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NETE0510 Data-link Layer and Protocols

NETE0510 Data-link Layer and Protocols. Dr. Supakorn Kungpisdan supakorn@mut.ac.th. Outline. Encoding Techniques Data Link Layer Fundamental Direct-link Protocols BSC, DDCMP, HDLC, PPP. Signal Encoding Techniques. Encoding Schemes. Bit rate and Baud rate.

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NETE0510 Data-link Layer and Protocols

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  1. NETE0510Data-link Layer and Protocols Dr. Supakorn Kungpisdan supakorn@mut.ac.th NETE0510: Communication Media and Data Communications

  2. Outline • Encoding Techniques • Data Link Layer Fundamental • Direct-link Protocols • BSC, DDCMP, HDLC, PPP NETE0510: Communication Media and Data Communications

  3. Signal Encoding Techniques NETE0510: Communication Media and Data Communications

  4. Encoding Schemes NETE0510: Communication Media and Data Communications

  5. Bit rate and Baud rate • Bit rate or data rate: number of bits transmitted over a period of time (bit per second) • Baud rate: number of symbols transmitted over a period of time (baud) NETE0510: Communication Media and Data Communications

  6. Non-return to Zero-Level (NRZ-L) • two different voltages for 0 and 1 bits • voltage constant during bit interval • no transition I.e. no return to zero voltage • such as absence of voltage for zero (0 V), constant positive voltage for one (+5V) • more often, negative voltage (-5 V) for one value and positive (+5 V) for the other NETE0510: Communication Media and Data Communications

  7. Problems of NRZ-L • Long sequence of 0s or 1s signal leads to “baseline wander” • Receiver maintains baseline of detecting 0 or 1 • Too many 0s or 1s cause this baseline to change • Clock synchronization • Frequent transitions of signals are necessary to enable clock recovery • Every clock cycle the sender transmits a bit and the receiver recovers a bit • Slightly difference in clocks at either sender and receivers may cause a problem NETE0510: Communication Media and Data Communications

  8. Non-return to Zero Inverted (NRZI) • nonreturn to zero inverted on ones • constant voltage pulse for duration of bit • data encoded as presence or absence of signal transition at beginning of bit time • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0 • example of differential encoding since have • data represented by changes rather than levels • More example of different encoding: Different Manchester • more reliable detection of transition rather than level • easy to lose sense of polarity NETE0510: Communication Media and Data Communications

  9. Manchester Encoding • has transition in middle of each bit period • transition serves as clock and data  no need to send clock • low to high represents one (1), high to low represents zero (0) • used by IEEE 802. • Both 0s and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver NETE0510: Communication Media and Data Communications

  10. Differential Manchester Encoding • midbit transition is clocking only • transition at start of bit period representing 0 • no transition at start of bit period representing 1 • this is a differential encoding scheme • used by IEEE 802.5 (Token Ring) NETE0510: Communication Media and Data Communications

  11. Problems of Manchester Encoding • Double the rate at which signal transition are made on the link  the receiver has half the time to detect each pulse of the signal. • Bit rate is half of baud rate  encoding is only 50% efficient NETE0510: Communication Media and Data Communications

  12. 4B/5B • Used together with other encoding techniques • Used in 100Base-TX transmission • The idea is to insert extra bits into the bit stream so as to break up long sequences of 0s and 1s • Every 4 bits of actual data are encoded in a 5-bit code  4B/5B • Each 5-bit code has no more than one leading 0s and no more than two trailing 0s. • No pair of 5-bit codes results in more than three consecutive 0s  solve problem of many consecutive 0s • Then the 5-bit codes are then transmitted using the NRZI encoding • NRZI already solved the problem of consecutive 1s so 4B/5B results in 80% efficiency NETE0510: Communication Media and Data Communications

  13. 4B/5B Encoding • 4-bit data is encoded into 5-bit code • “One leading 0, two trailing 0s” rule • 16 codes used, left the other 16 codes for other purposes • Code 00000  line is dead • Code 1111 line is idle • Code 00100  halt • 7 of them violates the rule • The other 6 represent control symbols NETE0510: Communication Media and Data Communications

  14. Outline • Encoding Techniques • Data Link Layer Fundamental • Direct-link Protocols • BSC, DDCMP, HDLC, PPP NETE0510: Communication Media and Data Communications

  15. Data Link Layer • Because of transmission errors, or because the receiver may need to regulate data rate, it is necessary to have a layer of control communication devices • Need layer of logic above Physical to manage exchange of data over a link • Frame synchronization: beginning and end of each frame must be recognizable • Flow control: sender must not send frames at a rate faster that the receiver can absorb them • Error control: correct bit errors • Addressing: identify involving communication parties • Control and data: receiver can distinguish control information from transmitted data • Link management: initiation, maintenance, and termination NETE0510: Communication Media and Data Communications

