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網路概論 Class 4 – Data Link Control Part II

網路概論 Class 4 – Data Link Control Part II. 授課老師 楊人順 2001/10/16-17. Presentation Outlines. Error Detection Techniques Cyclic redundancy check Error Control Error Control Technique Error Control Mechanism Data Link Protocols Multiplexing What is multiplexing ?

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網路概論 Class 4 – Data Link Control Part II

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  1. 網路概論Class 4 – Data Link Control Part II 授課老師 楊人順 2001/10/16-17 -1-

  2. Presentation Outlines • Error Detection Techniques • Cyclic redundancy check • Error Control • Error Control Technique • Error Control Mechanism • Data Link Protocols • Multiplexing • What is multiplexing ? • Frequency Division Multiplexing • Time Division Multiplexing • Code Division Multiple Access • Digital Subscriber Line -2-

  3. Cyclic redundancy check • CRC is the most common and powerful error-detecting codes. • Algorithm • Sender calculates a Frame Check Sequence (FCS) from a pattern. • Sender sends a frame with (Message + FCS). • Receiver then divides the incoming frame by the same pattern and, if there is no remainder, assume there was not error. • Modulo 2 Arithmetic: an exclusion-OR operation to replace the “Add” and “Subtract” • Definition • T = (k + n)-bit frame to be transmitted, with n < k • M = k-bit message, the fist k bits of T • F = n-bit FCS, the last n bits of T • P = pattern of n+1 bits; this is the predetermined divisor. -3-

  4. CRC Principle • Targets in receiver site • No error detected: T/P no remainder • Errors detected : T/P with as least one bit error. • A true : |P| = |F| + 1 • FCS generation -4-

  5. CRC Principle (con’t) • Error Detection • Example : pp. 204 • Another view of CRC : express all value as polynomials in a dummy variable X • E.g., M=110011, M(X)=X5 + X4 + X + 1, • P=11001, P(X) = X4 + X3 + 1 -5-

  6. Capacity of CRC • An error E(X) will only be undetectable if it is divisible by P(X). • The following error types are detectable : • All single-bit errors • All double-bit errors, as long as P(X) has as least three 1s • Any odd number of errors, as long as P(X) contains a factor (X + 1) • Any burst error for which the length of the burst is less than the length of the divisor polynomial; that is, less than or equal to the length of the FCS • Most larger burst errors • Undetectable error probability (r is the length of FCS) • A burst error of length r + 1, undetectable error probability is 1/2r–1 • A longer burst, undetectable error probability is 1/2r -6-

  7. Current CRC Versions • CRC-12, P(X)=X12 + X11 + X3 + X2 + X + 1 • Used for the stream of 6 bits characters, FCS is 12 bits • CRC-16, P(X)=X16 + X15 + X2 + 1 • Used for the stream of 8 bits characters, FCS is 16 bits • Popular for most applications • United States • CRC-CCITT, P(X)=X16 + X12 + X5 + 1 • Used for the stream of 8 bits characters, FCS is 16 bits • Popular for most applications • Europe • CRC-32, P(X)=X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10 + X8 + X7 + X5 + X4 + X2 + X + 1 • Some point-to-point synchronous transmission standards. -7-

  8. Error Control • Error types : lost frame, damaged frame • Error control techniques • Error detection : typically CRC is used • Positive acknowledgment (ACK) • Negative acknowledgment (NAK) • Error control mechanisms • Stop-and-wait ARQ (Automatic Repeat Request) • Go-back-N ARQ • Selective-reject ARQ • Piggybacked acknowledgment -8-

  9. Stop-and-Wait ARQ • Belong to Idle RQ (Fred pp.170) • Based on stop-and-wait flow control • Example : pp. 210 figure 7.8 • Note 1 : frame sequence 0,1 • Note 2 : frame loss time out • Link Utilization -9-

