FUNDAMENTAL CONCEPTS OF DATA COMMUNICATIONS (c22 Tomasi)

CHAPTER 22

FUNDAMENTAL CONCEPTS OF DATA COMMUNICATIONS

1)Often used to represent characters and symbols such as letters, digits and punctuation marks.

Another terms:

· character codes

· character sets

· symbol codes

· character languages

Data Communications Codes

2)Sometimes called the Telex Code, was the first fixed length character developed for machines rather than forpeople.

Baudot Code

3)A French postal engineer who developed the baudot code in 1875 and named after Emile Baudot, an early pioneer in telegraph printing.

Thomas Murray

4)Fixed-length source code.

Fixed Length Block Code

5)Stands for United States of America Standard Code for Information Exchange, better known as ASCII-63.

USASCII

6)The standard character set for source coding the alphanumeric character set that humans understand but computers do not (computers only understand 1s and 0s).

It is a seven bit fixed-length character set.

ASCII

7)Fixed-length source code.

Fixed Length Block Code

8)Extended binary coded decimal interchange-code, an eight bit fixed length character developed in 1962 by International Business Machines Corporation.

EBCDIC

9)Omnipresent block and white striped stickers that seem to appear or virtually every consumer item in the US and most of the rest of the world.

Bar Codes

10)It has spaces or gaps between characters. Each character is independent of every other character.

Discrete Code

11)It does not include spaces between characters. An example is Universal Product Code.

Continuous Code

12)It stores data in two dimensions in contrasts with conventional linear bar codes which stores data along only one axis.

2D code

13)It uses an alphanumeric code similar to ASCII code. It contains 9 vertical elements (5 bars & 4 spaces). It consists of 36 unique codes representing the 10 digits and 26 uppercase letters.

Other Names:

· Code 3 of 9

· 3 of 9 code

Code-39

14)A continuous code since there are no interchangeable spaces. Each UPC label contains a 12-digit number.

Universal Product Code

15)It consists of a 101 (bar-soace-bar) sequence, which is used to frame the 12 digit UPC number.

Start & Stop Guard Pattern

16)It separates the left and right halves of the label and consists of two long bars in the center of the label.

Center Guard Frame

17)Caused by electrical interference from natural sources

Classification of Data Com Errors:

· single bit

· multiple bit

· burst

Categories of Error Control :

· Error Detection

· Error Correction

Transmission Errors

18)Errors with only one bit within a given a given string is in error.

Single Bit Errors

19)Errors with two or more non-consecutive bits within a message.

Multiple-bit error

20)Errors when to or more consecutive bits within a given data string are in error. It can affect one or more characters within a message.

Burst Error

21)The theoretical (Mathematical) expectation of the rate at which errors will occur.

Probability of Error

22)The actual historical record of a system’s error perfor- mance.

Bit-Error Rate

23)The process of monitoring data transmission and deter mining when errors have occurred. It neither correct errors nor identify which bits are in error-they only indicate when an error has occurred.

Error Detection

24)Adding of bits for the sole purpose of detecting errors

Types of redundancy checks:

· vertical redundancy checking,

· checksum,

· longitudinal redundancy checking

· cyclic redundancy checking

Redundancy Checking

25)A form of error detection by duplicating each data

unit for the purpose of detecting errors.

Redundancy

26)An error detection bit.

Parity

28)The simplest error-detection scheme and is generally referred to as character parity.

Vertical Redundancy Checking (VRC)

29)An error detection bit in each character.

Parity Bit

30)The parity bit which is always a 1.

Marking Parity

31)The parity bit which is not sent or checked

Ignored Parity

32)Form of redundancy error checking where each character has a numerical value assigned to it.

Checksum

33)A redundancy error detection scheme that uses parity to determine if a transmission error has occurred with n a message.

Longitudinal Redundancy Checking (LRC)

34)An error occurred within a message.

Message Parity

35)The group pf characters that comprise a message

Block or Frame of Data

36)The bit sequence for the LRC.

Block Check Sequence (BCS) or Frame Check Sequence (FCS)

37)A convolution coding scheme that is most reliable redundancy checking technique for error detection. Almost 99.999% of all transmission errors are detected

Cyclic Redundancy Checking

38)Types of Error Messages.

Lost Message Damaged Message

39)One that never arrives at the destination or one that is damaged to the extent that it is unrecognizable.

Lost message

40)One that is recognized at the destination but contains one or more transmission errors.

