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Chapter 8

Data Communication

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Objectives

Explain various ways to connect DataTerminal Equipment (DTE) to DataCommunication Equipment (DCE).

Describe a null-modem cable.

Describe ASCII coding and Extended ASCIIcoding.

Explain serial transmission.

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Objectives (continued)

Explain parallel transmission.

Explain what a UART does.

Explain asynchronous transmission.Explain synchronous transmission.

Explain the error-detection technique used by

synchronous transmission.Explain the error-correction techniques used

by synchronous transmission.

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Objectives (continued)

Explain the term sliding window.

Explain ACK and NAK.

Explain baud rate and how it relates to bitrate.

Explain how a modem converts a data signalinto modulations of an analog signal.

Explain what a CSU/DSU is used for.

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Objectives (continued)

Explain why a modem must be used toconnect a PC to a regular phone line.

Explain how a 56-Kbps modem achieves that

speed.Explain how the end-to-end speed between

two PCs can be 115,000 bps when the speed

between the two modems is 28,800 bps.Explain the function of SS7.Explain how caller ID works.

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Data Communication

The primary difference between voice anddata lies in the signal used to conveyinformation.

The PSTN started life as an analog networkdesigned to handle analog voice signals.

When digital technology was integrated into

the switching matrix of class 5 SPC switchingsystems, the codec was moved from thechannel unit of the T1 carrier system to theline circuit of the switch.

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Data Communication

Several digital technologies have beendeveloped for voice communication thatmove the codec from the line circuit to thecustomers premises. Integrated Services Digital Network (ISDN)

Asymmetric Digital Subscriber Line (ASDL)

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Data Communication

Most signals are two-state digital signals. T1 carrier systems use alternating +3 V and

-3 V signals to represent a 1, and use 0 V to

represent a 0 The PC belongs to a category of equipment

called data terminal equipment (DTE).

The equipment used to connect a DTE to thePSTN is called data communicationequipment (DCE).

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Data Communication

The most common connection used toconnect DTE to DCE is an EIA-232 interface.

The EIA-232 interface cable comes in a 25-conductor and a 9-conductor cable.

The transmit lead and receive lead in thecable get their designation from the DTE end

of the cable.

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EIA-232 Interface

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Data Communication

A communication circuit will not work if thetransmitter is talking to another transmitter.

The two computers must be connected using

modems or a null-modem. The word modem is an acronym for

modulator/demodulator.

When data from a computer is fed into amodem, it modulates (changes) the analogsignal being sent to the other modem.

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RS-232 Null-Modem Cable

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8.1 Data and the Public

Switched Telephone Network

Because the PSTN has been evolving froman analog into a digital network, it is possibleto integrate data directly into the digital PSTNif the analog line circuit of a SPC switch isbypassed.

If a facility is to be used only for data an not

for voice, a channel service unit (CSU)/dataservice unit (DSU) is used to interface thedata circuit directly into the PSTN.

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Voice Circuit through Codec

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Data Blocked by Codec

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Modem Signal Passed by

Codec

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CSU/DSU Interface between

Customers Premises and C.O.

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Data and the PSTN

It is also possible to transmit data directlyover the local loop connecting the customerspremises to the central exchange with a

digital subscriber line circuit and ISDN. Cost is $100/month

Versus $20/month for an analog line circuit

The protocols used to transfer data are notthe same for ISDN and a modem.

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8.2 Data Communication and

Coding

Data communication is the transmission of datafrom one location to another.

The data transmitted can be numeric or

alphabetic. When data is transmitted, some code is used to

represent various characters or numbers sent

over the system. With the use of computers, a data

communication network has become essential.

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8.3 ASCII Coding

The standard binary code developed fortransmission of data is theAmericanStandard Code for Information Interchange

(ASCII).ASCII is the coding scheme used by a PC to store

information.

It uses 7 bits, each of which can be either a 1 or a0.

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Codes for Selected Decimal

Digits

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ASCII Code Chart

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Universal Asynchronous

Receiver/Transmitter (UART)

PCs contain a hardware device between thedata bus of the PC and the RS-232 serialport.

The UART will send a start bit (a 0, which is +3 Vor greater).

