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Data Link Layer CSE 434/598 Spring 2001

Data Link Layer CSE 434/598 Spring 2001. Data Link Layer. Bridges between the Network Layer, and the Physical Layer Refer Figure 3-1, and the OSI Stack. Virtual connection between the two sides’ data layers... Physical layer provides the capability of bit-transmission/reception

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Data Link Layer CSE 434/598 Spring 2001

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  1. Data Link LayerCSE 434/598Spring 2001

  2. Data Link Layer • Bridges between the Network Layer, and the Physical Layer • Refer Figure 3-1, and the OSI Stack. Virtual connection between the two sides’ data layers... • Physical layer provides the capability of bit-transmission/reception • Data Link layer will simply use them to transmit data • Why is this a big deal ? Source node simply puts bit out on the channel, and the destination receives them ... • Issues: Objectives such as reliability, packetizing, enhanced efficiencyPhysical media features such as noise, bit errors, finite data rate, non-zero propagation delay, ... • Functions of the Data Link layer • Well-defined service interface to the Network layers • Grouping of Physical layer’s bits into frames, i.e., packetizing • Accommodate transmission errors, and flow control (between Source and Destination)

  3. Connection Services • Provides a logical connection over the physical media • Three types of connections: • Unacknowledged connectionless • Acknowledged connectionless • Acknowledged connection-oriented • (Question: why isn’t the fourth combination, “Unacknowledged connection-oriented”, is considered ?) • Unacknowledged connectionless • Independent frames sent to the destination • No acknowledgment (neither per frame, nor per the entire message) • Cannot recover from lost frames, will not even know about frame loss !! • No connection setup (prior to message communication), no connection release (beyond the message delivery)

  4. Connection Services (contd...) • Unacknowledged connectionless • Source node simply emits the series of frames...(Without any care whether they reach or not) • Acceptable for media with very low error rate (e.g., Optical) • Also for applications where conducting Ackn is infeasible (due to delay) • Reliability can be augmented at higher OSI layers, e.g., Transport layer • Where must we use acknowledgement-services ? • Acknowledgement-mechanisms are needed for reliable data transfer • Ackn is typically provided at an event with moderate probability of failure • Neither too high, then the service will simply keep on Ackn-ing • Nor too low, then the service will incur the (unnecessary) overhead of Ackn • For noisy media: Ackn at Data Link layer (as well as in the higher layers) • For more reliable media: Start ackn-mechanisms at Transport layer...

  5. Acknowledged Services • Acknowledged Connectionless • No logical connection established, prior to data transmission • However, each frame is acknowledged individually • If a frame does not arrive within a time limit ==> re-transmit(Question: what if a frame is received multiple times ?) • Good for wireless media, a.k.a.. noise • The burden of Acknowledgement is acceptable, as without a minimum level of reliability from this layer, the upper layers will simply keep re-transmitting the same message for a very long (if not, infinite) time • Work out a typical example... • Not good for more reliable media • Too much overhead, since a minimum level of reliability is already built-in • Acknowledgment - per how many bits of information ? • Too few ==> do at Physical media • Modest ==> do at Data Link Layer; Large ==> at the Transport Layer...

  6. Acknowledged Connection-Oriented • Connection establishment • Source and destination nodes setup a logical channel, i.e., a form of virtual path between them • Before any data of the message is transmitted • Acquire the buffers, path variables and other resources necessary for the path • Data transmission • Frames are acknowledged • Frames are numbered, and guaranteed to be received once only • Provides a reliable bit-stream to the Network Layer • Connection release • Free up the variables, buffer space etc. for the connection • Complex message delivery services (e.g., multihop routing) ==>Connection-oriented services with Acknowledgments are preferred

  7. Framing • Basic element for provision of Acknowledgement • Entire message - too long to wait for an Acknowledgement • Requires the message to be fragmented ==> packet or frame concept • There a whole bunch of other reasons for creating frames...(e.g., self-routing, connectionless providers, alternate path finders, ...) • How to create frames ? • Break the message bit stream into fragments • Separated by time pauses ? • Too unreliable, or even demanding on the source node • Length count • Start/End character sequence • Start/End bit sequence • Physical layer coding

