1. Asynchronous and Synchronous Transmission Synchronization: - - PDF document

• Chap. 5 Data Communication Interface

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• 1. Asynchronous and Synchronous

Transmission

• Synchronization:

– Sender & receiver need to synchronize – Different levels of sync

• Clock sync: bit level
• Block sync: block or message level (character/word level)
• Asynchronous Transmission

– Bits are sent on a character-by-character basis – Independent clocks at sender & receiver (but matching reasonably) – Clock sync: receiver resync’s its clock every character – Block sync: each character is “bracketed” by start & stop bits – Advantage: simple & cheap, usable up to ~20 kbps – Disadvantage: not very efficient

1 Idle state

• f line Start

bit 5 to 8 data bits Odd,even,

• r unused

P bit stop Remain idle or next start bit 1-2 bit times

Character format

• Chap. 5 Data Communication Interface

2 start 100 200 300 400 500 600 700 800 93 186 279 372 465 558 651 744

Effect of timing error

• Synchronous Transmission

– Sender & receiver have identical clocks

• Separate clock line to carry clock signal from

sender

• “Self-clocking” transmission scheme

– Biphase encoding (e.g. Manchester) ← digital sig. – Using carrier signal ← Analog sig.

• Block sync:

– Special preamble & postamble bit patterns used to indicate start/end of a block (frame)

8-bit flag Control fields Data field Control fields 8-bit flag

Synchronous frame format

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• 2. Line Configurations
• Topology

– Physical arrangements of stations on a transmission medium – Point-to-point & Multipoint

• Full Duplex and Half Duplex

A B

Send and receive Only one way at a time

A B

Send receive simultaneously

Half-duplex Full-Duplex

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• 3. Interfacing
• Need a interface standard between DTE &

DCE which specify in detail

– mechanical – electrical – functional – procedural interface

• An example of physical layer protocol

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• V.24/EIA-232-E

– Interface standard between DTE and DCE – RS-232 was introduced in 1962 by EIA, and current version is EIA-232-E (5th version), and International version is ITU-T V.24

• RS: Recommended Standard
• EIA: Electronics Industry Association
• ITU: International Telecommunications Union (which has

superseded CCITT)

• ITU-T: ITU standard for Telecommunications

– Originally for modem, but can be used with digital networks – Official title for the standard is “Interface between DTE and DCE employing serial binary data interchange” – Mechanical Specification

• specifics of the connector, assignment of

interchange circuits to pins, size, dimensions, etc.

• 25-pin connector (known as D-connector)
• since not all 25 pins are necessary, 9-pin D-

connectors are also available

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Pin assignments – Electrical Specification

• NRZ-L

– Binary 1 (or OFF): -3V ~ -15V, – Binary 0 (or ON): +3V ~ +15V

• For less than 20 kbps and 15 meters

– Functional Specification

• Functional specification of each interchange circuit,

i.e., meaning of each circuit

• Circuits can be grouped into the categories of

data,control, timing, and ground

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7 for 9-pin connector

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– Loopback testing

• Useful fault-isolating tool
• Local loopback & remote loopbck

DCE ready Local loopback Remote loopback Test mode DCE ready Local loopback Remote loopback Test mode ON ON OFF ON ON OFF ON ON OFF OFF OFF ON Circuit Condition Circuit Local interface Remote interface

Local loopback Remote loopback Loopback circuit settings for V.24/EIA-232

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– Procedural Specification

• Legal sequence of events for a particular

application

• Example1: Asynchronous private line modem

(limited-distance modem)

– less than 2.5km apart DTE DCE

DTE ready DCE ready Request to send Clear to send Transmit data to DCE time

DTE Transmitting DTE Receiving

DTE DCE

Set Received-line signal detector Receive data from channel and send to DTE

• ver Received data

time

• Example 2: Modem over the telephone network

– Need additional circuit: ring indicator

Receive data

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V.24/EIA-232 dial-up operation

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– Null Modems

• Allow two DTEs communicate directly (with no

networks nor DCEs between them) DTE DTE