A symbol is a waveform, a state or a significant condition of the communication channel that persists, for a fixed period of time.
In the Navy, more than one flag pattern and arm can be used at once, so the combinations of these produce many symbols, each conveying several bits, a higher data rate.
In a modem, these may be sinewave tones with unique combinations of amplitude, phase and/or frequency.
In a digital modulation method provided by a modem, each symbol is typically a sine wave tone with a certain frequency, amplitude and phase.
It reduces the time required to send a given quantity of data over a limited bandwidth.
[2] Voiceband modem examples: In case of a baseband channel such as a telegraph line, a serial cable or a Local Area Network twisted pair cable, data is transferred using line codes; i.e., pulses rather than sinewave tones.
Common communication links such as 10 Mbit/s Ethernet (10BASE-T), USB, and FireWire typically have a data bit rate slightly lower than the baud rate, due to the overhead of extra non-data symbols used for self-synchronizing code and error detection.
More than two voltage levels are used in advanced techniques such as FDDI and 100/1,000 Mbit/s Ethernet LANs, and others, to achieve high data rates.
Some communication links (such as GPS transmissions, CDMA cell phones, and other spread spectrum links) have a symbol rate much higher than the data rate (they transmit many symbols called chips per data bit).
Representing one bit by a chip sequence of many symbols overcomes co-channel interference from other transmitters sharing the same frequency channel, including radio jamming, and is common in military radio and cell phones.
However, in spread spectrum systems, the term symbol may also be used at a higher layer and refer to one information bit, or a block of information bits that are modulated using for example conventional QAM modulation, before the CDMA spreading code is applied.
The disadvantage of conveying many bits per symbol is that the receiver has to distinguish many signal levels or symbols from each other, which may be difficult and cause bit errors in case of a poor phone line that suffers from low signal-to-noise ratio.
In that case, a modem or network adapter may automatically choose a slower and more robust modulation scheme or line code, using fewer bits per symbol, in view to reduce the bit error rate.
For example, in frequency-shift keying (FSK), the frequency of a tone is varied among a small, fixed set of possible values.
In a synchronous data transmission system, the tone can only be changed from one frequency to another at regular and well-defined intervals.
A more practical scheme is differential binary phase-shift keying, in which the carrier remains at the same frequency, but can be in one of two phases.
For example, a differential phase-shift keying system might allow four possible jumps in phase between symbols.
In telecommunication, concerning the modulation of a carrier, a significant condition is one of the signal's parameters chosen to represent information.
Information can be transmitted either during the given time interval, or encoded as the presence or absence of a change in the received signal.
[4] Significant conditions are recognized by an appropriate device called a receiver, demodulator, or decoder.
The decoder translates the actual signal received into its intended logical value such as a binary digit (0 or 1), an alphabetic character, a mark, or a space.
Each significant instant is determined when the appropriate device assumes a condition or state usable for performing a specific function, such as recording, processing, or gating.