DATA COMMUNICATIONS - Categories of Data Communications

DATA COMMUNICATIONS

The purpose of this Lesson is to discuss Data Communications.

Data Communications is the exchange of information between computers and other devices, including,

 Integrated Circuits,

Sub-sections,

PC Boards,

other computers,

peripherals

circuits &

controllers

2 Categories of

Data Communications

1. Local

The computer & devices are directly connected.

(hardwired or internal)

2. Remote

The computer & devices are indirectly connected.

(thru telephone or satellite etc.)

__________ ______ ____ _____ _______ ______ ______

These Categories of Data Communications include:

Parallel Interconnection Standards

& Serial Interconnection Standards

Parallel Standards are used for local communication.

Serial Standards are used for remote communication.

__________ ______ ____ _____ _______ ______ ______

Parallel Interconnection Standards include,

IEEE-488 Instrumentation Bus Standard,

S-100 Hobbyist Bus Standard

IBM-PC Bus Standard

Centronics Printer Connection Standard

And many others

 __________ ______ ____ _____ _______ ______ ______

Serial Interconnection Standards include,

Current Loop,

RS-232C,

RS-241,

XMODEM,

YMODEM, &

ZMODEM

__________ ______ ____ _____ _______ ______ ______

There are 3 Terms describing

Data Communications Capabilities

1. Simplex One Way Only (like a car radio)

2. Half-Duplex One Way at a Time (Police Radio)

3. Full-Duplex Both Ways Simultaneously (telephone)

__________ ______ ____ _____ _______ ______ ______

The rates of data transmission

are described in terms of;

BPS Bits Per Second (Data Bits only)

&/or Baud Rate Data & Sync. bits per sec

Pieces of information per sec

There are

2 major types of

information modulation

commonly used

AM Amplitude Modulation.

& 2. FM Frequency Modulation.

__________ ______ ____ _____ _______ ______ ______

For,

Amplitude Modulation AM

A single frequency is;

turned-on to indicate a '1',

turned-off to indicate a '0'

This Method resembles Telegraph Code

__________ ______ ____ _____ _______ ______ ______

And for,

Frequency Modulation FM

2 specific types of FM

FSK - Frequency Shift Keying, which uses,

1 tone for '1'

another for '0'

PSK - Phase Shift Keying, which uses,

1 phase-shift for '1'

another for '0'

__________ ______ ____ _____ _______ ______ ______

There are

2 Categories of

Serial Data Communication

1. Asynchronous

& 2. Sychronous

__________ ______ ____ _____ _______ ______ ______

Asynchronous (Below 10,000 bps)

In this category, one character is transmitted at a time, with synchronization included. (LSB 1st)

Each character is FRAMED by adding START & STOP bits, adding another 20% to the total time. (They're also called mark & space bits )

__________ ______ ____ _____ _______ ______ ______

 The START bit is the1st negative edge of the frame.

A STOP bit is represented by a HI for 2 time periods at the end of each character.

 __________ ______ ____ _____ _______ ______ ______

Synchronous (Up to 20,000 bps)

Each user synchronizes tranmission with the other.

 In this category, many characters are sent at a time in block form.

Each block is identified by start & end characters.

 IC'S commonly used in these circuits include;

a. MC6830 ACIA

b. 8250 UART

c. 8251 USART

. ..d. 8291.. IEEE-488

__________ ______ ____ _____ _______ ______ ______

Error Detection & Correction Methods

Retransmission

Just retransmit the data

& Parity Checking Methods

Bit-counting Techniques

__________ ______ ____ _____ _______ ______ ______

Parity Checking Methods include,

a. Check Sum

Parity sum sent with data

b. Cyclic Redundancy Checking

A Parity Sum quotient fraction is sent with data.

c. Hamming Code

Similar to Check Sum with added bits.

d. Matrix Sum

Parity Sum set into a matrix. (LRC/VRC)

NETWORKS.

The purpose of this Lesson is to discuss Networks.

A Computer NETWORK is the interconnection of several Computers for the purpose of sharing information &/or resources.

