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1/29

.

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2/29

(point-to-point

links)

(framing)

(error detection

and correction)

8/7/2019 Direct Link Networks Temp

3/29

8/7/2019 Direct Link Networks Temp

4/29

(bytes)

Binary Synchronous Communication (BISYNC)

IBM, 1960s

Digital Data Communication Message Protocol(DDCMP)

Digital Equipment Corporation DECNET.

Point-to-Point Protocol (PPP)

8/7/2019 Direct Link Networks Temp

5/29

Binary Synchronous Communication

(BISYNC)

SYN: Synchronization ( )

SOH: Start of Header ( )

STX: Start of Text ( )

ETX: End of Text ( )

CRC: Cyclic Redundancy Check ( )

BODY:

SYN SYN SOH ETX CRCHeader BODY

8 888 8 8 16

STX

:

:

8/7/2019 Direct Link Networks Temp

6/29

Point-to-Point Protocol (PPP)

Flag: Protocol: LCP, NCP, IP IPX

Payload

1500 bytes.

To PPP

.

Flag Address Control FlagChecksumProtocol Payload

8 168 8 816

8/7/2019 Direct Link Networks Temp

7/29

Point-to-Point Protocol (PPP)

.

LCP (Link Control Protocol):

NCP (Network Control Protocol): , .., IP.

Flag Address Control FlagChecksumProtocol Payload

8 168 8 816

8/7/2019 Direct Link Networks Temp

8/29

Digital Data Communication Message

Protocol (DDCMP)

SYN: Synchronization ( )

COUNT: bytes BODY

.

SYN SYN class CRCHeader BODY

8 42148 8 16

COUNT

COUNT ;

bytes COUNT

CRC .

.

8/7/2019 Direct Link Networks Temp

9/29

(bit streams)

High-level Data Link Control (HDLC) BS: Beginning Sequence 01111110

ES: Ending Sequence 01111110

01111110

.

01111110 ;

BS ESCRCHeader BODY

8 16 16 8

8/7/2019 Direct Link Networks Temp

10/29

bit-stuffing

01111110 ;

5 1, (

ES), 0 .

, 5 1, 0, bit-stuffing,

.

1, .

0, 01111110, .

8/7/2019 Direct Link Networks Temp

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- Synchronous OpticalNetwork (SONET)

.

.

: STS-1: 51.84Mbps STS-48: 2488.32Mbps 51.84Mbps

125s NRZ. -,

(scrabbled) 0 1.

8/7/2019 Direct Link Networks Temp

12/29

SONET STS-1

2 bytes .

byte-stuffing

90 bytes.

87 (87 bytes)

9

3 bytes

(payload)

8/7/2019 Direct Link Networks Temp

13/29

SONET

byte byte

o

.

(pointer)

.

STS-1

STS-1

STS-1

STS-3

270 bytes

125s

8/7/2019 Direct Link Networks Temp

14/29

(Error Detection)

(redundant information).

..

(two-dimensional parity) (check sum)

(Cyclic Redundancy

Check CRC).

8/7/2019 Direct Link Networks Temp

15/29

(Error Detection)

;

(retransmission):

(error correction codes).

;

8/7/2019 Direct Link Networks Temp

16/29

(Parity)

1 bit byte bit . bits (even parity)

(even parity) (odd parity)

(matrix) (even

parity)

1 2 3 4.

= 8 + n bits, n= dataBits/7 2

8/7/2019 Direct Link Networks Temp

17/29

1111101

0010110

0111000

0111101

1001011

1001010Bits

(parity bits)

8/7/2019 Direct Link Networks Temp

18/29

01101111

0111110110010110

10111000

10110101

01001011

11001010

:

!

8/7/2019 Direct Link Networks Temp

19/29

01101111

0111110110010110

10111000

10100101

01001011

11001010

:

8/7/2019 Direct Link Networks Temp

20/29

(checksum)

(words)

(ones complement addition).

(

.

,

.

,

.

.

8/7/2019 Direct Link Networks Temp

21/29

(Cyclic

Redundancy Check CRC).

.

(bit stream) M(x) (n+1)-bit n.

1001 M(x) = 1x3+0x2+0x1+1x0=x3+x0

: C(x) k .

P(x) n+k ( n+k+1 bits).

P(x) P(x) / C(x) 0.

.

, .

8/7/2019 Direct Link Networks Temp

22/29

(x) C(x) (x) C

(x). (x) C(x)

(x) C(x).

(x) / C(x)

C(x) (x). C(x) (x)

Exclusive-OR(XOR) .

8/7/2019 Direct Link Networks Temp

23/29

M(x).

C(x)

k. M(x) xk

T(x), T(x)= xkM(x).

(x) C(x)

R

(x).

R(x) (x) P(x) = (x)

- R(x)

.

8/7/2019 Direct Link Networks Temp

24/29

10011010.

:

C(x)= x3+x2+1

M(x) x3

10011010000

10 7 6 4

3 2 1

x x x x

x x

8/7/2019 Direct Link Networks Temp

25/29

10 9 8 7 6 5 4 3 2 1 00 0 0 0 0 0 0x x x x x x x x x x x 3 2 1x x 10 9 8 70 x x x x

x

.

8/7/2019 Direct Link Networks Temp

26/29

1 0 0 1 1 0 1 0 0 0 0 1 1 0 1

8/7/2019 Direct Link Networks Temp

27/29

:

1 0 0 1 1 0 1 0 1 0 1 1 1 0 1

8/7/2019 Direct Link Networks Temp

28/29

C(x)

x16+x15+x2+1

x16+x12+x5+1x32+x26+x23+x22+x16+x12+

+x11+x10+x8+x7+x5+x4+x2+x1+1

CRC-16

CRC-CCITTCRC-32

x8+x2+x1+1

x10+x9+x5+x4+x1+1

x12+x11+x3+x2+x1+1

CRC-8

CRC-10

CRC-12

CRC

8/7/2019 Direct Link Networks Temp

29/29

,

n bits bit = p ( bit).

(1-p)n

, !

( )

, , .

, (.. Forward error

correction code FEC).