X25



X.25 is broadly a CCITT-recommendation for the interface between a DTE and a DCE over a telephone connection (PSTN). Fundamentally X.25 describes the Layer 1 until 3 of the ISO of stack model, but the here viewed protocol-specific name means the Paket-Layer 3.
The here presented data only should represent a short overview over the types of X25-pakets. For detailed information, the ISO 8208 only is responsible.


broadly:

GFI

LCN

Typ

Parameter / Daten



GFIGeneral Format Identier
• bit-7 is Qualifer-bit
• bit-6 is Delivery Confirmation-bit
• bit-5..4 is Sequence scheme (01 for modulo-8 / 10 for modulo 128)
• bit-3..0 is Logical channel group number (LGN)
LCNLogical channel number (SVC / PVC)
TypType of the package
(1byte in modulo-8 / 2byte in modulo-128)
Parameter / Datendependent on the type


Types:

This overview treats exclusively the most frequently use variant in Germany at the representing modulo-8.


CALL Request

" Connection construction "

GFI

LCN

0Bh

xyh

number [/number]

facility

PID

user-data



0BhTyp: CALL Request
xyhLenght of number(s) - in nibble
number [/number]call-number [/source-number] - packed in nibble
facilityz.B.
00h - without
02h 01h 01h - reverse charching
PIDProtocol Identifier (4 byte)
user-datamax 16 bytes


CALL Accept

" Connection built "

GFI

LCN

0Fh

xyh

number [/number]

user-data



0BhTyp: CALL Accept
xyhLenght of number(s) - in nibble
number [/number]called-number [/source-number] - packed in nibble
user-datamax 16 bytes


I-frame

" Data package "

GFI

LCN

xx..xx0b

data



xx..xx0bTyp: I-frame
• bit-7..5 is number of last received I-frame+1
• bit-4 is more-bit to mark data will be continue
• bit-3..1 is number of this I-frame
• bit-0 must be zero
datamax 128 bytes


Receive-Ready

" ready "

GFI

LCN

x1h



x1hTyp: Receive-Ready
• bit-7..5 is number of last received I-frame+1
• bit-4 is poll/final flag


Receive-Not-Ready

" not ready "

GFI

LCN

x5h



x5hTyp: Receive-Not-Ready
• bit-7..5 up to number-1 is ok.
• bit-4 is poll/final flag


Reject

" repeat "

GFI

LCN

x9h



x9hTyp: Reject
• bit-7..5 up to number-1 is ok. - after this is to repeat
• bit-4 is poll/final flag


Clear request

" Connection finishes "

GFI

LCN

13h

cause

diag



13hTyp: Clear request
cause / diagsee ISO 8208 and DATEX-P (and manuals from TA's)


Clear confirmation

" Receipt connection finishes "

GFI

LCN

17h



17hTyp: Clear confirmation


Reset

" reset package numbering "

GFI

LCN

1Bh

cause

diag



1BhTyp: Reset
cause / diagsee ISO 8208 and DATEX-P (and manuals from TA's)


Reset confirmation

" Receipt Reset "

GFI

LCN

1Fh



1FhTyp: Reset confirmation


Interrupt

" sends Data despite FLOW "

GFI

LCN

23h

data



23hTyp: Interrupt
datamax 32 bytes


Interrupt confirmation

" Receipt INT-transport "

GFI

LCN

27h



27hTyp: Interrupt confirmation


Restart

" reset all canals "

GFI

LCN

FBh

cause

diag



FBhTyp: Restart
cause / diagsee ISO 8208 and DATEX-P (and manuals from TA's)


Restart confirmation

" Receipt restart "

GFI

LCN

FFh



FFhTyp: Restart confirmation


Diagnostic

" unspecific mistake in package"

GFI

LCN

F1h

diag

expan.



F1hTyp: Diagnostic
diag / expan.see ISO 8208 and DATEX-P (and manuals from TA's)


Registration Request

" Features agrees "

GFI

LCN

F3h

lenght

code



F3hTyp: Registration Request
lenght / codesee ISO 8208 and DATEX-P


Registration Confirmation

" Features answers "

GFI

LCN

F7h

lenght

code



F7hTyp: Confirmation Request
lenght / codesee ISO 8208 and DATEX-P





In the standard to X25, costly State-diagrams are represented to the respective transitions and mistake treatments. Getting a variation about a space-saving and simultaneously good to in the habit of state/change-mechanism is the implementation by means of a State-Table. The current status is used as line and the received Paket-Typ on that occasion as columns. The there deposited byte returns the necessary reaction and the low Nibble the new State in the upper Nibble. There sufficiently exceptions exists, however even further evaluations are around necessary about such a table, sees ISO 8208.