#parity bit

So now that I've actually made my way through the more popular protocols, let's go through the manual and see what they want us to know:

Firstly, there are many disruptions that a signal encounters on its way to the receiver - fading, interference, and noise.  And when it comes to digital communications, that means that erroneous bits may accidentally be added into the stream of data.  One way to measure this is the bit error rate (BER).

So some protocols adjust for this and monitor this.  For instance, some codes include a parity bit which is used to detect errors in a single character of data.  Specifically, parity bits are added to a sequence of bits to either make the number of 1's even or odd.  Example:

The original sequence of bits: 0011000

Even parity = 00011000 = 2 1's therefore even

Odd parity = 10011000 = 3 1's therefore odd

So my guess is that for every bit sent out, a parity bit is added to each sequence.  So if you modulated the bit stream to be even parity, and at the receiver side, it notices an odd parity, then it knows that there is an error in that particular sequence of bits.

So all of the digital modes that we've talked about are under the umbrella of packet radio.  And packet radio as we know use different methods of modulation, frequency shift keying, phase shift keying, etc. 

Frequency Shift Keying: Morse code being a good example, is where characters are represented by rapidly alternating audio tones (usually between two different specified frequencies).  Receiving modems and terminal node controllers then reassemble data from the packets.

Each of these packers consist of a header and data.  The header has information about the packet and the call sign of the destination station.  It also includes a checksum to allow the receiver to detect errors. 

And as we've talked before - if any errors are seen - the receiver sends a message back to the transmitter asking for the message again in a process known as a automatic repeat request (ARQ)

A checksum is a set of random data that is computed from a specific block of data and is placed, in this case, into the header.  If at anytime the integrity of the message is in question, the set of random data can be recomputed and compared to the stored one.  If they are similar to each other, then you can rest assured that the data you've received is accurate.

These packet stations can connect to each other directly and node stations help to act as routing centers for packet connections.  These nodes connect to other nodes and these form networks.  Stations can also use relays call digipeaters to connect to out-of-range stations.