Installation guide

88 DC 900-1564A

Synchronous Link Control (SLC) Programmer’s Guide

6.4.3 SS

This level of SLC service performs message segmentation and reconstruction opera-

tions for high-level network format messages. This requires that the SLC service control

the construction of the Transmission Sequence Identiﬁer and the Message Block Iden-

tiﬁer ﬁelds within each transmitted SLC block.

At the client interface, the

usProtCommand ﬁeld determines the message priority indica-

tion in the constructed Transmission Sequence Identiﬁer ﬁeld. The

DLI_PROT_SEND_NORM_DATA command is associated with a Transmission Sequence Iden-

tiﬁer ﬁeld that contains a low-priority indicator. The

DLI_PROT_SEND_PRIOR_DATA com-

mand is associated with a Transmission Sequence Identiﬁer ﬁeld that contains a high-

priority indicator.

The

iProtModifier ﬁeld determines the Acknowledge Message Label portion of the

Message Block Identiﬁer series for the message. The

DLI_PROT_SEND_NORM_DATA com-

mand allows

iProtModifier ﬁeld values of SLC_AML_A_ND through SLC_AML_G_ND for all

message lengths, and restricts

SLC_AML_H_ND_SBM through SLC_AML_T_NDH_SBM to single

block messages. The

DLI_PROT_SEND_PRIOR_DATA command allows iProtModifier ﬁeld

values of

SLC_AML_A_PD through SLC_AML_G_PD for all message lengths, and restricts

SLC_AML_H_PD_SBM through SLC_AML_T_PDH_SBM to single block messages.

The client application cannot send Link Control Blocks directly. However, the client can

use the

DLI_PROT_CONTROL command to request transmission of an SLC Link Control

Block that contains a STOP ﬂow or RESUME ﬂow request. Furthermore, the

Table 6–2: Additional Characteristics Indicator (ACI) and Extensions on

Low-level Networks

Additional Characteristics Indicator (ACI) — One character per ACI or ACI Extension

ACI_BASE_VALUE Always set

ACI_EXTENSION Extension of ACI present when set

ACI_CONTENT_MASK Mask to isolate (&) or ignore (&~) undeﬁned portion of ACI