  8. Framing (contd...) • Framing by character count • The length of the frame, as an integer, is immediately before the number of frames (refer Figure 3-3a) • Highly scalable in frame sizes • However, vulnerable to errors • If the length data, i.e., the integer denoting the #frames, is corrupted • Not only the current frame is incorrectly assimilated • But the subsequent frames also will be confused (refer Figure 3-3b) • Framing by special character sequence (Start, as well as Stop) • Specially coded Start character sequence, and Stop character sequence • Not as vulnerable to error as above (error cannot affect non-local frames) • Difficulty: if parts of the bit-stream coincides with the special Start/Stop codes • Solution: if so, repeat the special code once more (called Character Stuffing) • Question: give an analogy where else you might have seen similar stuffing !!

  9. Framing (contd...) • Character Framing: difficulty with binary data, e.g., any data other than 8-bit ASCII format • Solution: Bit-level Framing • Start/Stop bit pattern: A special 6-bit code (e.g., “01111110”) • If the data bit-stream happens to include the same bit-sequence: • Bit-stuffing: insert a “0” after the 5-th consecutive “1” • Analogous to character stuffing, except at the bit-level (Figure 3-5) • Media coding based frames • Available for non-binary media, where codes other than 1/0 are available(eqv: use 2 bits for transmission of a 1-bit data) • Example: 11 for bit = 1, and 00 for bit = 0 • Then, use 10 for Start bit, and 01 for Stop bit • Useful for added reliability in transmission...

  10. Error Control • Goal: All the frames must • Reach the destination • Reach once and only once • Reach in order (otherwise, may need a high-speed sorting) • Solution approaches: • Feedback, usually Acknowledgement (both positive and negative) • If a frame is not received, sender will re-transmit • Time-out, to indicate a completely vanished frame and/or Ackn • Sender waits a max-time, and if neither +ve nor -ve Ackn arrives ==> re-transmit the frame • Accommodates total bit-drop situations... • Frame sequence numbers • Due to re-transmission, and/or time-out ==> a frame may arrive the destination more than once. Frame numbers can allow discarding duplicate/stale frames...

  11. Flow Control • Issue: speed mismatch between sender and receiver • Transient mismatch • Can accommodate using buffers • However, if continued for longer time, buffer will overflow • Consistent mismatch • Flow Control: Receiver must send rate-control signals to the sender • Protocol contains well-defined rules regarding when a sender may transmit the next frame • Question: discuss flow control in • Connectionless services • With or without Ackn transmissions • Connection-Oriented services

  12. Coupling Model for the OSI Layers • Desired features: • Security • Real-time • Fault-tolerance, reliability • Question: • Which layers of the OSI stack should accommodate them • Explicitly, i.e., as a ‘must’ • Optional, i.e., as a “may be”, or “desired with some parameters”... • Not necessary, i.e., doing could be harmful type

  13. Error Detection and Correction • Goal: from a continued bit-stream of transmission, find out (on-the-fly) whether some of the bits have errors (e.g., a 0 got changed to a 1, or vice versa) • Size of the bit-stream • Potential error bit position (i.e., which one among all the bits is wrong) • How many errorneous bits ? • For each error, how much is the error ? (difference between current and actual) • Being binary, we are fortunate here. An errorneous-0 ==> 1, always • For r-ary systems, this is another uncertainty to deal with • Assumption • Bit errors occur independently, and randomly across the bit-stream

  14. Single Error vs. Bursty Errors • Burst error: The erroneous bits are correlated in bit-positions • Reflect the physical reasons behind failures (e.g., fault occurs due to a noise level, which affects a group of bits) • Is it any easier to detect (than single bit errors) ? Likely, yes. • Is it any easier to correct (than single bit errors) ? Certainly, not. • Burst error correction • Better to trash the entire bit stream, request the source to re-send • Too expensive to correct • Individual bit-error correction • Instead of re-send request to the source, correct upon the bit-stream • Approach: provide few extra error correcting bits