  16. Flow Control • ensure sending entity does not overwhelm receiving entity • by preventing buffer overflow • influenced by: • transmission time • time taken to emit all bits into medium • propagation time • time for a bit to traverse the link NETE0510: Communication Media and Data Communications

  17. Stop and Wait • source transmits frame • destination receives frame and replies with acknowledgement (ACK) • source waits for ACK before sending next • destination can stop flow by not send ACK • works well for a few large frames • Stop and wait becomes inadequate if large block of data is split into small frames NETE0510: Communication Media and Data Communications

  18. Sliding Windows Flow Control • allows multiple numbered frames to be in transit • receiver has buffer W long • transmitter sends up to W frames without ACK • ACK includes number of next frame expected • sequence number is bounded by size of field (k) • frames are numbered modulo 2k • giving max window size of up to 2k - 1 • must send a normal acknowledge to resume • Piggybacking: insert acknowledgement number field in data frame so that a frame can contain both sequence number and ACK numbers NETE0510: Communication Media and Data Communications

  19. Sliding Window Diagram NETE0510: Communication Media and Data Communications

  20. Error Control • detection and correction of errors such as: • lost frames • damaged frames • common techniques use: • error detection • positive acknowledgment (ACK) • retransmission after timeout • negative acknowledgement (NAK) & retransmission NETE0510: Communication Media and Data Communications

  21. Automatic Repeat Request (ARQ) • collective name for such error control mechanisms, including: • stop and wait • go back N • selective reject (selective retransmission) NETE0510: Communication Media and Data Communications

  22. 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 alternate numbering and ACK0 / ACK1 NETE0510: Communication Media and Data Communications

  23. Stop and Wait • see example with both types of errors • pros and cons • simple • inefficient NETE0510: Communication Media and Data Communications

  24. Go Back N • based on sliding window • if no error, ACK as usual • 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 NETE0510: Communication Media and Data Communications

  25. Go Back N - Handling • Damaged Frame • error in frame i so receiver rejects frame i • transmitter retransmits frames from i • Lost Frame • frame i lost and either • transmitter sends i+1 andreceiver gets frame i+1 out of seq and rejects frame i • or transmitter times out and send ACK with P bit set which receiver responds to with ACK i • transmitter then retransmits frames from i NETE0510: Communication Media and Data Communications

  26. Go Back N - Handling • Damaged Acknowledgement • receiver gets frame i, sends ack (i+1) which is lost • acks are cumulative, so next ack (i+n) may arrive before transmitter times out on frame i • if transmitter times out, it sends ack with P bit set • can be repeated a number of times before a reset procedure is initiated • Damaged Rejection • reject for damaged frame is lost • handled as for lost frame when transmitter times out NETE0510: Communication Media and Data Communications

  27. Selective Reject • also called selective retransmission • only rejected frames are retransmitted • subsequent frames are accepted by the receiver and buffered • minimizes retransmission • receiver must maintain large enough buffer • more complex logic in transmitter • hence less widely used • useful for satellite links with long propagation delays NETE0510: Communication Media and Data Communications

  28. Go Back N vsSelective Reject NETE0510: Communication Media and Data Communications

  29. Error Detection • Two-dimensional parity • Internet checksum • Cyclic Redundancy Check NETE0510: Communication Media and Data Communications

  30. Internet Checksum • Not used in the link layer • checksum for the internet protocol: • Sum 16-bit data blocks • Take one complement of the result to produce the checksum • E.g. calculate checksum of 5 and 3 • 5 (0101) + 3 (0011) = 8 (1000) • Checksum = -8 (0111) • Send 5, 3, -8 to receiver  send 010100110111 • Calculate 5+3+(-8) 0101 0011 0111 1111 NETE0510: Communication Media and Data Communications

  31. Outline • Encoding Techniques • Data Link Layer Fundamental • Direct-link Protocols • BSC, DDCMP, HDLC, PPP NETE0510: Communication Media and Data Communications

  32. BSC (or BISYNC) Protocol • Binary Synchronous Communication • Byte-oriented approach: view each frame as a collection of bytes (characters) rather than a collection of bits • Support a particular character set: ASCII, EBCDIC, and IBM’s 6-bit Transcode NETE0510: Communication Media and Data Communications

  33. Problem of BISYNC • ETX character may appear in the body of a frame • Solved by “character stuffing”  insert a special character called DLE (data-link-escape) character whenever it appears in the body NETE0510: Communication Media and Data Communications

  34. Point-to-point (PPP) Protocol • Designed to encapsulate IP inter-network data • Commonly run over dial-up modem links, but can be used on any leased line for point-to-point connections not supported by FR or ATM. • Have character stuffing • Several of the field sizes are negotiated rather than fixed  using LCP (Link Control Protocol) • Two sub protocols: LCP (Link Control Protocol) and NCP (Network Control Protocol) to negotiate options for a network-layer protocol running on top of PPP • IPCP (Internet Protocol Control Protocol) for IP • IPXCP for IPX, ATCP for AppleTalk NETE0510: Communication Media and Data Communications