It will then send the least significant data bit andeach higher-order data bit until the most

significant bit is sent. It then sends the parity bit. It then sends a stop bit (a 1, which is -3 V or less).

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8.4 Extended ASCII Coding

It was not long before we needed torepresent more than 128 different characters.

To represent more than 128 differentcharacters, an eight-level coding system wasdeveloped called Extended ASCII. The first 128 codes are the same as for regular

seven-level ASCII. The codes from 128 to 255 are used to represent

additional characters.

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Extended ASCII Code Chart

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EBCDIC

Extended Binary Coded Decimal Interchange

Code

An eight-level coding system developed byIBM.

Used in its mainframes.

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Unicode

A computing industry standard allowing computersto consistently represent and manipulate textexpressed in most of the world's writing systems.

Developed in tandem with the Universal CharacterSet Consists of a repertoire of more than 100,000

characters.

The most commonly used encoding is UTF-8 Uses 1 byte for all ASCII characters, which have the samecode values as in the standard ASCII encoding.

Uses up to 4 bytes for other characters.

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8.5 Serial vs. Parallel

Transmission of Data

Data can be transmitted one bit or severalbits at a time.

Serial transmission sends one bit at a timeover the same wire.

Parallel transmission sends several bits at atime, each bit is transmitted over its own wire.

Most printers that connect to a PC areparallel printers transferring 8 bits at a time. Note that in computers, an 8-bit byte is referred to

as a word.

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8.6 Asynchronous vs.

Synchronous Transmission

There are two ways to transmit data usingserial transmission:Asynchronous Transmission

Synchronous Transmission

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Asynchronous Transmission

Each character has its own synchronizinginformation.

The beginning of the character is designated by a bitcalled the start bit. The start bit is a bit whose value is a 0. A bit that has a value of 0 is called a space bit.

The end of a character is designated by a bit knownas the stop bit. This bit has a value of 1. A bit that has a value of 1 is known as a mark bit.

Asynchronous transmission is also called start-and-stop transmission.

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Synchronous Transmission

This type of transmission sends data as ablock of characters at a time (a typical blockcontains 128, 256, 512, or 1024 characters).

A header is placed at the start of the datasent.

A trailer is placed at the end of the data.

Synchronous transmission is typically usedon a dedicated circuit connecting two high-speed computers.

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8.7 Asynchronous

Transmission Error Checking

The simplest error-checking method is paritychecking.

ASCII uses 7 bits for coding allowing the 8thbit to be used for parity.

There are five types of parity: Odd total number of 1s is odd.

Even total number of 1s is even. Mark parity bit is always a 1.

Space parity bit is always a 0.

None used for Extended ASCII

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8.8 Synchronous Error

Detection and Correction

Cyclic Redundancy Checking (CRC)

Automatic Retransmission Request (ARQ)

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Cyclic Redundancy Checking

Parity checking provides an end-to-end check (PCto PC) on each byte transmitted between two PCs,but no error checking is performed.

Modems use a synchronous error-detectiontechnique called cyclic redundancy checking (CRC).

The sender calculates a CRC remainder (typically16- or 32-bits).

The receiver recalculates the CRC based on thereceived data and compares its CRC with that of thesender.

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Automatic Retransmission

Request

Error conditions are handled by requestingthat the originating modem retransmit thedata.

Discrete ARQ

Continuous ARQ

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Discrete ARQ

Also called stop-and-wait ARQ because aftersending a block of data, the transmitting end willwait for a signal from the receiving end before

sending another block of data. The signal from the receiving end will be either a

positive acknowledgment (ACK) that the data wasgood, or a negative acknowledgment (NAK) that it

was corrupted. On receipt of an ACK, the next block of data is sent.

If a NAK is received, the last block of data is retransmitted.

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Continuous ARQ

With the use of discrete ARQ, the transmitting modemspends a lot of time waiting for the acknowledgmentmessage.

Continuous ARQ eliminates the need for the transmitting

modem to wait for an ACK or NAK on each block of datasent. The transmitting modem keeps sending data (up to a

specified maximum number of blocks) until an ACK orNAK is received.

