COE 342: Data

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COE 342 Data Computer Communications
(T042)Dr. Marwan Abu-Amara

• Chapter 6
• Digital Data Communications Techniques

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Contents

• Asynchronous and Synchronous Transmission
• Line Configuration
• Topology point-to-point, multipoint
• Full/Half Duplex
• Interfacing
• V.24/EIA-232-F (formerly known as RS-232)
• ISDN Physical Interface

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Timing Requirements

• Reception of digital data requires sampling of
  received signal at receiver ? Sampling time
  should be known
• Clock drift (example)
• If a receiver clock drifts by 1 every sample
  time,
• Then for Tb 1msec, total drift after 50 bit
  times 50 X 0.01 0.5 msec
• Hence, instead of sampling at the middle of the
  bit time, the receiver will sample at the edge of
  the bit (i.e. receiver is out-of-synch with
  transmitter clock)
• For correct reception, receiver clock/carrier
  should be synchronized with transmitter

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Asynchronous Synchronous Transmission

• Timing problems require a mechanism to
  synchronize the transmitter and receiver
• Two solutions
• Asynchronous
• Synchronous

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Asynchronous

• To avoid the timing problem by NOT sending long,
  uninterrupted streams of bits.
• For that, data transmitted one character at a
  time
• 5 to 8 bits
• Timing only needs maintaining within each
  character
• The receiver has the opportunity to resynchronize
  at the beginning of each new character.
• Used for short stream of bits data transmitted
  one character (5 8 bits) at a time
• Synchronization is needed to be maintained for
  the length of short transmission
• Character is delimited (start end) by known
  signal elements start bit stop element

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Asynchronous Efficiency

• Efficiency transmit 1 start bit 8 bits of data
  2 stop bits
• ? Efficiency 8/11 72
• (or overhead 3/11 28)
• Good for data with large gaps (e.g. keyboard, etc)

Receive idle signal
Receive Start bit
Receive Stop element
S1 receiver in idle state S2 receive is
receiving character
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Asynchronous (diagram)
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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
• Overhead of 2 or 3 bits per char (20)
• Good for data with large gaps (keyboard)

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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
• Carrier frequency (analog)

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Synchronous Frame Format

• Typical Frame Structure

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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
• Requires two data paths (or echo canceling)

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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
• DCE transmits bits on medium
• DCE communicates data and control info with DTE
• Done over interchange circuits
• Clear interface standards required

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Characteristics of Interface

• Mechanical
• physical specification of connection
• type, dimensions, location of pins, etc
• Electrical
• Voltage level, timing, encoding
• Functional
• Data, control, timing, grounding
• specify functions that are performed such as Rx
  and Tx circuits
• Procedural
• Specification of sequence of event for
  transmitting data based on functional
  specification Sequence of events
• Two examplesV.24/EIA-232-F, and ISDN physical
  interface

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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
• Functional v.24
• Procedural v.24

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Mechanical Specification
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Electrical Specification

• Digital signals
• Values interpreted as data or control, depending
  on circuit
• More (-)ve than -3v is binary 1, more ()ve than
  3v is binary 0 (NRZ-L)
• Signal rate lt 20kbps
• Distance lt15m
• For control, more (-)ve than -3v is off, more
  ()ve than 3v is on

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Functional Specification

• (See table 6.1 in Stallings chapter 6)

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Local and Remote Loopback
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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 Receive
  Line Signal Detector and delivers data

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V.24/EIA-232-F Procedural Specification (1)

• Two terminals (computers) connected back to
  back through modems (no telephone network) as
  shown bellow
• Interchange Circuits Needed
• Signal Ground (102)
• Transmitted Data (103)
• Received Data (104)
• Request to Send (105)
• Clear to Send (106)
• DCE Ready (107)
• Received Line Signal Detector (109)

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V.24/EIA-232-F Procedural Specification (2)

• Sequence (DTE A sends a character to DTE B)
• When DCE A is ready (e.g. turned on)? DCE Ready
  (107) ON
• When DTE A has data to send ? Request to Send
  (105) ON
• DCE A responds ? Clear to Send (106) ON
• DTE A now sends data ? Transmitted Data (103)
• When DCE B receives data ? Received Line Signal
  Detector (109)
• DCE B deliver data to DTE B ? Received Data (104)

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Dial Up Operation (1)
1. For dial up operation we will also need DTE
Ready (108.2) Ring Indicator (125) 2. Example
Modem A rings Modem B , Modem B accepts the call
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Dial Up Operation (2)
Modem A confirms the connection to DTE A and also
generates a carrier for Modem B
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Dial Up Operation (3)
Data exchange phase DTE sends data to Modem A
Modem A modulate and transmit to Modem B Modem
B recovers data and sends to DTE B
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Null Modem
Good for short distance
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ISDN Physical Interface

• ISDN Terminology
• TE Terminal equipment (equivalent to DTE)
• NE Network equipment (equivalent to DCE or point
  of connection to network)

• Two pins for exchanging data and control for each
  direction (Note TE and NE need to make the right
  interpretation of signal!)

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ISDN Physical Interface

• Connection between terminal equipment TE
  (corresponds to DTE) and network terminating
  equipment NE (corresponds to DCE)
• Physical connection defined in ISO 8877
• Which define cables terminate in matching
  connectors with 8 contacts
• Transmit/receive carry both data and control