EIE325: Telecommunication TechnologiesMaciej J. Ogorza ł ek, PolyU, EIE Telecommunication Technologies Week 8 Flow Control Error Control

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Where are we?  Up until now: Physical Layer  Now: Data Link Layer Flow Control Error Control  Later… Synchronisation Multiplexing “sending signals over a transmission link” “sending data over a data communications link”

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait  Source transmits frame  Destination receives frame and replies with acknowledgement (ACK)  Source waits for ACK before sending next frame  Destination can stop flow by not send ACK  Works well for a few large frames

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Fragmentation  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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Question  If the data rate is 10 Mbps and transmission is over a distance of 100 m, how long will it take to send 1 Kb in eight 128 byte chunks with stop and wait FC? [assume v = 2×10 8 ms -1 ].

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait Link Utilization

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Sliding Window 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 2 k W = 2 k -1

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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)

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Automatic Repeat Request (ARQ)  Stop and wait  Go back N  Selective reject (selective retransmission)

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE 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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait ARQ

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait - Pros and Cons Simple Inefficient

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N  Based on sliding window  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

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N - Damaged Frame  Receiver detects error in frame i  Receiver sends rejection- i (REJ- i )  Transmitter gets rejection- i  Transmitter retransmits frame i and all subsequent

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N - Lost Frame (1)  Frame i lost and transmitter sends i+1  Receiver gets frame i+1 out of sequence  Receiver sends reject i  Transmitter goes back to frame i and retransmits

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N - Lost Frame (2)  Frame i lost and no additional frame sent  Receiver gets nothing and returns neither acknowledgement nor rejection  Transmitter times out and sends acknowledgement frame with P bit set to 1  Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )  Transmitter then retransmits frame i

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N - Damaged Acknowledgement  Receiver gets frame i and send acknowledgement ( i+1 ) which is lost  Acknowledgements are cumulative, so next acknowledgement ( i+n ) may arrive before transmitter times out on frame i  If transmitter times out, it sends acknowledgement with P bit set as before  This can be repeated a number of times before a reset procedure is initiated

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Go Back N - Damaged Rejection  Receiver sends rejection which is lost  As for lost frame (2)  (Transmitter times out and sends acknowledgement frame with P bit set to 1)

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Selective Reject  Also called selective retransmission  Only rejected frames are retransmitted  Subsequent frames are accepted by the receiver and buffered  Minimises retransmission  Receiver must maintain large enough buffer  More complex log at transmitter

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Performance Issues  What is the line utilisation for various flow control methods? Stop and Wait Sliding Window  What is the line utilisation for various ARQ schemes? Stop and Wait Selective Reject Go Back N Which is best ???

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait Flow Control  Assume stations A and B are communicating frames F 1,F 2,F 3,…,F n  Total transmission time T = nT F where T F is the time to transmit one frame and receive an acknowledgement  T F = t prop + t frame + t proc + t prop + t ack + t proc t prop : propagation delay t frame : time spent transmitting a frame t proc : processing time t ack :time spent transmitting an acknowledgement

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Stop and Wait Flow Control  T F = t prop + t frame + t proc + t prop + t ack + t proc = 2 t prop + t frame T = n (2 t prop + t frame )  T is the total time to transmit, the actual time spent transmitting is only nt frame  Line utilisation U = nt frame / T

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Line utilisation  Define line utilisation U as the ratio of transmission time to the time taken to transmit data  For Stop and Wait define a = t prop / t frame, then U = 1/(1+2 a )

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE The parameter a  We have defined a = t prop /t frame  Alternatively, define V : propagation speed (ms -1 ) d : transmission distance (m) R : data rate (bps) L : frame size (bits)

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Spread Spectrum  Analog or digital data  Analog signal  Objective: Spread data over wide bandwidth  Makes jamming and interception harder  Frequency hoping Signal broadcast over seemingly random series of frequencies  Direct Sequence Each bit is represented by multiple bits in transmitted signal Chipping code

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE General model for SS

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Spread Spectrum  Frequency hoping Signal broadcast over seemingly random series of frequencies  Direct Sequence Each bit is represented by multiple bits in transmitted signal Chipping code

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Direct Sequence

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Question  What is the bandwidth of a digital data stream encoded with direct sequence spreading?

EIE325: Telecommunication TechnologiesMaciej Ogorza ł ek, PolyU, EIE Generating noise  Need to generate same “noise” at source and destination  Computers are deterministic – generating true noise is not possible (i.e. no truly random numbers)  May use pseudo-random algorithm predetermined sequences (e.g. Gold sequences) chaos