Chapter 6 The Data Communications Interface

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William Stallings

Data and Computer

Communications

Chapter 6

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Asynchronous and Synchronous

Transmission

 _{Timing problems require a mechanism to} synchronize the transmitter and receiver  _{Two solutions}

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Asynchronous

 _{Data transmitted on character at a time}  _{5 to 8 bits}

 _{Timing only needs maintaining within each} character

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Asynchronous - Behavior

 _{In a steady stream, interval between characters}

is uniform (length of stop element)

 _{In idle state, receiver looks for transition 1 to 0}  _{Then samples next seven intervals (char length)}  _{Then looks for next 1 to 0 for next char}

 _Simple

 _Cheap

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Synchronous - Bit Level

 _{Block of data transmitted without start or} stop bits

 _{Clocks must be synchronized}  _{Can use separate clock line}

 _{Good over short distances}  _{Subject to impairments}

 _{Embed clock signal in data}  _{Manchester encoding}

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Synchronous - Block Level

 _{Need to indicate start and end of block}  _{Use preamble and postamble}

 _{e.g. series of SYN (hex 16) characters}

 _{e.g. block of 11111111 patterns ending} in 11111110

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Line Configuration

 _Topology

 _{Physical arrangement of stations on medium}  _{Point to point}

 _{Multi point}

 _{Computer and terminals, local area network}

 _{Half duplex}

 _{Only one station may transmit at a time}  _{Requires one data path}

 _{Full duplex}

 _{Simultaneous transmission and reception between two}

stations

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Interfacing

 _{Data processing devices (or data terminal} equipment, DTE) do not (usually) include data transmission facilities

 _{Need an interface called data circuit} terminating equipment (DCE)

 _{e.g. modem, NIC}

 _{DCE transmits bits on medium}

 _{DCE communicates data and control info} with DTE

 _{Done over interchange circuits}

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Characteristics of Interface

 _Mechanical

 _{Connection plugs}  _Electrical

 _{Voltage, timing, encoding}  _Functional

 _{Data, control, timing, grounding}  _Procedural

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V.24/EIA-232-F

 _{ITU-T v.24}

 _{Only specifies functional and procedural}

 _{References other standards for electrical} and mechanical

 _{EIA-232-F (USA)}  _RS-232

 _{Mechanical ISO 2110}  _{Electrical v.28}

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Electrical Specification

 _{Digital signals}

 _{Values interpreted as data or control,} depending on circuit

 _{More than -3v is binary 1, more than +3v} is binary 0 (NRZ-L)

 _{Signal rate < 20kbps}  _{Distance <15m}

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Functional Specification

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Procedural Specification

 _{E.g. Asynchronous private line modem}

 _{When turned on and ready, modem (DCE)}

asserts DCE ready

 _{When DTE ready to send data, it asserts}

Request to Send

 _{Also inhibits receive mode in half duplex}  _{Modem responds when ready by asserting}

Clear to send

 _{DTE sends data}

 _{When data arrives, local modem asserts}

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ISDN Physical Interface

 _{Connection between terminal equipment} (c.f. DTE) and network terminating

equipment (c.f. DCE)  _{ISO 8877}

 _{Cables terminate in matching connectors} with 8 contacts

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ISDN Electrical Specification

 _{Balanced transmission}

 _{Carried on two lines, e.g. twisted pair}

 _{Signals as currents down one conductor and}

up the other

 _{Differential signaling}

 _{Value depends on direction of voltage}

 _{Tolerates more noise and generates less}

 _{(Unbalanced, e.g. RS-232 uses single signal}

line and ground)

 _{Data encoding depends on data rate}

 _{Basic rate 192kbps uses pseudoternary}

 _{Primary rate uses alternative mark inversion}

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Foreground Reading

 _{Stallings chapter 6}