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

Asynchronous and Synchronous Transmission zTiming problems require a mechanism to synchronize the transmitter and receiver zTwo solutions yAsynchronous ySynchronous

Asynchronous zData transmitted on character at a time y5 to 8 bits zTiming only needs maintaining within each character zResynchronize with each character

Asynchronous (diagram)

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 zCheap zOverhead of 2 or 3 bits per char (~20%) zGood for data with large gaps (keyboard)

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)

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. block of patterns ending in zMore efficient (lower overhead) than asynchronous

Synchronous (diagram)

Line Configuration zTopology yPhysical 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)

Traditional Configurations

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

Interfacing

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

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

Mechanical Specification

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

Functional Specification z(See table in Stallings chapter 6)

Local and Remote Loopback

Procedural Specification zE.g. 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 zWhen data arrives, local modem asserts Receive Line Signal Detector and delivers data

Dial Up Operation (1)

Dial Up Operation (2)

Dial Up Operation (3)

Null Modem

Integrated Services Digital Network (ISDN) zSupport of voice and nonvoice applications zSupport for switched and non-switched applications zReliance on 64Kbps 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

ISDN Channel Structure

Conceptual View of ISDN Connection Features

ISDN Architecture

ISDN Physical Interface zConnection between terminal equipment (DTE) and network terminating equipment (DCE) zISO 8877 zCables terminate in matching connectors with 8 contacts zTransmit/receive carry both data and control

ISDN Physical Interface Diagram

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) and B8ZS or HDB3