
"What is that strange turkey gobbling sound I
just heard on
20 meters?"
Oh, just a bunch of hams with a taste for ALE!
What
is ALE?
ALE is the
humorous acronym
for the lofty-sounding
Automatic Link Establishment.
With the capability to call up a specific station, a group of stations,
a net... or just CQ, ALE is a versatile system for connecting operators
for voice, data, text, instant messaging, or image QSOs. A radio
operator
initiating a call, can within minutes have the ALE automatically pick the best
clear frequency. It signals the operators on
both ends, so they can begin communicating with each other immediately.
In this respect, it can eliminate the longstanding need for repetitive
calling on pre-determined time schedules and monitoring static on HF.
It
is a great tool for finding a clear frequency between stations in
real-time,
while avoiding guesswork, beacon listening, and complicated HF
prediction
charts altogether. These days, ALE is most commonly used for hooking up
operators for voice QSOs on SSB, but there are many other useful
features.
ALE
in Ham Radio
For the past 7 years,
a group of
Amateur Radio operators has joined together for experiments and
communications
using ALE and Selective Calling. The number of hams has grown from just
a handful active in 2001, to the hundreds of enthusiasts in it today.
Some
are following the traditional ham curiosity to explore interesting
aspects
of communications; others are developing dependable HF nets, or just
using
it to keep in touch with a circle of ham friends. The need to call up
emergency
nets or inter-operability and liason with government HF systems has led
many hams to adopt the government ALE standard, called FED-STD-1045
or MIL-STD 188-141. This standard caught on slowly in the ham
community,
initiated by a few operators with limited government surplus gear and
some
with expensive commercial equipment having embedded ALE or hardware controllers.
Now, with a ham HF transceiver, a computer as the controller, and an
appropriate
antenna system, hams can harness the power of ALE.
How
ALE Works
Each ham radio ALE
station uses the
operator's callsign as an address in the ALE controller.When
not
actively in a QSO with another station, each HF SSB transceiver
constantly
scans through a list of frequencies, listening for its callsign. To
reach
a specific station, the caller simply enters the callsign just like
dialing
a phone number. The ALE controller selects the best available frequency
and sends out brief selective calling signals containing the
callsigns.
When the distant scanning station detects the first few characters of
its
callsign, it stops scanning and stays on that frequency. The two
stations'
ALE controllers automatically
handshake to confirm that a link is
established and they are ready to communicate. The receiving station,
which
was muted up until now, will typically emit an audible alarm and visual
alert for the receiving operator of the incoming call. It also
indicates
the callsign of the linked station. The operators then can talk in a
regular
QSO. At the conclusion of the QSO, one of the stations sends a
disconnect
signal to the other station, and they each return their ALE stations to
the scanning mode. Some military / commercial HF transceivers are
available
with ALE options. Amateurs commonly use G4GUO's PCALE soundcard
software
interfaced to a ham transceiver via RS-232 CAT port, multi-frequency
antenna.
ALE
Using a Computer
as the Transceiver Controller Unit
In 2001, Charles
Brain G4GUO, wrote and released a soundcard PC program for ALE
appropriately named PCALE.
Charles is to be commended for his wonderful generosity and technical
achievement.
By doing this, he effectively opened up ALE to the non-professional
user
at low cost. The availability of PCALE in various
improving versions made it possible for more amateur radio
operators
to start experimenting with ALE.
Ham
Group for Coordinating
ALE
During 2001, several
hams in USA
started using PCALE, at the suggestion of Trish WA6UBE. Trish's
already-operational
Motorola Micom 2R transceiver (with embedded ALE) became one of the
center
points for testing and linking. In December 2001, from this core of
early
ALE operators, an egroup called
HFLINK
was founded by the author. The purpose of HFLINK would be to
further the development
of methods and coordination for use of ALE and Selective Calling in HF
Amateur Radio. The HFLINK egroup helped work out guidelines for use of
ALE under current FCC rules. The scanning frequencies essential for
effective
ALE operation evolved and became HFLINK channel lists.
