Data Modem

ALE Modem


Fig 1—Functional heirarchy of an automated HF station.

sage) or as a data message, it uses a small interval of air-time compared with the time required to send the same message by voice. Therefore, as more and more concepts for networking are developed, the system network throughput will increase.

The principal functions of the station, and the association with the reference model, are schematically shown in Fig 1. When we start adding features such as indirect route selection, connectivity tracking, connectivity (data) exchange (CONEX), automatic message exchange (AME), and automatic message exchange with store-and-for-ward message exchange (AME w/S&F) we extend into the network and higherlevel functions.

Physical Layer Functions

The major function of the controller that relates to the physical layer is that of transceiver and modem (data modem or ALE modem). The data modem used is generally of a MIL-STD-188-110A type, but other possibilities exist. The ALE modem employs 8-ary frequency shift keying (FSK) with 8-millisecond tones, where 3-bit symbols are sent at a rate of 125 per second, giving a raw data rate of 375 bits per second (BPS).

Data Link Layer (DLL) Function

Referring to Fig 1, we see that at the data-link layer, parallel paths exist where the system/user has the option of using a data modem or ALE modem.

a) Data-Link Layer/Data Modem— The data-link layer for the data modem is basically a pass-through function, where functions of the network layer act directly on the physical layer function.

b) Data-Link Layer /ALE Modem—The lowest layer of the DLL ALE Modem is the Forward Error Correction (FEC) function. Other sublayers add linking protection (optional), selective calling, an orderwire message section, channel selection, link establishment and termination, LQA collection and reporting, and polling functions.

• Forward Error Correction (FEC)— This sublayer adds forward error correction, using a Golay code, applied to the ALE words, and each word is sent three times to allow redundancy and majority voting.

• Linking Protection (optional)— Linking protection (LP) is a technique that protects the linking functions from unintentional or malicious interference by scrambling the ALE signaling exchanged among protected stations.14

• Selective Calling—Calls can be made using individual, net, or group station addressing or sounding. The fundamental address element in the ALE system is the individual station address. A net is a prearranged collection of stations called with a net call. A group is non-prearranged collection of stations where little or nothing is known about them except their individual addresses and scanned channels. Sounding is the ability to empirically test selected channels (and propagation paths) by providing a very brief, beaconlike, identifying broadcast which may be used by other stations to evaluate connectivity, propagation, and availability; and to select known working channels for possible later use for communications or calling.

• Message Passing—The link-layer message passing by an HF ALE radio, between communicators, might be in an automatic message display (AMD) message, data text message (DTM), data block message (DBM), or LQA mode. The AMD message appears on an 80-character frontpanel display device, the DTM is a standard-speed message mode, and the DBM mode message is highspeed (relative to AMD and DTM) with deep interleaving to penetrate HF channel long fades and large noise bursts, LQA concerns the automatic measurement of the quality of the ALE signal on link(s) between stations. The resultant LQA data is used to score the channels and to support selection of a "best" (or an acceptable) channel for calling and communication.

• Channel Selection—The primary function of the link-layer HF ALE controller is to monitor receiver channels, choose the best transmit channel, and link with the called station to carry out some function. Linking functions include linking for voice traffic, linking for the transmission of orderwire, and linking to allow transmission of data by an external modem.

• Link Establishment and Termination—The data-link function initiates a required response and acknowledgement to complete a three way handshake.

• Data Transfer—The data transfer function transfers data such as passive LQA data. Passive LQA data is data obtained from monitoring normal ALE traffic as well as soundings.

• Passive LQA—Evaluation of channel quality by measuring the characteristics of received signals is termed passive LQA because the local radio does not transmit a request for this data. Passive LQA data is obtained by listening to normal ALE traffic and soundings (see note 14).

• Polling—The polling data link functions are used to acquire current link quality data by using handshaking and exchanges with one or more stations.

• LQA Reporting—LQA reporting is the broadcast of data for purposes of updating the memory in other stations in the network.

Amateur Use of Data Link Layer ALE radios

A recent article in QSTs "Packet Perspective" column written by Stan Horzepa, WA1LOU, describes what may well be the first amateur band experiment using FED-STD-1045 ALE equipment. "The experiments, which began in June, 1992, were conducted to see how well ALE performs in the noise and interference typical on the amateur bands." (See note 4.) The article gives a brief synopsis of ALE as it exists today, as well as the results of these amateur band tests.

Federal HF ALE Networking

The federal program for advancing HF ALE radio into networking is presently in the definition stage. Protocols are being written that will extend the basic capabilities of the ALE controller into functions associated with the network and transport layers of the OSI model. As an example of the extension of networking to the functions of the ALE controller, we can look at the protocol necessary for providing star group networking functions for prearranged networks that have direct connectivity, The scanning call signal structure of a star group (one station to many) is illustrated in Fig 2 (see note 12).

The Federal HF ALE networking program is presently defining a series of controllers that extends from a very simple networking controller capable of only slave routing to a full-featured HF controller with the ability to hold messages until connectivity (direct or indirect) to the destination is achieved. In general, these controllers can be diagramed as shown in Fig 3.15

The FED-STD-1045 HF ALE radio system is not normally thought of as a packet-radio system. Packets are generally associated with internal packet-

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