ALE Protocols

When an operator directs the ALE system to establish a link to one or more destinations, the first step is selecting a suitable channel (or an ordered list of channels) to try. Then the stations execute an over-the-air protocol that brings them from their idle (scanning) state to a linked state; linked means that the stations are all tuned to the same channel and prepared to exchange traffic.

4.5.1 Frame Structure

Every ALE transmission is structured as shown in Figure 4.6.

• The address(es) of the station(s) to receive the frame are sent first so that every network member will know whether to stop scanning and receive the frame or resume scanning after reading the destination address( es).

• The station sending the frame is identified at the end of the frame.

• Any commands or messages are inserted in the optional center section of the frame.

When a called station is scanning, the calling station must capture that scanning receiver by sending the called station address repeatedly. This scanning call portion of the frame is the first part of any frame sent to a receiver that may be scanning (as seen in Figure 4.7). During the scanning call, only the first three characters of any ALE address are sent.

The next portion of the frame (always present) contains the complete address(es) of the called station(s), sent twice. This is termed the leading call. Finally, the conclusion of the frame contains the complete address of the calling station.

The contents and sequencing of frames differ for each type of calling protocol, as discussed below.

4.5.2 Individual Calling Protocol

An individual call sets up a point-to-point link from one calling station to one called station. The protocol comprises a three-way handshake (Figure 4.8). The calling station sends a call frame, which begins with a scanning call if the called station may be scanning. Every ALE word in the scanning call is a TO word. Use of the TO preamble indicates that the word contains the first three characters of the called station’s address.

Figure 4.6 ALE frame structure.

Figure 4.7 Example ALE frames.

Figure 4.8 Individual calling protocol.

The called station, if it is scanning, will arrive on the selected channel at some point during the scanning call, read its address in a TO word, and recognize that it is being called. Any other stations that read the TO word will find that the address does not match any of their self addresses and will depart to scan other channels.

The called station does not know which station is calling until the conclusion of the call. The conclusion begins with either a TIS or a TWAS word. The TIS preamble indicates that the calling station wishes to establish a link. A TWAS preamble identifies the calling station, so the link quality analysis of the call can be correctly entered in the receiving station LQA database, but indicates that the calling station will return to scanning after sending the frame.

The called station does not know the length of the calling station address until it has either received five ALE words in the conclusion, or the signal is lost after receiving at least one word of the conclusion¹. Thus (except in the case of five-word addresses), the called station must wait one T_rw past the end of the transmission to be sure that the transmission is ended.

After successfully receiving a call, the called station will send a response frame, which is addressed to the calling station. Before sending the response, the called station may need to tune its antenna coupler. The calling station allows some extra time for this (a programmable parameter). If the calling station does not receive a timely response, it automatically aborts the linking attempt and either returns to scan or retries the

call on another channel.

If the calling station receives a timely and correct response to its call, it now knows that the selected channel is propagating in both link directions. However, the called station does not know that the channel propagates back to the calling station. Therefore, the caller sends a third transmission, an acknowledgment, to the called station. This completes the link establishment protocol. At this point, the speakers are unmuted for voice traffic, or a data link protocol is engaged for passing data.

Once stations are linked, they both start a wait for activity timeout (nominally 30 seconds) that will return both stations to the idle (scanning) state if neither station transmits during the period of the timeout. This timeout is stopped whenever either station is transmitting, and restarted from 0 at the end of each transmission.

When a station returns to scan, it may announce this by sending a TWAS-concluded frame.

4.5.3 Net Calling Protocol

A net call establishes a multipoint link among multiple stations. Recall that a net address is programmed to be recognized by many stations. When a net address is called, using the same call frame structure as in an individual call, all of those stations will prepare to respond. How do we avoid collisions among those responses? By assigning those responses to individual time slots following the call.

When an address is programmed as a net address, it will be accompanied by a slot wait timer value (different for each net member) that determines how long the called station will wait after the end of the call frame before sending its response. A simple case of such slotted responses is shown in Figure 4.9.

The first slot after the end of the call, Slot 0, is always reserved (e.g., for tuning antenna couplers) and is not used for responses. After Slot 0, each net member responds in its assigned slot. After all of the slot times have passed, the calling station will send a collective acknowledgment (if any stations have responded). This completes the three-way handshake.

4.5.4 Group Calling Protocol

When a link is desired to a group of stations that is not preprogrammed, a group call may be used. This presents some interesting problems:

• A single address has not been programmed to refer to this group of stations, so the call must list their individual addresses. However, if a scanning station detects a call to an address that is not one of its self addresses, it will ignore that call and return to scanning without waiting to see if its address comes later in the list.

• We will need slotted responses, but slot wait times have not been assigned.

