This is the Review Notes in Digital and Data Communications System as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Summary of Important Digital and Data Communication Review Notes
Redundancy means the symbol are to be repeated
Optical fiber transmission media is not suitable to CSMA operation
Transducer is a device used to convert a time varying electrical quantity to an appropriate form
The bandwidth of U600 mastergroup is 2520 kHz
Hybrid data – refers to the combined digitally encoded signals transmitted with FDM signals as one composite baseband signal
L carrier systems – transmit frequency-division-multiplexed voice band signals over a coaxial cable for distances up to 4000 miles
Level 5 of Japanese PCM multiplex hierarchy has a channel capacity of 5760 VB channels
The line data rate (in Mbps) for level 4 of CEPT 30 + 2 PCM multiplex hierarchy is 139.264
The guardband between supergroup 18 and supergroup D25 is 56 kHz
Supergroup 17 has a carrier frequency (in kHz) of 2108
A radio channel is composed of 1800 VB channels
The guardband between supergroup 1 and supergroup 2 is 12 kHz
CCITT’s supermastergroup has 900 voice band channels
The transparency mechanism used with SDLC is called zero-bit insertion
2B + D – defines the composition of an ISDN basic access line
Synchronous digital hierarchy (SDH) – a digital network where voice, video, text and data are multiplexed into a single network for processing and are transmitted prior to use
T-1 carrier service has 1.544 Mbps digital bit rate
Non-ISDN equivalent can be connected to ISDN line by the use of terminal adapters
The baseband frequency of standard FDM basic supergroup is 312 to 552 kHz
The transmission signal coding for T1 carrier is Bipolar
The primary purpose of the data modem is to interface digital terminal equipment to analog communications channel
Data terminal equipment (DTE) – the line control unit (LCU) operates on the data in digital form
32 H – SYN character of EBCDIC code
Bit or clock synchronization ensures that the transmitter and receiver agree on prescribed time slot for the occurrence of a bit
2^n >= m + n + 1 – used to determine the number of Hamming bits in the Hamming code
Data means digital information
Network layer determines which network configuration is most appropriate
Ethernet is a baseband system using CSMA/CD operating at 10 Mbps
Channel accessing – mechanism used by a station to gain access to local area network
TDM multiplexing scheme is used by baseband transmission
Use of coaxial cables in interconnecting networks is limited to an overall length of 1500 m
Topology or architecture identifies how the stations are interconnected in a network
Local Area Network is a data communications network designed to provide two-way communications between a large variety of data communications terminal equipment within a relatively small geographic area
Start/stop mode is the mode of transmission in public data network in which data are transferred from source to the network then to the destination in an asynchronous data format
A seven-bit character can represent one of 128 possibilities
Channel capacity is defined to be the maximum rate at which information can be transmitted through a channel
Source coding – circuit that performs the inverse mapping and delivers to the user destination, a reproduction of the original digital source output
PSK digital modulation technique used in modems
Emile Baudot developed the fixed-length binary code for telegraphy
Alex Reeves – inventor of pulse-code modulation for the digital encoding of the speech signals
In 1950’s, computers and terminals start communicating with each other over long distance
PRNET – store-and-forward multiple-access network
Huffman code – a source code whose average world length approaches the fundamental limit set by the entropy of a discrete memoryless source
DCA promulgate communications-related military standards (MIL-STD)
P2 is the class of probability in error detection techniques that is known as residual error rate
RS 232 is normally an interface between DTE and DCE. The signal rate 20 kbps
TMS – digital switching concepts that can handle more channels
The most significant advantage of modular switch than time-and-space switch is flexible size
The biggest disadvantage of PCM systems is the larger bandwidth required
The reason why companding is employed in PCM systems is to protect small signals in PCM from quantizing distortion
Folding frequency – the highest frequency that can be processed at a sampling rate (fs) without aliasing
The overlapping of the original spectrum and the first translated component is known as aliasing
Full duplex – transmission sent in both directions simultaneously
Multiplexing in a time division multiplexer occurs based upon the position of data within a frame
Multiplexing – a technique that enables more than one data source to share the use of a common line
One of the reasons whey FDM is being replaced by TDM is because noise is amplified with voice when an FDM system is used
In pulse modulation the carrier is a periodic train of pulses
The process that uses the orthogonality of sines and cosines that makes possible to transmit and receive to different signals simultaneously on the same carrier frequency is Quadrature multiplexing
Pulse code modulation technique is a digital transmission system
Bandwidth utilization is not an advantage of digital transmission
Dynamic range is the ratio of the largest possible magnitude to the smallest possible magnitude that can be decoded by the DAC
Companding – a process where the higher amplitude analog signals are compressed prior to transmission then expanded at the receiver
Delta Modulation (DM) uses a single bit PCM code to achieve a digital transmission of analog signal