Using
ALE Features
Under FCC Rules
USA hams can legally
use ALE's Selective
Calling and Alerting features in the phone subbands,
and all of the features of ALE in the
data subbands.
One of the aspects of operation using "new technology" in Amateur Radio
is navigating the complexities of arcane, and sometimes contradictory
FCC
rules regarding modes, bauds, and subbands. Currently, many
countries
of the world have reasonable, easily understood bandwidth-based Amateur
Radio rules where most of the protocol and features of ALE can be
utilized
freely. The USA Amateur Radio Service has been somewhat hamstrung in
this
respect, due to our more antiquated mode-based rules.
There
are several rule-changing proposals in progress which seek to take the
USA Amateur Radio Service from its present mode-based rules
to the more modern bandwidth-based rules. So, US
operators
might someday in the future, enjoy the freedom that other hams of the
world
already have to explore communications technology.
ALE
as a Phone
Mode
Currently, USA
Amateur Radio Service
operators are legally utilizing ALE and SELCAL in the "phone bands".
Transmissions
fall under the FCC rule's statement: "Incidental tones for the purpose
of selective calling or alerting or to control the level of a
demodulated
signal may also be considered phone."
ALE controller units
use handshake
signals to set up communication links between stations... The 1st
station
initiates the calling, the 2nd station ACKs (acknowledges) the
reception
of the call, and the 1st station ACKs the 2nd station's ACK... in other
words, it is a 3-way exchange of signalling to set up the communication
to be in a linked status. Within this 3-way linking signal
exchange,
there are parts of the ALE protocol that can include "incidental" types
of very limited text/status sequences and Link Quality signals, AMDs,
or
MOTDs.
The question then is:
"Where do we draw the
line between
Automatic Link Establishment operation as a PHONE mode, and make the
conceptual
jump to DATA mode under the FCC rules?"
The complex language
of FCC rules
are attempts at drawing clear lines between what is allowed and not
allowed,
but technology often turns these clear lines fuzzy. If the "spirit of
the
rules" are to be our guide, once the selective call or ALE exchange
happens,
and the LINKED status is established, the use of the ALE or selcall
modem
should cease and voice should commence.
Limiting
Procedures
for Operation as a Phone Mode
When the ALE or Selcall
system is
used as a "PHONE" emission under that rule, (in the "phone bands"),
operators
then probably need to limit the transmissions to:
1. ANYCALLS or NETCALLS
(the ALE selective calling
equivalent of CQ or a call for a group of stations)
2. ALLCALLS (the ALE
alerting equivalent
of a QST)
3. INDIVIDUAL CALLS (ALE
direct call to another station)
4. Selective calling
signals to
set up a voice or image contact for QSO with other station(s).
5. Alerting another
station to talk
on voice.
6. Alerting another
station to QSY.
7. Sending an alerting
or selective
calling signal to break the link.
Linked
in the Phone
SubBands
Perhaps the ALE protocol
and features
themselves provide an easy way for us as operators to make the fuzzy
line
between PHONE and DATA a little clearer! In the ALE protocol, there is
a point during the "handshake" between two stations when the ALE
software
or the ALE controller tells the user that the station is LINKED.
It
means that you have succeeded with Selective Calling of the other
station(s).
We can use this LINKED status indicator as a dividing line between what
the FCC defines as PHONE and DATA operation. If we use the AMD or data
modem after the indicator says we are LINKED, we are probably stepping
out of the PHONE mode and into DATA mode.
The possible allowable
exceptions
to this are:
1. An AMD to alert the
other station
to QSY or start scanning for a selective call on another channel.
2. An AMD or CALL to
attempt to
call yet another station.
3. An AMD to alert
another station
to talk on voice.
4. The limited
orderwire
that gets a free ride as part of the selective calling and alerting
signalling.
5. Some bands such as
160 meters
which do not have phone subbands.