  15. Error Detection vs. Correction ? • Detection is preferred when • Error probability is too low (making steady overhead of an error correcting code unacceptable) • Bursty error takes place; error bits are localized (grouped) • Correction is preferred when • Re-transmission requests are not acceptable, due to reasons such as • Speed of information delivery • Simplex nature of the link, i.e., cannot send back a re-transmit request • Error bits are not localized, or clustered • A small, and evenly spread out number of errors for every block of data • Basis of error-correcting codes is retained (refer following discussion)

  16. Idea of Error-Detecting and Correcting Code • Message of m bits, add r redundant or check bits • Total message is of n bits, with n = m + r • Information in the message: 1 among 2^m possibilities • Information available in the transmitted bit-stream: 1 in 2^n • Goal: Map this (1 in 2^m) information onto the (1 in 2^n) transmitted bit-stream, with large error-locator distances • Error locator distance • Distance between the corresponding transmitted bit-streams for consecutive message bit streams • If distance is d+1, then we can detect upto d-bit failure • If distance is 2d+1, then we can correct upto d-bit failure

  17. Error Locator Distance A valid message, m-bit data A particular transmission, with error • Not all the transmitted bit-streams are valid message streams • Error makes the transmitted bit-stream deviate from the valid positions • Correction: simply choose the nearest valid bit stream • Assumption • Error < threshold; else jumping to other slots likely Correction distance Correction: n-bit adjacency, not linear

  18. Examples • Single bit parity • Every alternate bit-pattern is a valid data • “Alternate” according to the Hamming distance, not integer count sequence • Example with even parity • 2-bit message data: (00, 01, 11, 10) become (000, 011, 110, 101) with parity • 3-bit transmitted data sequence • 000, 001, 011, 010, 110, 111, 101, 100 • An error locator distance = 2 • Try odd parity: (00, 01, 11, 10) become (001, 010, 111, 100) • 3-bit transmitted data sequence: 000, 001, 011, 010, 110, 111, 101, 100 • Hence, single bit parity can do upto 1-bit error detection, and no correction • Background (assumed): Hamming distance and Hamming codes

  19. 1-bit Correcting Codes • Message = m bits, total transmission = m+r = n bits • Total 2^m valid message bits • Desired error locator distance - at least 2 • Since, when 1-bit occurs, the code will not equate (wrongly !) to another valid message stream’s code • Thus, every valid message bit pattern must leave out all its Hamming distance-1 codes unused • Each of the 2^m codes will require n Hamming distance-1 codes to be free • Each one of the 2^m codes occupy (n+1) codes on the code space • Hence, (n+1). 2^m <= 2^n, Or, (m+r+1) <= 2^r • For a given m, we can find a lower bound on r • m = 8, r >= 4 • Question: extend this logic for k-bit correcting codes...

  20. Hamming 1-bit Correcting Code • n bits are positioned as 1, 2, ..., n • Bits that are at powers of 2 positions, i.e., 1, 2, 4, 8, .. are check bits (i.e., r) --- the other bit positions are data bits (i.e., m) • Bit-value at check bit position p (p = 2^i) • Will consider the bit-values (NB: data bits, eventually) from all bit positions j, where the binary expression of j include a 2^i • Implement an odd (or, even) parity over all such positions j • Example: Check bit at position 2 will address all data bits which has a ‘1’ at the 2^1 bit position of their bit-id • Example: Data bit at position 11 (=1+2+8) will contribute the check bits at positions 1, 2 and 8 • Continue...

