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Asynchronous and Synchronous Transmission. Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous. Asynchronous. Data transmitted on character at a time 5 to 8 bits Timing only needs maintaining within each character

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asynchronous and synchronous transmission
Asynchronous and Synchronous Transmission
  • Timing problems require a mechanism to synchronize the transmitter and receiver
  • Two solutions
    • Asynchronous
    • Synchronous
  • Data transmitted on character at a time
    • 5 to 8 bits
  • Timing only needs maintaining within each character
  • Resync with each character
asynchronous behavior
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)
synchronous bit level
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)
synchronous block level
Synchronous - Block Level
  • Need to indicate start and end of block
  • Use preamble and postamble
    • e.g. series of SYN (hex 16) characters
    • e.g. block of 11111111 patterns ending in 11111110
  • More efficient (lower overhead) than async
flow control
Flow Control
  • Ensuring the sending entity does not overwhelm the receiving entity
    • Preventing buffer overflow
  • Transmission time
    • Time taken to emit all bits into medium
  • Propagation time
    • Time for a bit to traverse the link
stop and wait
Stop and Wait
  • Source transmits frame
  • Destination receives frame and replies with acknowledgement
  • Source waits for ACK before sending next frame
  • Destination can stop flow by not send ACK
  • Works well for a few large frames
  • Large block of data may be split into small frames
    • Limited buffer size
    • Errors detected sooner (when whole frame received)
    • On error, retransmission of smaller frames is needed
    • Prevents one station occupying medium for long periods
  • Stop and wait becomes inadequate
sliding windows flow control
Sliding Windows Flow Control
  • Allow multiple frames to be in transit
  • Receiver has buffer W long
  • Transmitter can send up to W frames without ACK
  • Each frame is numbered
  • ACK includes number of next frame expected
  • Sequence number bounded by size of field (k)
    • Frames are numbered modulo 2k
sliding window enhancements
Sliding Window Enhancements
  • Receiver can acknowledge frames without permitting further transmission (Receive Not Ready)
  • Must send a normal acknowledge to resume
  • If duplex, use piggybacking
    • If no data to send, use acknowledgement frame
    • If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)
error detection
Error Detection
  • Additional bits added by transmitter for error detection code
  • Parity
    • Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
    • Even number of bit errors goes undetected
cyclic redundancy check
Cyclic Redundancy Check
  • For a block of k bits transmitter generates n bit sequence
  • Transmit k+n bits which is exactly divisible by some number
  • Receive divides frame by that number
    • If no remainder, assume no error
    • For math, see Stallings chapter 7
error control
Error Control
  • Detection and correction of errors
  • Lost frames
  • Damaged frames
  • Automatic repeat request
    • Error detection
    • Positive acknowledgment
    • Retransmission after timeout
    • Negative acknowledgement and retransmission
automatic repeat request arq
Automatic Repeat Request (ARQ)
  • Stop and wait
  • Go back N
  • Selective reject (selective retransmission)
stop and wait21
Stop and Wait
  • Source transmits single frame
  • Wait for ACK
  • If received frame damaged, discard it
    • Transmitter has timeout
    • If no ACK within timeout, retransmit
  • If ACK damaged,transmitter will not recognize it
    • Transmitter will retransmit
    • Receive gets two copies of frame
    • Use ACK0 and ACK1
high level data link control
High Level Data Link Control
  • HDLC
  • ISO 33009, ISO 4335
hdlc station types
HDLC Station Types
  • Primary station
    • Controls operation of link
    • Frames issued are called commands
    • Maintains separate logical link to each secondary station
  • Secondary station
    • Under control of primary station
    • Frames issued called responses
  • Combined station
    • May issue commands and responses
hdlc link configurations
HDLC Link Configurations
  • Unbalanced
    • One primary and one or more secondary stations
    • Supports full duplex and half duplex
  • Balanced
    • Two combined stations
    • Supports full duplex and half duplex
hdlc transfer modes 1
HDLC Transfer Modes (1)
  • Normal Response Mode (NRM)
    • Unbalanced configuration
    • Primary initiates transfer to secondary
    • Secondary may only transmit data in response to command from primary
    • Used on multi-drop lines
    • Host computer as primary
    • Terminals as secondary
hdlc transfer modes 2
HDLC Transfer Modes (2)
  • Asynchronous Balanced Mode (ABM)
    • Balanced configuration
    • Either station may initiate transmission without receiving permission
    • Most widely used
    • No polling overhead
hdlc transfer modes 3
HDLC Transfer Modes (3)
  • Asynchronous Response Mode (ARM)
    • Unbalanced configuration
    • Secondary may initiate transmission without permission form primary
    • Primary responsible for line
    • rarely used
frame structure
Frame Structure
  • Synchronous transmission
  • All transmissions in frames
  • Single frame format for all data and control exchanges
frequency division multiplexing
Frequency Division Multiplexing
  • FDM
  • Useful bandwidth of medium exceeds required bandwidth of channel
  • Each signal is modulated to a different carrier frequency
  • Carrier frequencies separated so signals do not overlap (guard bands)
  • e.g. broadcast radio
  • Channel allocated even if no data
analog carrier systems
Analog Carrier Systems
  • AT&T (USA)
  • Hierarchy of FDM schemes
  • Group
    • 12 voice channels (4kHz each) = 48kHz
    • Range 60kHz to 108kHz
  • Supergroup
    • 60 channel
    • FDM of 5 group signals on carriers between 420kHz and 612 kHz
  • Mastergroup
    • 10 supergroups
synchronous time division multiplexing
Synchronous Time Division Multiplexing
  • Data rate of medium exceeds data rate of digital signal to be transmitted
  • Multiple digital signals interleaved in time
  • May be at bit level of blocks
  • Time slots preassigned to sources and fixed
  • Time slots allocated even if no data
  • Time slots do not have to be evenly distributed amongst sources
  • No flag or SYNC characters bracketing TDM frames
  • Must provide synchronizing mechanism
  • Added digit framing
    • One control bit added to each TDM frame
      • Looks like another channel - “control channel”
    • Identifiable bit pattern used on control channel
    • e.g. alternating 01010101…unlikely on a data channel
    • Can compare incoming bit patterns on each channel with sync pattern
adsl design
ADSL Design
  • Asymmetric
    • Greater capacity downstream than upstream
  • Frequency division multiplexing
    • Lowest 25kHz for voice
      • Plain old telephone service (POTS)
    • Use echo cancellation or FDM to give two bands
    • Use FDM within bands
  • Range 5.5km
  • High data rate DSL
  • Single line DSL
  • Very high data rate DSL