3. Data Communications

3.1 Basic concepts of data communications and networking

Data Communications System

Transmitter – transmit data to another medium.

Receiver – receive data from a transmitter.

Medium of transfer – the medium for transfer of data.

Data Communications System

Transmitter ReceiverMedium

TV transmission station

Antenna Microwave

Sender RecipientPost Office

Letter by post

TV programme

Communications Model

Source TransmissionsystemTransmitter Receiver Destination

Source System Destination System

Workstation Modem Public TelephoneNetwork

Modem Server

Simplex Transmission Simplex communication means that

communication can only flow in one direction and never flow back the other way.

Data

Half-duplex Transmission Half-duplex data transmission means

that data can be transmitted in both directions on a signal carrier, but not at the same time.

Data

Full-duplex Transmission Full-duplex data transmission means

that data can be transmitted in both directions on a signal carrier at the same time.

Data

Real life examples Simplex transmission

– Pager Half-duplex transmission

– Telephone, facsimile Full-duplex transmission

– Dual Carriageway

Data Transmission Rate Data transmission rate: bps, Kbps,

Mbps bps – bits per second Kbps – kilo-bits per second Mbps – mega-bits per second Bps – bytes per second 1 Byte = 8 bits

Serial Transmission The transfer of discrete signals one after

another. Bits travel sequentially along the same wire. Send information over a single line one bit

at a time, as in modem-to-modem connections.

Parallel Transmission The simultaneous transmission of a

group of bits over separate wires. The transmission of 1 byte (8-bits) with

computers.

Relatively fast Limited distance before data is lost As short as possible (no longer than

15 feet) As the length of cable increases so

does the danger of cross-talk.

Parallel Transmission

Serial Transmission Not as fast as parallel transmission Can transmit data for longer

distances

Asynchronous Transmission

In modem communication, a form of data transmission in which data is sent one character at a time. In addition, a parity bit is usually used for error checking.

Avoid timing problem by not sending long, uninterrupted streams of bits.

Start bitData bits Parity bit

Stop bit

The coding of a typical character sent in asynchronous transmission

Asynchronous Transmission

0

1

Idle state of line

Startbit

Stop

5 to 8 data bits

Odd, even,or unused

Pbit

1 – 2 bit times

Remain idleor next start bit

Character format

Asynchronous Transmission

8-bit asynchronous character stream

1

Startbit

Stopbit

Unpredictable time intervalBetween characters

1 1 1 0 0 0 1 0 0 1 0 1 1 0 0

Stopbit

Startbit

Asynchronous Transmission

Effect of timing error

Transmitter timing

Start

Stop1 2 3 4 5 6 7 8

0

0 100 200 300 400 500 600 700 800

93 186 279 372 465 558 651 744 Receiver timing

Assumptions: data rate of 10 kbps 0.1 ms each bit.The receiver is off by 7% or 0.007 ms per bit-timeThe receiver samples the incoming character every 0.093 ms(based on the transmitter’s clock).

Asynchronous Transmission

Advantages:– simple– cheap

Disadvantages:– requires an overhead of 2 – 3 bits per

character (start and stop bits) (>=20%)– cannot send large blocks or bits between

start and stop bits with great cumulative timing error

Synchronous Transmission

Data transfer in which information transmitted in block (frames) of bits separated by equal time intervals

A block of bits is transmitted in a steady stream without star and stop codes

Synchronous Transmission

Method 1:– Provide a separate clock line between

transmitter and receiver– The other side uses these regular pulses as a

clock– This technique works well over short distances

Method 2:– Embed the clocking information in the data

signal

Synchronous Transmission

8-bitflag

8-bitflag

Controlfields

ControlfieldsData fields

preamble postamble

Synchronous frame format

Synchronous Transmission

Advantage:– For sizable/large blocks of data, synchronous

transmission is far more efficient that asynchronous.

– The control information, preamble, and postamble are typically less than 100 bits.

– E.g. 48 bits of control, preamble, and postamble with 1000-character block of data, each frame consists of 48 bits of overhead and 8000 bits of data, so % overhead = 48/8048 x 100% = 0.6%

Data Transfer Directed from PC to PC

Direct Cable Connection– A null modem cable allows you to

connect your PC to another nearby PC or serial device using its modem protocol.

– A null modem cable is limited to 30 feet in length.

– A null modem cable is sometimes called crossover cable.

Crossover Cable A crossover cable is a cable that

is used to interconnect two computers by "crossing over" (reversing) their respective pin contacts.