  21. Example (1-bit Correcting Code) • 7 bit codes, i.e., m=7 • Hence, r = 4 • NB: (m+r+1) <= 2^r • Total 11 bit code • 1, 2, 4 and 8-th positions are Check Bits • 3, 5, 6, 7, 9, 10, 11-th bit positions are data bits • Check bit at position 2^i implements an even parity over all the bit-positions that require i-th bit = 1 in their respective ids Example: Data: 1101101 1 1 0 1 1 0 1 Position 1 11 1 1 1 0 1 0 1 0 1 0 1

  22. Discussion • Question: explain why this approach works ? • Check bit at 2^i position, caring about data bits from positions who require bit-i = 1 in their id ---- whats the catch ? Why does it work ? • Construct the mathematics, i.e., the bit-arithmetic behind this • Special Project 2 • Due in 2 weeks, turn in an explanation of the above • Assumption: no consultation to the original paper(the answer/explanation should be your own !) • Credit: 2 absolute bonus points • Thus, if you do this project (correctly !), and do everything else, you may score upto 102 (out of 100) • Can be useful for borderline grades...

  23. Bursty Error • Significantly long bit-stream gets corrupted • Naive approach • Devise an correcting code to correct upto 100’s or 1000’s of errorneous bits • Too much bit overhead. It will be cheaper to perhaps re-transmit • A better approach • 2D structuring of the bit-stream • Bits that are temporal-adjacents get distributed to become far apart in transmission, i.e., not transmission-adjacents • Error-correcting codes are designed per temporal adjacency • A bursty error in transmission will (most likely) affect a small number of bits within each temporal code-block • Concern: delay of the individual block of bits

  24. Bursty Error Handling using 2D Data Ordering • Design question: how to associate temporal adjacencies to transmission adjacencies ? • Objective: Given a timing deadline, and maximum bursty error length, how to arrive at the 2D data structure ? adjacency per Correcting code adjacency per Transmission sequence

  25. Correction vs. re-Transmission • Timeliness • Re-transmission likely to be slower • Correcting codes may become slower too, if too many overhead bits are involved • Traffic overhead • Re-transmission imposes a conditional overhead • Correcting codes impose a constant overhead • Types of errors handled • Re-transmission can handle bursty errors • Correcting codes may be able to handle bursty errors, but typically designed for small number of isolated errors

  26. Error Detecting Code • Correcting code is a must in some designs • e.g., simplex channel (thus, no room for Ackn & re-transmission) • Otherwise, typical design choice: error detection and re-transmission • More effective for fewer bit errors, i.e., low probability of failure • Traditional error detecting approaches • Parity - single bit (even, or odd) • Essentially, implements a (#1’s in the bit stream) mod 2 = 0 policy • Multiple (r) bit parity • Implements (#1’s in the bit stream) mod (2^r) = 0 • Checksum • Row-column organization of the data bits • Checksum implements parity within each row

  27. Other Error Detection Approaches • Polynomial codes (CRC, cyclic redundancy code checker ) • Sender and receiver must agree upon a degree-r generator polynomial, G(x) • Typical G(x): x^16 + x^12 + x^5 + 1; or, x^16 + x^ 15 + x^2 + 1; ... • Corresponding bit patterns: co-efficient bit-string for the Polynomial • Checksum computation (message = M(x)) • Attach r-bits, all equal to ‘0’, at the lsb end of M(x) • Divide [ M (x) 000...0 ] by G(x); use modulo-2 division • Subtract the remainder from [ M (x) 000...0 ] ===> this is the Check-summed frame to send • Receiver end • Divide (modulo-2) the received frame by G(x). If remainder != 0, its an error.

