网路概论课件

1、网路概论網路概論網路概論Class 4 Data Link Control Part II授課老師楊人順2001/10/16-17网路概论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 Div

2、ision Multiplexing Code Division Multiple Access Digital Subscriber Line网路概论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

3、 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 la

4、st n bits of T P = pattern of n+1 bits; this is the predetermined divisor.网路概论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 generationj k l m frameion transmiss the; 2 FCSremainder : quotient,

5、: ; 2left bits toshift ; 2FMTRFRQPRQPMnMMnnn网路概论CRC Principle (cont) Error Detectionu v w 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 0 add) 2 (Modulo ; then ssion,in transmi occured

6、errors no If2 ; pattern divide frame received the; )(2RRQPRRQPRRQPTPRQPPRPRQPTTPRFMPTnn网路概论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

7、 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 F

8、CS) A burst error of length r + 1, undetectable error probability is 1/2r1 A longer burst, undetectable error probability is 1/2r网路概论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 str

9、eam 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 +

10、X + 1 Some point-to-point synchronous transmission standards.网路概论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 Re

11、peat Request) Go-back-N ARQ Selective-reject ARQ Piggybacked acknowledgment网路概论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 Utilizationspeedlight 0.67 : distance, : ,/y

12、probabiliterror frame theis where,211VdRLVdTime onTransmissiTime nPropagatioaPaPU网路概论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 Utilizationsize W

13、indow theis where,21 121121 211 WaWWPPaPWaWaPPU网路概论An Example of Go-back-N ARQ网路概论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 f

14、ield = n bits, then window size = 2n-1 (why ?) Link Utilizationsize Window theis where,21 21121 1WaWaPWaWU网路概论An Example of Selective-Reject ARQ网路概论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 A

15、ccess 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

16、on ISDN LAPD provides data link control over D channel网路概论Data Link Protocols (cont) 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 Co

17、ntrol) layers Use CRC-32 Frame Relay (will be explained in Class 13) Asynchronous Transfer Mode (ATM) (will be explained in Class 13)网路概论Multiplexing What is Multiplexing ? Multiplexing Types Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM)Synchronous TDMAsynchronous (Statistic

18、al) TDM Code Division Multiplexing (CDM)网路概论FDM v.s. TDM网路概论FDM System Separated Frequency Band Un-shareable Channel Narrow Band Channel Simultaneously Transmission Cable-TV网路概论Synchronous TDM Shared Frequency Band Un-shareable Time-slot Fixed Time-slot allocation Digital Carrier System, ISDN and Te

19、lephone Network 网路概论An Example of Synchronous TDM网路概论Digital Carrier Systems网路概论DS-1 Transmission Format网路概论ISDN Primary Access Frame Formats网路概论Synchronous TDM v.s. Statistical TDM网路概论s1(t)s2(t)s(t) = s1(t) +s2(t)d1(t)+c1(t)s2(t)c2(t)s1(t)+d2(t)LPFLPFSimplified Scheme of CDMA (uplink)网路概论Digital Su

20、bscriber 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 Subscribe

21、r 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.网路概论Digital Subscriber Line (cont) Signal

22、 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 f

23、ollowing tentative (試驗性的) allocation:POTS: 0 4 kHzISDN: 4 80 kHzUpstream: 300 700 kHz1.Downstream: 1 MHz网路概论Comparison of xDSL Alternative网路概论ADSL Channel Configuration网路概论Group Discussions网路概论F7IaLdPgSjVnYq$t*w-A1D4G8JbNeQhTlWoZr%u(y+B2E6H9KcOfRjUmXp!s&v)z0C4F7IaMdPgSkVnYq$t*x-A1D5G8JbNeQiTlWo#

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