Subject: Re: Lecture 2 notes
From: Rajmohan Rajaraman (rraj@ccs.neu.edu)
Date: Mon Oct 08 2001 - 21:23:53 EDT
This is a clarification on Problem 5 of Problem Set 1, based on a
question raised by a student.
> For problem 5, "...codewords that consist of binary bits and contain the
> same number of 1s." What does "same number" mean? What does
> "two-out-five" mean? Can you give it a more detailed description?
>
The statement "...codewords that consist of binary bits and contain
the same number of 1s" means that all the words that are transmitted
over the channel have the same number of 1s. Suppose the DLC layer
receives x-bit words and is using the "y-out-of-z" code (y < z) .
Then, every word that is handed to the DLC layer will be first
transformed to a z-bit word in which y of the bits are 1. The
receiver, on getting any data detects errors by checking whether the
received word has y out of z bits as 1. If this is not true, then the
received data has errors. Of course, the encoding at the sender must
satisfy the property that every x-bit word is transformed to a unique
z-bit codeword so that the decoding at the receiver can be done
correctly in the event there are no errors.
The "two-out-five code" should read as "two-out-of-five code". By the
two-out-of-five code, we mean that every codeword is a 5-bit word in
which exactly 2 of the bits are 1s.
Hope this clarifies any doubts. Best,
Rajmohan.
This archive was generated by hypermail 2b28 : Mon Oct 08 2001 - 21:24:17 EDT