PAM Programming Manual

Link-Level Interfaces

PAM Programming Manual—142482

3-4

Port Type Identifiers (Ethernet and Token Ring)

The ZPAM^Val^Port^Type identifier is a required input parameter for all primitives in

the L2S Formatting Library. It is used to determine how the fields of the SDUs are to be

formatted and interpreted. The two valid port type identifiers defined for level-2 services

(L2S) is ZPAM^Val^Port^Type^LLC2 (IEEE 802.2 Link Level Control, Type 2). The

IEEE 802.2 Logical Link Control, Type 2 (LLC1) LAN access interface provides

unacknowledged, connection-oriented logical link control support for both Ethernet

(IEEE 802.3) drivers at the physical level. (ZPAM^Val^Port^Type^LLC2 has the same

value as ZLAM^Val^Port^Type^LLC2 for backwards compatibility with TLAM

applications.)

Link-Level Data Transfer

Applications can send and receive level-2 protocol data units (PDUs) through ports. The

level-2 services provide a path between an application process running on the host and

an equivalent link-level user on a node of the attached LAN.

The host-based application formats buffers containing aggregate SDUs (Aggr^SDUs)

using the L2S or L2TR Formatting Library primitives (described under Formatting

Library for Ethernet on page 3-6) and sends the aggregate SDU to the port. Outbound

aggregate SDUs associated with a port are queued and delivered to the adapter in first-

in, first-out (FIFO) order.

An active inbound aggregate SDU is associated with each port associated with an

adapter. When the L2 address of an inbound PDU matches the L2 address of the port,

the PDU is converted to an SDU and added to the port’s inbound aggregate SDU.

Inbound aggregate SDUs are delivered to the host when one of the following occurs:

•

When a newly-arrived SDU causes the inbound aggregate SDU to be larger than the

port’s configured maximum request size, the inbound aggregate SDU is delivered to

the host. The newly-arrived SDU becomes the first SDU in the next inbound

aggregate SDU.

•

When a newly-arrived SDU causes the number of individual SDUs in the inbound

aggregate SDU to equal the port’s configured data forward count (DFC), the

inbound aggregate SDU is delivered to the host, even if the inbound aggregate SDU

is smaller than the port’s maximum request size.

•

Inbound aggregate SDUs are delivered to the host when DFT of the port expires,

regardless of whether the inbound aggregate SDU is smaller than Max^Req^Size of

the port or the number of SDUs in the inbound aggregate SDU has reached the DFC

of the port.

•

Receipt of an XID or Test PDU immediately triggers the delivery of an inbound

aggregate SDU, regardless of any other circumstance.

Another inbound aggregate SDU is begun for a port as soon as any of the above

conditions causes the previous inbound aggregate SDU to be sent to the application.

PAM receives SDUs from the adapter and creates aggregate SDUs which it forwards to