PAGE LAISSEE EN BLANC INTENTIONNELLEMENT
DATA LINK LAYER
4 Elements for layer-to-layer communication
4.1 General
Com m unications between layers and, for this R ecom m endation, between the data link layer and the layer m anagem ent are accomplished by m eans o f primitives.
Primitives represent, in an abstract way, the logical exchange o f inform ation and control between the data link and adjacent layers. They do not specify or constrain im plem entations.
Primitives consist of com m ands and their respective responses associated with the services requested o f a lower layer. The general syntax o f a prim itive is:
XX — G eneric nam e — Type: Parameters
where XX designates the interface across which the prim itive flows. For this Recom m endation, XX is:
— DL for com m unication between layer 3 and the data link layer;
— PH for com m unication between the data link layer and the physical layer;
— M DL for com m unication between the layer m anagem ent and the data link layer; or
— MPH for com m unication between the m anagem ent entity and the physical layer.
Fascicle VI. 10 — Rec. Q.921 31
4.1.1 Generic names
The generic nam e specifies the activity that should be perform ed. Table 6/Q.921 illustrates the primitives defined in this Recom m endation. Note that not all prim itives have associated parameters.
TABLE 6/Q.921
Primitives associated with Recommendation Q.921
G eneric name
Type Param eters
Message unit contents Request Indication Response Confirm Priority
indicator
Message unit
L 3 «-► L 2
DL-ESTABLISH X X - X -
-DL-RELEA SE X X - X -
-DL-DATA X X - - - X Layer 3
peer-to-peer message
D L -U N IT DATA X X - - - X Layer 3
peer-to-peer message M +—> L 2
MDL-ASS1GN X X - - - X TEI value, CES
M D L-REM OV E X - - - X TEI value, CES
M D L-ERR O R - X X - - X Reason for error message
M D L -U N IT DATA X X - - - X M anagem ent function
peer-to-peer message
M D L-X ID X X X X - X C onnection m anagem ent
inform ation L 2 — L I
PH -DATA X X - - X X D ata link layer
peer-to-peer message
PH-ACTIVATE X X - - -
-PH -DEA CTIV A TE - X - - -
-M —- L I
M PH-ACTIVATE - X - -
-M PH -DEA CTIV ATE X X - - -
-M PH -IN FO R -M A T IO N - X - - - X C onnected/disconnected
L3 — L2 L2 *— LI M —+ L2 M «— LI
Layer 3 /d a ta link layer boundary D ata link layer/physical layer boundary M anagem ent e n tity /d a ta link layer boundary M anagem ent entity/physical layer boundary
32 Fascicle VI.10 - Rec. Q.921
The primitive generic names that are defined in this Recommendation are:
4.1.1.1 D L -E S T A B L IS H
The DL-ESTABLISH prim itives are used to request, indicate and confirm the outcom e o f the procedures for establishing m ultiple fram e operation.
4.1.1.2 D L -R E L E A S E
The D L-RELEASE prim itives are used to request, indicate and confirm the outcom e o f the procedures for term inating a previously established m ultiple fram e operation, or for reporting an unsuccessful establishm ent attem pt.
4.1.1.3 D L-D ATA
The DL-DATA primitives are used to request and indicate layer 3 messages which are to be transm itted, or have been received, by the data link layer using the acknowledged inform ation transfer service.
4.1.1.4 D L -U N IT DATA
The D L -U N IT DATA prim itives are used to request and indicate layer 3 messages which are to be transm itted, or have been received, by the data link layer using the unacknow ledged inform ation transfer service.
4.1.1.5 M D L -A S S IG N
The M DL-A SSIGN primitives are used by the layer m anagem ent entity to request that the data link layer associate the TEI value contained within the message unit of the prim itive with the specified C onnection E ndpoint Suffix (CES), across all SAPIs. The M D L-A SSIG N prim itive is used by the data link layer to indicate to the layer m anagem ent entity the need for a TEI value to be associated with the CES specified in the prim itive message unit.
4.1.1.6 M D L -R E M O V E
The M DL-REM O VE prim itives are used by the layer m anagem ent entity to request that the d ata link layer remove the association o f the specified TEI value with the specified CES, across all SAPIs. The T E I and CES are specified by the M D L-REM O V E prim itive message unit.
4.1.1.7 M D L -E R R O R
The M D L-ER R O R prim itives are used to indicate to the connection m anagem ent entity th at an error has occurred, associated with a previous m anagem ent function request or detected as a result o f com m unication with the data link layer peer entity. The layer m anagem ent entity may respond with an M D L -E R R O R prim itive if the layer m anagem ent entity cannot obtain a TEI value.
4.1.1.8 M D L -U N IT D ATA
The M D L -U N IT DATA prim itives are used to request and indicate layer m anagem ent entity messages which are to be transm itted, or have been received, by the data link layer using the unacknow ledged inform ation transfer service.
4.1.1.9 M D L-X ID
The M D L-X ID primitives are used by the connection m anagem ent entity to request, indicate, respond and confirm the outcome o f the actions for the use o f the X ID procedures.
4.1.1.10 PH-DATA
The PH-DATA primitives are used to request and indicate message units containing frames used fo r data link layer peer-to-peer com m unications passed to and from the physical layer.
Fascicle VI.10 — Rec. Q.921 33
4.1.1.11 P H -A C T IV A T E
The PH-ACTIVATE prim itives are used to request activation o f the physical layer connection or to indicate that the physical layer connection has been activated.
4.1.1.12 P H -D E A C TIV A TE
The PH-D EA CTIV A TE prim itive is used to indicate that the physical layer connection has been deactivated.
4.1.1.13 M PH-A C TIVA TE (see Appendix III)
The M PH -ACTIVATE prim itive is used to indicate that the physical layer connection has been activated.