__________ ______ ____ _____ _______ ______ ______

4 Basic Network Types,

which link many microcomputers together:

1. LANs Local Area Networks;

2. MANs Metropolitan Area Networks

3. WANs Wide Area Networks

4. SNAs System Network Architectures;

Let's discuss each of these one at a time

 __________ ______ ____ _____ _______ ______ ______

LANs.

Local Area Networks usually connect computers together by Common Data Transmission Medium, that are in a single building or on a single campus. (Less than 10 Kilometers.)

In NETWORK discussions, the term FILE-SERVER often arises.

A FILE-SERVER is, simply, a dedicated Microcomputer, whose function is to SERVE the members of a Network.

MANs.

Metropolitan Area Networks are capable of Hi-Speed Data Communications and connect computers together, that are within the same Metropolitan Area (10 to 80 Kilometers.)

WANs.

Wide Area Networks connect computers together by common carrier, that may be very far apart in Different Cities, States, or even different Countries. (Over 80 Kilometers.)

SNAs.

System Network Architectures links PC's to mainframe computers. (IBMs mainframe standard)

There are several of these business-dedicated Networks in use in the world, but, needless to say, they are not in the public domain.

 __________ ______ ____ _____ _______ ______ ______

The Network Topology

describes HOW Computers are connected together to exchange information.

__________ ______ ____ _____ _______ ______ ______

The 3 connection-types are called TOPOLOGYs

The 3 most common means of network inter-connection are,

a. BUS Topology

Computers share a bus, where each may drop-off or pick-up information.

b. STAR Topology

(Parallel) (Token Ring)

All computers are connected to a central brain or coordinator.

c. RING Topology

(Series) (Cascaded Star)

Computers are daisy-chained together.

__________ ______ ____ ________________

A Network, besides the Connections and users themselves, consists of,

HARDWARE including

Network Interface Cards,

& Network Connecting Cables

SOFTWARE including

Protocol Software

Network Operating System Software,

& Network Software

to run end-user programs

 __________ ______ ____ _____ _______ ______ ______

Let's discuss them, one at a time

NETWORK HARDWARE

Network Interface Cards

The purpose of Network Interface Cards is to provide the physical connection for the computer to the Network.

 The Network Interface Card must,

1. Get Computer Data

2. Format Computer Data

3. Encode Computer Data

4. Get access to the Network Cable

5. Establish the lines of communications by providing the HANDSHAKING.

__________ ______ ____ _____ _______ ______ ______

 

The 3 ways to move Data between the Computer & the Network Interface Card are,

DMA

I/O

& Shared Memory

__________ ______ ____ _____ _______ ______ ______

The Network Interface Card & its Software determine Network SPEED.

 At the other end, the transceiver waits for the Data & the whole process happens in reverse. The Data is

1. Demodulated

2. Decoded

3.Converted (from Serial, back to Parallel)

4. Depacketized &

5. Made READABLE

__________ ______ ____ _____ _______ ______ ______

Connecting Cables

Network InterConnection is often done by Cable, although Microwave Transmission IS used.

 These connections are accomplished by,

Coaxial Cable,

Twisted Pair,

Fiber Optics

Each one has its advantages & disadvantages

 __________ ______ ____ _____ _______ ______ ______

NETWORK SOFTWARE.

Protocol Software,

Protocol is the set of rules, provided by Software, that governs communication between computers.

Protocols govern, Data & Handshaking,

1. Format

2. Timing

3. Sequence

4. Error Control

__________ ______ ____ _____ _______ ______ ______

Protocols are divided into 7 layers

by the Open System Interconnection (OSI) Model to allow different Protocol Standards to work together. These are,

LAYER PURPOSE

1. Physical .Defines Cable Characteristics

2. Data-Link .Data Blocks,Sync,Error & Flow Control

3. Network .Establishes,Maintains,Terminates Connection

4. Transport .Checks for Data OK

5. Session .Establishes Conversations

6. Presentation ..Code Conversion,Data Reformat

7. Application .Supplies Functions to Computer

__________ ______ ____ _____ _______ ______ ______

Network Operating System Software,

A Network Operating System makes a collection of independent Computers act as ONE System.

