Bits, Bytes, and Baud

Little knowledge about Bits, Bytes, and Baud

Let's begin by examining the differences between bits, bytes, and baud. The word bit is an acronym for binary digit, and is usually indicated by a lower case b. A binary digit indicates 2 electronic states, either an "on" state or an "off" state, and is sometimes referred to as 1s or 0s.

A byte is labeled with an upper case B, and usually 8 bits in length. A byte could be more than 8 bits, so we can precisely call an 8-bit word an octet. Also, there are two "nibbles" in a byte. A nibble is defined as a 4-bit word, which is half a byte.

Bit rate, or throughput, is equal to bits per second (bps) and is associated with the speed of the signal through a given medium. For example, this signal could be a baseband digital signal or perhaps a modulated analog signal conditioned to represent a digital signal. One type of modulated analog signal is QPSK. QPSK stands for Quadrature Phase Shift Keying.

This is a modulation technique, which manipulates the phase of the signal by 90 degrees to create 4 different signatures as shown in Figure 1. We call these signatures "symbols", and their rate is referred to as baud. Baud equates to symbols per second.

Figure 1 - QPSK Diagram

QPSK signals have four different symbols; four is equal to 2^2. The exponent will give us the theoretical number of bits per cycle (symbol) that can be represented, which equals 2 in this case. The four symbols represent the binary numbers: 00, 01, 10, and 11. Therefore, if I use a symbol rate of 2.56 Msymbols/s to transport a QPSK carrier, it would be referred to as 2.56 Mbaud and the theoretical bit rate would be 2.56 Msymbols/s * 2 bits/symbol = 5.12 Mbps. This is further explained later in the document.

You may also be familiar with the term PPS, which stands for packets-per-second. This is a way to qualify the throughput of a device based on packets regardless of whether that packet contains a 64 B or a 1518 B Ethernet frame.

Source : www.cisco.com