The line speed of Bell System T1 carrier system is 1.544 Mbps
T2 lines carry 96 channels
A super group has 60 channels
The output frequency at the channel combiner of channel 7 is 80 – 84 kHz
The carrier frequency of the supergroup D25 is 2652 kHz
Data in video is not a hybrid data
Baudot Code was the first fixed-length 5-bit character code
CCITT V.26 modem has a modulation rate of 1200 bauds
A smart modem accepts commands from the terminal via RS232 interface
An advantage of PTM over PAM is much better noise immunity
ISO adapted the seven-layer OSI model in 1983
Amplitude shift keying is also known as ON/OFF keying
FSK modulation system is used in telephony
FSK – modulation used by asynchronous data
PCM systems require large bandwidth
Shanon-Hartley theorem sets the limit on the maximum capacity of a channel with a given noise level
Quantizing noise is present in PCM system
Quantizing noise is the difference between the original and reconstructed signal
The reason why companding is employed in PCM system is to protect small signals in PCM from quantizing distortion
FDM – band of frequencies in a transmission medium allotted to each communication channel on a continuous time basis
ARQ – error control used in high frequency radio data transmission
Sampling, quantizing and coding are the steps to follow to produce a PCM signal
Network topology identifies how the difference stations in a multipoint system are interconnected
Baseband – a transmission where data are inputted directly on the cable
Broadband – a transmission where data are inputted inside the carrier wave
Polling – a transmission system for a multidrop network
CSMA/CD – before attempting to transmit data, each station has to listen to the channel
Synchronous modems cost more than asynchronous modems because they must contain clock recovery circuits
When one station is designated as master and the rest of the stations are considered slaves, massage handling is polling
Master – the computer that initiates information transfer
Message switching – a store and forward switching
LAN is used to connect computers in the same building or in same area
WAN – two or more LAN linked together over a wide geographical area
OSI system allows different types of network to be linked together
OSI consist of 7 layers of interconnection
Session layer determines if the user can send or receive based on whether they can send and receive simultaneously or alternately
A bridge that interconnects LAN having identical protocols at the physical and data link layers
A router interconnects LAN having identical protocols at the physical data link and network layers
A gateway interconnects LAN that have totally different protocols and format
Parallel data – all bits in a character can be sent/received simultaneously
Serial data – the bits in a character which are sent/received one at a time
DTE – a system that performs parallel-to-serial and serial-to-parallel conversion of a data link
EIA stands for Electronic Industry Association
An acoustic modem converts a computer digital signal into audio tones
Echoplex mode of transmission achieves less than full-duplex but more than half-duplex
Line protocol – rules governing the transmission of digital information
Codes must be agreed upon in advance between sender and receiver
The standard ASCII has 132 characters including 32 control characters
The Baudot code requires shift characters to provide sufficient combinations
The corrections and accuracy of the transmitted message content is determined by the sender and the receiver
Framing is concerned with the boundaries between characters
Asynchronous transmission is less efficient but simpler
Digital transmission provides a higher level of signal quality than analog transmission because repeaters regenerate digital pulses and remove distortion
Digital to analog converter in synchronous modems send signals to the equalizer
Binary codes are transformed in modem into Gray code
Synchronous modems cost more than asynchronous modem because they must contain clock recovery circuits
The best type of data communications test equipment is a protocol analyzer
The data rate of the ISDN Basic access B channel is 64 kbps
The data rate of the ISDN Basic access D channel is 16 kbps
Baudot code uses 5 bits per symbol
3 bits are there to present 8 combinations
Intelligent terminal can be programmed to perform new functions
EBCDIC character code is used without parity bit
Modem is used when the host computer and the terminal are in separate locations
Front end processor is a data communications hardware that assists the host computer in handling input and output tasks
ASCII means terminals using asynchronous transmission in ASCII
Bit – a unit of information used in data communications
Transmission of binary signals requires more bandwidth than analog
The modulation rate of the CCITT V.26 modem is 1200 bauds
The digital-to-analog converter in a synchronous modem sends signal to the equalizer
Unknown – the number of bits that are zeros in each symbol when one is transmitting odd parity coded symbols
FSK – a digital modulation technique that results in two different frequencies representing binary 1 and 0
The input to the Digital-to-Analog Converter of a PCM decoder circuit is a series of bits
The output of the DAC at a PCM decoder circuit is a parallel output of binary-coded digits
Serial printer prints one character at a time
Delta modulation is a 1-bit differential PCM system
Delta modulation – alternative way of digitizing analog signals
One dit is equal to 3.32 bits
Information theory was developed by Shannon
A quadratic signaling has 4 possible states
Bit is the smallest unit of information in binary transmission system
Physical layer is the lowest layer in the ISO protocol hierarchy
