Asynchronous and Synchronous Transmission • Timing problems require a mechanism to synchronize the transmitter and receiver • Two solutions —Asynchronous Chapter 6 —Synchronous Digital Data Communications • Transmission Errors: Detection and Correction Techniques Asynchronous Asynchronous (diagram) • Data transmitted on character at a time —5 to 8 bits • Timing only needs maintaining within each character • Resynchronize with each character • Parity check – number of ones including the parity bit must be even (even parity) or odd (odd parity) 1
Asynchronous - Behavior Synchronous - Bit Level • In a steady stream, interval between characters • Block of data transmitted without start or stop is uniform (length of stop element) bits • In idle state, receiver looks for transition 1 to 0 • Clocks must be synchronized • Then samples next seven intervals (char length) • Can use separate clock line • Then looks for next 1 to 0 for next char —Good over short distances —Subject to impairments • Simple • Embed clock signal in data • Cheap —Manchester encoding • Overhead of 2 or 3 bits per char (~ 20% ) —Carrier frequency (analog) • Good for data with large gaps (keyboard) Synchronous - Block Level Synchronous (diagram) • 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 2
Line Configuration Traditional Configurations • 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 Data Communications Interfacing 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, NIC • DCE transmits bits on medium • DCE communicates data and control info with DTE —Done over interchange circuits —Clear interface standards required 3
Characteristics of Interface ISDN Physical Interface Diagram • Mechanical —Connection plugs • Electrical —Voltage, timing, encoding • Functional —Data, control, timing, grounding • Procedural —Sequence of events ISDN Physical Interface ISDN Electrical Specification • Balanced transmission • Connection between terminal equipment (c.f. — Carried on two lines, e.g. twisted pair DTE) and network terminating equipment (c.f. — Signals as currents down one conductor and up the other DCE) — Differential signaling • ISO 8877 — Value depends on direction of voltage — Tolerates more noise and generates less • Cables terminate in matching connectors with 8 — (Unbalanced, e.g. RS-232 uses single signal line and ground) contacts — Data encoding depends on data rate • Transmit/receive carry both data and control — Basic rate 192kbps uses pseudoternary — Primary rate uses alternative mark inversion (AMI) and B8ZS or HDB3 4
Transmission Error Error Detection Process • An error occurs when a bit is altered between transmission and reception • Single bit errors — One bit altered — Adjacent bits not affected — White noise • Burst errors — Length B — Contiguous sequence of B bits in which first last and any number of intermediate bits in error — Impulse noise — Fading in wireless — Effect greater at higher data rates Error Detection Cyclic Redundancy Check • Additional bits added by transmitter for error • For a block of k bits transmitter generates n bit detection code sequence • Parity • Transmit k+ n bits which is exactly divisible by some number —Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones • Receive divides frame by that number —Even number of bit errors goes undetected —If no remainder, assume no error 5
Error Correction Process Error Correction Diagram • Correction of detected errors usually requires data block to be retransmitted (see chapter 7) • Not appropriate for wireless applications —Bit error rate is high • Lots of retransmissions —Propagation delay can be long (satellite) compared with frame transmission time • Would result in retransmission of frame in error plus many subsequent frames • Need to correct errors on basis of bits received Error Correction Process Working of Error Correction • Each k bit block mapped to an n bit block ( n > k ) • Add redundancy to transmitted message — Codeword • Can deduce original in face of certain level of — Forward error correction (FEC) encoder error rate • Codeword sent • E.g. block error correction code • Received bit string similar to transmitted but may —In general, add ( n – k ) bits to end of block contain errors • Gives n bit block (codeword) • Received code word passed to FEC decoder • All of original k bits included in codeword — If no errors, original data block output —Some FEC map k bit input onto n bit codeword such — Some error patterns can be detected and corrected that original k bits do not appear — Some error patterns can be detected but not corrected — Some (rare) error patterns are not detected • Results in incorrect data output from FEC 6
Flow Control • Ensuring the sending entity does not overwhelm the receiving entity —Preventing buffer overflow • Transmission time Chapter 7 —Time taken to emit all bits into medium Data Link Control Protocols • Propagation time —Time for a bit to traverse the link Model of Frame Transmission 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 7
Fragmentation Stop and Wait Link Utilization • 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 Window s Flow Control Sliding Window Diagram • 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 2 k 8
Example Sliding Window 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 Cyclic Redundancy Check • Additional bits added by transmitter for error • For a block of k bits transmitter generates n bit detection code sequence • Parity • Transmit k+ n bits which is exactly divisible by some number —Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones • Receive divides frame by that number —Even number of bit errors goes undetected —If no remainder, assume no error • CRC types: CRC-16, CRC—CCITT, CRC-32 9
Automatic Repeat Request Error Control (ARQ) • Detection and correction of errors • Stop and wait • Lost frames • Go back N • Damaged frames • Selective reject (selective retransmission) • Automatic repeat request —Error detection —Positive acknowledgment —Retransmission after timeout —Negative acknowledgement and retransmission Stop and Wait - Stop and Wait Diagram • 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 10
Stop and Wait - Pros and Cons Go Back N (1) • Simple • Based on sliding window • Inefficient • If no error, ACK as usual with next frame expected • Use window to control number of outstanding frames • If error, reply with rejection —Discard that frame and all future frames until error frame received correctly —Transmitter must go back and retransmit that frame and all subsequent frames Go Back N - Damaged Frame Go Back N - Lost Frame (1) • Receiver detects error in frame i • Frame i lost • Receiver sends rejection- i • Transmitter sends i+ 1 • Transmitter gets rejection- i • Receiver gets frame i+ 1 out of sequence • Transmitter retransmits frame i and all • Receiver send reject i subsequent • Transmitter goes back to frame i and retransmits 11
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