  28. Types of Errors Detected • Single bit errors • Remainder (after dividing by G(x)) = x^i, with i-th bit as error • If G(x) contains two or more terms, it will never divide x^i • Hence, all single bit errors will be detected • Two, independent, bit errors • Here, E(x) = x^i + x^j = x^j ( x^{i-j} + 1 ) • G(x) will not divide (x^i + x^j), if a sufficient condition holds that • G(x) does not divide (x^k +1), for all k values from 1 to max. frame length • Example: G(x) = x^15 + x^14 + 1 does not divide any (x^k + 1) for k <= 32,768 • Odd number of bit errors (e.g., 3 errors, 5 errors, 7 errors, ...) • Known result: A polynomial with odd #terms, is not divisible by (x+1) • Thus, select G(x) such that (x+1) is a factor of it • Then, G(x) will never divide a polynomial with odd #terms

  29. Features and Why does it work ? • Features • Detects all single and double errors, all errors with an odd #bits, most burst errors, ... • Principle of division and checking remainder = 0 or not • Analogy: the multi-dimensional circular dial approach • Consecutive “correct” positions in the dial refers to the codes which will yield a remainder = 0, when divided by G(x) • NB: a special case is with x=2, i.e., G(x) = numeric value for the bit-string of G(x) • Division is really one (clean) way to select 1-correct, and (q-1)-incorrect positions; if we are dividing by q

  30. Unrestricted Simplex Protocol • Data transmitted in one direction only • Both the transmitting and receiving Network Layers are always ready • Assume large enough buffer space • Sender’s mode • Infinite loop, pumping out data onto the link as fast as possible • Fetch a packet from the upper layer (i.e., sender user) • Construct a frame, putting the data and a header • Transmit the frame onto the link • No ackn involved • Frames are not assigned to any sequence numbers

  31. Receiver of Unrestricted Simplex Channel • Receiver’s mode • Wait for the arrival of an (undamaged) frame • Receive the frame, remove header and extract the data bits • Propagate the data bits to the receiver user • Problems... • Sender’s rate can become much faster than what receiver can receive in time • Eqv: Line is as fast as the sender node can pour data into it • No provision for acknowledgement and error-handling Receive Data Data ready Receiver Sender Transmit

  32. Acknowledgement per Frame • Synchronous transmission • If the transmitter, receiver and the channel could agree on a common speed • Not applicable with widely varying delays @ the channel, ... • Tuning to the worst case speeds • Utilization falls off, and is a conservative estimate only • Other solution approach: provide “ready-for-next” type of feedback to sender • Control the sender, not to send the next frame if • the receiver is not ready yet • the transmission link is not free yet, ... • Receiver’s feedback to the Sender • After having recd the current frame, send a reverse frame (dummy, eqv. Ack)

  33. Stop and Go Protocol • Sender: must wait for an Ack before transmitting the next frame • Receiver: must send back a dummy-frame, as an Ackn, to the sender after receiving a frame • Features: • Rate control achieved • Half-duplex or better channel required • Overhead of the reverse frame’s transmission into each forward packet’s journey Ackn-reverse Data ready Receiver Sender Receive Data Transmit

  34. Protocol for Noisy Channels • Transmission line can be noisy with partial (or, total) failure • Frames can get damaged, or lost completely • Employ error detection approaches ==> find out if an error took place • What if ? • A negative Ackn. is sent when a failure occurs • The sender could re-transmit • Keep repeating until the packet is successfully transmitted • Ackn is sent only for successful (i.e., error free) transmissions • Faulty, or erroneous transmissions would not receive an Ackn-back • Implement a time-out, and if no Ackn is received ==> re-transmit • Problem: does not prepare for missing Ackn signals

  35. Sequencing through Frames • Error in Acknowledgement signal • Data (i.e., forward) transmission is error free • Receiver (after computing Checksum etc. and finding no error) sends Ackn • Reverse channel is noisy • the Ackn signal is corrupted (but reach the sender) • Or, the Ackn signal is completely lost, i.e., dropped • Sender would re-transmit the frame (anticipating forward transmission error) • Receiver has multiple copies of the same frame ==> consistency problem • Frame sequence number • Assign a serial id# to each frame • Receiver can check, if multiple copies of the same frame arrived • In fact, a time-stamp along with frame id# can help in resolving staleness also

