ALE-Automatic Link Establishment-(2G)
Mil-Std-188-141b
Introduction:Through this document we will study the second generation (2G) basic ALE system . We will
start with basic definitions .Then we will look at the layered structure of the System. Also we will
study the requirements needed for having a basic ALE system. After that we will return back to
ALE sublayers and discuss them in details.
Definitions:
ALE:-Automatic Link establishment- it’s a method used mainly in medium Frequencies –MF- and
High Frequencies –HF- for automatic establishing of a communication link between Transceivers
without a need for skillful operator to do so.
Available State:-it’s when the receiver is in the scanning state over the prescribed channels and
the link is not yet established or being established.
Linking State:- It enters this state when the receiver receives a calling frame that contains the
receiver’s ID. A Completion of this state is achieved by 3 way handshakes. If the link cannot be
established the receiver returns back to the Available State.
Linked State:- We enter this state when the Transceivers succeed in establishing a link
.Additional 3 way handshakes are carried on in this state for signaling to occur. The Transceiver
remains in this state until link termination occurs and the receiver returns back to the Available
state.
ALE Layers;The ALE system extends
in two main layers .These
two layers are the
Physical Layer and The
Data Link Layer. Within
these two main layers ALE
system is divided into sub
layers. The details of
these layers are shown in
the following diagram.
ALE General Requirements:1-A Modem with FEC encoding/decoding and supports 8-ary FSk.
2-An ALE protocol that supports selective calling, channel evaluation and being able to pass data
messages.
3-ALE Addresses to identify individual stations or collections of Stations.
4-Scanning Capability: The Transceiver should be Capable of repeatedly scanning channels
stored in the memory. The Transceiver should stop scanning when any of the following events
occurs: i-ALE controller decide to stop scan-when entered a linking or linked state-,ii-Manual
input to stop scan.
5-Calling: when requested by the operator the radio system should execute the ALE protocol.
6-Channel Evaluation: The radio System should be capable of measuring the quality of the signal
at ALE reception and make link Quality analysis –LQA-.
7-Selectable Scanning Rate: from two, five or ten channels per second.
ALE Modem Waveform-Physical Layer- :The waveform should be able to pass through the Audio pass band of the standard SSB radio
equipment.
The waveform should be 8-ary FSK-frequency Shift Keying- with 8 Orthogonal tones ,one
tone/symbol. Each tone represents 3 bits of data .
The incoming bits shall be encoded and interleaved .
Transition from bits should be phase continuous and shall be at waveform maxima or minimazero slope-.
Tone Frequency
750Hz
1000Hz
1250Hz
1500Hz
1750Hz
2000Hz
2250HZ
2500Hz
Corresponding Bits
000
001
011
010
110
111
101
100
Tone rate: is 125 tone/secs (symbol/sec), with 8msecs period for each tone-3 bits-.The
Transmitted bit rate is 375bits/sec.
Accuracy:-The tones produced should be within +/- 1Hz .At RF all Transmitted tones should be
within the range of 2 db in amplitude. The bit rates should be within ten parts per million.
FEC-Forward Error Correction- Sublayer:For enhanced performance of stations over various RF channels .The ALE system makes a
combined use of Forward Error Correction-FEC-, Interleaving-for protection against burst noiseand redundancy. These functions are performed within the Transmit encoder and the receive
decoder-Modem-.
Golay Coding:Golay Code (24,12) is used for FEC. Its Generator polynomial is
g(x) = x11 + x9 + x7 + x6 + x5 + x + 1
The generator matrix G, derived from g(x), shall contain an identity matrix I12 and a parity matrix
P as shown in the following figure.
I12
G=
P
100
000
000
000
:
101
011
100
011
010
000
000
000
:
111
110
010
010
001
000
000
000
:
110
100
101
011
000
100
000
000
:
110
001
110
110
000
010
000
000
:
110
011
011
001
000
001
000
000
:
011
001
101
101
000
000
100
000
:
001
100
110
111
000
000
010
000
:
101
101
111
000
000
000
001
000
:
010
110
111
100
000
000
000
100
:
001
011
011
110
000
000
000
010
:
101
110
001
101
000
000
000
001
:
010
111
000
111
Generator matrix for (24, 12) Golay code.
Encoding.
Encoding uses the fundamental formula x = uG, where the code word x shall be derived from
the data word u and the generator matrix G. Encoding is performed using the G matrix by
summing (modulo-2) the rows of G for which the corresponding information bit is a "1.
The corresponding parity check matrix H shall contain a transposed matrix pT and an identity
matrix I12 as shown in the next figure.
PT
H=
I12
111
110
010
010
:
100
000
000
000
011
111
001
001
:
010
000
000
000
110
001
110
110
:
001
000
000
000
011
000
111
011
:
000
100
000
000
110
010
001
111
:
000
010
000
000
100
111
010
101
:
000
001
000
000
101
101
111
000
:
000
000
100
000
010
110
111
100
:
000
000
010
000
001
011
011
110
:
000
000
001
000
000
101
101
111
:
000
000
000
100
111
100
100
101
:
000
000
000
010
101
011
100
011
:
000
000
000
001
Parity-check matrix for (24, 12) Golay code.
Decoding.
 Decoding will implement the equation: s = y HT where y = x + e is a received vector which is the
modulo-2 sum of a code word x and an error vector e, s is a vector of "n - k" bits called the
syndrome. Each correctable/detectable error vector e results in a unique vector s. Because of
this, s is computed according to the equation above and is used to index a look-up of the
corresponding e, which is then added modulo-2 to y to give the original code word x. Flags are
set according to the number of errors being corrected. The uses of the flags are described later.
If s is not equal to 0 and e contains more ones than the number of errors being corrected by
decoding mode, a detected error but not corrected is indicated and the appropriate flag is set.
