Can someone help me with the following sentence: “The chances against successful transmission are a thousand to one."  Does this mean that one transmission out of a thousand will fail or on the contrary will be successful?
 
And one more thing. What's the meaning of the sentence below? Could you explain it to me?
"It didn't even work all that well for the gentry above them."

Thanks in advance.

I agree with Clive, but I'd like to expand upon it a bit; the key word in "The chances against successful transmission are a thousand to one" is against.

If the sentence had read "The chances of successful transmission are a thousand to one",then it would have been ambiguous, unless against had been added to the very end of the sentence.

In British culture at least, betting odds are expressed in terms of odds against, and odds on. So, if you go into your local betting shop in the High Street of any English town, and put ten pounds (£10) on a horse that has quoted odds of 50-1 against,  then your horse doesn't have much chance of winning the race, but you'll win a lot of money if it does.

Conversely, if you stake your £10 on a horse with quoted odds of 50-1 on, then it's almost certain to win the race, but your winnings are going to be very small indeed.

Incidentally, the equivalent term for odds that are equal, i.e. neither odds against nor odds on, is evens.

Also, incidentally, regarding the term gentry, if the source of your sentence "It didn't even work all that well for the gentry above them." is a British one, then the society being referred to is probably going to be one divided into the nominal social classes Aristocracy- Gentry-Middle Classes-Working Classes, although nowadays such divisions don't have quite the meaning that they might have done a hundred years ago; British society is (mercifully) a lot more fluid now.