Electrical Interface

1
Chapter 2

• Electrical Interface

2
Introduction

• 2? data? ???? ?? ???? ??? ????? ??
• ??? ????? ?? ??? ?? attenuated distorted
• Figure 2.1
• attenuation? distortion? ??? ?? ??
• ????? ??
• data? bit rate
• ????

3
????

• Transmission line
• a pair of conductors or wires
• electromagnetic wave
• glass fiber
• ????? ??
• bps? ?? ??

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(No Transcript)
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Two-wire open lines

• Simplest transmission medium
• 50m ??
• 19.2kbps ??
• The complete set of wires
• multicore cable
• flat ribbon cable

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Two-wire open lines (cont.)

• Crosstalk
• cross-coupling of electrical signals between
  adjacent wires in the same cable
• ??, ??? ??

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Twisted Pair lines

• ??? ?? ???? ??? ?? twisted pair ??
• Any interface signal is picked up by both wires
  reducing its effect on the difference signal
• 1Mbps ??? bit rate
• 100m ??

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Twisted Pair lines (cont.)

• ??
• UTP(Unshielded Twisted Pairs)
• ????? ?? ??
• STP(Shielded Twisted Pairs)
• ?? ?? ??? ?? ??

9
Coaxial cable

• Skin effect
• Twisted-pair? ?? ?? ??
• increases bit rate ? the current flowing tends to
  flow on the outer surface of the wire ? less of
  the available cross-section ? increases
  electrical resistance ? higher attenuation
• ?? m ??
• 10Mbps ??
• point-to-point multipoint topologies? ??

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Optical fiber

• Optical fiber
• in form of a fluctuating beam of light
• a much wider bandwidth
• hundreds of Mbps
• immune to electromagnetic interference and
  crosstalk
• useful in electronically noisy environments

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Optical fiber

• Transmitter
• uses LED or LD
• Receivers
• uses photodiode or photo transistor

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Optical fiber (cont.)

• Transmission mode
• depending on the type and width of core material
  used
• multimode stepped index fiber
• multimode graded index fiber
• monomode fiber

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Satellites

• Transmitted
• using electromagnetic (radio) waves through free
  space in satellite system

14
Terrestrial microwave

• Communication links
• when it is impractical or too expensive to
  install physical transmission media
• for example, across a river or perhaps a swamp or
  desert

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Radio

• It is used in place of fixed-wire links over
  modest distances using ground-based transmitters
  and receivers

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Attenuation and distortion sources

• The various attenuation and distortion effects
  that can degrade a signal during transmission

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Attenuation

• Signal attenuation
• As a signal propagates along a transmission
  medium its amplitude decreases
• Amplifiers (Repeaters)
• to restore the received signal to its original
  level
• Equalizers
• to equalize the attenuation across a defined band
  of frequencies

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Delay distortion

• Delay distortion
• the rate of propagation of a sinusoidal signal
  along a transmission line varies with the
  frequency of the signal
• Intersymbol interference
• as the bit rate increases, so some of the
  frequency components associated with each bit
  transition are delayed and start to interfere
  with the frequency components associated with a
  later bit.

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Noise

• Line noise level
• there are random perturbations on the line even
  when no signal is being transmitted
• Signal-to-noise ration (SNR, ratio S/N)
• the ration of the average power in a received
  signal, S, to the power in the noise level , N

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Signal types

• Modem
• When we use telephony service
• to convert the electrical signals output by the
  source DTE into a form analogous to the signals
  used to convey spoken messages
• on reception, to convert signals back into a form
  suitable for use by the destination DTE

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Coaxial cable signals

• Two ways
• Baseband mode
• all the available bandwidth is used to derive a
  signal high bit rate transmission path
• Broadband mode
• the available bandwidth is divided to derive a
  number of lower bandwidth subchannels on one cable

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Baseband mode (cont.)
23
TDM

• Time-division multiplexing
• to share the available capacity of a based
  transmission channel
• two types
• Synchronous
• Each user has access to the channel at precisely
  defined time intervals
• Asynchronous
• Each user has random access to the channel and,
  on acquiring access, is the sole user of the
  channel for the duration of transmission

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Broadband

• FDM(Frequency-division multiplexing)
• using the broadband mode, multiple transmission
  channels are derived from a single distribution
  cable
• requires a device, a radio frequency(RF) modem

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Optical fiber signals

• A variety of forms of optical signal encoding
• based on a bipolar encoding scheme
• produces a three level optical output
• makes it suitable for operating the cable from DC
  up to 50 Mbps
• is used mainly in a point-to-point mode

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Optical fiber signals (cont.)
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Satellite and radio

• Access control methods
• Random access
• all stations compete for a transmission channel
  in a random way
• Fixed assignment
• Both the channel frequency and the channel time
  slot are preassigned to each ground or cordless
  station

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Satellite and radio (cont.)

• Demand assignment
• when a station wishes to transmit data, it first
  requests channel capacity from a central site
  which assigns the required capacity to the
  requesting station

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Aloha

• Definition
• the oldest type of access control method
• be first used to control access to a single
  satellite channel

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Aloha (cont.)

• Pure Aloha
• operation
• when a station has a message to transmit, it
  simply transmits it. If a second station starts
  to transmit while the first station is
  transmitting its message, both transmissions are
  corrupted and a collision occurs
• less than 20 of the available capacity

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Aloha (cont.)

• Slotted Aloha
• establish a synchronous time clot structure
• constrain all transmissions to be carried out in
  these time slots
• a transmission can corrupt another transmission
  only in the same time slot
• utilization in excess of 30 are possible

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Aloha (cont.)

• Preassigned FDMA
• with fixed assignment, both the channel frequency
  and/or channel time slot are preassigned to each
  station in advance
• be easier than time slot assignment
• demand-assigned TDMA
• much better channel utilization
• using a demand assignment access control method

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Signal propagation delay

• Transmission propagation delay
• time delay for a signal to propagate from one end
  of a transmission medium to the other
• Round-trip delay
• the time delay between the first bit of a block
  being transmitted by the sender and the last bit
  of its associated acknowledgement being received

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Public carrier circuits

• When we wish to transmit data between two DTEs in
  different establishments
• The solution is widely used
• switched circuits
• leased circuits

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Analog PSTN circuits

• When we wish to transmit data using existing
  analog PSTN transmission lines
• we must convert the electrical signals output by
  the source DTE into a form that is acceptable to
  the PSTN

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Analog PSTN circuits (cont.)

• Modulator / Demodulator
• we must convert the binary data into a form
  compatible with a speech signal at the sending
  end of the line and reconvert this signal back
  into its binary form at the receiver

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Digital leased circuits

• Digital leased circuits
• be used not only to provide a direct connection
  between two DTEs but also as the basis of most
  private data network
• normally, be used when a single organization or
  enterprise has a very high level of intersite
  traffic
• ISDN(Integrated services digital network)
• without modem

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Voice digitization

• Nyquist sampling theorem
• to convert signals into digital form
• amplitude must be sampled at a minimum rate of
  greater than twice the highest frequency component

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Voice digitization (cont.)

• Pulse amplitude modulated(PAM) signal
• the sampled signal is converted into a pulse
  stream
• Quantizing
• PAM signal is converted into an all-digital form
• Pulse code modulated)PCM) signal

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Multiplexing

• TDM
• circuits that link exchanges carry multiple calls
  concurrently in a digital form
• Aggregate link bit
• North America and Japan
• 24 voice channel
• 1.544Mbps
• ITU-T recommendation
• 30 channel
• 2.048Mbps

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Multiplexing
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Physical layer interface standards

• Transmission media and the associated electrical
  signals
• be used to transmit a binary data stream between
  two DTEs
• physical layer interface standards

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EIA-232D
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Null modem
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X.21
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ISND interface
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Modem summary