FCC
Rules for ALE
in the Phone Bands
Here are some excerpts
of present
Amateur Radio Service FCC rules permitting the use of ALE and
SELCALL
for Selective Calling and Alerting in the PHONE sub-bands.
Please
note the last sentence in
§97.3(c)(5) which is the definition
of Phone mode. At the bottom are the URLs for
§2.201
of FCC Rules which define emission type designators.
"§97.3
Definitions.
...[extraneous text
deleted]
(c) The following terms
are used
in this Part to indicate emission types. Refer to §2.201 of the
FCC
Rules, Emission,
modulation and
transmission characteristics,
for information on emission type designators.
....[extraneous text
deleted](5)
Phone. Speech and other sound emissions havingdesignators with A, C, D,
F, G, H, J or R as the firstsymbol; 1, 2 or 3 as the second symbol; E
as
the thirdsymbol. Also speech emissions having B as the first symbol; 7,
8 or 9 as the second symbol; E as the third symbol. MCW for the purpose
of performing the station identification procedure, or for providing
telegraphy
practice interspersed with speech.
Incidental tones for the purpose
of selective calling or alerting or to control the level of a
demodulated
signal may also be considered phone."
[---emphasis added]
Does
the ALE RF
Signal Meet the FCC "300 Baud Rule" for Data mode?
Yes. The ALE (MIL-STD
188-141B or
FED-STD-1045) signal is transmitted at
125 baud. It is legal
under FCC
Rules for use in the Amateur Radio
Service for DATA MODE
transmission
on HF in the DATA sub-bands. Current FCC Rules in USA allow DATA modes
on HF up to 300 baud (300 symbols per second). Symbol rate (baud) is
the
number of state changes the transmitted
signal makes per second.
The ALE
signal is 8FSK (8 Frequency Shift Keyed). 8 discrete tone frequencies
are
spaced 250 Hz apart from 750 Hz to 2500 Hz at audio baseband. Each
symbol
represents three bits of data, resulting in an over-the-air data rate
of
375 bits per second (b/s) using 125 baud.
What
is the Bandwidth
of the ALE Signal? Does it meet FCC specifications?
The FCC does not
currently specify
an occupied bandwidth limitation for DATA emission, other than the
stipulation
that bandwidths must not be excessive for the data rate used.
The ALE modem using
MIL-STD 188-141B
or FED-STD-1045 does not use excessive bandwidth to obtain sufficient
signal
quality error rates to establish linking through difficult HF noise
conditions.
The bandwidth is similar to PACTOR-III which is in current use in the
Amateur
Service. The ALE signal bandwidth is suitable for transmission via HF
transceivers
through a standard SSB audio/RF chain with an IF bandwidth of about 2.5
kHz. The 8aryFSK tones take up about 1750 Hz, but when they are keyed
rapidly,
the bandwidth spreads out. In practice, for demodulating, amateurs have
been able to use receive IF filters as narrow as about 2200 Hz.
The signal is
somewhat resistant
to noise, QRM, passband tilt, group delay, and phase distortion. It can
be run through typical SSB transceivers with speech compressors on
transmit
and various AGC characteristics on receive. However, DSP Noise
Reduction
receive circuits tend to be counterproductive.
What
subbands should
be used with the ALE modem for DATA mode?
Current FCC Rules
specify that the
data signal may be transmitted anywhere in the allocated DATA mode
sub-bands.
According to the IARU
Region 2 Bandplan,
there are certain voluntary sub-bands dedicated to PACTOR and
similar-bandwidth
digital modes. Since the 1045 ALE modem uses a similar bandwidth as
PACTOR-III,
the HFLINK
group has recently
adopted channel
frequencies within the PACTOR sub-bands. HFLINK's current channel list
includes these "DATA" channels.
