digital data transmission dte-dce interface modems …csnotes.upm.edu.my/kelasmaya/web.nsf... ·...

Lecture notes SKR 3200 17 August 2009

Idawaty Ahmad 1

CHAPTER 4TRANSMISSION OF DIGITAL DATA

8/17/200TRANSMISSION OF DIGITAL DATAINTERFACE AND MODEM

Digital Data TransmissionDTE-DCE InterfaceModems

09Idaw

aty Ahm

ad : idawaty@ode s @

fsktm.upm

.edu.my

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INTRODUCTION

How do we encoded data from the generatingdevice (PC) to the next device (modem) before it is

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device (PC) to the next device (modem) before it issent over a communication link (telephone line)?The answer is interfaceinterface – a bundle of wires, a sortof mini-communication linkBecause an interface links two devices notnecessarily made by the same manufacturer, itscharacteristics must be defined and standardsstandards mustmust

Idawaty A

hmad : idaw

aty@fsk

bebe establishedestablished..The characteristics: mechanical spec, electricalspec, functional spec.

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ktm.upm

.edu.my


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DIGITAL DATA TRANSMISSION8/17/2009

Transmission of data wiring data streamDo we send one bit at a time, or do we group bitsDo we send one bit at a time, or do we group bitsinto larger groups, if so how?The transmission of binary data across a link can beaccomplished either in parallel mode or serialmode.In parallel mode – multiple bits are sent with eachclock pulse

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In serial mode – one bit is sent with each clockpulse. Occurs in two ways: synchronous andasynchronous.

PARALLEL TRANSMISSION

Binary data (consist of 1s,0s) may be organized intogroups of n bits each.

8/17/2009g pConcepts: “use nn wires to send nn bits at one time”Each bit has its own wire, and all n bits one groupcan be transmitted with each clock pulse from onedevice to another.Advantage:

Speed – increase transfer speed by factor of n over serialDisadvantage:

Cost – requires n communication lines (wires) just totransmit the data stream.

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Figure 2 Parallel transmission

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SERIAL TRANSMISSION

Concepts: “one bit follows another in single wire”Required only one communication channel (wire)

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Required only one communication channel (wire)rather than nn to transmit data between twocommunicating devices.Advantage:

Reduces the cost of transmission – by using only one wireSince communication within devices is parallel,conversion devices are required at the interfaceqbetween the sender and the line (parallel-to-serial)and between the line and the receiver (serial-to-parallel).

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Figure 3 Serial transmission

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Figure 1 Data transmission

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SERIAL TRANSMISSION: ASYNCHRONOUSTRANSMISSION

Timing of a signal is unimportant.Instead, information is received and translated by agreed-upon

tt

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patterns.Patterns are based on grouping the bit stream into bytes.Each group consist of 8 bits, is sent along the link as a unit.Without a synchronizing pulse, the receiver cannot use timing topredict when the next group will arrive.To alert the receiver to the arrival of a new group, an extra bit isadded to the beginning of each byte.

Start bit – usually 0;To let receiver know that the byte is finished, one or moreadditional bits are appended to the end of byteadditional bits are appended to the end of byte.

stop bits – usually 1s.Each byte size is increased to at least 10 bits. Gap between bytetransmission is varies.

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Figure 4 Asynchronous transmission

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In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte There may be a gap between each byte

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byte. There may be a gap between each byte.Called asynchronous because, at the byte level, sender andreceiver do not have to be synchronized.But within each byte, the receiver must still be synchronizedwith the incoming bit stream.When the receiver detects a start bit, it sets a timer and beginscounting bits as they come in. After n bits, the receiver looksfor a stop bit.The addition of stop and start bits and the insertion of gapsThe addition of stop and start bits and the insertion of gapsinto the bit stream make asynchronous transmission slow.Cheap and effective – choice for low speed communicationE.g: Connection of a terminal to a computer.

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I h i i d 1 biI h i i d 1 bi

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Asynchronous here means “asynchronous at theAsynchronous here means “asynchronous at the

In asynchronous transmission, we send 1 start bit In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between the end of each byte. There may be a gap between

each byte.each byte.

Asynchronous here means asynchronous at the Asynchronous here means asynchronous at the byte level,” but the bits are still synchronized; byte level,” but the bits are still synchronized;

their durations are the same.their durations are the same.12


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SERIAL TRANSMISSION: SYNCHRONOUSTRANSMISSION

The bit stream is combined into longer “frames”, which maycontain multiple bytes.

i i i i i i

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Each byte is introduced onto the transmission link without agap between it and the next one.It is the responsibility of the receiver to group the bits.Timing becomes very important.Because the accuracy of the received information iscompletely dependent on the ability of the receiving devicesto keep an accurate count of the bits as they come.Advantage:

Speed – without extra bits, gap, it is faster than asynchronoustransmission

More useful for high-speed applications like the transmissionof data from one computer to another.

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Figure 5 Synchronous transmission

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SERIAL TRANSMISSION: SYNCHRONOUSTRANSMISSION

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In synchronous transmission, In synchronous transmission, we send bits one after another without start/stop we send bits one after another without start/stop

bits or gaps. bits or gaps. It is the responsibility of the receiver to group the It is the responsibility of the receiver to group the

bits.bits.

