Measurements of Speed
Bits per second (bps), baud rate and bandwidth are all measures commonly used to describe the speed of communication. Unfortunately many references use these terms incorrectly. The most common error is to use all three terms interchangeably to mean bits per second. In this section we consider the technical meaning of each of these measures, together with their relationship to each other.
Bits per second
Bits per second is the rate at which binary digital data is transferred. For instance a speed of 2400bps, means 2400 binary digits can be transferred each second. Notice bps means bits per second not bytes per second. If a measure refers to bytes a capital B should be used, and if it refers to bits then a lower case b should be used; for example kB means kilobyte and kb means kilobit, similarly MB means megabyte whilst Mb means megabit. It is customary to refer to bits when describing transmission speeds. Consider an Ethernet network based on the Fast Ethernet 100 Base-T standard. This network is able to transfer data at a maximum speed of approximately 100Mbps. Now imagine we wish to transfer a 15MB video from one machine to another. 15MB = 15× 8 Mb = 120Mb, therefore the transfer should take approximately 1.2 seconds. In reality the transfer will take significantly longer due to the overheads required to create the frames at the source and decode the frames at the destination. Also the headers and trailers added by each communication protocol involved have not been included in our calculation, yet they too must be transferred.
Baud rate
Baud rate is a measure of the number of distinct signal events occurring each second along a communication channel. A signal event being a change in the transmission signal used to represent the data. Technically each of these signal events is called a baud, however commonly the term baud is used as a shortened form of the term baud rate. Most modern communication systems represent multiple bits using a single signal event. For example, a connection could represent 2 bits within each baud by transmitting say +12 volts to represent the bits 11,+6 volts for 10, -6 volts for 01, and –12 volts for 00. If this connection were operating at 1200 baud then 2400bps could be transmitted. This example is trivial, in reality various complex systems are used where up to 4, 6, 8 or more bits are represented by each baud. In these situations different waveforms or symbols are needed to represent each bit pattern. The number of different symbols required doubles for each extra bit represented, for example to represent 4 bits requires 2^4 = 16 different symbols whilst 5 bits requires 22× 16 = 32 different symbols. Altering or modulating the amplitude, frequency and/or phase of the signal produces these different symbols.
As most high-speed data communication is restricted to a particular range of frequencies, most encoding systems use a combination of amplitude and phase modulation.The time taken for each individual symbol to travel (or propagate) along the medium from the transmitter to the receiver can also affect transmission times. In regard to the transmission of individual data packets this is relatively insignificant. It only becomes significant over longer distances, particularly when each data packet must be acknowledged before the next one can be sent. The speed at which waves propagate from transmitter to receiver approaches the speed of light – the speed of light (3×10^8m/s) is only achieved as waves travel through a vacuum. In copper wire and other mediums speeds of around 2×10^8m/s are more realistic. In any case the speed of the wave is incredibly fast. At a speed of 2×10^8m/s, travelling the 20,000km around to the other side of the Earth takes one tenth of a second.
Bandwidth
The term bandwidth is often used incorrectly, people make statements such as “video requires much more bandwidth than text” or “my bandwidth decreases as more people use the Internet”. Statements such as these are incorrect; they are using bandwidth when they really mean speed or bps. Bandwidth is not a measure of speed at all; rather it is the range of frequencies used by a transmission channel. Presumably misunderstandings have occurred because the theoretical maximum speed does increase as the bandwidth of a channel increases. However, it is simply impossible for the bandwidth of most channels to change during transmission. Each channel is assigned a particular range of frequencies when it is first setup; unless you run a high-speed Internet company or are creating your own hardware transmitters and receivers, then altering bandwidth is really beyond your control. So what is bandwidth? It is the difference between the highest and the lowest frequencies used by a transmission channel. Frequency is measured in hertz(Hz), meaning cycles per second. Each cycle is a complete wavelength of an electromagnetic wave, so 20Hz means 20 complete wavelengths occur every second. As frequency is expressed in hertz then so to is bandwidth. For example, standard telephone equipment used for voice operates within a frequency range from about 200Hz to 3400Hz, so the available bandwidth is approximately 3200Hz. As high-speed connections routinely use bandwidths larger than 1,000Hz or even 1,000,000Hz, bandwidth is usually expressed using kilohertz (kHz) or megahertz (MHz). For example 3200Hz would be expressed as 3.2kHz.All signals need to be modulated in such a way that they remain within their allocated bandwidth. This places restrictions on the degree of frequency