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