1 Chapter 6 The Data Communications Interface

2 Data Flow: Simplex zTransmits in only one direction zrarely used in data communications ze.g., receiving signals from the radio or TV station zthe sending station has only one transmitter, the receiving station has only one receiver

3 Simplex Illustration

4 Data Flow: Half Duplex zdata may travel in both directions, but only in one direction at a time zprovides non-simultaneous two-way communication zcomputers use control signals to negotiate when to send and when to receive zthe time it takes to switch between sending and receiving signals is called turnaround time

5 Half Duplex Illustration

6 Data Flow: Full Duplex zcomplete two-way simultaneous transmission zfaster than half-duplex communication because no turnaround time is needed

7 Full Duplex Illustration

8 zWhile OSI model is increasingly out of favor in application development, it is still very useful in understanding networking in a conceptual context

9 ISO’s Open Systems Interconnection (OSI) zApplication Layer zPresentation Layer zSession Layer zTransport Layer zNetwork Layer zData Link Layer zPhysical Layer

10 Physical Layer zRefers to transmission of unstructured bits over physical medium zDeals with characteristics of and access to the physical medium

11 Data Link Layer zProvides for reliable transfer of information across physical link zIncludes: ytransmission of blocks of data (“frames”) ysynchronization yerror control yflow control

12 Asynchronous & Synchronous Transmission zTiming problems require a mechanism to synchronize the transmitter and receiver zTwo solutions exist yAsynchronous ySynchronous zBoth methods are concerned with timing issues zHow does the receiver know when the bit period begins and ends? zSmall timing difference becomes more significant over time if no synchronization takes place between sender and receiver zSynchronization occurs on the data link layer

13 Asynchronous Transmission zUsed in serial communication zData transmitted 1 character at a time zCharacter format is usually 1 start & 1+ stop bits, plus data of 5-8 bits zCharacter may include parity bit zTiming needed only within each character zResynchronization is accomplished with each start bit zUses simple, cheap technology zWastes 20-30% of bandwidth

14 Synchronous Transmission zUsed in parallel communication zLarge blocks of bits transmitted without start/stop codes zSynchronized by a clock signal or clocking data zData framed by preamble (opening)/ postamble (closing) bit patterns zMore efficient than asynchronous zOverhead typically below 5% zUsed at higher speeds than asynchronous

15 Synchronization Choices zLow-speed terminals and PCs commonly use asynchronous transmission yinexpensive zLarge systems and networks commonly use synchronous transmission yoverhead too expensive; efficiency necessary yerror-checking more important

16 Digital Interfaces zThe point at which one device connects to another zStandards define what signals are sent, and how zSome standards also define the physical connector to be used

17 Generic Communications Interface Illustration

18 DTE and DCE

19 RS-232 and RS-449 zIt is a physical protocol to interface computers with modems yspecify mechanical, electrical, functional, and procedural interface Computer or Terminal Modem Protective Ground (1) Transmit (2) Receive (3) Request to Send (4) Clear to Send (5) Data Set Ready (6) Common Return (7) Carrier Detect (8) Date Terminal Ready (20)

20 RS-232C (EIA 232C) zEIA’s “Recommended Standard” (RS) zSpecifies mechanical, electrical, functional, and procedural aspects of the interface zUsed for connections between DTEs and voice-grade modems, and many other applications EIA-Electronics Industries Association

21 Mechanical Specifications z25-pin connector with a specific arrangement of leads zDTE devices usually have male DB25 connectors while DCE devices have female zIn practice, fewer than 25 wires are generally used in applications

22 V.24/EIA-232-F zITU-T v.24 zOnly specifies functional and procedural yReferences other standards for electrical and mechanical zEIA-232-F (USA) yBased on RS-232 yMechanical aspects are defined by ISO 2110 yElectrical v.28 yFunctional v.24 yProcedural v.24 ITU-International Telecommunication Union ISO-International Standards Organization

23 DB-25 Female DB-25 Male RS-232 DB-25 Connectors DB Connector-Data Bus Connector

24 RS-232 DB-25 Pinouts

25 RS-232 DB-9 Connectors zLimited RS-232

26 RS-422 DIN-8 zFound on Macs, some IBM compatible computers, network processor panels DIN-8 MaleDIN-8 Female DIN Connector-Deutsche Industrie Norm Connector

27 Electrical Specifications zSpecifies signaling between DTE and DCE zUses NRZ-L encoding yVoltage < -3V = binary 1 yVoltage > +3V = binary 0 yVoltage could be as high as  25 volts zRated for >20Kbps and <15M ygreater distances and rates are theoretically possible, but not necessarily wise

28 RS-232 Signals (Asynch) Even Parity Odd Parity No Parity See ASCII Table 3.1, Page 83

29 Functional Specifications zSpecifies the role of the individual circuits zData circuits in both directions allow full- duplex communication zTiming signals allow for synchronous transmission (although asynchronous transmission is more common)

30 Procedural Specifications zMultiple procedures are specified zSimple example: exchange of asynchronous data on private line yProvides means of attachment between computer and modem ySpecifies method of transmitting asynchronous data between devices ySpecifies method of cooperation for exchange of data between devices

31 See Table 6.1, Page 184 For the older RS-232-C standard, some of the pin definitions are: Pin NumberName (function) 2TD (Transmitted Data) 3RD (Received Data) 4RS (Request to Send) 5CS (Clear to Send) 6DSR (Data Set Ready) 20DTR (Data Terminal Ready) 8CD (Carrier Detect) 21SQ (Signal Quality detector)

32 Dial Up Operation (1)

33 Dial Up Operation (2)

34 Dial Up Operation (3)

35 Limited Distance Modem Example (Point-to-Point) zOnly a few circuits are necessary: ySignal Ground (7) yTransmitted Data (2) yReceived Data (3) yRequest to Send (4) yClear to Send (5) yDCE Ready (6) yReceived Line Signal Detector [Carrier Detect] (8) zAdditional circuits necessary sometimes: yDTE Ready(20) yRing Indicator (22)

36 Null Modem Cable SG DTR DSR RTS CTS CD TD RD SG DTR DSR RTS CTS CD TD RD zAllows DTE to DTE direct communication

37 EIA-232-D znewer version of RS-232-C adopted in 1987 zimprovements in grounding shield, test and loop-back signals zthe popularity of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace

38 RS-449 zan EIA standard that improves on the capabilities of RS-232-C zprovides for a 37-pin connection, cable lengths up to 200 feet, and data transmission rates up to 2 million bps zequates with the functional and procedural portions of R-232-C ythe electrical and mechanical specifications are covered by RS-422 and RS-423