-1- 網路概論 Class 4 – Data Link Control Part II 授課老師 楊人順 2001/10/16-17.
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Transcript of -1- 網路概論 Class 4 – Data Link Control Part II 授課老師 楊人順 2001/10/16-17.
-1-
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
-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.
-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
frameion transmiss the; 2
FCS
remainder : quotient, : ; 2
left bits toshift ; 2
FMT
RF
RQP
RQ
P
M
nMM
n
n
n
-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
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
-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
-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.
-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
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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
-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
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
-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)
-16-
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
-23-
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)
-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.
-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 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
-28-
Comparison of xDSL Alternative
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ADSL Channel Configuration
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Group Discussions