Network Operating Systems provide,

1. Resource Sharing Capabilities

2. Data Integrity

3. Security

4. Administrative Tools

5. Troubleshooting Tools

6. Multi-Network Support

__________ ______ ____ _____ _______ ______ ______

Network Operating Systems

being used include,

Novell NetWare

Banyan VINES

Sitka

DCA

ArcNet

EtherNet

__________ ______ ____ _____ _______ ______ ______

Of course, any of the dedicated Application Program software, that were discussed in earlier Lessons, can be used to process information, after it has been recovered by the System.

__________ ______ ____ _________

MODEMs

The purpose of this Lesson is to discuss MODEMs

MODEM stands for

MODulator/DEModulator.

A modem is a device used by computers to communicate with each other over standard telephone lines.

 Modems convert digital signals from the computer into audio frequencies within the bandwidth of a telephone.

__________ ______ ____ _____ _______ ______ ______

MODEM TYPES.

MODEMs are available as either,

1. INTERNAL

A PC Board that plugs into an Expansion Slot

or 2. EXTERNAL

A separate Unit, that can be switched or moved as needed.

 __________ ______ ____ _____ _______ ______ ______

Modems are also classified by speed

The speed of a MODEM can be given as Bits Per Second (bps) or as its Baud Rate.

Bits Per Second refers to Data Bits only

Baud rate is bps including start & stop bits

It is also defined as the total number of Flux Changes Per Second.

__________ ______ ____ _____ _______ ______ ______

Let's see the SPEED Classifications.

MODEM SPEED CLASSIFICATIONS

Lo spd = 1 - 600 bps (No Longer Used)

Med spd 600 - 2400 bps .. (Common

Hi spd = 24K-30,000 bps (Most Common)

Above 19,000 bps a wide-band Modem & channel is required.

Using special data packaging techniques, the BPS actually realized may be much higher.

 We'll see HOW that can be, but first,

There are other considerations involved with MODEM information exchange. They are

1. Protocols

2. Handshaking &

3. Type of Communication

__________ ______ ____ _____ _______ ______ ______

 Terms

Synchronous or Asynchronous

Half or Full Duplex, etc

FM, QAM, Bits, Dibits, or Tribits

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PROTOCOLS.

A Protocol is the established procedure for exchanging Data along with instructions that coordinate the process.

Most Protocols are error-correcting and use one or more of the error-correcting techniques already discussed.

The most-used of these include,

PROTOCOL - TRANSMISSION TECHNIQUE

XMODEM..1/2 Duplex PSK.

YMODEM 1/2 Duplex PSK

MODEM Full Duplex QAM

KERMIT Full Duplex QAM

BLAST Full Duplex QAM

JMODEM Full Duplex QAM

BiMODEM Full Duplex QAM

Sealink Full Duplex QAM

Telink 1/2 Duplex PSK

Almost all of these use CRC & Checksum Error Detection and will automatically request retransmission upon Error Detection.

 __________ ______ ____ _____ _______ ______ ______

MODEMs

TECHNIQUE OF TRANSMISSION.

For MODEMs, AM was never used. FM Techniques like FSK & PSK were initially used often.

The original FSK standard was the Kansas City Standard, in which 1040 Hz is a & 2240 is a  It is no longer used by MODEMs.

Half-Duplex MODEMs use PSK Techniques

1 Phase Shift is a , Another is a

__________ ______ ____ _____ _______ ______ ______

QAM, Dibits & Tribits

Nowadays, Full Duplex MODEMs use QAM.

QAM stands for Quadrature Amplitude Modulation.

It is a type of Phase Modulated FM, in which a Dibit, (2 Bits), are encoded together.

Quadrature means 90 degrees out of phase

Binary 00,01,10,& 11 or Decimal 0,1,2,& 3 are encoded by 1 of 4 Phase Shifts.

Tribits are encoded, similarly. Remember,

There are 2 Categories of Serial Data Communication Which MODEMs use,

1. Asynchronous

& 2. Sychronous

 __________ ______ ____ _____ _______ ______ ______

Asynchronous (Below 10,000 bps)

In this category, one character is transmitted at a time, with synchronization included (LSB 1st).

