1 / 51

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.

mac
Download Presentation

NETE0510 Data-link Layer and Protocols

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 per second) NETE0510: Communication Media and Data Communications

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

  14. 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

  15. 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

  16. 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

  17. 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

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

  19. 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

  20. 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

  21. 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

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

  23. 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

  24. 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

  25. 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

  26. 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

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

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

  29. 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

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

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

  32. 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

  33. 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 IPCP (Internet Protocol Control Protocol) for network control • IPXCP for IPX, ATCP for AppleTalk NETE0510: Communication Media and Data Communications

  34. 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

  35. LCP • Negotiation of size is conducted by LCP • 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

  36. 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

  37. 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

  38. 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

  39. 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, eg 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

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

  41. 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

  42. 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

  43. 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 NETE0510: Communication Media and Data Communications

  44. 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

  45. 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 • seq number in S- and I-frames usually 3 bits • can extend to 8 bits as shown below NETE0510: Communication Media and Data Communications

  46. 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

  47. 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 NETE0510: Communication Media and Data Communications

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

  49. 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

  50. HDLC Operation Example NETE0510: Communication Media and Data Communications

More Related