CS601 - Data Communication - Lecture Handout 21

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DTE-DCE Interface

  • There are 4 basic functional units involved in communication of data:
    • A DTE and DCE on one end
    • A DTE and DCE on the other end

 

  • DTE: Any device that is a source of or destination of digital data
  • DCE: Any device that transmits/receives signal through network

DTE-DCE Interface

  • The DTE generates the data and passes it along with any control information to a DCE
  • The DCE converts the signal to a format appropriate to the TX medium and introduces it onto the network link
  • When the signal arrives at the receiving end this process is reversed

DTE

  • DTE includes any unit that functions either as a source of or as a destination for binary digital data
  • At the physical layer, it cab a terminal, microcomputer, computer , printer or any other device that generates or consumes digital data
  • DTEs do not often communicate with each other directly with each other
  • Think of DTE as your Brain.
  • Your and your friends brains are DTEs
  • The vocal chords or mouth are DCEs
  • Air is TX Meduium

DCE

  • DCE includes any functional unit that transmits or receives data in the form of an analog or digital signal through a network
  • At the physical layer, a DCE takes data generated by a DTE, converts it to the appropriate signal and then introduces it to the comm link
  • Commonly used DCEs at the physical layer include MODEMS
  • In any n/w , a DTE generates digital data and passes it on to a DCE, the DCE converts the data to a form acceptable to the TX medium and sends the converted signal to another DCE on the network
  • The second DCE takes the signal off, converts it to a suitable form for its DTE and delivers it
  • To make this communication possible, the sending and receiving DCEs must use the same modulation method

The two DTEs need not be coordinated with each other but they need to be coordinated with their respective DCEs and the DCEs must be coordinated so that data translation occurs w/o loss of integrity

Standards

Standards

Many standards have been developed to define the connection b/w the DTE and a DCE

  • Though the solution differ, each standard provides a model for mechanical, electrical and functional characteristics of the connection
  • Electronic Industries Organization (EIA) and ITU-T

EIA 232 Interface

  • Standard developed by EIA
  • Defines Mechanical, Electrical and Functional characteristics of the interface b/w DTE and a DCE
  • Originally issued in 1962 as the RS 232 standard
  • Revised several times, recent version EIA 232-D
  • Defines not only the type of connectors to be used but also the specific cable and plugs and the functionality of each pin

Mechanical Specifications

  • EIA 232 standard defines interface as a 25-wire cable with a male and a female DB 25 pin connector attached to either end.
  • The length of the cable may not exceed 15 meters(50 feet)
  • A DB 25 connector is a plug with 25 pin or receptacles each of which is attached to a single wire with a specific function
  • With this design, EIA has created the possibility of 25 separate interactions b/w a DTE and a DCE
  • Fewer are actually used but standard allows for future inclusion of functions
  • EIA 232 calls for a 25 wire cable terminated at one end by by a male connector and at the other end by a female connector
  • Male refers to a plug with each wire in the cable attaching to a pin
  • Female refers to a receptacle with each wire in cable connecting to a metal tube or sheath

Electrical Specifications (Sending data)

  • EIA 232 states that data must be tx as binary 1’s and 0’s using NRZ-L encoding with 0 defines as a positive and 1 defined as a negative voltage
  • However rather than defining a single range bounded by highest and lowest amplitudes, EIA 232 defines two distinct ranges , one for +ve voltages and one for -ve

Electrical Specifications (Sending data)

  • The receiver accepts any voltage that falls within these ranges as valid signals
  • To be recognized as data, the amplitude of the signal must fall b/w 3 and 15 volts or b/w –3 and –15 volts
  • Degradation of noise will in misinterpretation of bits
  • A square wave is shown to be converted into a curve by noise and it covers many voltages
  • If the rx were looking for a fixed voltage, or only for pulses that held a single voltage for their entire duration,degradation would have made it unrecoverable.