____________________

Each character is FRAMED by adding START & STOP bits, adding another 20% to the total time. (They're also called mark & space bits.)

____________________

The START bit is the 1st negative edge of the frame.

A STOP bit is represented by a HI for 2 time periods at the end of each character.

__________ ______ ____ _____________

Synchronous (Up to 20,000 bps)

a. Each data handler person synchronizes with the other

b. In this category, many characters are sent at a time in block form.

Each block is identified by start & end characters

__________ ______ ____ ______________

STEPS IN MAKING CONTACT

In order for a Computer to exchange information with another computer, CONTACT must be established between them.

To establish contact,

1. Initialize the MODEM

2. Dial the telephone number

3. Wait for answer carrier

4. Send originate carrier

5. Report MODEMs locked-on

& TO ANSWER AN INCOMING CALL

1. Initialize the MODEM

2. Answer on specified ring

3. Provide the answer carrier

4. Listen for an Originate carrier

5. Report MODEMs locked-on

In order to establish the lines of communication between one computer & another, an orderly process must be used to ensure that data is transmitted only when the other party is READY for it.

This process is known better as,

HANDSHAKING

__________ ______ ____ _____ _______ ______ ______

MODEM CONTROL SIGNALS

MODEM Control Signals include,

Abbrev Name Meaning

DTR Data Terminal Ready Computer Ready

DSR Data Set Ready MODEM Ready

DCD Data Carrier Detect Remote MODEM on-line & ready

RI Ring Indicator Modem senses ringing

RTS Request To Send Computer to MODEM,OK/Send?

CTS Clear To Send MODEM to Computer,Go Ahead

TD Transmit Data Actual Outbound Data

RD Receive Data Incoming Data

The MODEM may need DTR from the Computer before permitting Data Exchange.

The Communications Program may need DSR from the MODEM before permitting Data Exchange.

 The computer sends RTS and only after the MODEM responds with CTS can a computer transmit data.

Even though that's what's supposed to happen, in reality, many office computer systems connect to MODEMs that only have 3 wires. For,

Transmit Data

Receive Data &

Ground

__________ ______ ____ _____ _______ ______ ______

MODEM CIRCUITRY.

A MODEM has several sections, some optional

The vital circuits are,

Section Ckt Purpose

I/O INTERFACE ..,,For V & I level adjustment

OUT MODULATOR D-A Conversion & VCO output

IN DEMODULATOR A-D Conversion & VCO Phase Reference

I/O .FILTERS To remove unwanted noise

I/O AMPLIFIERS To ensure adequate Gain

I/O TRANSFORMER Telephone Line Isolation, Z-Matching

 __________ ______ ____ _____ _______ ______ ______

SENDING DATA

Information to be sent comes from the Computer, and is Level-adjusted by the INTERFACE Circuit.

It is then passed to the MODULATOR Circuit.

 The Modulator converts the DIGITAL Data into an Analog Voltage which is used to vary the Frequency of the Voltage Controlled Oscillator (VCO).

 The VCO Output is, then, FILTERED, AMPLIFIED & applied to the Telephone Line Isolation TRANSFORMER, where it travels to its destination.

__________ ______ ____ _____ _______ ______ ______

RECEIVING DATA

Received information comes through the Telephone Line Isolation Transformer, is FILTERED & AMPLIFIED before being applied to the DEMODULATOR.

 The job of the DEMODULATOR is to convert the Analog Signal back into Usable Digital Data.

 This is performed by comparing the phase & frequency of the incoming signal to the phase & frequency of the local VCO.

 This Phase Comparator Output is then applied to the INTERFACE circuit, which Holds it for Computer Processing.

IC'S commonly used in Telecommunication circuits include;

IC # .. TYPE USED for,

a. MC6830 ACIA PORTs

b. UART PORTs

c. NS16450 UART MODEMs

d. NS16550 UART MODEMs

c. USART PORTs

d. IEEE-488 Parallel Instrumentation