64 kbps is the basic speed rate of digital system
Protocol – a formal set of conventions governing the formatting and relative timing of message exchange between two communications system
Null modem – a device that connects 2 data terminal equipment (DTE) directly by emulating the physical connections of a data communications equipment (DCE)
Impact printer strikes a ribbon against the paper to produce character image
Modem is referred to as data communications equipment
Common channel signaling – a signaling method relating to a multiplicity of circuits is conveyed over a single channel by labeled messages
Ring – a data highway in LAN, this type of topology allows workstations to be connected to a common line where all messages pass stations on the way to their destination
A packet format has an error detecting code at the end of packet framing
X.75 – it is a protocol used to connect the other packet switching network
Novel Netware is a network operating system within several buildings in compound
FSK – used by Bell 103/113 series for full duplex, 2 transmission speed of 0 to 300 bps
Channel capacity is directly proportional to bandwidth
Quantizing level does not affect noise in a channel
Bisync is a character oriented protocol
STX character signifies the start of the test for Bisync
In synchronous data the clock pulse is used to time the sending and receiving of the characters
All bits in a character are sent and received in one at a time in serial port
Bridge can only interconnect LANs having identical protocols at the physical and data link layers
OSI stands for Open System Interconnection
ISO stands for International Standardization Organization
Message switching is store and forward network
Packet switching is hold and forward network
Polling is an invitation by the primary to a secondary equipment to transmit a message
Bit rate is the rate of change at the input of the modulator
Morse code – a code that uses three unequal length symbols, dot, dash and space to encode a character
BAUDOT - A 5-bit character code
EBCDIC – an 8-bit character code
Modem – an equipment that interfaces the data terminal equipment to the analog transmission line
Protocol – set of rules governing the orderly exchange of data information
Public data network (PDN) – a communication network designed for transferring data from one point to another
Half-duplex – radio communications between points using a single share frequency
RS232C interface had 25 numbers of pins
A modem is sometimes referred to as Data Communication Equipment
Adaptive equalizers – provide post equalization to the received analog signal
The receive equalizer in a synchronous modem is called an adaptive equalizer
Compromise equalizer settings typically affect amplitude delay
Training sequence (a bit pattern used to synchronize the receive modem) accomplishes activate RLSD
When asynchronous data are used with synchronous modems this is called isochronous transmission
H factor is defined as a figure of merit used to express the degree of modulation achieved in an FSK modulator
Low-band channel in the 103 modem occupies a passband from 300 to 1650 Hz
Low speed modems generally have bit rates of 2400 bps
Medium speed modems have bit rates of 2400 – 4800 bps
High speed modems have bit rates of 9600 bps
Line conditioning – a means of improving the quality of a private-line circuit by adding amplifiers and equalizers to it
A voice-grade circuit using the PTN has an ideal passband from 0 to 4 kHz
The digital-to-analog converter in a synchronous modem is called an adaptive equalizer
The minimum-quality circuit available using the PTN is called basic voice channel (VC)
Direct distance dialing (DDD) network is commonly called dial-up network
The line control unit (LCU) operates on the data when it is in digital form and is therefore called data terminal equipment (DTE)
With asynchronous data, each character is framed between a start and a stop bit
With synchronous data, rather than frame each character independently with start and stop bits, a unique synchronizing character called a SYN character is transmitted at the beginning of each message
A logic 0 is used for the start bit
All stop bits are logic 1
Bit or clock synchronization ensures that the transmitter and receiver agree on a prescribed time slot for the occurrence of a bit
Characters that must be transmitted other than data are called overhead
In the United States, the most common CRC code is CRC – 16
Parallel transmission is also called parallel-by-bit and serial-by-character
Data means digital information
Go-ahead sequence 01111111
Flag field are used to achieve character synchronization in SDLC
DISC – a command that places a secondary station in the normal disconnect mode
Asynchronous – character oriented protocol
Network layer determines which network configuration is most appropriate
ISO 7809 – a standard that combines previous standards 6159 (E) (unbalanced) and 6256 (E) (balanced) and outlines the class of operation necessary to establish the link-level protocol
Ethernet is a baseband system using CSMA/CD operating at 10 Mbps
Manchester – a type of digital encoding technique used to detect collision in CSMA/CD
With CSMA/CD, a station monitors the line to determine if the line is busy
Channel accessing describes the mechanism used by a station to gain access to local area network
Broadband uses FDM
TDM – a typical multiplexing scheme used by baseband transmission
Baseband transmission uses the connecting medium as a single-channel device
Broadband transmission uses the connecting medium as a multi-channel device
The use of coaxial cables in interconnecting networks is limited to an overall length of approximately 1500 m