Damaged Message

41)It includes enough redundant information with each transmitted message to enable the receiver to determine hen an error has occurred.

Examples:

· Parity bits

· block and frame check characters

· cyclic redundancy characters

Error-Detecting Codes

42)It includes sufficient extraneous information along with each message to enable the receiver to determine hen an error has occurred and which bits is in error.

Two primary methods for error correction:

· Retransmission

· Forward Error Correction

Error-correcting Codes

43)When a receive station requests the transmit station to resend a message when the message is received in error.

Retransmission

44)A two-way radio term which automatically a retransmission f the entire message.

Types of ARQ:

· Discrete

· Continuous

Automatic Repeat Request (ARQ) or Automatic Retransmission Request

45)The recipient of data sends a short message back to the sender acknowledging receipt of the last transmission.

Types of acknowledgements:

· Positive

· Negative

Acknowledgement

46)A receive station becomes the transmits station such as when acknowledgments are sent or when retransmission are sent in response to a negative acknowledgment.

Line Turnarounds

47)It uses acknowledgments to indicate the successful or unsuccessful reception of data.

Discrete ARQ

48)It can be used when messages are divided into smaller lock or frames that are sequentially numbered and transmitted in succession, without waiting for acknowledgments between blocks.

Continuous ARQ

49)The sending station does not receive an acknowledgment after a predetermined length of time.

Retransmission Time-Out

50)The destination station asynchronously requests the retransmission of specific frame of data and still be able to reconstruct the entire message once all frames have been successfully transported through the system.

Selective Repeat

51)The error-correction scheme that detects and corrects transmission errors when they are received without requiring a retransmission.

Forward Error Correction (FEC)

52)A mathematician who was an early pioneer in the development of error-detection and correction procedures, developed the Hamming Code while working at Bell Telephone Laboratories.

Richard W. Hamming

53)An error-correcting code used for correcting transmission errors in synchronous data streams. It requires the addition of overhead to the message, consequently increasing the length of a transmission.

Hamming Code

54)Inserted into a character at random locations.

Hamming Bits

55)The combination of the data bits and the hamming bits.

Hamming Code

56)It means to harmonize, coincide, or agree in time.

Synchronize

57)Involves identifying the beginning and end of a character with in a message.

Character Synchronization

58)Its literal meaning is “without synchronism”. In Data Com, it means “without a specific time reference”

Asynchronous

59)Asynchronous communications is called as such because each data character is framed between start and stop bits.

Start-stop Transmission

60)A condition when the transmit and receive clocks are substantially different

Clock Slippage

61)It occurs when the transmit clock is substantially lower than the receive clock.

Under slipping

62)Occurs when the transmit clock is substantially higher than the receiver clock.

Overslipping

63)It involves transporting serial data at relatively high speeds in groups pf characters.

Synchronous Data

65)Plain old Telephone system

POTS

66)It is comprised of three basic elements:

transmitter (source)

· transmission path (data channel)

· receiver (destination)

3 fundamental components of endpoints:

· data terminal equipment

· data communications equipment

· serial interface

Data Communications System

67)It can be virtually any binary digital device that generates transmits, receives, or interprets data messages. It is where information originates or terminates.

Data Terminal Equipment (DTE)

68)Devices used to input, output and display information such as keyboards, printers

and monitors

Terminal

69)Basically a modern-day terminal with enhance computing capabilities

Client

70)High-powered, high capacity mainframe computers that support terminals.

Hosts

71)It functions as modern-day host.

Servers

72)A general term use to describe equipment that interfaces data terminal equipment to a transmission channel, such as a digital T1 carrier or an analog telephone circuit. It is a signal conversion device, as it converts signals from a DTE to a form more suitable to be transported over transmission channel.

Types of DCE:

· channel service units (CSUs)

· Digital service units (DSUs)

· data modems

Data Communications  Equipment (DCE)

73)Another term for DCE.

Data Circuit-terminating Equipment (DCTE)

74)A DCE used to interface a DTE to an analog telephone circuit commonly called POTS.

Data Modem

75)It controls data flow between several terminal devices and the data communications channel.

Cluster Controller

76)Line control units at secondary stations.

Station Controllers (STACOs)

77)Universal Asynchronous Receiver/transmitter it is designed for asynchronous data transmission.

UART

78)A special purpose UART chip manufactured byMotorola.

