8/7/2019 Direct Link Networks Temp
1/29
.
8/7/2019 Direct Link Networks Temp
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).