No, it's wrong, but not necessarily fraudulent! I've known some reputable companies push the C and E closer together, to save space on the product label.
14 yo girl electrocuted trying to charge mobile in bath
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There is, actually at the end of each page.
The process starts with the sender dialling, the receiver answering, and the modems training (at this point, the modems will select the fastest speed they can both do) - the screech sometimes heard before the machines turn the speaker off. The two ends then exchange information on capabilities and what's desired - and agree the transmission scheme to be used. Along the line, the machines also exchange identity - the number (often incorrect) that gets reported on the printouts and reports.
Then the sender sends a start of page message, followed by the page data, and an end of page message. The receiver acknowledges each end of page and the sender carries on. There is no error detection or correction as standard, I can't recall if it's optional.
After the last page, there's a final handshake.
In terms of immediacy, many (most ?) machines these days will scan the document into memory and only then start sending. Some start sending as soon as they've got a page scanned, but most I've used scan the whole thing first. Older machines had a very limited memory, and so would "scan while they send". To know if it's been sent you need to refer to the machine logs, or trust that it'll print a failure page if it didn't go - or specifically set the machine to real-time mode.
Thanks Simon. So as I thought, the receiver is acknowledging the end of page, not that the transmission of the page has been successful.
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Yes, there is an optional error-correction mode in which data octets are sent with checksums, and the receiving end will request a retransmission of any corrupted octets.
Something I've tried to impress upon my step-daughter when she's been arranging things and furiously sending text messages and waiting for replies. I think I've finally managed to get it into her head that if you're trying to work out details urgently, it's better to just call the person than send an SMS message and hope for the best, since message can take hours or - as you say - even days to arrive! (And some just disappear never to be seen again.)
Even if the option is implemented though, that will only detect octets that are invalid, not octets that are wrong, but valid?
Like any checksum method, it works pretty well, but yes, there are certain combinations of errors which will get through undetected since certain such combinations will result in exactly the same checksum value as the original, valid data.
Traditional Telex never did anything like that. Once the link was established it was basically just the transmitter contacts at one end connected over one side of the circuit to the receiving magnet at the other, the other side of the circuit providing the identical communication path in the opposite direction. Sending contacts and receiving magnets were replaced with electronics in later years.
Telex did have the answerback code, sent by the answering station upon answering a call so that the caller could verify that he'd reached the correct number - Sort of the Telex equivalent of answering a phone call with your number, except that answerback codes were different from the actual dialed numbers.
Oh well, seems my telex training was wrong then.
Perhaps you're thinking of Telex over some particular type of link, rather than the basic Telex system per se?
No, standard telex.
In many applications of checksums, in which the checksum is calculated across a substantial number of bits (or bytes) the probability of a combination of errors resulting in an apparently correct checksum can be 'vanishingly small'.
However, if (is this what you mean?) you are talking about the checksum of a single octet/byte, which can only take 9 possible values (0-8), then that probability becomes much greater, particularly for some octets/checksums.
Kind Regards, John
I've been through exactly the same, but it was not just 'send an SMS message'. I sometimes watched in bewilderment as maybe 20 SMS messages were sent backwards and forwards over a very short period of time when they were trying to 'arrange something'!
Kind Regards, John
Definitely no per-character handshaking there, even when transmission was over RF carrier systems.
Which is why it's become so widely used as a pretty robust error-checking protocol. The fax ECM octets aren't individual bytes, but octets of multiple collected fields of data. It's been years since I looked at the full protocol, so I'm afraid I can't recall the details off the top of my head, but each checksum covers a large enough amount of data to be quite a reliable method.
If you want to go down to one of the most basic and unreliable checking methods, consider the use of a single parity bit in each data word, as was common once for ASCII transmission of messages (7-bit data plus parity) or for an elementary check for memory errors (say 8 or 16 bits of data plus parity bit). In that very basic scheme the parity check will flag a single bit in error, but if two bits are in error it will be completely missed. It can basically detect only an odd number of erroneous bits.
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