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

dependent on the type

Type of the package
(1byte in modulo-8 / 2byte in modulo-128)

Logical channel number (SVC / PVC)

   General 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)

 

Typen:

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


" Connection construction "

GFI

LCN

0Bh

xyh

number [/number]

facility

PID

user-data

max 16 bytes

      Protocol Identifier (4 byte)

z.B.
00h - without
02h 01h 01h - reverse charching

call-number [/source-number]
packed in nibble

Lenght of number(s)
in nibble

CALL Request

 

" Connection built "

GFI

LCN

0Fh

xyh

number [/number]

user-data

max 16 bytes

called-number [/source-number]
packed in nibble

Lenght of number(s)
in nibble

CALL Accept

 

" Data package "

GFI

LCN

xx..xx0b

data

max 128 bytes

   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

I-frame

 

" Ready "

GFI

LCN

x1h

   bit-7..5 is number of last received I-frame+1
   bit-4 is poll/final flag

Receive-Ready

 

" not ready "

GFI

LCN

x5h

   bit-7..5 up to number-1 is ok.
   bit-4 is poll/final flag

Receive-Not-Ready

 

" repeat "

GFI

LCN

x9h

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

Reject

 

" Connection finishes "

GFI

LCN

13h

cause

diag

see ISO 8208 and DATEX-P (and manuals from TA’s)

Clear request

 

" Receipt connection finishes "

GFI

LCN

17h

Clear confirmation

 

" Package numbering puts back "

GFI

LCN

1Bh

cause

diag

see ISO 8208 and DATEX-P (and manuals from TA’s)

Reset

 

" Receipt Reset "

GFI

LCN

1Fh

Reset confirmation

 

" Data despite FLOW sends "

GFI

LCN

23h

data

max 32 bytes

Interrupt

 

" Receipt INT-Transport "

GFI

LCN

27h

Interrupt confirmation

 

" all canals puts back "

GFI

LCN

FBh

cause

diag

see ISO 8208 and DATEX-P (and manuals from TA’s)

Restart

 

" Receipt restart "

GFI

LCN

FFh

Restart confirmation

 

" unspecific mistake in package "

GFI

LCN

F1h

diag

expan.

see ISO 8208 and DATEX-P (and manuals from TA’s)

Diagnostic

 

" Features agrees "

GFI

LCN

F3h

lenght

code

see ISO 8208 and DATEX-P

Registration Request

 

" Features answers "

GFI

LCN

F7h

lenght

code

see ISO 8208 and DATEX-P

Registration Confirmation


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.