William Stallings Data and Computer Communications Chapter 6

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

Asynchronous and Synchronous Transmission z Timing problems require a mechanism to synchronize the transmitter and receiver z Two solutions y. Asynchronous y. Synchronous

Asynchronous z Data transmitted on character at a time y 5 to 8 bits z Timing only needs maintaining within each character z Resync with each character

Asynchronous (diagram)

Asynchronous - Behavior z In a steady stream, interval between characters is uniform (length of stop element) z In idle state, receiver looks for transition 1 to 0 z Then samples next seven intervals (char length) z Then looks for next 1 to 0 for next char z Simple z Cheap z Overhead of 2 or 3 bits per char (~20%) z Good for data with large gaps (keyboard)

Synchronous - Bit Level z Block of data transmitted without start or stop bits z Clocks must be synchronized z Can use separate clock line y. Good over short distances y. Subject to impairments z Embed clock signal in data y. Manchester encoding y. Carrier frequency (analog)

Synchronous - Block Level z Need to indicate start and end of block z Use preamble and postamble ye. g. series of SYN (hex 16) characters ye. g. block of 1111 patterns ending in 11111110 z More efficient (lower overhead) than async

Synchronous (diagram)

Line Configuration z Topology y. Physical arrangement of stations on medium y. Point to point y. Multi point x. Computer and terminals, local area network z Half duplex y. Only one station may transmit at a time y. Requires one data path z Full duplex y. Simultaneous transmission and reception between two stations y. Requires two data paths (or echo canceling)

Traditional Configurations

Interfacing z Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities z Need an interface called data circuit terminating equipment (DCE) ye. g. modem, NIC z DCE transmits bits on medium z DCE communicates data and control info with DTE y. Done over interchange circuits y. Clear interface standards required

Characteristics of Interface z Mechanical y. Connection plugs z Electrical y. Voltage, timing, encoding z Functional y. Data, control, timing, grounding z Procedural y. Sequence of events

V. 24/EIA-232 -F z ITU-T v. 24 z Only specifies functional and procedural y. References other standards for electrical and mechanical z EIA-232 -F (USA) y. RS-232 y. Mechanical ISO 2110 y. Electrical v. 28 y. Functional v. 24 y. Procedural v. 24

Mechanical Specification

Electrical Specification z Digital signals z Values interpreted as data or control, depending on circuit z More than -3 v is binary 1, more than +3 v is binary 0 (NRZ-L) z Signal rate < 20 kbps z Distance <15 m z For control, more than-3 v is off, +3 v is on

Functional Specification z (See table in Stallings chapter 6)

Local and Remote Loopback

Procedural Specification z E. g. Asynchronous private line modem z When turned on and ready, modem (DCE) asserts DCE ready z When DTE ready to send data, it asserts Request to Send y. Also inhibits receive mode in half duplex z Modem responds when ready by asserting Clear to send z DTE sends data z When 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

ISDN Physical Interface Diagram

ISDN Physical Interface z Connection between terminal equipment (c. f. DTE) and network terminating equipment (c. f. DCE) z ISO 8877 z Cables terminate in matching connectors with 8 contacts z Transmit/receive carry both data and control

ISDN Electrical Specification z Balanced transmission y. Carried on two lines, e. g. twisted pair y. Signals as currents down one conductor and up the other y. Differential signaling y. Value depends on direction of voltage y. Tolerates more noise and generates less y(Unbalanced, e. g. RS-232 uses single signal line and ground) y. Data encoding depends on data rate y. Basic rate 192 kbps uses pseudoternary y. Primary rate uses alternative mark inversion (AMI) and B 8 ZS or HDB 3

Foreground Reading z Stallings chapter 6 z Web pages from ITU-T on v. specification z Web pages on ISDN