  10. Go-Back-N ARQ • Belong to continues ARQ • Based on sliding window flow control • Algorithm : pp. 209 - pp.211 • Example in next slide • If sequence number field = n bits, then window size = 2n – 1 (why ?) • Link Utilization -10-

  11. An Example of Go-back-N ARQ -11-

  12. Selective-Reject ARQ • Belong to continues ARQ • Based on sliding window flow control • Algorithm : only re-transmitting the frame with • Negative acknowledgement (SREJ) • Time out loss • Example in next slide • If sequence number field = n bits, then window size = 2n-1 (why ?) • Link Utilization -12-

  13. An Example of Selective-Reject ARQ -13-

  14. Data Link Protocols • High-Level Data Link Control (HDLC) • HDLC is the basis for many other important data link control protocols • ISO 3009, ISO 4335 • Link Access Procedure, Balanced (LAPB) • LAPB was issued by ITU-T • X.25 packet-switching network interface standard, it provides p-t-p link control between a user system and a packet-switching network node • Link Access Procedure, D-Channel (LAPD) • LAPD was issued by ITU-T as part of its set of recommendations on ISDN • LAPD provides data link control over D channel -14-

  15. Data Link Protocols (con’t) • Logical Link Control (LLC) • LLC is part of IEEE 802 family of standards • LLC control the link operation over a LAN • Link control functions in the case of LLC are divided between two layers: LLC and MAC (Media Access Control) layers • Use CRC-32 • Frame Relay (will be explained in Class 13) • Asynchronous Transfer Mode (ATM) (will be explained in Class 13) -15-

  16. Multiplexing • What is Multiplexing ? • Multiplexing Types • Frequency Division Multiplexing (FDM) • Time Division Multiplexing (TDM) • Synchronous TDM • Asynchronous (Statistical) TDM • Code Division Multiplexing (CDM) -16-

  17. FDM v.s. TDM -17-

  18. FDM System • Separated Frequency Band • Un-shareable Channel • Narrow Band Channel • Simultaneously Transmission • Cable-TV -18-

  19. Synchronous TDM • Shared Frequency Band • Un-shareable Time-slot • Fixed Time-slot allocation • Digital Carrier System, ISDN and Telephone Network -19-

  20. An Example of Synchronous TDM -20-

  21. Digital Carrier Systems -21-

  22. DS-1 Transmission Format -22-

  23. ISDN Primary Access Frame Formats -23-

  24. Synchronous TDM v.s. Statistical TDM -24-

  25. Simplified Scheme of CDMA (uplink) s(t) = s1(t)+s2(t) d1(t)+c1(t)s2(t) s1(t) LPF LPF s2(t) c2(t)s1(t)+d2(t) -25-

  26. Digital Subscriber Line • Asynchronous Digital Subscriber Line (ADSL) • Provides high-speed digital data rate transmission over ordinary telephone wire • Uses FDM to exploit the 1-MHz capacity of twisted pair • Each carrier (frequency) is use QAM (ASK + PSK) analog signaling technique • High Data Rate Digital Subscriber Line (HDSL) • To cope the problems of high bandwidth (1.5 MHz) and short distance (1km) in T1 line • HDSL uses 2B1Q coding scheme to provide a data rate of up to 2Mbps over two twisted pair lines within a bandwidth about 196kHz and enables distance range 3.7km. -26-

  27. Digital Subscriber Line (con’t) • Signal Line Digital Subscriber Line (SDSL) • Uses only one twisted pair line. • Echo cancellation is used to achieve full-duplex transmission over a signal pair • Very High Data Rate Digital Subscriber Line (VDSL) • VDSL does not use echo cancellation but provides separate bands for different services, with the following tentative (試驗性的) allocation: • POTS: 0 – 4 kHz • ISDN: 4 – 80 kHz • Upstream: 300 – 700 kHz • Downstream:  1 MHz -27-

  28. Comparison of xDSL Alternative -28-

  29. ADSL Channel Configuration -29-

  30. Group Discussions -30-

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