Asynchronous Communications Interface Adapter (ACIA)

79)It means that an asynchronous data format is used and no clocking information transferred between the DTE and the DCE.

Asynchronous Data Transmission

80)An n-bit data register that keeps track of the status of the UART’s transmits and receive buffer registers.

Status Word

81)Transmit shift register has completed transmission of data character.

Transmit Buffer Empty (TBMT)

82)Set when a received character has a parity error in it.

Receive Parity Error (RPE)

83)Set when a character is received without any or with improper number of stop bits.

Receive Framing Error an (RFE)

84)Set when a character in the receive buffer register is written over by another receive character.

Receiver Overrun (ROR)

85)A data character has been received and loaded into the receive data register.

Receive Data Available (RDA)

86)The difference in time between the beginning of a start bit and when it is detected.

Detection Error

87)It is used for synchronous transmission of data between a DTE and a DCE.

Functions of USRT:

· Serial to parallel and parallel to serial data conversions

· Error detection by inserting parity bits in the transmitter and checking parity bits in the receiver.

· Insert and detect unique data synchronization (SYN) characters

· Formatting data in the transmitter and receiver.

· Provide transmit and receive status information to the CPU.

· Voltage-level conversion between the DTE and the serial interface and vice versa.

· Provide a means of achieving bit and character synchronization.

Universal Synchronous Receiver/transmitter (USRT)

88)It should provide the ff:

· A specific range of voltages for transmit and receive signal levels

· Limitations for the electrical parameters of the transmission line.

· Standard cable and cable connectors

· Functional description of each signal on the interface.

Serial Interface

89)In 1962, standardized the interface equipment between data terminal equipment and data communications equipment.

Electronics Industries Association (EIA)

91)It means “Recommended Standards”

RS

92)The official name of the RS-232 interface

Interface Between Data

Terminal Equipment and

Data Communications

Equipment Employing

Data Communications

Equipment Employing

Serial Binary Interchange

93)In, 1969, the third revision which was published and remained the industrial standard until 1987.

RS-232C

94)Sometimes referred to as the EIZ-232 standard Versions D and E of the RS-232 standard changed some of the pin designations.

RS-232D

95)It is a sheath containing 25 wires with a DB25P-compatible male connector (plug) on one end and a DB25S-compatible female connector (receptacle) on the other end.

Two full-duplex channels:

· Primary data-actual information

· secondary data-diagnostic information and handshaking signals

RS-232 Cable

96)It is designed for transporting asynchronous data between a DTE and a DCE or between DTEs .

9-pin Version of RS-232

97)It is designed for transporting either synchronous or asynchronous data between a DTE and a DCE.

25 pin Version

98)It is designed exclusively for dial-up telephone. It is used for transporting asynchronous data between a DTE and a DCE when the DCE is connected directly to a standard two-wire telephone line attached to the public switched telephone network.

EIA-561

99)It converts the internal voltage levels from the DTE and DCE to RS-232 values.

Voltage-Leveling Circuits

100)A voltage leveler wherein its output signals onto the cable.

Driver

101)It accepts signals from the cable.

Terminator

102)Protective ground, frame ground, or chassis ground. FUNCTIONS OF RS-232 PINS

Pin 1

103)Transmit data or send data.

Pin 2

104)Receive data (RD or RxD)

Pin 3

105)Request to send (RS or RTS)

Pin 4

106)Clear to send.(CS or CTS)

Pin 5

107)Data set ready or modem ready.(DSR or MR)

Pin 6

108)Signal ground or reference ground.

Pin 7

109)Unassigned and non-EIA specified often held at +12V

Pin 8

110)Receive line signal detect, carrier detect or data carrier detect (RLSD, CD or DCD)

Pin 9

111)Unassigned and often held at -12 Vdc for test purposes

Pin 10

112)Secondary receive line signal detect, secondary carrier detect or secondary data carrier detect (SRLSD, SCD, or SDCD)

Pin 12

113)Secondary clear to send.

Pin 13

114)Secondary transmit data or secondary send data

Pin 14

115)Transmission signal element timing or serial Clock transmit.

Pin 15

116)Secondary received data

Pin 16

117)Receiver signal element timing or serial clock receive

Pin 17

118)Unassigned is used for local loopback signal

Pin 18

119)Secondary request to send

Pin 19

120)Data terminal ready.

Pin 20

121)Signal quality detector.