4.1.1.14 M P H -D E A C T IV A T E (see Appendix III)
The M PH -D EA CTIV A TE primitives are used to request deactivation o f the physical layer connection or to indicate that the physical layer connection has been deactivated. The —REQ U EST primitive is for use by the netw ork side system m anagem ent entity.
4.1.1.15 M PH -IN F O R M A TIO N
The M PH -IN FO R M A T IO N prim itive is for use by the user side m anagem ent entity, and provides an indication as to whether the term inal is:
— connected; or
— disconnected or unable to provide sufficient power to support the TEI m anagem ent procedures.
4.1.2 Primitive types
The prim itive types defined in this R ecom m endation are:
4.1.2.1 R E Q U E S T
The REQ U EST prim itive type is used when a higher layer or layer m anagem ent is requesting a service from the lower layer.
4.1.2.2 IN D IC A T IO N
The IN D IC A T IO N prim itive type is used by a layer providing a service to inform the higher layer or layer m anagem ent.
4.1.2.3 R E S P O N S E
The R ESPO N SE prim itive type is used by layer m anagem ent as a consequence o f the IN D IC A T IO N prim itive type.
4.1.2.4 C O N F IR M
The C O N FIR M prim itive type is used by the layer providing the requested service to confirm that the activity has been completed.
Figure 7/Q.921 illustrates the relationship of the prim itive types to layer 3 and the data link layer.
4.1.3 Parameter definition
4.1.3.1 Priority indicator
Since several SAPs may exist on the network side or user side, protocol messages units sent by one SAP may contend with those of other service access points for the physical resources available for message transfer.
The priority indicator is used to determ ine which message unit will have greater priority when contention exists.
The priority indicator is only needed at the user side for distinguishing message units sent by the SAP with a SAPI value o f 0 from all other message units.
34 Fascicle VI.10 - Rec. Q.921
p r o to c o l
FIGURE 7/Q.921
Relationship of the primitive types to layer 3 and the data link layer
4.1.3.2 Message unit
The message unit contains additional layer-to-layer inform ation concerning actions and results associated with requests. In the case of the DATA primitives, the message unit contains the requesting layer peer-to-peer messages. For example, the DL-DATA message unit contains layer 3 inform ation. The PH-D A TA message unit contains the data link layer frame.
Note - The operations across the data link layer/layer 3 boundary shall be such that the layer sending the DL-DATA or D L-U N IT DATA prim itive can assume a tem poral order of the bits within the message unit and that the layer receiving the prim itive can reconstruct the message with its assumed tem poral order.
&
Fascicle VI.10 — Rec. Q.921 35
4.2 Primitive procedures
4.2.1 General
Primitive procedures specify the interactions between adjacent layers to invoke and provide a service. The service primitives represent the elements o f the procedures.
In the scope o f this Recom m endation the interactions between layer 3 and the data link layer are specified.
4.2.2 Layer 3 — Data link layer interactions
The states o f a data link connection endpoint may be derived from the internal states of the data link layer entity supporting this type of a data link connection.
D ata link connection endpoint states are defined as follows:
a) Broadcast data link connection endpoint:
— information transfer state.
b) Point-to-point data link connection endpoint:
— link connection released state;
— awaiting establish state;
— awaiting release state;
— link connection established state.
The primitives provide the procedural m eans to specify conceptually how a data link service user can invoke a service.
This section defines the constraints on the sequences in which the prim itives may occur. The sequences are related to the states at one point-to-point data link connection endpoint.
The possible overall sequences o f primitives at a point-to-point data link connection endpoint are defined in the state transition diagram , Figure 8/Q.921. The link connection released and link connection established states are stable states whilst the awaiting establish and awaiting release states are transition states.
The model illustrates the behaviour o f layer 2 as seen by layer 3. This model assumes that the prim itives passed between layers is im plem ented by a first in first out queue. In this m odel, “collisions” o f R EQ U EST and IN D IC A T IO N primitives can occur thereby illustrating actions that seem to be in conflict with the actual layer 2 protocol description. In some im plem entations, these collisions could occur.
36 Fascicle VI.10 — Rec. Q.921
D L - U N I T D A T A - R E Q U E S T /
I N D I C A T I O N
I N D I C A T I O N
FIG U R E 8/Q.921
State Transition Diagram for sequences o f Primitives at a point-to-point D ata Link Connection as seen by Layer 3 (Note 1)
Fascicle VI.10 — Rec. Q.921 3 7
Note 1 — If the data link layer entity issues a DIsESTA BLISH -IN D IC A TIO N (this applies to the case of data link layer initiated or peer system initiated re-establishment), DL-RELEA SE-CON FIRM or D L-RELEASE-INDICATION, this indicates the discard of all the data link service data units representing D U D A TA -REQ U ESTs.
Note 2 — This primitive notifies layer 3 of link re-establishment.
Note 3 lA This primitive will occur if a DL-RELEASE-REQ U EST collides with a DL-RELEASE-INDICATION.
Note 4 - This primitive will occur if a D ISESTABLISH-REQUEST collides with a D IsESTABLISH-IND ICA TION . Note 5 — This primitive will occur if a DIsRELEASE-REQUEST collides with a DIsESTABLISH-INDICATION.
Note 6 — This primitive will occur if a DLsESTABLISH-REQUEST (this applies to the case of layer 3 initiated re-establishment) collides with a D Is RELEASE-INDICATION. Since this D Is RELEASE-IND ICATIO N is not related to the DISESTABLISH-REQUEST, the data link layer will establish the link and issue a DIsESTABLISH-CONFIRM.
Notes to Figure 8/Q . 921 :