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Chapter 6 The Data Communications Interface

Chapter 6 The Data Communications Interface. Data Flow: Simplex. Transmits in only one direction rarely used in data communications e.g., receiving signals from the radio or TV station the sending station has only one transmitter, the receiving station has only one receiver.

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Chapter 6 The Data Communications Interface

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  1. Chapter 6The Data Communications Interface

  2. Data Flow: Simplex • Transmits in only one direction • rarely used in data communications • e.g., receiving signals from the radio or TV station • the sending station has only one transmitter, the receiving station has only one receiver

  3. Simplex Illustration

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

  5. Half Duplex Illustration

  6. Data Flow: Full Duplex • complete two-way simultaneous transmission • faster than half-duplex communication because no turnaround time is needed

  7. Full Duplex Illustration

  8. While 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) • Application Layer • Presentation Layer • Session Layer • Transport Layer • Network Layer • Data Link Layer • Physical Layer

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

  11. Data Link Layer • Provides for reliable transfer of information across physical link • Includes: • transmission of blocks of data (“frames”) • synchronization • error control • flow control

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

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

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

  15. Synchronization Choices • Low-speed terminals and PCs commonly use asynchronous transmission • inexpensive • Large systems and networks commonly use synchronous transmission • overhead too expensive; efficiency necessary • error-checking more important

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

  17. Generic Communications Interface Illustration

  18. DTE and DCE

  19. RS-232 and RS-449 • It is a physical protocol to interface computers with modems • specify mechanical, electrical, functional, and procedural interface Protective Ground (1) Computer or Terminal Modem 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. EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional, and procedural aspects of the interface Used for connections between DTEs and voice-grade modems, and many other applications RS-232C (EIA 232C) EIA-Electronics Industries Association

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

  22. V.24/EIA-232-F ITU-International Telecommunication Union • ITU-T v.24 • Only specifies functional and procedural • References other standards for electrical and mechanical • EIA-232-F (USA) • Based on RS-232 • Mechanical aspects are defined by ISO 2110 • Electrical v.28 • Functional v.24 • Procedural v.24 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. Limited RS-232 RS-232 DB-9 Connectors

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

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

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

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

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

  31. See Table 6.1, Page 184 For the older RS-232-C standard, some of the pin definitions are: Pin NumberName (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)

  32. Dial Up Operation (1)

  33. Dial Up Operation (2)

  34. Dial Up Operation (3)

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

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

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

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

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