MIL-STD-187-721D
4. GENERAL REQUIREMENTS
4.1 Data link sublayers.
The MIL-STD-188-141 data link layer contains three sublayers: a lower sublayer concerned with error correction and detection (forward error correction (FEC) sublayer), an upper sublayer containing the ALE protocol (ALE sublayer); and an optional protection sublayer in between, as shown on figure 1. Within the FEC sublayer are redundancy and majority voting, interleaving,
and Golay coding applied to the 24-bit ALE words that constitute the service-data-unit, in terms of the International Organization for Standardization (ISO) model. The ALE sublayer specifies protocols for link establishment, data communication, and rudimentary link quality analysis (LQA) based on the capability of exchanging ALE words. Linking protection (LP) is placed in the intermediate "protection" sublayer so that it may make full use of the error correcting power of
the FEC sublayer while intercepting unauthorized attempts to communicate with the local ALE
protocol entity to establish links or otherwise disrupt operations.
FIGURE 1. Conceptual model of data link layer protocols in MIL-STD-188-141.
4.2 Linking protection. See MIL-STD-188-141.
4.3 Time of day (TOD) synchronization. See MIL-STD-188-141.
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