1. William Stallings Data and Computer Communications Chapter 6 The Data Communications Interface

2. Asynchronous and Synchronous Transmission zTypically, data transmitted serially over transmission medium. zFor receiver to sample incoming bits properly, it must know yArrival time yDuration of each bit zTo receive bits correctly, timing of transmitter and receiver needs to be synchronized. zTwo solutions for synchronization yAsynchronous ySynchronous

3. Timing Problems – Example zAssume sender transmits a stream of bits at a rate of 1 Mbps. yBit duration =1us zAssume that receiver’s clock has a 1% drift over transmitter’s clock. yFirst bit is sampled at 0.51us instead of 0.5us ySecond bit is sampled at 1.52us instead of 1.5us yBit#50 is sampled at 50us instead of 49.5us zAfter 50 or more samples, the receiver will be in error as it will be sampling in the wrong bit time

4. Asynchronous zAvoid timing problems by not sending long uninterrupted stream of bits. zData transmitted one character at a time y5 to 8 bits zTiming or synchronization only needs maintenance within each character zResynchronize at the beginning of each new character

5. Asynchronous (diagram)

6. Asynchronous - Behavior zIn a steady stream, interval between characters is uniform (length of stop element) zIn idle state, receiver looks for transition 1 to 0 zThen samples next seven intervals (char length) zThen looks for next 1 to 0 for next char zSimple and cheap zOverhead of 2 to 3 bits per char (~20%) zGood for data with large gaps (keyboard)

7. Synchronous - Bit Level zBlock of data transmitted without start or stop bits zClocks must be synchronized zCan use separate clock line yGood over short distances ySubject to impairments zEmbed clock signal in data yManchester encoding yCarrier frequency (analog)

8. Synchronous - Block Level zNeed to indicate start and end of block zUse preamble and postamble ye.g. series of SYN (hex 16) characters ye.g. 8-bit flag 01111110 zMore efficient (lower overhead) than asynchronous

9. Line Configuration zTopology: physical arrangement of stations on medium yPoint to point yMulti point xComputer and terminals, local area network zHalf duplex yOnly one station may transmit at a time yRequires one data path zFull duplex ySimultaneous transmission and reception between two stations yRequires two data paths (or echo canceling)

10. Traditional Configurations

11. Interfacing zData processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities zNeed an interface called data circuit terminating equipment (DCE) ye.g. modem, NIC zDCE transmits bits on medium zDCE communicates data and control info with DTE yDone over interchange circuits yClear interface standards required

12. Interfacing

13. Characteristics of Interface zMechanical yConnection plugs zElectrical yVoltage, timing, encoding zFunctional yData, control, timing, grounding zProcedural ySequence of events for transmitting data

14. 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) yRS-232 yMechanical: ISO 2110 yElectrical: V.28 yFunctional: V.24 yProcedural: V.24

15. Mechanical Specification

16. Electrical Specification zDigital signals used zValues interpreted as data or control, depending on circuit zMore negative than -3v is binary 1, more positive than +3v is binary 0 (NRZ-L) zSignal rate < 20 kbps zDistance < 15 m zFor control, more negative than -3v is off, more positive than +3v is on

17. Functional Specification zCircuits can be grouped into four categories yData yControl yTiming yGround zData group yOne data circuit in each direction: Transmitted data (Pin 2), Received data (Pin 3) yTwo secondary data circuits useful for half-duplex transmission: Secondary Transmitted Data (Pin 14), Secondary Received data (Pin 16)

18. Functional Specification z16 control circuits, 10 of which related to transmission of data over primary channel zFor Asynchronous transmission, six of these circuits are used yRequest to send (Pin 4), Clear to send (Pin 5), DCE Ready (Pin 6), DTE Ready (Pin 20), Ring Indicator (Pin 22), Received Line Signal Detector (Pin 8) zFor Synchronous transmission, 4 more control signals are used ySignal Quality Detector (Pin 21), Data Signal Rate Selector (Pin 12 & 23), Ready for Receiving (Pin 4)

19. Functional Specification zThree control signals used to control secondary channel ySecondary Request to Send (Pin 19), Secondary Clear to Send (Pin 13), Secondary Received Line Signal Detector (Pin 12) zThree control signals used for loopback testing yRemote Loopback (Pin 21), Local Loopback (Pin 18), Test Mode (Pin 25)

20. Local and Remote Loopback

21. Functional Specification zThree timing signals provide clock pulses for synchronous transmission yTransmitter Signal Element Timing (Pin 15) yTransmitter Signal Element Timing (Pin 24) yReceiver Signal Element Timing (Pin 17) zWhen DCE is sending data over Received Data Circuit to DTE, it sends 1-0 and 0-1 transitions on Receiver Signal Element Timing timed to the middle of each received data signal element. zWhen DTE is sending data to DCE either DTE or DCE can provide the synchronizing clock.

22. Functional Specification z(See table in Stallings chapter 6)

23. Procedural Specification zExample: Asynchronous private line modem zWhen turned on and ready, modem (DCE) asserts DCE Ready zWhen DTE ready to send data, it asserts Request to Send yAlso inhibits receive mode in half duplex zModem responds when ready by asserting Clear to send zDTE sends data over Transmitted Data line zWhen data arrives from remote modem, local modem asserts Receive Line Signal Detector and delivers data on Received Data line.

24. Procedural Specification zAsynchronous private line modem is common for connecting two devices over a short distance within a building zTo transmit data over telephone network, two additional pins are required yDTE Ready yRing Indicator zFor short distances, it is possible for two DTEs to talk directly without DCEs, using Null Modem.

25. Dial Up Operation (1)

26. Dial Up Operation (2)

27. Dial Up Operation (3)

28. Null Modem

29. Integrated Services Digital Network (ISDN) zSupport of voice and nonvoice applications zSupport for switched and non-switched applications zReliance on 64 Kbps connections zProvides a set of channels at a single interface yB channel: 64 Kbps xCircuit switching, packet switching, dedicate yD channel: 16 Kbps xControl signaling (call setup) and some data

30. ISDN Channel Structure

31. Conceptual View of ISDN Connection Features

32. ISDN Architecture

33. ISDN Physical Interface zConnection between terminal equipment (TE) and network terminating equipment (NT) zPhysical connection defined in ISO 8877 zCables terminate in matching connectors with 8 contacts zTransmit/receive carry both data and control

34. ISDN Physical Interface Diagram

35. ISDN Electrical Specification zBalanced transmission yCarried on two lines, e.g. twisted pair ySignals as currents down one conductor and up the other yDifferential signaling yValue depends on direction of voltage yTolerates more noise and generates less zUnbalanced, e.g. RS-232 uses single signal line and ground) zData encoding depends on data rate zBasic rate 192kbps uses pseudoternary zPrimary rate uses Alternative Mark Inversion (AMI) with B8ZS (1.544 Mbps) or HDB3 (2.048 Mbps)