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
- 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
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
- 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 (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
- 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
- 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?
- 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
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