MIL-STD-187-721 Interoperability And Performance Standard For Advanced Adaptive Hf RadioThe purpose of this document is to describe the technical parameters for adaptive high frequency HF radio that are more advanced than those described in MIL-STD-188-141. The advanced technology contained in this standard will guide the planning of military adaptive HF radio network technology into the 21st century. To fully understand and utilize this document, it is necessary to refer to portions of MIL-STD-188-141.

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MIL-STD-187-721D

An available data capacity sample shall be computed and histogrammed each time the ALE controller successfully sends a message to another station. (If possible, also compute available data capacity samples when messages are sent by other data link controllers.) See table XII for the applicable symbology. Available data capacity samples require the measurements for the symbols listed in table XII for each message sent.

TABLE XII. Available data capacity symbols.

Symbol	Meaning
Tb	Time when the first transmission related to sending the message began (e.g., the call to establish a link for sending the message).
Tf	Time when the (final) data link ACK arrived, indicating that the message had been completely received by the receiving station, and could be discarded at the sending station.
Lm	Number of bits in the message body, exclusive of data link layer overhead bits.
Ls	Sum of the Lms for all other messages (to any destination) waiting to be sent after the current message, as of time Tb.
Lf	Sum of the Lms for all other messages (to any destination) waiting to be sent as of time Tf, including messages not in the queue at time Tb.

The effective data rate R for the message shall be computed as follows:

R L m .

Tf Tb

Station utilization U shall be computed as follows:

U L f L s .

The available data capacity sample X shall be computed as:

X R(1

U) .

For example, consider a 300-character message to be sent as a data text message. Lm in this case is 2100 bits. Assume that two other messages were waiting when link establishment to send this message began, and that they contained a total of 480 characters, so Ls = 3360 bits. Link establishment succeeds on the first channel tried, and the message is sent in the ALE acknowledgment. However, a fade during the message transmission causes an ARQ NAK, a (successful) retransmission and an ACK. This requires a total of 100 seconds = Tf - Tb. During