Pin 21

122)Ring indicator (RI)

Pin 22

123)Data signal rate selector (DSRS)

Pin 23

124)Transmit signal element timing or serial clock transmit-DTE

Pin 24

125)Unassigned. It is sometimes used as a control signal from the DCE to the DTE to indicate that the DCE is in either the remote or local loop back mode.

Pin 25

126)

It specifies a 37-pin primary connector DB37 and a 9 pin secondary connector DB9 for a total of 46 pins which provides more functions, faster data transmission rates and spans greater distances than the RS-232.

Primary goals of RS-449:

· Compatibility with the RS-232 interface standard

· Replace the set of circuit names and mnemonics

· Provide separate cables and connectors

· Reduce crosstalks

· offer higher data transmission

· longer distances over twisted pair cables

· loopback capable

· improve performance and reliability specify a standard connector

Two categories:

· Category I

· Category II

RS-449 Serial Interface

127)Used by the DTE to request a local loopback from the DCE.

10 CIRCUITS IN RS-449 1.Local Loopback

128)Used by the DTE to request a remote loopback from the distant DCE.

2.Remote Loopback

Allows the DTE to select the DCE’s transmit and

receive frequencies.

3. Select frequency

129)Used by DTE to signal the DCE that a test is in progress.

4.Test Mode

130)Common return wire for unbalanced signals propagating from the DCE to the DTE

5. Receive Common

131)Used by the DTE to signal the DCE whether it is operational

6. Terminal in Service

132)Used by the DTE to request that the DCE switched to standby equipment.

7. Select Standby

133)Used with a modem at the primary location of a multipoint data circuit.

8. New Signal

136)It was intended to operate at data rates between 20 kbps and 2 Mbps using the same DB25 connector

RS-530 Serial Interface

137)It is used to interface computers, computer networks

to analog transmission media

Alternate names:

· datasets

· dataphones

· modems

Data Communications Modem

138)A contraction derived from the words Modulator and Demodulator.

Primary Block of a Modem:

· Serial interface Circuit

· Modulator Circuit

· Bandpass filter and equalizer circuit

· telco interface circuit

· demodulator circuit

· carrier and clock generation circuit

Modem

139)Data communications modems designed to operate over the limited bandwidth of the PSTN.

Voice-band Modem

140)It is able of transporting higher bit rates.

Broadband Modem

141)Digital to analog converter.

DAC

142)Analog to digital converter.

ADC

143)It is a rate of change of signals on the transmission medium after encoding and modulation have occurred

Baud

144)Refers to the rate of change of a digital information signal.

Bit Rate

145)It is classified as low-speed voice-band modems

Asynchronous Modems

146)Synchronous data transported by asynchronous modems.

Isochronous Transmission

147)It uses PSK or quadrature amplitude modulation to transport data.

Synchronous Modems

148)A special, internally generated bit pattern in transmit modem.

Training Sequence

149)Located in the transmit section of a modem and provide pre-equalization

Compromise Equalizers

150)Located in the receiver section of a modem where they provide post-equalization to the received signals

Adaptive Equalizer

151)The first internationally accepted standard fro 9600bps data transmission rate.

ITU-T V.29

152)It is intended to provide synchronous data transmission over four-wire leased lines.

V.29 Standard

153)Five bits.

Quin bits

154)A technique for full-duplex operation over two wire switched telephone lines.

Echo Cancellation

155)It address asynchronous-to synchronous transmission conversions and error control that includes both detection and correction. It specifies a new protocol called Link Access Procedures for Modems.

V.32 Specification

156)It is the next generation data transmission with data rates of 28.8 Kbps without compression possible using V.34.

V.34 Innovations:

· Nonlinear coding

· multidimensional coding and constellation shaping

· Reduced complexity

· precoding of data

· line probing

V.fast

157)An enhanced standard adopted by ITU in 1996.

It adds 31.2 kbps and 33.6 kbps to the V.34 specification.

V.34+

158)Developed by ITU-T in February 1998 during a meeting in Geneva, Switzerland. It defines an asymmetrical data transmission technology where the upstream 33.6kbps and downstream of 56kbps.

V.90 Recommendation

159)A new modem standard in 2000 which offers 3 improvements over V.90 that can be achieved only if both the transmit and receive modems and the internet Service Provider(ISP) are compliant.

It offers:

· upstream transmission rate of 48 kbps

· faster call setup capabilities

· incorporation of a hold option

V.92 Recommendation