Link
Quality Analysis
For operation as an
ALE system, the
HF communications system normally has a number of channels assigned to
it throughout the HF spectrum. The system works much like a telephone
in
that each radio in a network is assigned an address (ID). When not in
use,
each radio receiver constantly scans through its assigned frequencies
(channels),
listening for calls addressed to it. To reach a specific station, the
caller
simply enters an ID just like dialing a phone number. It takes a lot of
time for the radio to go through the sequence of calling a station on
every
one of the assigned channels. But there are several ways the ALE system
can be programmed to decrease the time it takes, over simple random
scanning
and sequential channel calling. Methods for decreasing the time by
using
a "smarter" way of predictive or synchronized linking can be applied.
An
ALE system utilizing (Link Quality Analysis) capability uses periodic
sounding
and linking signals between other stations in the network, to predict
which
channel is clear to call on at any given time.
Here’s how it works
in an adaptive
system using LQA. Once or twice every hour, each station in a network
will
attempt to "sound" out each channel by sending a short transmission of
its own ID on each of its channels. All stations will receive the
sounding
transmissions and measure the signal quality on each channel for each
other
station IDs it receives. These signal quality "scores" are stored in a
complex matrix of:
-
STATION ID
-
TIME DATE STAMP
-
CHANNEL NUMBER
-
SIGNAL QUALITY LEVEL
When a call is initiated
to a station,
the radio automatically checks its LQA matrix “memory” to make a
determination
of the most probable best quality channel for the call to the desired
station,
based upon its record of recent Link Quality data it has logged on that
station. It then makes its first attempted linking call on that most
probable
channel. If the link cannot be established, it will try again on the
next
best frequency in the matrix, and so on, until a link is established.
Since
HF clear frequencies can change very quickly, typical ALE systems using LQA
make
use of recently measured soundings within the past few hours.
ALE Automatic Link
Establishment
(MIL-STD-188-141; FED-STD-1045)
B. Crystal RF
Laboratory measurements
Spectrum Analyzer
Image
Conditions
of Transmitter
-
Icom
756pro
-
IF
bandwidth 2.6 kHz (No
speech compressor)
-
RF
Power Output 70Watts
-
signal
center freq 29.000MHz.
-
Audio
source: PCALE
-
Audio
path: Soundcard
to Rigblaster to mic input.
-
Test
transmission time
240 seconds
|
Spectrum
Analyzer
-
Horizontal
graticule 0.5kHz/div
-
Vertical
graticule 10dB/div
-
IF
bandwidth 30Hz
-
Video
bandwidth 30Hz
-
14
and 30 seconds sweep
time
-
Peak
hold
-
Markers
set delta mode
at -23dBc +/- 0.6dB
-
Measured
-23dBc bandwidth
= 2157Hz
|
Comments
-
It
is easy to see the
sharp spikes at the centers of a few of the tone carriers (above). Upon
close inspection of the audio waveform (below) using 16bit sampled
audio,
and looking at the excerpt of the specification (below) for the ALE
system,
notice that the 8-ary FSK tone frequencies are shifted at the peaks of
the sinewaves. Perhaps this is the cause of the spectrum spikes. In any
case, this spectrum analysis shows that a properly adjusted amateur
transceiver
running soundcard- generated ALE signals can transmit a clean signal
and
easily operate within an occupied bandwidth of less than 2.5kHz,
similar
to a voice transmission.
|
Photo below:
Audio waveform
showing the shift
in frequency of the 8-ary FSK signal transmitted by ALE
Excerpt of
Specification for MIL-STD-188-141;
FED-STD-1045 signal:
A.5.1 ALE modem
waveform.
18 A.5.1.1
Introduction.
The ALE waveform
is designed
to pass through the audio passband of standard SSB radio
equipment.
This waveform shall provide for a robust, low-speed, digital modem
capability
used for multiple purposes to include selective calling and data
transmission.
This section defines the waveform including the tones, their meanings,
the timing and rates, and their accuracy.
19
20 A.5.1.2 Tones.
The waveform shall
be an 8-ary
frequency shift-keying (FSK) modulation with eight orthogonal tones,
one
tone (or symbol) at a time. Each tone shall represent three bits
of data as follows (least significant bit (LSB) to the right):
· 750 Hz 000
· 1000 Hz 001
· 1250 Hz 011
· 1500 Hz 010
· 1750 Hz 110
· 2000 Hz 111
· 2250 Hz 101
· 2500 Hz 100
The transmitted
bits shall
be encoded and interleaved data bits constituting a word, as described
in paragraphs A.5.2.2 and A.5.2.3. The transitions between tones
shall be phase continuous and shall be at waveform maxima or minima
(slope
zero).
