Synchronous Operation

As in the previous generations of ALE systems, all available 3G HF receivers continuously scan an assigned list of calling channels, listening for 2G or 3G calls⁴. However, 2G ALE is an asynchronous system in the sense that a calling station makes no assumption about when a destination station will be listening to any particular channel. The 3G HF system includes a similar asynchronous mode; however, synchronous operation is the preferred mode for 3G networks, as it will usually provide better speed of linking⁵ and efficiency of spectrum use.

When operating in synchronous mode, all scanning receivers in a 3G ALE network change frequency at the same time, to within a relatively small timing uncertainty (see Figure 5.11). The current dwell channel of every station in a network can always be computed from the time of day and the address of the desired station. Thus, a synchronous 3G call doesn’t require the extended 2G ALE scanning call to capture a scanning receiver; instead, a very short call on the known dwell channel will suffice to reach a receiver that is not linked on some other channel.

Figure 5.11 Synchronous scanning.

Note that it is not necessary that all stations monitor the same calling channel at the same time. By assigning groups of network members to monitor different channels in each scanning dwell (Figure 5.12), simultaneous calls directed to different member stations will be distributed in time or frequency, which greatly reduces the probability of collisions among 3G ALE calls. This is especially important under high-traffic conditions. The set of stations that monitor the same channels at the same time is called a dwell group.

Of course, in some applications it is valuable for all stations in a network to be in the same dwell group so that they overhear calls to each other. They can thereby track when other stations will be linked and when they will be unavailable for calls.

Synchronous scanning permits rapid completion of a call, since no scanning call is required. However, when a link is required to use a specific frequency (e.g., if only that frequency is propagating), an asynchronous system may link faster. This is because the synchronous system must wait for the called receiver to dwell on the desired frequency, and this requires, on average, waiting through half of the scan cycle.

Figure 5.12 Dwell groups.

5.3.2 3G Frequency Management

The automatic channel selection (ACS) function of 3G ALE is invoked to select channels for calling and traffic. A well-conceived ACS function would be aware of various attributes for each channel in the pool, such as the IONCAP predicted SNR, recent sounding information, recent occupancy information, and latency until the channel is next scanned.

3G networks can operate using a single pool of frequencies for both calling and traffic (as in previous generations of HF ALE). However, large networks with a large pool of frequencies can make more efficient use of the spectrum by segregating channels used for calling from those used for traffic.

Calling channels are used in an unscheduled manner to make contact with other stations. Ideally, calling channels would always be vacant, and therefore available for immediate use by any station that needs to set up a link. However, traffic channels are engaged after the parties to a link are already in contact, and can therefore be managed to achieve very high utilization.

Thus, large 3G networks with heavy traffic and a large number of channels available will generally operate more effectively in such a trunking mode (i.e., with separate calling and traffic channels). Smaller or lightly loaded networks, on the other hand, may forego the complexity of trunked operation and employ a small pool of channels for both calling and traffic, just as in previous generations of ALE.

5.3.3 3G-ALE Addressing

One of the functions of the subnetwork layer in Figure 5.1 is translation of upper-layer addresses (e.g., IP addresses) into the addressing scheme used in the local subnet. The addresses used in 3G-ALE PDUs are 10-bit binary numbers. For comparison, even the shortest 3-character addresses used in 2G ALE provide over 15 bits of name space. Although the 3G name space is much smaller than that available with 2G ALE, 3G networks may nevertheless have up to 1000 stations, which even the U.S. DoD agreed was sufficient for their largest contemplated HF networks. Also note that while the user-friendly ASCII call signs used as station addresses in 2G ALE may still be used in a 3G network—they must now be translated by the

equipment to and from the small binary 3G addresses used over the air.

In NATO, HF stations are addressed using a 13-bit network number and a 10-bit address within that network. The latter field is called the point/multipoint address because it may refer either to a single station or to a collection of stations sharing a single address. The 3G ALE PDUs naturally accommodate the 10-bit point/multipoint addresses of the NATO addressing structure. The 13-bit network number is used as follows to ensure that cross-network addressing ambiguity is avoided:

• The network number of the called station (or collective) is used in the linking protection (LP) applied to the 3G ALE PDU, as shown in Figure 5.13. It is replicated to match the length of the LP encryption key in use in the network, and then exclusive-ored with that key for use as the key in the LP algorithm. Stations that receive this protected PDU will attempt to decrypt it using their local network number(s). If the network number is not the same as that of the called station, the decryption will fail, and the receiver will ignore the PDU. This scheme thus ensures that the network number is used to disallow links to unintended networks just as effectively as if the network number bits were sent explicitly.

Figure 5.13 NATO-mode addressing.

• The network number of the calling station is not used during link setup. The same scheme cannot be used to mix the calling network number into the over-the-air PDU because the called station(s) do not know a priori which networks may call them. Any necessary authentication of the caller is therefore deferred until after link setup.