  36. Range of Frame id# • How many bits for the Frame id# • As few as possible • What is minimum ? • Stop and Go model, implemented for each frame • Distinguish between a frame, and its immediate successor • If a frame #m is damaged, then Ackn-for-m would be missing • Sender will re-transmit frame #m • Distinction required between “m+1” and ‘m’ • Thus, a 1-bit sequence number could be adequate • Provide an window of tolerance between transmision and reception • Not “stop and go” per frame level • “Go” until Ackn for the last W frames are not received • Frame id# should have sufficient number of bits to reflect the number W

  37. Piggybacking • Full Duplex Communication environment • Forward • Step 1-F: Node A (sender) sends data to node B (receiver) • Step 2-F: Node B sends Ack back to Node A • Node A implements a time-out on Ack signals, and re-transmits (using frame id#’s) • Reverse • Step 1-R: Node B (sender) sends data to node A (receiver) • Step 2-R: Node A sends Ack back to Node B • Node B implements a time-out on Ack signals, and re-transmits (using frame id#’s) • Piggybacking: Combine steps (2-F, and 1-R), likewise combine steps (1-F, and 2-R)

  38. Features of Piggybacking • Advantages • Reduction in bandwidth • Ack signals get (almost always) a free ride on the reverse transmitting frames • Disadvantages • Delay in the Ack signals • has to necessarily wait for the reverse data transmission • How long should the sender wait for an Ack to arrive ? • When would the sender decide to re-transmit • How to implement the Time-Out policy

  39. Sliding Window Protocol • Provides a mechanism for transmitter and receiver to remain synchronized, even when • frames get lost, mixed and/or corrupted • premature time-outs take place • Sending Window • sender node maintains a set of (consecutive ?) sequence numbers, corresponding to the frames that it can transmit at a given time • other frames cannot be transmitted, at that particular time • Receiver Window • maintains a set of sequence numbers of the frames that it can accept(other frame id#’s cannot be received at that time) • need not be the same size, or hold identical bounds as of the sender window

  40. Sliding Window Protocol (contd...) • Sender’s Window • Represents frames sent out, but not received Ack yet • Current window: (low, ..., high) • A new packet is sent out • Updated window: (low, ..., high+1) • Maximum window size based upon the buffer space, and tolerable max-delay • An Ack is received • Updated window: (low+1, ..., high) • Circular window management • Receiver’s Window • Represents frames the receiver may accept • Any frame outside this range is discarded without any Ack

  41. Window Attributes • Receiver’s Window • Current window: (low, ..., high) • A frame (id# = low) is received • Updated window: (low+1, ..., high+1) • Request generation of an Ack signal for the arrived frame • Circular window management • No other frame may be received (analyze this !!) • Receiver’s window always stays at the same size (unlike Sender) • Example: figure 3-12 • A 1-bit Sliding Window Protocol • Sender transmits a frame, waits for Ack, and send the next frame...

  42. 1-Bit Sliding Window Protocol • Sender’s window: maximum size = 1 • Essentially, a Stop-and-Go protocol • Added features: frame id#, window synchronization • Refer Figure 3-13 • Ignore the transmission time, i.e., basically assumes a small transmission delay • If the sender (node A) and receiver (node B) are in perfect synchronization • Case 1: Node B starts its first frame only after receiving node A’s first transmission • Case 2: Nodes A and B simultaneously initiate their transmissions

  43. Synchronization in 1-bit Sliding Window Protocol • Case 1: every frame is accepted • Reason: B’s first transmission had waited until A’s first tranmission reached B ==> B’s first transmission could bring with it the Ack for A’s first transmission • Otherwise, B’s first transmission would not bring with it A’s ACK • A will re-transmit, and keep re-transmitting until some packet from B brings with it an Ack for A’s first transmission • Leading to Case 2 • Case 2: a large fraction of the transmissions are replicates • Refer Figure 3-14 (b) • Message format: (frame sequence #, ack for the frame with sequence #, frame or the actual data packet)