Topology or architecture identifies how the stations are interconnected in a network
Local Area Network – a data communications network that is designed to provide two-way communications between a large variety of data communications terminal equipment within a relatively small geographic area
2B1Q encoding – the type of encoding used in the transmission of data on an ISDN line between a customer’s premises and a carrier’s central office
Start/stop mode – a mode of transmission in public data network in which data are transferred from source to the network to the destination in an asynchronous data format
IEEE 802.3 standard is CSMA/CD
IEEE 802.4 standard is Token passing for bus topology
IEEE 802.5 standard is Token passing for ring topology
The data rate of the ISDN Basic access B channel is 64 kbps
The data rate of the ISDN basic access D channel is 16 kbps
2B + D equation defines the composition of an ISDN Basic access line
The number of channels on which different operations can occur simultaneously on one ISDN Basic access line is 3
Digital telephones and integrated voice-data workstations are examples of TE type of ISDN equipment
X.25 standard have 3 OSI layers covered
Physical layer – an Open System Interconnection (OSI) layer which has the responsibility for the physical transportation of the bits of data from one end of a point-to-point link to the other
Handshaking – a technique involving signaling by both ends of a link to ensure correct data transfer
XON/XOFF is a method of flow control
V.14 – a recommendation which allows a synchronous modem to be used to transmit asynchronous characters, without error control
The scrambler in a synchronous modem is found in the modular section
V.24 – A CCITT recommendation which gives the definition for the interchange circuits between DTE’s and DCE’s
Binary codes are sometimes transformed in modems into Gray code
A Western Electric 2911 modem operates with a carrier frequency of 1800 Hz
Flag – a special bit pattern used to denote the start and end of a data link layer frame
A bipolar violation occurs when two successive pulses have the same polarity and are separated by a zero level
Flow control – the process of starting and stopping the terminal output to avoid loss of characters by the receiving device
Buffering refers to temporarily storing data to allow for small variations in device speeds
The reason why many cables have RS-232 connectors with some wires crossed is because asynchronous modems reverse the direction of transmitted and received data from the standard
RS-232, RS-530, RS-449, V.21 and V.24 are examples of standards for interfaces between terminals and modems
Request To Send (RTS) / Clear To Send (CTS) is the way the DTE indicates that is ready to transmit data, or the way the DCE indicates that it is ready to accept data
Extended command sets supported by modern modems use different commands to control many advance modem features
A smart modem accepts commands from the terminal via the RS-232 interface
Communications protocols always have a set of symbols
The Baudot code uses 5 bits per symbol
ZMODEM protocol adjusts its block size based on the line error rate
DEREP-RB is not a common DNA repeater
Unknown is the number of bits that are zeros when transmitting odd-parity coded symbols
X.200 – standard for the 7-layer model for Open System Interconnection
Application-independent interfaces option is a characteristic of a LAN
Twin lead is not a common LAN medium
Cable TV system is not a local area network (LAN)
V-series – CCITT standard concentrating on data communications over the telephone network
Fiber optic transmission cannot be provided in a broadband LAN
PBX – a small telephone switching system that can be used as a LAN
Digital PBX – a LAN architecture that can be expanded to the greatest total system bandwidth
Bus is considered as the fastest LAN topology
Data communications refers to the transmission of computer data
Protocol is a procedure or rule that defines how data is to be transmitted
XMODEM is a popular PC protocol
ASCII is the most widely used data communications code
The ASCII code has 7 bits
Both analog and digital method are used in data communications
Telephone is not a primarily a type of data communications
Mark and space refer respectively to binary 1 and binary 0
A modem converts both analog signals to digital and digital signals to analog
I.120 – an I-series recommendation which describe what an ISDN is
Slow speed modems use FSK digital modulation method
When the data signal is transmitted directly over the medium, this is called baseband transmission
Broadband is the technique of using modulation and FDM to transmit multiple data channels of a common medium
Noise cause bit errors in data transmission
The other name for parity is VRC
QAM is the combination of ASK and QPSK
I-series – a CCITT standard that refers to various aspects of Integrated Service Digital Network (ISDN)
XNOR is not a part of a QAM modulator
Redundancy is not a commonly used method of error detection
Block check character (BCC) is produced by a longitudinal redundancy check (LRC)
Balanced modulators is the basic modulator / demodulator circuit in PSK
The main reason why serial transmission is preferred over parallel transmission is because it requires only a single channel
30 stations per segment – limitation of 10base-2 network
Optical fiber transmission media is not readily suitable to CSMA operation
Entropy refers to the measurement of an uncertainty
Logical is not an important characteristic of the physical layer
The bit rate of B-channel is 64 kbps
V.26bis – defines a 2400-bps, PSK, half-duplex modem operating at 1200 baud
V.100 – describe the interconnection techniques between PDNs and PSTNs
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