TRUYỀN SỐ LIỆU VÀ MẠNG Ch02 data communications

Data Communication and
Networking
Dr. –Ing. Vo Que Son
Email:

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

1


Content
Chapter 1: Medium of PHY Layer
Wired and Wireless Media
Physical layer standards: RS232, RS422, RS485
Line Coding
Digital modulation/demodulation
Channel parameters
Gaussian noise and BER

Chapter 2: Data Communications
Asynchronous transmission
Synchronous transmission
Channel Coding
Data Compression
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

2


Coding schemes
 EBCDIC (Extended Binary Coded Decimal Interchange
Code):
 Invented by IBM
 8-bit character encoding used mainly on IBM mainframe and
IBM midrange computer operating systems.

 ASCII (American Standards Committee for Information
Interchange): defined by ITU-T
 character-encoding scheme originally based on the English
alphabet that encodes 128 specified characters - the numbers 09, the letters a-z and A-Z, some basic punctuation symbols,
some control codes that originated with Teletype machines, and
a blank space - into the 7-bit binary integers.
 ASCII codes represent text in computers, communications
equipment, and other devices that use text. Most modern
character-encoding schemes are based on ASCII, though they
support many additional characters.
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

3

EBCDIC Table

 Data characters

 Control characters

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

4


ASCII Table
 Data characters
 Control characters

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

5

ASCII Table
 ASCII unprintable
characters:

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

6


Network Topology
 Bus

 Point-to point

 Star

 Multipoint

 Ring

 Mesh

Telecomm. Dept.
Faculty of EEE

DCN

HCMUT

7


Communication types
 Simplex communication is
permanent uni-directional
communication (e.g. Radio, TV)
 Half duplex: A half duplex link
can communicate in only one
direction, at a time. Two way
communication is possible, but
not simultaneously. E.g. talkback radio, Citizen Bands radio
 Full duplex communication is
two-way communication
achieved over a physical link that
has the ability to communicate in
both directions simultaneously.
E.g. Telephone system
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

8


Transmission modes

 The transmission of binary data across a link can be
accomplished in either parallel or serial mode. In parallel
mode, multiple bits are sent with each clock tick. In serial
mode, 1 bit is sent with each clock tick. While there is only one
way to send parallel data, there are three subclasses of serial
transmission: asynchronous, synchronous, and isochronous.

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

9


Parallel transmission
 Bits are transmitted in bus at the same time
 High speed over short distance
 Example: PC-printer

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

10

Serial transmission
 Bits are transmitted over one line sequentially
 Low speed over long distance
 Asynchronous and Synchronous transmission

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

11


Timing
 Timing problems require a mechanism to synchronize the
transmitter and receiver
 Timing of bits is the key!
 How can we control the timing?
System A

System B

…11010010

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

12


Asynchronous transmission
 Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the
same.
 Each character of data is treated independently

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

13


Synchronous transmission
 In synchronous transmission, we send bits one after another
without start or stop bits or gaps. It is the responsibility of the
receiver to group the bits.
 For sending large blocks of data
 Control schemes
 Character-oriented
 Bit-oriented

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

14


Asynchronous Transmission
Asynchronous
Data transmitted one character at a time
5 to 8 bits
Timing only needs to be maintained within each character
Transmitter and Receiver clocks need not be in sync
Resynchronize at the beginning of each character

Behavior:
In a steady stream, interval between characters is uniform
• Length of stop element

In idle state
• Receiver looks for transition 1 to 0

Then samples next seven intervals
• Char length

Then looks for next 1 to 0 for next char
Telecomm. Dept.
Faculty of EEE

DCN

HCMUT

15


Asynchronous Transmission
Diagram
Advantages:
Simple
Cheap
Overhead of 2 or 3
bits per character
(~20%)
Good for data with
large gaps (e.g.
keyboard)
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

16


Asynchronous Transmission
 How can the receiver detect bits?
 How can the receiver receive the whole byte?
 How can the receiver receive the whole frame?

Common synchronization techniques
Bit synchronization
Byte synchronization
Frame synchronization

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

17


Asynchronous Transmission
Bit Synchronization
Clock rate at the
receiver is N times
clock rate at
transmitter
When detecting the
start bit transition,
using N/2 and N
counters to sample
bits until the end of
the character

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

18


Asynchronous Transmission
If N is larger, the
sampling is more
precise
Usually, N=16

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

19


Asynchronous Transmission
 Byte synchronization: the configuration of transmission must
be the same at the transmitter and receiver
 Start bit: 1
 Data bits: 4,5,6,7,8
 Parity bit: 0, 1 (even or odd)
 Stop bit: 1, 1.5, 2

Telecomm. Dept.

Faculty of EEE

DCN
HCMUT

20


Asynchronous Transmission
Frame synchronization:
If Blocks of printable characters
Encapsulate complete block between two special (nonprintable) transmission control characters
• STX : Start-of-text
• ETX : End-of-text

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

21


Asynchronous Transmission
If Binary data

e.g., Contents of a file
containing a compiled
program

Use of single ETX not
sufficient to indicate the
end of a file

• One of the bytes might be the
same as an ETX character

Solution: STX and ETX are
preceded by another
transmission control
character
• Data link escape (DLE)

Character or byte stuffing
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

22


Example
 DTE A transmits to DTE B a sequence of characters:

T S L DLE
 The message is transmitted in Asynchronous mode using
RS232 (8-bit data, 1 parity bit, 1 stop bit), R=1200 bps and
characters are in ASCII code

 Show the data frame? (If transmitting: T S L DLE ETX? What is
the transmitted frame?)
 Determine the time to transmit this data frame (ignoring the
processing time) and the effficiency?
DLE STX

T

S

L

DLE DLE DLE ETX

T = (9 x 11)/1200 = 82.5 ms
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

23


Synchronous Transmission
Disadvantages of Asynchronous transmission
Bit synchronization is too difficult when increasing bit
rate
High overhead

Synchronous transmission
Data has been transmitted in blocks
Two approaches
• Character-oriented
• Bit-oriented

Both use the same bit synchronization methods
Major difference is in how they achieve character &
frame synchronization
Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

24


Synchronous Transmission
Bit level: Block of data transmitted without start or
stop bits
Tx and Rx clocks must be synchronized!!!

Option 1 -- Can use separate clock line
Good over short distances
Subject to impairments

Option 2 -- Embed clock signal in data
Manchester encoding
Carrier frequency (analog)

Telecomm. Dept.
Faculty of EEE

DCN
HCMUT

25