1. 3.1 Basic concepts of data communications and networking

2. Data Communications System
Transmitter – transmit data to another medium.
Receiver – receive data from a transmitter.
Medium of transfer – the medium for transfer of data.

3. Data Communications System
Transmitter
Receiver
Medium
Sender
Recipient
Post Office
Letter by post
TV programme
TV transmission
station
Antenna
Microwave

4. Communications Model Source Receiver Transmitter Destination
Transmission
system
Transmitter
Receiver
Destination
Source System
Destination System
Workstation
Modem
Public Telephone
Network
Server

5. Simplex Transmission
Simplex communication means that communication can only flow in one direction and never flow back the other way.
Data

6. 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

7. 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

8. Real life examples Simplex transmission Half-duplex transmission
Pager
Half-duplex transmission
Telephone, facsimile
Full-duplex transmission
Dual Carriageway

9. 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

10. 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.

11. Parallel Transmission
The simultaneous transmission of a group of bits over separate wires.
The transmission of 1 byte (8-bits) with computers.

12. Parallel Transmission
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.

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

14. 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 bit
Data bits
Parity bit
Stop bit
The coding of a typical character sent in asynchronous transmission

15. Asynchronous Transmission1
Idle state
of line
Start
bit
Stop
5 to 8 data bits
Odd, even,
or unused P
1 – 2 bit times
Remain idle
or next start bit
Character format

16. Asynchronous Transmission1
Start
bit
Stop
Unpredictable time interval
Between characters
8-bit asynchronous character stream

17. Asynchronous Transmission
Transmitter timing
Start
Stop 1 2 3 4 5 6 7 8
100
200
300
400
500
600
700
800 93
186
279
372
465
558
651
744
Receiver timing
Effect of timing error
Assumptions: data rate of 10 kbps  0.1 ms each bit.
The receiver is off by 7% or ms per bit-time
The receiver samples the incoming character every ms
(based on the transmitter’s clock).

18. 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

19. 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

20. 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

21. Synchronous Transmission
8-bit
flag
Control
fields
Data fields
preamble
postamble
Synchronous frame format

22. 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%

23. 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.

24. 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.

25. Diagrams on RJ-11 and RJ-45 Interfaces

26. 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.

27. (Reverse Order for Socket)
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

28. (Reverse Order for Socket)
RS-232C
Pin Settings for Plug
(Reverse Order for Socket) 1 2 3 4 5 14 15 16 17
9 pins

29. RS-232C 1 – PG Protective ground 2 – 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 CD
13 – SCS Secondary CTS

30. RS-232C 14 – STD Secondary TD 15 – TC Transmit clock
16 – SRD Secondary RD
17 – RC Receiver clock
18 –
19 – SRS Secondary RTS
20 – DTR Data terminal ready (4 for 9-pin)
21 – SQD Signal quality detector
22 – RI Ring indicator (9 for 9-pin)
23 – DRS Data rate select
24 – XTC External clock
25 –

31. 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

32. 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

33. Data Modem Modulation Demodulation Digital signal Analogue signal PC
Public Telephone
Network PC
Modem
Digital signal
Analogue signal
Demodulation

34. Data Modem Baud Rate Bit Rate
This refers to the number of signals per one second transmitted
Bit Rate
The bit rate is multiplied by the bits per signal

35. 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

36. 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

37. Abbreviations found in an External ModemHS
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

38. Abbreviations found in an External ModemRD
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

39. 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

40. Bandwidth Frequency Latency
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.

41. Bandwidth
f = 1 /  
(wave length)

42. Bandwidth
High frequency
Wide bandwidth

43. Bandwidth
Low frequency
Narrow bandwidth

44. 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

45. Bandwidth Voice-grade phone line Human voice
Range of freq.: 300 to 3000 Hz
Bandwidth = 3000 – 300 = 2700
Human voice
Range of freq.: 20 to Hz
Bandwidth = 19980

46. E.g. Video transmission times
Pixels per screen (640*480)
Bytes per pixel
Total bits per screen
Frames per second
Total bits per second