If a NAK is received there are two ways the transmittercan respond. Go-Back-N Selective Repeat (Selective ARQ)

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8.9 Flow Control

Modems are continually storing and retrievingdata from their memory.

Some type offlow controlis necessary toprevent the memory buffers from overflowingand still keep enough data in memory toallow for retransmission.

There are two basic types of flow control: Software flow control

Hardware flow control

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Software Flow Control

Although both types of flow control use softwareprograms to control the flow of data across aconnection, the type based on what activates or

terminates flow control as software operation.Activation and deactivation can be done through

specific messages sent between the twodevices. The messages are special characters.

We refer to this as X-on/X-off software flow control.

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Hardware Flow Control

Hardware flow control uses a physical lead or wirebetween devices to activate flow control.

Between a PC and a modem, two wires are used toprovide a physical path for signals to the flow controlsoftware. Request-to-Send (RTS) Clear-to-Send (CTS)

When the PC has data to send, it sends an RTSsignal to the modem.

When a modem has approached a full-buffercondition, it will remove the CTS signal.

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8.10 Serial Ports and UARTs

The Universal Asynchronous Receiver/Transmitter (UART) is a parallel-to-serial andserial-to-parallel converter.

The UART between the parallel data bus andthe EIA-232 connector converts paralleltransmitted data from the computer into serialtransmission.

It also converts the received serial data from theEIA-232 interface into parallel transmission forthe computer.

H d Fl C t l RTS

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Hardware Flow Control RTS

and CTS

PC t M d S d d

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PC-to-Modem Speed and

Modem-to-Modem Speed

8 11 B d R t d M d l ti

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8.11 Baud Rate and Modulation

of the Carrier

Modems communicate with each other via audiofrequency waves modulated by digital input from a PC.

The audio signal between two modems is called the

carrier signal. The modulation to the audio signal can be accomplished

by changing the amplitude, frequency, or phase of theaudio signal.

How fast these signal changes occur is called the baudrate.

Baudrefers to the number of changes occurring in asignal.

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V.32bis Constellation Pattern

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V.34 Quarter-Superconstellation

with 240 Signal Points

8 12 C ti t th

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8.12 Connections to the

Internet

8 13 P k t S it hi

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8.13 Packet Switching

Networks

In a circuit switched network, we establish acircuit path through the PSTN for a call.

Thepublic data network (PDN) uses apacketswitched network (PSN).

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Connectionless Networks

In the PSN a call does not have to remain on thesame path through the network.

The Packet Assembler/Disassembler (PAD)assembles data from customers into packets.

The PAD sends the data packet to a Data SwitchingExchange (DSE).

The DSE checks the address of the intendedreceiver and determines which route to take throughthe PDN.

This type of switching is called a connectionlessnetworkbecause a connection is not maintained forthe duration of the call.

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Connection-Oriented Networks

It is also possible to establish connection-oriented packet circuits.

Switched virtual circuits and permanentvirtual circuits can be established in the PDNto achieve the equivalent of a circuit switchedfacility.

A virtual circuit allows all the packets to usethe same route.

8 14 Signaling S stem 7

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8.14 Signaling System 7

Network

Signaling System 7 (SS7) network is thepacket data network used by the PSTN.

All toll switches and many class 5 localswitching centers connect to the SS7network.

SS7 can only be used by computer-controlled

switching systems. The SS7 network is necessary for ISDN and

Custom Local Area Signaling Services

(CLASS).

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8.15 Caller Identification

A service provided by CLASS (others includeselective call forwarding and selective callrejection).

When a customer with caller ID receives acall: The CPU of the receiving switch will receive the

originating number over the SS7 network. The CPU will then cause this number to be sent

over the local loop to the receiving telephone.

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8.16 Summary

Data communication is the transmission ofcharacters, numbers, graphics, and symbolsusing digital signals.

The transmission between a PC an a modemis asynchronous transmission, where a startbit precedes each character and a stop bit

follows each character.Modems contain sophisticated error-detection

and error-correction techniques.

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Summary (continued)

Baud rate is defined as the number of times asignal changes.

The PDN is a network of data switches thatallows the public to transmit data in the formof packets.