Communication between layers is accomplished with primitives. In an abstract way, primitives represent the logical exchange of information and control between the data link and adjacent layers; they do not specify or constrain implementations.
Primitives consist of commands and their respective responses associated with the services requested from a lower layer. The general syntax of a primitive is:
XXX - Generic name - Type: Parameters
Where “XXX” designates the interface across which the primitive flows. For this Standard, “XXX” is:
a) DL for communications between Layer 3 and the data-link layer;
b) LM for communications between the data-link layer and the link multiplexer; c) PH for communications between the link multiplexer and the physical layer; and d) MDL for communications between Layer 3 and the layer management.
5.1. Layer 3 Entity <—> Management Data-link State Machine
Communication between the Layer 3 Entity and the Management Data-link State Machine is characterized by three primitives:
• MDL-NEGOTIATE Request. The Layer 3 entity uses this primitive to request the Data-link State Machine to notify/negotiate.
• MDL-NEGOTIATE Confirm. The Management Data-link State Machine uses this primitive to notify the Layer 3 entity that notification/negotiation is complete.
• MDL-ERROR Indicate. The Management Data-link State Machine uses this primitive to notify the Layer 3 entity that notification/negotiation has failed.
5.2. Management Data-Link State Machine <—> Link Multiplexer State Machine
Communication between the Management Data-link State Machine and the Link Multiplexer State Machine is characterized by two primitives:
• LM-DATA Request. The Management Data-link State Machine uses this primitive to pass frames of any type (XID, UI, etc.) to the Link Multiplexer State Machine.
• LM-DATA Indication. The Link Multiplexer State Machine uses this primitive to pass frames of any type (XID, UI, etc.)to the Management Data-link State Machine.
5.3. Layer 3 Entity <—> Data-Link State Machine
Communication between the Layer 3 Entity and the Data-link State Machine is characterized by thirteen primitives:
• DL-CONNECT Request. The Layer 3 entity uses this primitive to request the establishment of a AX.25 connection.
• DL-CONNECT Indication. The Data-link State Machine uses this primitive to indicate that a AX.25 connection has been requested.
• DL-CONNECT Confirm. The Data-link State Machine uses this primitive to indicate that a AX.25 connection has been made.
• DL-DISCONNECT Request. The Layer 3 entity uses this primitive to request the release of a AX.25 connection.
• DL-DISCONNECT Indication. The Data-link State Machine uses this primitive to indicate that a AX.25 connection has been released.
• DL-DISCONNECT Confirm. The Data-link State Machine uses this primitive to indicate that a AX.25 connection has been released and confirmed.
• DL-DATA Request. The Layer 3 entity uses this primitive to request the transmission of data using connection-oriented protocol. If necessary, this frame is examined and acted upon by the segmenter. • DL-DATA Indication. The reassembler uses this primitive to indicate reception of Layer 3 data using
connection oriented protocol.
• DL-UNIT-DATA Request. The Layer 3 entity uses this primitive to request the transmission of data using connectionless protocol. If necessary, this frame is examined and acted upon by the segmenter. • DL-UNIT-DATA Indication. The reassembler uses this primitive to indicate reception of Layer 3 data
using connectionless protocol.
• DL-ERROR Indication. The Data-link State Machine uses this primitive to indicate when frames inconsistent with this protocol definition have been received. This includes short frames, frames with inconsistent parameter values, etc. The error indications are discussed in the SDL appendices.
• DL-FLOW-OFF Request. The Layer 3 entity uses this primitive to temporarily suspend the flow of incoming information.
5.4. Data-Link State Machine <—> Link Multiplexer State Machine
Communication between the Data-link State Machine and the Link Multiplexer State Machine is characterized by six primitives:
• LM-SEIZE Request. The Data-link State Machine uses this primitive to request the Link Multiplexer State Machine to arrange for transmission at the next available opportunity. The Data-link State Machine uses this primitive when an acknowledgement must be made; the exact frame in which the acknowledgement is sent will be chosen when the actual time for transmission arrives.
• LM-SEIZE Confirm. This primitive indicates to the Data-link State Machine that the transmission opportunity has arrived.
• LM-RELEASE Request. The Link Multiplexer State Machine uses this primitive to stop transmission.
• LM-EXPEDITED-DATA Request. The data-link machine uses this primitive to pass expedited data to the link multiplexer.
• LM-DATA Request. The Data-link State Machine uses this primitive to pass frames of any type (SABM, RR, UI, etc.) to the Link Multiplexer State Machine.
• LM-DATA Indication. The Link Multiplexer State Machine uses this primitive to pass frames of any type (SABM, RR, UI, etc.) to the Data-link State Machine.
5.5. Link Multiplexer State Machine <—> Physical State Machine
Communication between the Link Multiplexer State Machine and the Physical State Machine is characterized by eight primitives:
• PH-SEIZE Request. The Link Multiplexer State Machine uses this primitive before each transmission to request access to the radio channel.
• PH-SEIZE Confirm. The Physical State Machine uses this primitive to confirm that the channel has been seized.
• PH-RELEASE Request. The Link Multiplexer State Machine uses this primitive to release the radio channel.
• PH-QUIET Indication. The Physical State Machine uses this primitive to indicate that the channel is not busy.
• PH-BUSY Indication. The Physical State Machine uses this primitive to indicate that the channel is busy.
• PH-EXPEDITED-DATA Request. The Link Multiplexer State Machine uses this primitive to request transmission of each digipeat or expedite data frame.
• PH-DATA Request. The Link Multiplexer State Machine uses this primitive to request transmission of each normal frame.
• PH-DATA Indication. The Physical State Machine uses this primitive to provide incoming frames to the link multiplexer.
5.6. Physical State Machine <—> Hardware
Communication between the Physical State Machine and the Hardware is characterized by five primitives: • Acquisition of Signal. The hardware uses this primitive to notify the Physical State Machine that
modem synchronization, flag fill or frame structure have been detected.
• Loss of Signal. The hardware uses this primitive to notify the Physical State Machine that modem synchronization, flag fill or frame structure have been lost.
• Frame. The hardware uses this primitive and the Physical State Machine to pass frames to send or that have been received.