PACTOR :An Overview of a New and Effective HF Data Communication Protocol

3 Pages·1992·0.26 MB·English

by  Gwyn Reedy, W1BEL

PACTOk
wf e odiwanvearN envAO FaHtaD
EfEixtive Commnnication Protocol
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DataGommllnicatio~ ruetalna oidar Only the MA!STER corrects his Receive
LlsiIlgHFfre hasrecentlybecome
Pb-
more effective and enjoyable due to a new A
long series of tests conducted by
co-cation protocol called PACTOR
DL6MAA and DF4W have shown that
PACTOR was developed by two
for operation in rapidly changing
enterprising German amateu~~,DIhMAA
conditions, it is not a good policy to adjust
and DF4KJL This arkle is based on the
packet length automatically. Simulations
information provided by these gentlemen
on-the-air testing the optimum
in their various writings.
HF packet length to be about one second.
PACTOR Features To compensate for varying conditions,
PACTOR varies the number of data
tub seodton
PACTOR was designed to overcome the characters in the data block,
egnah cyna
shortcomings exhibited by both packet and of the synchronous timing
ANlTOR in HF operation while remaining parameters. PACTOR determines the
affordable for the average amateur broper baud rate to use based on the
operator. ‘accuracy of bit transitions and the link
error statistics.
- Error-free data transmission (less than 1
x 10-S) Data blocks have CRC-16 checking as is
done in AX25 packet. This is much more
True binary data transmission
robust than the parity bits FEC used in
Efficient use of channel capacity AMTOR.
Good interference tolerance The data field of the PACTOR packet can
contain any digital information; the format
Requires only 600 Hz channel bandwidth
of the codes is specified in the status byte.
Complete visibility of sending and At the present trme the choice is between
receiving callsigns 8 bit ASCII and Huffman compressed 7 bit
ASCII.
Whv PACTOR?
Authorization
HF propagation is characterized by
multipath propagation which induces ‘bit The US FCC regulations specify that
stretching’ and phase distortions, fading, Baudot or ASCII codes may be used for
and
impulse noise, interference by other data transmission, The 8 bit ASCII text
stations, among other obstacles to transmitted by PACTOR is closer to erup‘
comIIlunication, ASCII’ than the bit-stuffed HDLC used by
so there should be no
AX25 packet,
The PACTOR mode is similar to AMTOR
ytilage lfo siht .edomehT
question of the
which is good for ordinary HF
compressed PACTOR data mode uses
commumcation. Both use half duplex
ASCII characters encoded in a channel-
ARQ; packets (blocks) of data carrying the
capacity-enhancing format which still
information are acknowledged with short
meets the intent of the regulations. The
‘receipt’ signals by the receiving station.
regulations regarding amateur
When errors occur, the receiver can
transmissions require that there be no
request the repetition of a packet with
intent to obscure the data transmitted. The
relevant control signals. *
emehcs si dehsilbupsa
Huffman encoding
PACTOR uses a MASTER/SLAVE
an appendix to this article. It seems
phasing like AMY’TOR. The SLAVE clock tganhitdocne
reasonable to me to consider
1s synchronized to the h4ASTER timing.
62
for spectrum efficiency using a widely throughput in good conditions is up to four
pubhshed encoding scheme shows no times as greaL During the poorest of
mtent to encrypt the content of the data conditions, throughput is considerably
transmkion and is therefore allowable better than AMTOR because of the
under US regulalions. CRC-16 error checking and Memory-ARQ
capabilities.
ecnamrofreP
:xidneppA ROTCAP namffuH
PACTOR achieves good throughput code
during poor HIF conditions by a variety of
tech.quesXheactualbaudrateiskept l!itdbm coding is relativelv indifferent to
low-thesameasAMTORThisisone differences be6een red &d theoretical
thirdtherateoftyphNWAX25packet alphabet character fpequencies, so that
operation. From another point of view, similar good results are obtained in