21 A.5.1.3
Timing.
The tones shall be
transmitted
at a rate of 125 tones (symbols) per second, with a resultant period of
8 ms per tone. Figure A-5 shows the frequency and time
relationships.
The transmitted bit rate shall be 375 bits per second (b/s). The
transitions between adjacent redundant (tripled) transmitted words
shall
coincide with the transitions between tones, resulting in an integral
49 symbols (or tones)
per redundant
(tripled) word. The resultant single word period (Tw) shall be
130.66...
ms (or 16.33... symbols), and the triple word (basic redundant format)
period (3 Tw) shall be 392 ms.
22 A.5.1.4
Accuracy.
At baseband audio,
the generated
tones shall be within +1.0 Hz. At rf, all transmitted
tones
shall be within the range of 2.0 dB in amplitude. Transmitted
symbol
timing, and therefore, the bit and word rates shall be within ten parts
per million.

figure A-5 - ALE Timing
and Coding.
|
.
.End of page.
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2003, 2004, 2005, 2006 HFpack, Inc. All Rights Reserved.
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for the purpose of hyperlinking to the main page of the HFLINK.COM
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All photos on the HFLINK website are property or copyrighted by HFpack,
Inc. or when used by permission, are copyright by their respective
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No part of the HFLINK website or HFLINK Group
Forum
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digital selective calling system automatic
link establishment
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Other term data
for informational
and educational reference purposes:
This site gives
possible references
to the following terms which may be useful to those who are searching:
HFLINK -
Communications Equipment
- HF VHF UHF Antennas FH ALE Manpack Milisec Broadband Dipole Portable
Base Mobile Voice HF SSB FM Data Automatic Link Establishment Selective
Calling Broadband Antenna. HFLINK is at the forefront of military,
government,
commercial, and other HF - VHF and UHF radio systems that are highly
flexible,
interoperable, and capable of simultaneously supporting diverse mission
requirements, special operations, emergency communications systems,
homeland
security, HF SSB, VHF FM, FH frequency hopping radios, ALE Automatic
Link
Establishment Systems, broadband and wideband communications.
©Copyright
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trademarks of HFLINK. The overall respective look, feel, and trade
dress
of these entities are trademarks and intellectual property of
HFLINK.
The HFLINK Logo may be used by non-commercial or commercial websites
only
for the purpose of including an http hyperlink to the main web index
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of the HFLINK.com website. Capture of the HFLINK website or any part of
it within the frame of another website is not authorized and not
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All photos, reproductions, screen captures, or use of images on the
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All links to the HFLINK web site must be to the main index page
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TFD, terminated folded dipole, tilted terminated folded dipole,
terminated
dipole antenna, broadband terminated dipole antenna, broadband
terminated
antenna, broadband vertical antenna, broadband terminated vertical
antenna,
wideband antenna, broadband portable antenna, broadband HF antenna, HF
SSB antenna, marine HF SSB antenna, marine antenna, sailboat antenna,
HF
SSB antenna, HF broadband folded dipole antennas for
conventional,
NVIS, and ALE use, ALE antenna, FH antenna, AC-1.8-30, ACS-1.8-30,
Frequency
Hopping antenna, frequency agile antenna, Amateurs.
This site gives
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HFLINK Logo may be used by non-commercial or commercial websites only
for
the purpose of including an http hyperlink to the main web index page
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All photos, reproductions, screen captures, or use of images on the
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All links to the HFLINK web site must be to the main index page
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All copyrights and trademarks used on this page are trademarks and
copyrights
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selcal
australia australian standard CCIR 493-4, CCIR SELCALL ITU-R 493 ITU
standard
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