Chapter 6 The Data Communications Interface 1 Data

Chapter 6 The Data Communications Interface 1

Data Flow: Simplex z. Transmits 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 2

Simplex Illustration 3

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 4

Half Duplex Illustration 5

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

Full Duplex Illustration 7

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

ISO’s Open Systems Interconnection (OSI) z. Application Layer z. Presentation Layer z. Session Layer z. Transport Layer z. Network Layer z. Data Link Layer z. Physical Layer 9

Physical Layer z. Refers to transmission of unstructured bits over physical medium z. Deals with characteristics of and access to the physical medium 10

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

Asynchronous & Synchronous Transmission z Timing problems require a mechanism to synchronize the transmitter and receiver z Two solutions exist y. Asynchronous y. Synchronous z Both methods are concerned with timing issues z How does the receiver know when the bit period begins and ends? z Small timing difference becomes more significant over time if no synchronization takes place between sender and receiver z Synchronization occurs on the data link layer 12

Asynchronous Transmission z Used in serial communication z Data transmitted 1 character at a time z Character format is usually 1 start & 1+ stop bits, plus data of 5 -8 bits z Character may include parity bit z Timing needed only within each character z Resynchronization is accomplished with each start bit z Uses simple, cheap technology z Wastes 20 -30% of bandwidth 13

Synchronous Transmission z Used in parallel communication z Large blocks of bits transmitted without start/stop codes z Synchronized by a clock signal or clocking data z Data framed by preamble (opening)/ postamble (closing) bit patterns z More efficient than asynchronous z Overhead typically below 5% z Used at higher speeds than asynchronous 14

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

Digital Interfaces z. The point at which one device connects to another z. Standards define what signals are sent, and how z. Some standards also define the physical connector to be used 16

Generic Communications Interface Illustration 17

DTE and DCE 18

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

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

Mechanical Specifications z 25 -pin connector with a specific arrangement of leads z DTE devices usually have male DB 25 connectors while DCE devices have female z. In practice, fewer than 25 wires are generally used in applications 21

V. 24/EIA-232 -F ITU-International Telecommunication Union z. ITU-T v. 24 z. Only specifies functional and procedural y. References other standards for electrical and mechanical z. EIA-232 -F (USA) y. Based on RS-232 y. Mechanical aspects are defined by ISO 2110 y. Electrical v. 28 ISO-International Standards y. Functional v. 24 Organization y. Procedural v. 24 22

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

RS-232 DB-25 Pinouts 24

RS-232 DB-9 Connectors z. Limited RS-232 25

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

Electrical Specifications z. Specifies signaling between DTE and DCE z. Uses NRZ-L encoding y. Voltage < -3 V = binary 1 y. Voltage > +3 V = binary 0 y. Voltage could be as high as 25 volts z. Rated for >20 Kbps and <15 M ygreater distances and rates are theoretically possible, but not necessarily wise 27

RS-232 Signals (Asynch) Even Parity Odd Parity No Parity See ASCII Table 3. 1, Page 83 28

Functional Specifications z. Specifies the role of the individual circuits z. Data circuits in both directions allow fullduplex communication z. Timing signals allow for synchronous transmission (although asynchronous transmission is more common) 29

Procedural Specifications z. Multiple procedures are specified z. Simple example: exchange of asynchronous data on private line y. Provides means of attachment between computer and modem y. Specifies method of transmitting asynchronous data between devices y. Specifies method of cooperation for exchange of data between devices 30

See Table 6. 1, Page 184 For the older RS-232 -C standard, some of the pin definitions are: Pin Number Name (function) 2 TD (Transmitted Data) 3 RD (Received Data) 4 RS (Request to Send) 5 CS (Clear to Send) 6 DSR (Data Set Ready) 20 DTR (Data Terminal Ready) 8 CD (Carrier Detect) 21 SQ (Signal Quality detector) 31

Dial Up Operation (1) 32

Dial Up Operation (2) 33

Dial Up Operation (3) 34

Limited Distance Modem Example (Point-to-Point) z Only a few circuits are z Additional circuits necessary: necessary sometimes: y. Signal Ground (7) y. DTE Ready(20) y. Transmitted Data (2) y. Ring Indicator (22) y. Received Data (3) y. Request to Send (4) y. Clear to Send (5) y. DCE Ready (6) y. Received Line Signal Detector [Carrier Detect] (8) 35

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

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 37

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 38