Electrical Specifications (Control & Timing)

  • Only 4 wires out of 25 in EIA 232 are used for data functions.
  • Remaining 21 are reserved for functions like Control, Timing, Grounding and Testing
  • Any of the functions is considered ON if it transmits a voltage of atleast +3 and OFF if it tx a voltage with a value less than -3
  • The specification for control signals is conceptually reversed from that for data transmission
  • A positive voltage means ON and a negative voltage means OFF

Electrical Specifications (Control & Timing)

Electrical Specifications (BIT rate)

  • A final imp o ortant function of electrical specifications is definition of Bit Rate
  • EIA 232 allows for a maximum bit rate of 20Kbps although in practice it is often is exceeded

Functional Specifications

  • Two different implementations of EIA 232 are available:
    • DB 25
    • DB 9
  • DB 25 connector defines the functions assigned to each of the 25 pins in the DB 25 connector

Functional Specifications

Functional Specifications

  • Figure shows the ordering and functionality of each pin of a male connector
  • Female connector will be a mirror image of the male so that pin 1 in the plug matches tube 1 in the receptacle and so on
  • Each comm function has a mirror or answering function for traffic in the opposite direction to allow for full duplex operation
  • For Example, pin 2 is for transmitting data and pin 3 is for receiving data
  • Pin 9 and 10 are for future use
  • Pin 11 is yet unassigned

 

  • DB9 IMPLEMENTATION
    • Most of the pins in DB 25 implementation are not necessary ina single asynchronous connection
    • A simpler 9 pin version of EIA 232 is shown in the figure

Functioning Step

  • Step 1: Preparation
    • Preparation of the interface between DTE and DCE
  • Step 2: Readiness
    • Checks if all four devices are ready
  • Step 3: Set up
    • –Set up the connection between two DCEs
  • Step 4: Data Transfer
    • DTE -> DCE -> DCE -> DTE
  • Step 5: Clearing
    • Turning OFF the connection

Functioning Example

Functioning Example

  • Step1: Preparation of interfaces for TX. The two grounding circuits,1(shield) and 7 (ground) are active b/w the two devices
  • Step 2 ensures that all four devices are ready for TX. First the sending DTE activates pin 20 and sends a Dte ready message to its DCE . DCE answers by activating pin6 . Same sequence is performed by remote computer
  • Step3: sets up the physical connection b/w the sending and the receiving modems.
  • First the DTE activates pin 4 and sends its DCE a request to send message. The DCE transmits a carrier signal to the idle receiving modem
  • When receiving modem detects the carrier signal, it activates pin 8, telling its computer that a TX is about to begin .
  • After tx the carrier signal, sending DCE activates pin 5 sending its DTE a clear to send message. The remote computer and modem do the same step
  • Step4: Data transfer procedure
  • Initiating computer transfers its data stream to its modem over circuit 2accompanied by the timing pulse of circuit 24.
  • Modem converts digital data to an analog signal and sends it over the network
  • Responding modem retreives the signal, converts it back to digital and passes it to DTE via circuit 3 and timing pulse of 17
  • Step5: Once both computers have completed their transmission, both computers deactivate their request-to-send circuits , modems turn off their carrier signals, their received line signal detectors and their clear to send circuits

NULL MODEM

  • Suppose you need to connect two DTEs in the same building, for example two workstations
  • Modems are not needed to connect two compatible devices directly
  • The TX never needs to cross analog lines, such as telephone lines and therefore does not need to be modulated
  • But you do need an interface to handle the exchanging , just as EIA 232 DTE- DCE cable does
  • The solution is a NULL Modem
  • A null modem provide DTA –DTE interface w/o DCEs
  • But why use a Null Modem
  • If all you need is the interface, why not just a a standard EIA 232 cable?

NULL MODEM

  • Part a shows a connection using a telephone network
  • Part b shows what happens when we use the same connections between two DTEs
  • The receive circuit is void because it has been isolated completely for the TX
  • The tx cct 2 endsup full of collision noise

NULL MODEM Crossing Connections

  • Whereas EIA 232 DTE-DCE interface cable has a female connector at the DTE and a male connector at the DCE end, a null modem has female connectors at both ends to allow it to connect to the EIA 232 DTE ports which a male

NULL MODEM Crossing Connections

Summary

  • DTE-DCE Interface
  • DTE-DCE Interface Standards
  • EIA-232
  • Null Modem

Reading Sections

  • Section 6.2,6.3, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan