CS3502, Data and Computer Networks: the physical layer4

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CS3502,Data and Computer Networksthe physical
layer-4

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Synchronization

• to transport bits from X to R, R must know when
  X is transmitting, in order to correctly
  interpret the signals 2 standard ways are
  synchronous and asynchronous.
• asynchronous transmission
• small groups of bits (5-10 bits)
• each small group synchronized separately
• simple signaling (NRZ)
• short distances only eg, PC to printer
• start and stop bits mark the bit group
• how much overhead? how efficient is this?

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synchronization

• synchronous transmission
• start, end of data marked by flag byte (01111110)
• flag pattern must not appear inside frame
  bit-stuffing takes care of this
• encoding -gt need self clocking codes
• exercise give a FSM for bit stuffing for the
  flag 01110, and to unstuff bits at the receiver
• what is the overhead (efficiency)?

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interfacing

• this means translating from 1 physical protocol
  to another
• digital devices usually have a very limited data
  transmission/reception capability - not able to
  transmit onto a network directly
• examples
• digital to analog (modems)
• digital to digital (PC to LAN)
• 4 parts of standard interface
• mechanical
• electrical
• functional
• procedural

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interfacing EIA -232 standard

• terminology
• DTE - data terminal equipment -the device which
  we wish to connect to the network
• generic term for data source, data terminal
  (sink), or both
• examples PC, computer terminal, workstation
• DCE - data circuit terminating equipment - the
  device which interfaces with the network
• creates, maintains and terminates connection with
  network
• signal conversion and coding
• example modem

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interfacing EIA -232 standard

• 25 pin connector most apps. dont use all
• signal/line types data, control, timing, ground.
  (note Table 5.1 list)
• 15 meters max distance
• 3 to 25 volts for 0 -3 to -25 V for 1.
• unbalanced/asymmetric connection (circuit
  completed by ground).
• 1 data line each way, so full duplex possible
• more details in textTanenbaum p114.

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interfacing ISDN physical connector

• standard for ISDN connections (Integrated
  Services Digital Network)
• ISDN basic data rate 144 Kbps
• symmetric - this gives better electrical
  properties
• more logic, less circuits 8 pins
• 2 data pins each way 4 data pins
• date circuits carry both data and control
  information
• other pins for power sources

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multiplexing

• problem a transmission line operates at 1.544
  Mbps, but 1 connection needs only 64 Kbps so
  rest is wasted.... since 1.544 Mbps costs about
  2K/ month.
• solution share the link among many users, each
  paying only their part.
• purpose to utilize as much of the line as
  possible
• 3 techniques FDM, synch TDM, statistical TDM

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multiplexing FDM

• analog signals with high bandwidth
• TV Cable channels broadcast radio voice trunks
  
• must have Wlink gt ??wi i.e.,link capacity
  greater than sum of channels.
• main carrier is a composite of many subcarriers.
  each subcarrier may be modulated with 1 channel
• example a carrier has a total bandwidth of 240
  MHz, from 54 to 294 MHz. Subcarriers are centered
  every 6 MHz each forming 1 channel.
• guard band necessary to avoid interference

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multiplexing FDM

• FDM problems
• crosstalk - can occur between neighboring
  channels, if overlap too close
• intermodulation noise - possible on high capacity
  links over distance
• noise, clarity - over distance, analog signals
  more vulnerable than digital gradually being
  replaced in most areas.
• switching - not as efficient with analog signals

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multiplexing TDM

• two types synchronous and statistical
• synchronous TDM
• digital data
• signal - usually digital can be analog signal
  coded digitally
• data rate of link must be greater than sum of
  inputs
• similar to timesharing computers
• example T1 multiplexer
• standards DS0, DS1 (T1), DS3 (T3) OCn EC1

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multiplexing TDM

• synchronous TDM
• time slot to each input line
• 1 slot for synchronization
• unused time slots lost
• slot size 1 bit or 1 byte, in general
• physical layer no error or flow control
• Q how much buffer space needed?
• Q what capacity needed for 24 voice channels?
  how many voice channels possible on a T3 line?
  OC3? OC12? how many T1 lines on an OC12? OC48?

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statistical TDM

• another way of assigning time slots
• if input rates irregular, varied, synch TDM could
  be wasteful stat. TDM could be more efficient
• slots are assigned dynamically, as needed
• requires more intelligence more of a data link
  layer function
• frames must have more control information
• show fields of a possible frame
• more overhead than synch. TDM closer to a MAC
  type protocol

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comparison stat and synch TDM

• synch TDM
• fixed number slots per round
• can waste slots
• timing simpler, fixed
• format simpler
• stat TDM
• variable number slots per frame
• doesnt waste slots
• more overhead, complexity similar to data link
  function
• Q how much buffer space needed for stat TDM?

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stat TDM - buffer space summary

• average input rate ? must be less than link
  capacity ?? but ? may exceed ??temporarily.
• buffer space stores temporary overflows
• buffer size must be estimated based on expected
  input rates and their arrival distribution. Given
  these we can calculate buffer size (queue
  length) but in reality never can be completely
  sure.
• link utilization is given by ????????? a
  standard queueing formula
• as ??approaches 1, queue (buffer) size becomes
  very large, quickly approaching infinity as ?
  reaches or exceeds 1
• utilization ? of no more that 0.8 is good rule of
  thumb

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the voice channel and telephone system

• basic telephone network designed to deliver
  quality voice service
• voice emits analog signal - sound waves - from 30
  to 10,000 Hz. Human ears detect up to 20K Hz.
• most energy in 200-3500 Hz range Standard analog
  voice channel is 4000 Hz. This key number
  selected many years ago by phone company.
• standard PCM digital voice channel is 64 Kbps.
• most local telephone loops still analog
• all long distance in US is digital majority is
  fiber.

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the voice channel and telephone system

• voice not very sensitive to most noise and
  distortion for this and other reasons, local
  telco loops not well suited to modern data
  networks
• However, the local telco networks are one of few
  comm. links between homes, businesses and rest of
  the world
• Structure of U S Telephone networks /companies
• local loops last mile and telcos
• long distance networks and companies
• network equipment

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video channels and the cable TV system

• TV cable system established only recent decades
• switching equipment designed for broadcast TV
• standard TV - needs 6 MHz per channel
• copper coaxial cables capable of 500 MHz carry
  many TV channels.
• these cables have capacity to carry thousands of
  voice channels and/or high speed data -- but need
  appropriate switching equipment at home office,
  and in homes
• already becoming a reality . Will threaten
  existence of old telcos. (note pending merger of
  ATT, TCI)