The group calling protocol addresses these concerns as follows:

• The scanning call portion of the call frame carries the first three characters of each called station address in THRU words (not TO words). However, we cannot repeat preambles when the ALE words are not logically identical, so THRU preambles must alternate with REP preambles (as shown in Figure 4.7). A scanning station that decodes THRU or REP words when it synchronizes to an incoming ALE transmission is obliged to continue to decode ALE words until it encounters the first three characters of a self address or it has seen a complete cycle of addresses.

Figure 4.9 Slotted responses.

• The leading call portion of a group call uses TO and REP words, and includes the full addresses of all called stations (sent twice, as seen in Figure 4.7).

• Response slots are computed on the fly: the last station named in the leading call will respond in Slot 1, the preceding station in Slot 2, and so on in reverse order of the list in the leading call. (As in the net call, Slot 0 is not used for responses.)

As in the net call, the calling station will send an acknowledgment after the last response slot.

4.5.5 Other One-to-Many Calling Protocols

Three other one-to-many calling protocols are defined for the ALE system:

• An Allcall uses an address of the form @?@. All stations that receive an All-call stop and listen, but do not respond. If some character other than “?” is placed between the two “@” characters, the call is a selective Allcall. Only stations whose address ends in the character between the two “@”

characters are called, and should stop and listen. Other stations ignore the selective Allcall.

• An Anycall uses an address of the form @@?. A selective Anycall uses an address with some character other than “?” after the two “@” characters, and only stations whose address ends in that character are being called. Others ignore the selective Anycall.

• A Wildcard call is any call other than an Allcall or an Anycall that contains a “?” in the address.

Anycalls and Wildcard calls use slotted responses, but responding stations cannot compute slot assignments for these types of calls. Instead, sixteen slots are always present, and responding stations choose one of those slots at random for sending the response. An unused Slot 0 immediately follows the end of the call, and the response slots (numbered 1 through 16) follow Slot 0. As usual, an acknowledgment follows the last response slot.

4.5.6 Timing

The timing characteristics of the radio, antenna coupler, and so on affect the operation of the protocols.

Programmable parameters are provided for many of these times (detailed in MIL-STD-188-141, Appendix A [5]). It is important in programming an ALE network to set these parameters identically in all network member ALE systems.

4.5.7 ALE Performance Requirements

The purpose in defining the ALE standard was to ensure interoperability among government HF radio

systems, but users also desired minimum performance requirements. These fall into two categories:

occupancy detection and linking probability.

4.5.7.1 Occupancy Detection Requirements

A potential problem in any automated communication system arises when it may interfere with other users of the medium. The early implementations of the ALE standard all listened before transmitting on the calling channel, but some only detected (and deferred to) ALE signaling. As a result, these systems earned the scorn of users whose voice conversations were interrupted by the infamous warbling tones of the ALE modem.

When a radio operator on Air Force One (the U.S. presidential aircraft) complained of such interference, the military standards committee quickly added requirements for ALE systems to reliably detect voice traffic, as well as ALE and data modem transmissions, and to defer transmitting on occupied channels.

These requirements are listed in Table 4.4, and are tested using standard recordings of all three types of traffic.

All ALE systems are required to listen for a dwell time of two seconds before transmitting a call or a sound, and to detect occupancy with the probabilities listed in Table 4.4. False detection probability cannot exceed 1%.

4.5.7.2 Linking Probability Requirements

The ability of ALE systems to establish links is tested in three channel conditions, as listed in Table 4.5. The unusual specifications for multipath spread were selected to preclude nulling of any of the ALE tones (which would occur if the usual 0.5 and 2.0 ms settings were used in standard Watterson model channel simulators).

4.5.8 Orderwire Functions

Numerous control and messaging functions have been defined that use ALE CMD words sent in the optional message section of an ALE frame. Very few of these are mandatory, and so are not universally implemented. Two of the mandatory functions are illustrative of the capabilities:

Table 4.4 Occupancy Detection Requirements

Table 4.5 Linking Requirements (SNR in 3 kHz)

• The LQA command, carried in a single CMD word, allows stations to report to each other the pseudo-BER, SNR, and multipath that they have measured on transmissions from each other. This may be useful when links are nonreciprocal (propagate better in one direction than the other), often due to local interference on some channels.

• Automatic message display (AMD) provides a low-overhead operator-to-operator text message capability using the ALE signaling. An ALE system that receives an AMD message is required to display it to the operator and to store it for later review. AMD messages are limited to 90 characters, using an alphabet of upper-case letters, digits, and the punctuation symbols.

The sending station may optionally insert its address before an especially long message section. This quick ID uses a FROM word (extended as usual with DATA and REP words as needed).