.25 (), OS1. / /:
1 (, OSI) - / .21 ;
2 ( , OSI) - LAPB ;
3 ( , OSI) - .25 (PLP).
OSI .4-3. /, LAPB ( ) - .25 ( ). (SLP) (MLP) , . .
.4-3. .25 OSI
1
1 .25 , , , DTE DCE. , , .
DTE/DCE .25 .. : .21, X.21bis, V.nn .31. . .4.7.
2
2 .25 , DTE DCE. .. :
LAP - ( .25 1988, LAPB);
LAPB - ( . );
HDLC - , .. - .25.
LAPB , , HDLC.
LAPB
2 - LAPB DCE DTE - (SLP MLP) . ( . X.1), 2,4 2048 /, 64 /.
.25 (SLP), (MLP) .
m =8 8. 2 . .25 ().
m =128. (LAPB) 128 ( - 2 ), HDLC - . .25.
|
|
|
=32768. (LAPB) 32768 ( -128 ). .25.
LAPB .4-1. I-IDLC.
4-1. LAPB
| F | A | C | FCS | F | |
| 1 | 1 | 1/2/4 | S/U | 2 | 1 |
F - : "01111110"; .. : ; , ( ); - -, : "10111110";
- 8 - - -;
- / DTE/DCE / ( ); : 1 ( U) ( I) =8, 2 =128, 4 =32768;
- (S) (U) I- ; LAPB 3, LAPB " "; , I, : 128 ( ), : 16, 32, 64, 256, 512, 1024, 2048 4096 ; - 10- ;
FCS - CRC 16- .