Interleaving and deinterleaving: the basic word bits W1 (most significant bit (MSB)) through W24 (LSB), and resultant
Golay FEC bits G1 through G24 (with G13 through G24 inverted), shall be interleaved,
before transmission using the pattern shown in following figure. The 48 interleaved bits
plus a 49th stuff bit S49, (value = 0) shall constitute a transmitted word and they shall be
transmitted as A1, B1, A2, B2... A24, B24, S49 using (16+1/3) symbols (tones) per word
(Tw). At the receiver, and after 2/3 voting, the first 48 received bits of the majority word
(including remaining errors) shall be deinterleaved as shown in the following figure and
then Golay FEC decoded to produce a correct(ed) 24-bit basic word (or an uncorrected
error flag). The 49th stuff bit (S49) is ignored.
Redundant words.
Each of the transmitted 49-bit (or 16+1/3 symbol) (Tw) words shall be sent redundantly (times 3)
to reduce the effects of fading, interference, and noise. An individual (or net) routing word
(TO...), used for calling a scanning (multichannel) station (or net), shall be sent redundantly as
long as required in the scan call (Tsc) to ensure receipt.. At bit time intervals (approximately
Tw/49), the receiver shall examine the present bit and past bit stream and perform a 2/3
majority vote, on a bit-by-bit basis, over a span of three words. See tables A-VI and A-VII. The
resultant 48 (ignoring the 49th bit) most recent majority bits constitute the latest majority word
and shall be delivered to the deinterleaver and FEC decoder.
Bit and word decoding.
ALE Word Structure-ALE Protocol Layer-:The ALE word is composed of 3 bits for preamble and 21 bits for 3 characters-7bits/character.The Total number of bits is 24.For FEC Golay encoding the word is divided into two halves with
12 bits for each half. The preamble is what determines the type of the Data carried in the word.
What carried in the word might be a destination address-TO preamble, a Source address –TIS
preamble ,data –Data preamble or any other preambles shown next. There are 8 types of word
determined by the preamble.
ALE word types (preambles).
Word
Code Bits
Functions
Significance
THRU
001
multiple (and indirect present multiple direct destinations for group calls
routing
(and future indirect relays, reserved)
TO
010
direct routing
CMD
110
ALE system-wide station (and operator) orderwire
orderwire control and
for coordination, control, status, and special
status
functions
FROM
100
identification (and
indirect routing)
identification of present transmitter without
termination (and past originator and relayers,
reserved)
TIS
101
terminator and
identification
continuing
identification of present transmitter, signal
terminations, protocol continuation
TWAS
011
terminator and
identification of present transmitter, signal and
identification quitting protocol termination
DATA
000
extension and
information
extension of data field of the previous ALE work,
or information defined by the previous CMD
REP
111
duplication and
information
duplication of the previous preamble, or
information defined by the previous CMD
P3
P2
Type
MSB
W1
present direct destination for individual and net
calls
P1
LSB
W2
W3
Address Words:TO:
 The TO word (010) is used as a routing designator which determines the address of the
destination station which will directly receive the call. TO shall be used in the individual
call protocols for single stations and in the net call protocols for multiple net-member
stations which are called using a single net address. The TO word itself shall contain the
first three characters of an address. For extended addresses, the additional address
words (and characters) shall be contained in alternating DATA and REP words, which
shall immediately follow. The sequence shall be TO, DATA, REP, DATA, and REP, and
shall be only long enough to contain the address, up to a maximum capacity of five
address words (15 characters).
THIS IS (TIS):
The TIS word (101) is used as a routing designator which indicates the address of the present
calling (or sounding) station which is directly transmitting the call (or sound). Except for the use
of TWAS, TIS is used in all ALE protocols to terminate the ALE frame and transmission. It shall
indicate the continuation of the protocol or handshake, and shall direct, request, or invite
(depending on the specific protocol) responses or acknowledgments from other called or
receiving stations. The TIS shall be used to designate the call acceptance sound. The TIS word
itself shall contain the first three characters of the calling stations address. For extended
addresses, the additional address words (and characters) shall be contained in alternating DATA
and REP words which shall immediately follow, exactly as described for whole addresses using
the TO word and sequence. The entire address (and the required portion of the TIS, DATA, REP,
DATA, REP sequence, as necessary) shall be used only in the conclusion section of the ALE frame.
WAS should not be used in the same frame as TIS, as they are mutually exclusive.
THIS WAS (TWAS):
The TWAS word (011) is used as a routing designator exactly as the TIS, with the following
variations. It shall indicate the termination of the ALE protocol or handshake, and shall reject,
discourage, or not invite (depending on the specific protocol) responses or acknowledgments
from other called or receiving stations. The TWAS is used to designate the call rejection sound.
TIS shall not be used in the same frame as TWAS, as they are mutually exclusive.
THRU:
The THRU word (001) is used in the scanning call section of the calling cycle only with group call
protocols. The THRU word is used alternately with REP, as routing designators, to indicate the
address first word of stations that are to be directly called. Each address first word shall be
limited to one basic address word (three characters) in length. A maximum of five different
address first words shall be permitted in a group call. The sequence shall only be alternations of
THRU, REP. The THRU shall not be used for extended addresses, as it will not be used within the
leading call section of the calling cycle. When the leading call starts in the group call, the entire
group of called stations shall be called with their whole addresses, which shall be sent using the
TO preambles and structures.
NOTE: 1. the THRU word is also reserved for future implementation of indirect and relay
protocols, in which cases it may be used elsewhere in the ALE frame and with whole addresses
and other information. Stations designed in compliance with this non relay standard should
ignore calls to them which employ their address in a THRU word in other than the scanning call