AMTOR and PACTOR bits are three German and Englglish plain text. The
times as long as 300 baud HF packet bits, compression factor attained with ASCII
thus providing much increased protection amoW to about 1.7, resulting in an
from the bit smearing caused by multipath average of 4.5 bits per character. A retaerg
propagation, compression factor would require
considering the statistical relationships
A doubling of the throughput compared to
sretcarahc vokraM(
AMTOR results ffrom sending longer between the individual
encoding).
blocks of data (but still short enough to
cope with most fades) thus reducing the Code in order of frequency, LSB tnes(
percentage of overhead carried. In first) on the left:
addition, the ability to automatically
Character ASCII
double the data content of each block EMYfIIlm
under favorable conditions provides a space 32 10
considerable increase in efficiency. e 101 011
n 110 0101
Finally, encoding ASCII text (7 bit
characters) using Huffhan codes increases i 105 1101
throughput by an average of at least 70 r 114 1110
percent. t 116 00000
S 115 00100
Memory-ARQ ’ d 100 00111
a 97 01000
A sigpificant feature of PACTOR is
u 117 1111.1
Memory-ARQ. Copies of the repeated
1 108 000010
reception of the same packet which fails
h 104 000100
the CRC are aggregated in memory and
103 000111
are summed individually for each bit. The g
aggregate of all unsuccessful transmissions m 109 oololl
is decoded which effectively increases the <CR> 13 001100
signal to noise ratio by about 15 dB. This <LF> 10 00’1101
PACTOR feature is hardware dependent 0 111 OlOOlO
and prevents the proper implementation of c 99 010011
PACTOR as a software- only upgrade to b 98 0000110
packet or Ah4TOR equipment. f 102 0000111
119 0001100
The combination of the above factors W
provides a protocol which can provide a D 68 0001101
throughput nearly equal to HI? packet in k 107 0010101
122 1100010
the best of conditions, and much better Z
throughput than packet during typical 46 1100100
conditions. Compared to AMTOR, the 44 1100101
63
S 83. 11’11011 39 11or001101:110
A 65 00’101001 w- 95 111100001z00
E 69 llooooo0 49. 38 111100111001
P 112 11m10 + 43 111100111110
V 118 11m11 > 62 111100111111
0 48 l1.sDoolll @ ooo101011-moo
F 70 lmolloo 36 4#)0101011:1001
B 66 11001111 < 60 ooo101011:1010
C 67 lllloool X 88 ooo101.0111011
I 73 11’110010 # 35 001~01ooo1 lo1 1
T 84 11’110100 Y 89 001010001z0101
0 79 ocm01ooo 59 111100001’10100
P 80 ooo101100 934 11110000110101
II
1 49 00’101oooo 91 001010001l01000
I
R 82 1m oooo10 93 001010001101001
( 40 110011011 127 110001101111000
) 41 110011100 126 110001101111001
L 76 110011101 125 110001101’1111010
N 78 1111ooooo 124 110001101’111011
Z 90 111100110 123 110001101111100
{
M 77 111100110 Y 96 110001~01111101
9 57 ooo1010010 A 94 110001101111110
W 87 ’ cKMI1010100 32 110001101111111
>su<
5 53 ~01010101 <GS> 29 11.1100001101100
Y l21 0001010110 9 ESC > 27 111100001101101
2 50 0001011010 <EM> 25 111100001101110
3 51 0001011011 <CAN> 24 111100001-101111
4 52 01)010111oQ <ETB> 23 111100001110000
6 54 0001011101 exN> 22 111100001110001
7 55 0001011110 <NAK> 21 111100001110010
8 56 ooo1011.111 <DC4> 20 111l00001110011
H 72 0010100010 <DC3> 19 111100001110100
J 74 1100000110 <DC2> 18 111100001110101
U 85 1100000111 cDCl> 17 111100001110110
V 86 1100011000 <DLE> 16 111100001110111
<FS> 28 1100011001 <RS> 30 113100001111000
X 120 1100011010 <SI> 15 111100001111001
K 75 1100110100 <so> 14 111100001111010
3 63 1100110101 4cl+ 12 113100001111011
-- 61 1111000010 <VT> 11 113100001111100
4 113 1111010110 <HT > 9 11l100001111101
Q 81 1111010111 CBS> 8 113.100001111110
J 106 00010100110 <BEL> 11-l.100001111111
G 71 00010100111 <ACK> 6 1111.00112000000
45 oco10101111 <EN@ 5 11n100111000001
58 00101000111 <EOT> 4 11l100111000010
?. 33 11110011101 <ETX> 3 1111001-l1000011
47 11110011110 4TX> 2 113100111000100
/
15 42 001010001100 <SOH> 1 1’1.:1100111000101
34 1l0001101100 CNUL> 0 11.1200111000110
37 110001101101 <SUB> 26 11l100111000111
64