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網路概論Class 4 – Data Link Control P

art II授課老師楊人順

2001/10/16-17

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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

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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.

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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

frameion transmiss the; 2

FCS

remainder : quotient, : ; 2

left bits toshift ; 2

FMT

RF

RQP

RQ

P

M

nMM

n

n

n

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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

0 add) 2 (Modulo ; then

ssion,in transmi occured errors no If

2 ;

pattern divide frame received the; )(2

RRQP

RRQ

P

RRQ

P

T

P

RQ

PP

R

P

RQ

P

T

TP

RFM

P

T

n

n

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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

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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.

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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

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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

speedlight 0.67 : distance, : ,/

/

yprobabiliterror frame theis

where

,21

1

VdRL

Vd

Time onTransmissi

Time nPropagatioa

P

a

PU

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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

size Window theis where

,21

121

1

21 21

1

W

aWWPPa

PW

aWaP

P

U

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An Example of Go-back-N ARQ

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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

size Window theis where

,21

21

1

21 1

W

aWa

PW

aWU

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An Example of Selective-Reject ARQ

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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

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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)

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Multiplexing

• What is Multiplexing ?

• Multiplexing Types– Frequency Division Multiplexing (FDM)– Time Division Multiplexing (TDM)

Synchronous TDMAsynchronous (Statistical) TDM

– Code Division Multiplexing (CDM)

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FDM v.s. TDM

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FDM System

• Separated Frequency Band• Un-shareable Channel• Narrow Band Channel• Simultaneously Transmission• Cable-TV

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Synchronous TDM

• Shared Frequency Band• Un-shareable Time-slot• Fixed Time-slot allocation• Digital Carrier System, ISDN

and Telephone Network

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An Example of Synchronous TDM

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Digital Carrier Systems

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DS-1 Transmission Format

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ISDN Primary Access Frame Formats

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Synchronous TDM v.s. Statistical TDM

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s1(t)

s2(t)

s(t) = s1(t) +s2(t) d1(t)+c1(t)s2(t)

c2(t)s1(t)+d2(t)

LPF

LPF

Simplified Scheme of CDMA (uplink)

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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.

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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 b

ands for different services, with the following tentative (試驗性的 ) allocation:POTS: 0 – 4 kHz ISDN: 4 – 80 kHzUpstream: 300 – 700 kHzDownstream: 1 MHz

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Comparison of xDSL Alternative

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ADSL Channel Configuration

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Group Discussions