Lost Word Detection System /LOWDS/ provides special identification for each error detection message transmitted from receiver to transmitter. The message is identified as an original message or an n-times retransmitted message so the receiver can detect where a retransmission request was not fulfilled and activate an alarm

Bokros, P.

Burstein, A.

Hewitt, E. D.

English

NASA Technical Reports Server

July 1966	 Brief 66-10287 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U. S. space 
program and to encourage their commercial application. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Detection System Ensures Positive Alarm Activation in Digital Message Loss 
tflconrrttg 
Word 
IC	 No 
Penny ..-----------------------------.-..-.--..--. 
OK? 
Yes
Lass 
Than 
Word A	 Yen Repet1toont	
Equal 
Repeat? Anad ID 
No. 
No Greaten 
Than 
Is 
Repeat	 No Alarm Count 
-0? 
Yes 
is Receiver	 Yes 
Storage Delay 
uny y 
No 
nd Word OnTo - 
Receiver Reject 
Storage Word
Counter	 ­ 	 Alarm 
No 
Increment 
Repeat 
Counter 
:1 
Send 
Retransmit 
Request 
Send No Enor 
Response 
The problem:	 of errors in the transmission from receiver to trans-
In many data transmission systems which use the 	 mitter, request for error retransmission could be im-
block- or word-segmented data group, the transmit- 	 properly decoded as an erase request. This would 
ting terminal of the system is required to store the 	 result in the loss of the message from both the trans-
message long enough for it to be examined for error 	 mitting and receiving stations, and this loss of message 
by the receiving terminal. After the examination, the 	 might go undetected, despite the elaborate message 
receiving terminal transmits one of two possible mes-	 bookkeeping schemes. 
sages to the transmitting terminal, instructing it either 
to retransmit the message (if an error was detected), 	 The solution: 
or to erase it from storage (if there was no error), and 	 A proposed LOWDS (Lost Word Detection System) 
permit the next message to be transmitted. As a result	 that provides additional special identification for each 
(continued overleul) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19660000286 2020-03-11T17:39:47+00:00Z
message. Each transmitted message contains a Mes-
sage Identity portion, which identifies the message as 
either an original (not previously transmitted) Pies-
sage, or as an n-times retransmitted message. The re-
ceiver can then detect any case where a request for 
retransmission was not fulfilled, and activate an 
alarm, indicating digital message loss as a result of an 
undetected error in a response message from the re-
ceiver to the transmitter. 
How it's done: 
At the receiving terminal the error checking cir-
cuits examine the incoming message from the trans-
mitting terminal. If the message contains an error, a 
retransmit request is generated and sent back to the 
transmitting terminal. At the same time the receiving 
terminal notes the fact that following a given period 
the message must be retransmitted. This retransmis-
sion must have a message identity designating it as the 
first requested retransmission. If a message with any 
other message identification is received, it indicates 
that the original message has been lost and activates 
an alarm. 
In detail, the incoming word is first checked for 
parity. If the parity check is good the word is ex-
amined to see if it is a repeat. If the ID serial number 
is "00", then the word is a new one and the repeat 
counter (which keeps track of the number of repeat 
requests sent by the feceiving terminal) is checked to 
see if a repeat was reuested. If a repeat was requested 
then a word has been lost and the alarm is activated. 
If the repeat counter also reads "00", then the word 
is a new word which is accepted by the receiver, and a 
signal is sent to the transmitting terminal, acknowl-
edging an error free reception. The transmitter is then 
free to transmit the next word and erase the previous 
word from its storage. 
If parity is good and the word is a repeat, then the 
repeat counter is compared to the ID serial number of 
the word (which shows which order of repeat it is). If 
the number in the repeat counter is greater than the 
ID serial number, then a word has been lost and the 
alarm is activated. If the number in the repeat counter 
is less than the ID serial number, a repeat word was
sent when a new word was requested. The word is 
rejected and a no-error-detected response is sent to 
the transmitting station. 
If the number in the repeat counter is equal to the 
ID serial number, then the word received is the proper 
order of repeat and is accepted by the receiver, a no-
error response is sent to the transmitting terminal, 
and the repeat counter is cleared. 
Notes: 
1. This system is part of a digital data link system 
providing computer-to-computer data transfer 
where an undetected loss of any message cannot be 
tolerated. 
2. The Message Identity portion of the message and 
the two responses from the receiver to the trans-
mitter (a no-error-detected response or a request 
for retransmission) may be protected by standard 
error correction codes to further decrease the prob-
ability of an undetected communication error. 
3. If the equipment to which the receiver is attached 
is too busy to accept an error free message, the 
receiver can then call for a retransmission of the 
message. This action inserts a delay in the system 
to permit the busy state to terminate. This feature 
requires very little additional equipment since it 
makes use of the standard LOWDS retransmission 
facility, and eliminates the requirement for a one-
word storage buffer. 
4. Inquiries concerining this innovation may be di-
rected to:
Technology Utilization Officer 
Western Operations Office 
150 Pico Boulevard 
Santa Monica, California 90406 
Reference: B66-10287 
Patent status: 
No patent action is contemplated by NASA. 
Source: Paul Bokros, Albert Burstein, 
and Edward D. Hewitt 
of Radio Corporation of America 
under contract to 
Western Operations Office 
(WOO-208) 
Brief 66-10287	 Category 01


Detection system ensures positive alarm activation in digital message loss

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server