LAPD, Link Access Protocol - Channel D, a Layer 2 protocol is defined in CCITT Q.920/921 as well as ETS 300 125. LAPD works exclusively in the Asynchronous Balanced Mode. X31 in the D-chanal (package service) is one of the typical applications.
The here presented data only serve as short overview over the types of LAPD-pakets. For detailed information, the ETS 300 125 only is responsible.
broadly:
Flag | SAPI | TEI | CF | Parameter / Daten | FCS | Flag |
7Eh 7Eh
dependent on the type
Control Field
Type of the package Frame Check Sequence
|---------------------| Address Field
Terminal endpoint identifer
bit-0 EA2-Second Address Extension bit which is always set to 1.
Service access point identifer
bit-1 C/R-Command/Response bit.
(Frames from the user with this bit set to 0 are command frames,
as are frames from the network with this bit set to 1.
Other values indicate a response frame.)
bit-0 EA1-First Address Extension bit which is always set to 0.
SAPI – Service Access Point Identifier:
0 | Used by CCITT signaling |
1 | Not defined |
16 | Used for paket-data communication |
32 | Used for internal loop-tests only (national !!!) |
63 | Used for management(TEI) in all services |
for X31 in the D-chanal is valid pronto on that occasion: SAPI = 16 (as far as on TEI-Management)
TEI - Terminal Endpoint Identifier:
0-63 | Used by non-automatic TEI assignment user equipment. |
64-126 | Used by automatic TEI assignment equipment. |
127 | Used for a broadcast connection meant for all Terminal Endpoints. |
broadly usually for X31(not pronto): TEI=1
The numbering takes place after Modulo-128.
Types:
" set asyncronous balanced mode extended "
Flag | SAPI | TEI | 7Fh | FCS | Flag |
bit-4 poll-bit
SABME
" disconnected mode "
Flag | SAPI | TEI | 1Fh | FCS | Flag |
bit-4 final-bit
DM
" disconnect "
Flag | SAPI | TEI | 53h | FCS | Flag |
bit-4 poll-bit
DISC
" unnumbered acknowledgement "
Flag | SAPI | TEI | 73h | FCS | Flag |
bit-4 final-bit
UA
" frame reject "
!!! it should not be used!!!
Flag | SAPI | TEI | 87h | data-field | FCS | Flag |
see ETS 300 125 for data-field
bit-4 final-bit
FRMR
" exchange identification node "
!!! it should not be used!!!
Flag | SAPI | TEI | AFh | data-field | FCS | Flag |
see ETS 300 125 for data-field
bit-4 poll/final-bit
XID
" reject "
Flag | SAPI | TEI | 09h | xxh | FCS | Flag |
bit-7..1 receive sequence number
bit-4 poll/final-bit
REJ
" receive not ready "
Flag | SAPI | TEI | 05h | xxh | FCS | Flag |
bit-7..1 receive sequence number
bit-4 poll/final-bit
RNR
" receive ready "
Flag | SAPI | TEI | 01h | xxh | FCS | Flag |
bit-7..1 receive sequence number
bit-4 poll/final-bit
RR
" data-frame "
Flag | SAPI | TEI | xx..xx0b | xxh | data | FCS | Flag |
bit-7..1 receive sequence number
bit-0 poll -bit
bit-7..1 send sequence number
bit-0 must be zero
I-frame
" unnumbered-information "
Flag | SAPI | TEI | 03h | [data-field] | FCS | Flag |
optional (for example: TEI-management)
see ETS 300 125
bit-4 poll-bit
UI
In the standard to LAPD, 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 ETS 300 125.