuitry
 a parallel interface matches the internal hardware well

19. Serial Transmission
 Serial transmission
 sends one bit at a time
It may seem that anyone would choose parallel transmission
for high speeds
 However, most communication systems use serial mode
There are two main reasons
(1)serial networks can be extended over long distances at less cost
(2)using only one physical wire means that there is never a timing
problem caused by one wire being slightly longer than another
Sender and receiver must contain a hardware that converts
data from the parallel form used in the device to the serial
form used on the wire

20. Serial Transmission
The hardware needed to convert data between an
internal parallel form and a serial form can be
straightforward or complex
In the simplest case, a single chip that is known as a
Universal Asynchronous Receiver and Transmitter
(UART) performs the conversion
A related chip, Universal Synchronous-Asynchronous
Receiver and Transmitter (USART) handles conversion
for synchronous networks

21. Timing of Serial Transmission
Serial transmission mechanisms can be divided into
three broad categories (depending on how transmissions
are spaced in time):
 Asynchronous transmission can occur at any time
with an arbitrary delay between the transmission of two
data items
 Synchronous transmission occurs continuously
with no gap between the transmission of two data items
 Isochronous transmission occurs at regular intervals
with a fixed gap between the transmission of two data
items
21

22. Asynchronous Transmission
 It is asynchronous if the system allows the physical medium to be idle for
an arbitrary time between two transmissions
 The asynchronous style of communication is well-suited to applications
that generate data at random
 (e.g., a user typing on a keyboard or a user that clicks on a link)
 The disadvantage of asynchrony arises from the lack of coordination
between sender and receiver
 While the medium is idle, a receiver cannot know how long the medium will
remain idle before more data arrives
 Asynchronous technologies usually arrange for a sender to transmit a few
extra bits before each data item
 to inform the receiver that a data transfer is starting
 extra bits allow the receiver to synchronize with the incoming signal
 the extra bits are known as a preamble or start bits

25. Synchronous Transmission
A synchronous mechanism transmits bits of data continually
 with no idle time between bits
 after transmitting the final bit of one data byte, the sender transmits a bit
of the next data byte
The sender and receiver constantly remain synchronized
 which means less synchronization overhead
On a synchronous system
 each character is sent without start or stop bits
 Synchronous transmission:
 A bit stream is segmented into relative large groups/blocks many
characters or bytes
 Add control bits at the beginning and end of each block
 Frame = H_control_bits + characters (data_bits) + T_control_bits
 No gap between two characters in a data block
25

26. Asynchronous and Synchronous Transmission
Asynchronous and Synchronous Transmission
1 0110001 0 1 1001100 0 1 0011101 0 1 1011100 0
Sender Receiver
independent
Stopwtch.ani Stopwtch.ani
Con_bits 0110001
... 0110001 1001100 0011101 1011100 Con_bits
Sender Receiver
synchronized
Stopwtch.ani

27. Asynchronous Serial Transmission
(RS232 Example)
 Because no signal lines are used to convey clock (timing) information, this method
groups data together into a sequence of bits (five to eight), then prefixes them with a
start bit and a stop bit. This is the method most widely used for PC or simple terminal
serial communications.
 In asynchronous serial communication, the electrical interface is held in the mark
position between characters. The start of transmission of a character is signaled by a
drop in signal level to the space level. At this point, the receiver starts its clock. After
one bit time (the start bit) come 8 bits of true data followed by one or more stop bits at
the mark level.
 The receiver tries to sample the signal in the middle of each bit time. The byte will be
read correctly if the line is still in the intended state when the last stop bit is read.
 Thus the transmitter and receiver only have to have approximately the same clock
rate. A little arithmetic will show that for a 10 bit sequence, the last bit will be
interpreted correctly even if the sender and receiver clocks differ by as much as 5%.
 It is relatively simple, and therefore inexpensive. However, it has a high overhead,
in that each byte carries at least two extra bits: a 20% loss of line bandwidth.

28. Synchronous Serial Transmission (PS2 Example)
 The PS/2 mouse and keyboard implement a bidirectional synchronous serial protocol.
 The bus is "idle" when both lines are high (open-collector). This is the only state where
the keyboard/mouse is allowed begin transmitting data. The host has ultimate control
over the bus and may inhibit communication at any time by pulling the Clock line low.
 The device (slave) always generates the clock signal. If the host wants to send data, it
must first inhibit communication from the device by pulling Clock low. The host then
pulls Data low and releases Clock. This is the "Request-to-Send" state and signals the
device to start generating clock pulses.
 Summary: Bus States
Data = high, Clock = high: Idle state. Data is transmited 1 byte at a time:
Data = high, Clock = low: Communication Inhibited. •1 start bit. This is always 0.
Data = low, Clock = high: Host Request-to-Send
•8 data bits, least significant bit first.
•1 parity bit (odd parity - The number of 1's
in the data bits plus the parity bit always add
up to an odd number. This is used for error
detection.).
•1 stop bit. This is always 1.
•1 acknowledge bit (host-to-device
communication only)

29. Simplex Transmission and Duplex
Simplex Transmission and Duplex
Transmission
Transmission
Direction of data
Simplex Device A Device B
Transmission
One can send and the other can receive
Direction of data at time 1
Half Duplex Device A Device B
Transmission
Direction of data at time 2
Both can send and receive but in different time
Direction of data all the time
Full Duplex Device A Device B
Transmission
Both can send and receive simultaneously

30. Simplex
In simplex mode, the communication is unidirectional.
Only one of the two devices on a link can transmit; the other can only
receive
Keyboards and traditional monitors are examples of simplex devices
key-board can only introduce input; the monitor can only accept output.
The simplex mode can use the entire capacity of the channel to send data
in one direction

31. Half-duplex
In half-duplex mode, each station can both transmit and receive, but
not at the same time.
When one device is sending, the other can only receive, and vice versa
ln a half-duplex transmission, the entire capacity of a channel is taken
over by whichever of the two devices is transmitting at the time.
In half-duplex, the entire capacity of the channel is taken over by the
transmitting (sending).
Walkie-talkies and CB (citizens band) radios are both half-duplex
systems

32. Full-duplex
In full-duplex mode both stations can transmit and receive
simultaneously
In full-duplex mode, signals going in one direction share the capacity of
the link with signals going in the other direction.
This sharing can occur in two ways: either the link must contain two
physically separate transmission paths, one for sending and the other
for receiving; or the capacity of the channel is divided between signals
traveling in both directions.
One common example of full-duplex communication is the telephone
network. When two people are communicating by a telephone line, both
can talk and listen at the same time

33. Communication Standards and Related Organizations
Communication Standards and Related Organizations
Communications need standards for inter-operations of different devices
Standard Organizations:
- ISO (International Standards Organization): ISO number
- ITU (International Telecommunication Union): V.num & X.num
- EIA (Electronic Industries Association): EIA-num
- IEEE (Institute of Electronics Engineers): IEEE.num
- ANSI (American National Standards Institute): ASCII, etc.
- IETF (Internet Society and Internet Engineering Task Force): RFC num
- W3C (World Wide Web Consortium): HTTP, HTML, XML, …
- WAP Forum (Wireless Application Protocol): WAP-num