  35. PPP Frame Format • Flag: 011111110 • Protocol: identify high-level protocol e.g. IP or IPX • Payload: size is negotiated, default at 150 bytes • Checksum: 2-4 bytes long NETE0510: Communication Media and Data Communications

  36. LCP • Negotiation of payload size is conducted by LCP • Frame payload size can be negotiated, 1500 byte by default • LCP sends control messages encapsulated in PPP frames • Such messages are denoted by an LCP identifier in the PPP Protocol field • Sizes are changed based on the information contained in those control messages NETE0510: Communication Media and Data Communications

  37. PPP (cont’d) • PPP tends to be reserved for dialup or a mixed-vendor environment, whereas HDLC is the default for T1 serial connections NETE0510: Communication Media and Data Communications

  38. DECNET’s DDCMP • Byte-Counting Approach: include number of bytes contained in a frame as a field in the frame header • Transmission error can corrupt the COUNT field  framing error NETE0510: Communication Media and Data Communications

  39. High-level Data Link Control (HDLC) • Previously known as Synchronous Data Link Control (SDLC) protocol • Most popular data link control (L2) protocol • Form the basis of ISDN and FR protocols and services • specified as ISO 33009, ISO 4335 • station types: • Primary - controls operation of link • Frames issued are called “commands” • Secondary - under control of primary station • Frames issued are called “responses” • Combined - issues commands and responses • link configurations • Unbalanced - 1 primary, multiple secondary • Balanced - 2 combined stations NETE0510: Communication Media and Data Communications

  40. HDLC Transfer Modes • Normal Response Mode (NRM) • unbalanced config, primary initiates transfer, secondary may only transmit data in response to a command • used on multi-drop lines, e.g. host + terminals • Asynchronous Balanced Mode (ABM) • balanced config, either station initiates transmission, has no polling overhead, widely used • Asynchronous Response Mode (ARM) • unbalanced config, secondary may initiate transmit without permission from primary, rarely used NETE0510: Communication Media and Data Communications

  41. HDLC Frame Structure • synchronous transmission of frames (no start-stop bits required) • single frame format used trailer header NETE0510: Communication Media and Data Communications

  42. Flag Fields and Bit Stuffing • delimit frame at both ends with 01111110 sequence • The sequence is transmitted any time that the link is idle to keep clock synchronized • “bit stuffing” used to avoid confusion with data containing flag sequence 01111110 • 0 inserted after every sequence of five 1s • if receiver detects five 1s it checks next bit • if next bit is 0, it is deleted (was stuffed bit) • if next bit is 1 and seventh bit is 0, accept as the end-of-frame flag • if sixth and seventh bits 1, sender is indicating abort (error occurred) NETE0510: Communication Media and Data Communications

  43. Flag Fields and Bit Stuffing (cont’d) • In both bit stuff and character stuffing, the size of a frame is dependent on the data being sent in the payload. • Not possible to make all the frames exactly the same size NETE0510: Communication Media and Data Communications

  44. Address Field • identifies secondary station that sent or will receive frame • usually 8 bits long • may be extended to multiples of 7 bits • LSB indicates if is the last octet (1) or not (0) • not needed for point-to-point link • all ones address 11111111 is broadcast • Allow primary station to broadcast a frame for reception by all secondaries NETE0510: Communication Media and Data Communications

  45. Control Field • different for different frame type • Information (I-) frame - data transmitted to user (next layer up) • Flow and error control piggybacked on information frames • Supervisory (S-) frame - ARQ when piggyback not used • Unnumbered (U-) frame - supplementary link control • first 1-2 bits of control field identify frame type NETE0510: Communication Media and Data Communications

  46. Control Field (cont’d) • use of Poll/Final bit depends on context • in command frame is P bit set to1 to solicit (poll) response from peer • in response frame is F bit set to 1 to indicate response to soliciting command • Sequence number in S- and I-frames usually 3 bits • can extend to 8 bits as shown below NETE0510: Communication Media and Data Communications

  47. Information & FCS Fields • Information Field • in information and some unnumbered frames • must contain integral number of octets • variable length • Frame Check Sequence Field (FCS) • used for error detection • either 16 bit CRC or 32 bit CRC NETE0510: Communication Media and Data Communications

  48. HDLC Operation • consists of exchange of information, supervisory and unnumbered frames • have three phases • initialization • by either side, set mode & seq • data transfer • with flow and error control • using both I & S-frames (RR, RNR, REJ, SREJ) • disconnect • when ready or fault noted Selective reject Go Back N reject NETE0510: Communication Media and Data Communications

  49. HDLC Commands and Responses NETE0510: Communication Media and Data Communications

  50. HDLC Operation Example Set sync balanced mode Send seq no Receive seq no busy A must respond with RR or RNR Unnumbered acknowledgement Return from busy condition Disconnect NETE0510: Communication Media and Data Communications

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