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DTE-DCE INTERFACE

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Data Terminal Equipment (DTE) and Data Circuit-TerminatingEquipment (DCE)There 4 basic functional units involved in the communicationdata:

DTE and DCE at one end and DTE and DCE at the other andThe DTE generates the data and passes them to a DCE.The DCE converts the signal to a format appropriate to thetransmission medium and introduces it onto the network link.When the signal arrives at the receiving end this process is

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When the signal arrives at the receiving end, this process isreversed.


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Figure 6 DTEs and DCEs

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Figure 7 DTE – DCE interface

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DTE

DTE includes any unit that functions either as a source of or asd ti ti f bi di it l d t

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a destination for binary digital data.It can be a terminal, computer, printer, fax machine .. Or anyother device that generates or consumes digital data.DTE do not often communicate directly with another one; theygenerate and consume information but need an intermediaryto be able to communicateAnalogy: our brain does when we talk. Brains –DTE, mouth/ear– DCE, air/telephone line –transmission medium

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DTE-DCE

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DTE-DCE8/17/2009


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DCE

DTE includes any unit that transmits or receives datain the form of an analog or digital signal through a

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in the form of an analog or digital signal through anetwork.@ PL – a DCE takes data generated by a DTE,converts them to an appropriate signal, and thenintroduces the signal onto the communication link.Commonly used DCEs include modemsTo make communication possible, both thesending and receiving DCEs must use the samemodulating method (e.g.FSK).

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STANDARDS

Many standards have been developed to defineth ti b t DTE d DCE

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the connection between a DTE and a DCEEach standard provides a model for mechanical,electrical, functional characteristics of theconnectionEIA and ITU-T are the most active organization havebeen involved in developing DTE-DCE interfacestandards.EIA standards – EIA-232, EIA-442, EIA-449ITU-T standards – V series and X series

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Figure 10 EIA-232

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Figure 11 Data Pins

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Figure 12 Control Pins

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Figure 13 Timing Pins

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Figure 14 Other Pins

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Figure 15 Synchronous Full-Duplex Transmission

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Figure 16 Synchronous Full-Duplex Transmission (cont..)

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MODEMS

The most familiar type of DCE.

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Internal/external modem attached to your PC -converts the digital signal generated by the PC intoan analog signal to be carried by a public accessphone line.Modem stands for modulator/demodulatorA modulator converts a digital signal into anA modulator converts a digital signal into ananalog signal using ASK, FSK, PSK or QAMA demodulator converts an analog signal into adigital signal.

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Figure 17 Modem concepts

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Figure 18 Bandwidth for Telephone Line

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MODEMS

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A telephone line has a bandwidth of almost 3000Hz(It carry frequencies between 300Hz to 3300Hz) –( y q )voice transmissionBandwidth of telephone line for data transmission is2400Hz (600Hz to 3000Hz)Modem Standards: (Bell Modem, ITU-T modem)

Bell Modem:The first commercial modems were produced by the BellTelephone Company in the early1970s.E g: 103/113 Series Earliest model Operates in full duplex

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E.g: 103/113 Series. Earliest model. Operates in full-duplexmode over two-wire switched telephone lines. Transmission issynchronous, using FSK modulation. The data rate is 300 bps.Others: 202 Series, 212 Series, 201 Series, 208 Series, 209 Series.


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MODEMS

ITU-T Modem StandardsM f th t l d il bl

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Many of the most popular modems available arebased on standards published by the ITU-T.Can be divided into two groups: essentially equivalentto Bell series modems and those are not.E.g. of equivalent to Bell – V.21 similar to Bell modem103Others: V.22bis, V.32, V.32bis…

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TRADITIONAL MODEMS AND 56K MODEMS

Traditional modems have a limitation on thed t t 33 6Kb

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data rate - max 33.6KbpsNew modem have a bit rate of 56,000 bps(called 5656KK modemmodem)

AsymmetricalDownloading (flow of data from the Internet Provider tothe PC) – a maximum of 56Kbps) pUploading (flow of data from the PC to the InternetProvider) - can be a maximum of 33.6 Kbps

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TRADITIONAL MODEMS

Traditional modems:

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Sending data from a computer at site A toanother computer at site B and vice versa.Result:

Uploading – max data rate is 33.6 KbpsDownloading – max data rate is 33.6 Kbps.

Limiting factor is the quantization step using PCM.

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Figure 19 Traditional modems

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Telekom

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PCM (Pulse Code Modulation)

8/17/2009Quantization is the process of approximating a continuous range of values (or a very large set of possible discrete values) set of possible discrete values) by a relatively small set of discrete symbols or integer values.

An analog-to-digital converter is a device which converts continuous signals to discrete digital numbers. The reverse operation is performed by a digital to analog converter

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by a digital-to-analog converter(DAC).

56K MODEMS

56K modems:At Internet provider site (which use digital

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At Internet provider site (which use digitalsignaling) does not have to pass through a PCMconverter - quantization is eliminated in onedirection, so the data rate can be increased to56Kbps.Result:

U l di d t t i till 33 6 Kb ( llUploading – max data rate is still 33.6 Kbps (as welltraditional modem).Downloading – max data rate is now 56 Kbps.

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56K MODEM8/17/2009

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Figure 20 56K modems

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Telekom

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8/17/2009Idaw

aty Ahm

ad : Semester 2 :

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