Either an RS-232C or an registered jack (e.g. RJ-45) connection is possible.

Diagrams on RJ-11 and RJ-45 Interfaces

RS-232C Your computer modem uses one

of your PC's serial connections or COM ports.

Serial communication between your PC and the modem and other serial devices adheres to the RS-232C (Recommended Standard-232 Current Version) standard.

RS-232C

Pin Settings for Plug(Reverse Order for Socket)

1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23 24 25

25 pins

RS-232C

Pin Settings for Plug(Reverse Order for Socket)

1 2 3 4 5

14 15 16 17

9 pins

RS-232C1 – PG Protective ground2 – TD Transmitted data (3 for 9-pin)3 – RD Received data (2 for 9-pin)4 – RTS Request to send (7 for 9-pin)5 – CTS Clear to send (8 for 9-pin)6 – DSR Data set ready (6 for 9-pin)7 – SG Signal ground (5 for 9-pin)8 – CD Carrier detect (1 for 9-pin)9 – + voltage (testing)10 – - voltage (testing)11 – 12 – SCD Secondary CD13 – SCS Secondary CTS

RS-232C14 – STD Secondary TD15 – TC Transmit clock16 – SRD Secondary RD17 – RC Receiver clock18 – 19 – SRS Secondary RTS20 – DTR Data terminal ready (4 for 9-pin)21 – SQD Signal quality detector22 – RI Ring indicator (9 for 9-pin)23 – DRS Data rate select24 – XTC External clock25 –

Possible reasons to connect two computers directly to

each other

Playing a game competitively (one person at each computer) with fast response time

Testing one computer by examining its behavior at the other computer

Saving the cost of a hub when you want to interconnect two devices in the same home or office

Data Modem

Modem – Modulator and Demodulator

Modulator – convert digital signal (data in PC) to analogue signal (data via telephone line)Demodulator – convert analogue signal to digital signal

Data Modem

Digital signal

Modem

Analogue signal

PC

Public TelephoneNetwork

Analogue signal

Digital signal

Demodulation

ModemPC

Modulation

Data Modem Baud Rate

– This refers to the number of signals per one second transmitted

Bit Rate– The bit rate is multiplied by the bits per

signal

Sources of errors during data transmission

Attenuation– Signal grows weak over distance

White noise– Caused by molecular movement

Impulse noise– Caused by electrical interference

Cross-talk– Caused by interference from adjacent lines

DCE and DTE DTE

– Data Terminal Equipment which is the ultimate source or final destination of data messages

DCE– Data Circuit-Terminating Equipment

which connects the DTE to the communication circuits

Abbreviations found in an External Modem

HS– The High Speed light

indicates that your modem is currently operating at its highest available transmission rate

AA– The Auto Answer light

indicates that your modem will automatically answer any incoming calls. This features allows access to your system while it is unattended

CD– The Carrier Detect light

goes on when your modem has successfully made a connection with a remote computer

OH– The Off-Hook light goes

on whenever your modem takes control of the phone line

Abbreviations found in an External Modem

RD– The Receive Data light

flickers each time the modem receives data from the remote computer

SD– The Send Data light

flashes whenever the modem sends data to the remote computer

TR– The Terminal Ready light

goes on when the modem detects a DTR (Data Terminal Ready) signal from your communications software. This signal informs your modem that a communications program is loaded and ready to run

MR– The Modem Ready light

lets you know that your modem is turned on and ready to operate

Bandwidth The capacity at which you can transfer

data is called bandwidth Typical telephone line: 33,600 kilobits

per second (33.6 Kbps) Cable TV: 10 megabits per second (10

Mbps) – almost 300 times the capacity of the normal phone connection

Bandwidth Frequency

– Frequency is measured in the number of cycles of change per second, or hertz (Hz).

Latency– In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another.

Bandwidth

(wave length)

f = 1 /

Bandwidth

High frequencyWide bandwidth

Bandwidth

Low frequencyNarrow bandwidth

Bandwidth Bandwidth is used to mean

– How fast data flows on a given transmission path

– The width of the range of frequencies that an electronic signal occupies on a given transmission medium

Any digital and analog signal has a bandwidth

Bandwidth Voice-grade phone line

– Range of freq.: 300 to 3000 Hz– Bandwidth = 3000 – 300 = 2700

Human voice– Range of freq.: 20 to 20000 Hz– Bandwidth = 19980

E.g. Video transmission times

Pixels per screen (640*480) 307200

Bytes per pixel 8

Total bits per screen 2457600

Frames per second 30

Total bits per second 73728000