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Communications Programming Concepts

DLC8023 Protocol Support

IEEE 802.3 Ethernet data link control (DLC8023) supports the system network architecture (SNA) logical link control (LLC) protocol and state tables as described in the Token-Ring Network Architecture Reference, which contains the local area network (LAN) IEEE 802.2 LLC standard. Additional name-discovery services have been added for establishing remote connections.

Station Types

A combined station supports a balanced (peer-to-peer) configuration on a logical point-to-point connection and allows either station to initiate asynchronously the transmission of commands at any response opportunity. The data source in each combined station controls the data sink in the other station. Data transmissions flow as primary commands; acknowledgments and status flow as secondary responses.

Response Modes

Both asynchronous disconnect mode (ADM) and asynchronous balanced mode extended (ABME) are supported. ADM is the default whenever a link session is initiated and is switched to ABME only after the set asynchronous balanced mode extended (SABME) command sequence is complete. Once operating in the ABME command mode, information frames containing user data can be transferred. The ABME command mode then remains active until the LLC session terminates, which occurs due to either the disconnect (DISC) command packet sequence or a major link error.

IEEE 802.3 Data Packet

All communication between a local and a remote station is accomplished by the transmission of a packet that contains the IEEE 802.3 headers and trailers as well as an encapsulated LLC link protocol data unit (LPDU).

The DLC8023 Frame Encapsulation figure,Figure 1-6, illustrates the DLC8023 data packet:

Figure 1-6. DLC8023 Frame Encapsulation. This diagram shows the DLC8023 data packet containing the following: preamble (8 bytes), destination address (6 bytes), source address (6 bytes), LPDU length (2 bytes), LPDU, Pad (together with LPDU consist of m bytes), and CRC (4 bytes). A second line shows that LPDU consists of the following: DSAP address, SSAP address (together with DSAP address consist of 2 bytes), control field [1 (2) byte], and the information field (p bytes).

Figure A0C0e410 not displayed.

The IEEE 802.3 data packet consists of the following fields:

LPDU LLC Link Protocol Data Unit
DSAP Destination Service Access Point (SAP) address field
SSAP Source SAP Address Field
CRC Cyclic Redundancy Check or frame1-check sequence
m bytes Integer value greater than or equal to 46 and less than or equal to 1500
p bytes Integer value greater than or equal to 0 and less than or equal to 1496

Note: Preamble and CRC are added and deleted by the hardware adapter.

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