  44. Added Features of Sliding Window • Staying synchronized even when frames get mixed • The windows of sender (as well as receiver) have strict sequential patterns... • Pre-mature time-outs cannot create infinite transmission loop • Without the window markers: • A pre-mature time-out can cause a re-transmission • Before the Ack of the (2nd) re-transmission arrives, another pre-mature time-out can happen ==> 3rd re-transmission • Infinitely repeating... • With the window markers: • Receiver node will update its window at the first reception • Subsequent receptions will simply be rejected • Sliding window protocols: much more resilient to pathological cases

  45. Sliding Window Protocols with Non-Negligible Transmission Delay • Example: a 50 Kbps satellite channel with 500 msec transmission delay - transmit a 1000 bit packet • time 0 to 20 msec: the packet is being poured onto the channel • time 20 msec to 270 msec: forward journey to the satellite • time 270 msec to 520 msec: return journey to the ground station • Bandwidth utilization = 20 / 520 ==> about 4% !! • Rationale • Sender node had to wait for receiving an Ack of the current frame, before sending out the next frame... • Window size = 1 • Approach for improving utilization: increase the Window size

  46. Sliding with Larger Windows • Example: a 50 Kbps satellite channel with 500 msec transmission delay - transmit a 1000 bit packet • time 0 to 20 msec: the packet is being poured onto the channel • time 20 msec to 270 msec: forward journey to the satellite • time 270 msec to 520 msec: return journey to the ground station • But, now the sender does not wait until 520-th msec before transmitting the second frame, third frame and so on • At time units: 20, 40, 60, ..., 500, 520, 540, ... msecs the sender transmits successive packets • Ack1 arrives about when Packet26 has just been sent out • “26” is arrived as 520 / 20, i.e., round trip transmission delay / single packet delay • Bandwidth utilization reaching near 100%

  47. Pipelining • Channel capacity = b bits/sec • Frame size = L bits • Round-trip propagation delay = R sec • Efficiency with Stop-and-Go protocol • (L/b) / [ (L/b) + R ] = L / ( L + bR ) • If L < bR, efficiency will be less than 50% • In principle, Pipelining can always help • In practice, unless R is beyond a threshold, Pipelining isn’t worth the trouble

  48. Pipelining with Unreliable Communication • What happens in a Packet-intermediate is lost or corrupted ? • SenderReceiverpacket 1packet 2packet 3 recd Packet 1packet 4 recd Packet 2packet 5 recd Packet 3..... • Difficulties • Large number of succeeding frames, sent in pipelined format, will now arrive at the receiver with one (or, few) missing front packets • This is a problem for the Network Layer @ receiver, since it wants the packets/frames in order. Also, the receiver application will pause. • Problem for the sender, since it has to slide back...

  49. Two Solutions • Problem Instance • A series of packets, 1, 2, 3, ..., N is received @ the Receiver node • Receiver finds that packet 1 is faulty • After a while, sender receives the negative Ack (or has a time-out, if no Ack is sent, or Ack is lost in reverse journey) • Solution 1: “go back N” • Receiver discards packets 2 thru N (although they were correct) • Sender will, sooner or later, re-transmit Packet 1 • Receiver waits for the re-transmitted (hopefully, correct) Packet 1 • Sender will follow up with Packets 2 through N again • Refer Figure 3-15(a) • Corresponds to a Receiver Window Size = 1

  50. Two Solutions (contd...) • Performance concern in “go back N” • Why discard Packets 2 thru N, even if they were correctly transmitted ? • For simplicity, easier network management, ... • But, certainly not being performance conscious • A performance aware system might • Hold packets 2 thru N in some temporary buffer • Request the sender to re-transmit (a correct version of) Packet 1 • Upon receiving Packet 1 (new copy), re-construct the packet sequence • Packet 1 (new), Packet 2 (old), Packet 3 (old), ..., Packet N(old), Packet N+1 (new), Packet N+2 (new), ... • This is basically a selective repeat, where packet 1 was repeated in above • Solution 2: Selective Repeat • Store all the correct Packets, following the bad one

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