2.3.5 Signaling Speed
It is important to distinguish between two related but fundamentally different measures of transmission rate: the baud rate (or symbol rate) and the bit rate (or data rate). Although the two terms are often used interchangeably in everyday conversation, they describe different characteristics of a communications system and should not be confused.
The baud rate is the number of signaling symbols transmitted each second. Each symbol represents one distinct signaling event, such as a change in voltage level on a cable or a particular combination of amplitude and phase on a radio carrier. The unit of symbol rate is the baud, named after the French telegraph engineer Émile Baudot.
The bit rate, by contrast, is the number of bits of information transmitted each second and is measured in bits per second (bps). Since a symbol may represent one or more bits, the bit rate is not necessarily equal to the baud rate.
The two rates are identical only when each transmitted symbol represents a single bit. This is the case for simple binary signaling, where there are only two possible symbols corresponding to the binary values ‘0’ and ‘1’. In such systems, a signaling rate of 1,000 baud also corresponds to a data rate of 1,000 bps.
Many modern communication systems, however, employ M-ary signaling, in which each symbol can represent several bits simultaneously. For example, a signaling scheme with four distinct symbols can represent two bits per symbol because there are four possible binary combinations: 00, 01, 10, and 11. If symbols are transmitted at a rate of 1,000 baud, the corresponding bit rate is therefore 2,000 bps.
Similarly, if a signaling scheme employs sixteen distinct symbols, each symbol represents four bits because (24 = 16). At the same symbol rate of 1,000 baud, the bit rate increases to 4,000 bps. In general, an M-ary signaling scheme conveys log2 M bits per symbol, giving the relationship:
where Rb is the bit rate (bps), Rs is the symbol rate (baud), and M is the number of signaling levels or symbols.
Increasing the number of bits represented by each symbol allows more information to be transmitted without increasing the symbol rate. This improves bandwidth efficiency because the same communication channel can carry a higher data rate. However, the symbols become more closely spaced, making them harder for the receiver to distinguish in the presence of noise and interference. Higher-order signaling therefore requires a better-quality communications channel and a higher signal-to-noise ratio to achieve reliable transmission.
This trade-off between data rate, bandwidth efficiency, and noise performance is one of the central themes of modern communications engineering. We return to it in Chapter 6 when we examine digital modulation techniques such as Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM), which routinely transmit several bits in every transmitted symbol.
It is also useful to distinguish these quantities from the information rate, introduced in the next section. The bit rate describes the total number of transmitted bits, whereas the information rate describes only the useful information delivered to the destination after allowing for framing, synchronization, error-control coding, and other protocol overheads. Consequently, the information rate is usually lower than the raw bit rate of the communication channel.
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