modulation that can be used. As a consequence most modulation systems rely on amplitude and phase modulation. For example, most current connections to the Internet use Quadrature Amplitude Modulation (QAM), this system represents different bit patterns by altering only the amplitude and phase of the wave. 16QAM uses 16 different symbols to represent 4 bits/symbol, 64QAM uses 64 different symbols to represent 6 bits/symbol and 256QAM uses 256 different symbols representing 8 bits/symbol. Amplitude, phase and frequency are related; altering one has an effect on each of the others. Increasing the available frequency range (bandwidth) results in a corresponding increase in the total number of unique amplitude and phase change combinations (symbols) that can accurately be represented and detected. In general, it is true that the speed of data transfer increases as the bandwidth is increased. It is difficult to discuss bandwidth without mentioning the related term ‘broadband’. Broadband, is a shortened form of the words broad and bandwidth. As is the case with numerous computer related terms there are various accepted meanings. In common usage broadband simply refers to a communication channel with a large bandwidth. However, the term is also used in reference to a physical transmission medium that carries more than one channel. In essence, the total bandwidth is split into separate channels that each use a distinct range of frequencies. Using either meaning, most long distance Internet connections and both ADSL (Asymmetrical Digital Subscriber Line) and cable are examples of broadband technologies. They all deliver high data rates (theoretically in excess of 5Mbps) by splitting the total bandwidth into separate communication channels. The opposite of broadband is baseband. Baseband connections include Ethernet, 56kbps modem links and 128kbps ISDN links where a single communication channel is used. The term “narrowband” refers to a single channel that occupies a small bandwidth, such as traditional voice telephone lines.
Bits per second (bps), baud rate and bandwidth are all measures commonly used to describe the speed of communication. Unfortunately many references use these terms incorrectly. The most common error is to use all three terms interchangeably to mean bits per second. In this section we consider the technical meaning of each of these measures, together with their relationship to each other.
Bits per second
Bits per second is the rate at which binary digital data is transferred. For instance a speed of 2400bps, means 2400 binary digits can be transferred each second. Notice bps means bits per second not bytes per second. If a measure refers to bytes a capital B should be used, and if it refers to bits then a lower case b should be used; for example kB means kilobyte and kb means kilobit, similarly MB means megabyte whilst Mb means megabit. It is customary to refer to bits when describing transmission speeds. Consider an Ethernet network based on the Fast Ethernet 100 Base-T standard. This network is able to transfer data at a maximum speed of approximately 100Mbps. Now imagine we wish to transfer a 15MB video from one machine to another. 15MB = 15× 8 Mb = 120Mb, therefore the transfer should take approximately 1.2 seconds. In reality the transfer will take significantly longer due to the overheads required to create the frames at the source and decode the frames at the destination. Also the headers and trailers added by each communication protocol involved have not been included in our calculation, yet they too must be transferred.
Baud rate
Baud rate is a measure of the number of distinct signal events occurring each second along a communication channel. A signal event being a change in the transmission signal used to represent the data. Technically each of these signal events is called a baud, however commonly the term baud is used as a shortened form of the term baud rate. Most modern communication systems represent multiple bits using a single signal event. For example, a connection could represent 2 bits within each baud by transmitting say +12 volts to represent the bits 11,+6 volts for 10, -6 volts for 01, and –12 volts for 00. If this connection were operating at 1200 baud then 2400bps could be transmitted. This example is trivial, in reality various complex systems are used where up to 4, 6, 8 or more bits are represented by each baud. In these situations different waveforms or symbols are needed to represent each bit pattern. The number of different symbols required doubles for each extra bit represented, for example to represent 4 bits requires 2^4 = 16 different symbols whilst 5 bits requires 22× 16 = 32 different symbols. Altering or modulating the amplitude, frequency and/or phase of the signal produces these different symbols.
As most high-speed data communication is restricted to a particular range of frequencies, most encoding systems use a combination of amplitude and phase modulation.The time taken for each individual symbol to travel (or propagate) along the medium from the transmitter to the receiver can also affect transmission times. In regard to the transmission of individual data packets this is relatively insignificant. It only becomes significant over longer distances, particularly when each data packet must be acknowledged before the next one can be sent. The speed at which waves propagate from transmitter to receiver approaches the speed of light – the speed of light (3×10^8m/s) is only achieved as waves travel through a vacuum. In copper wire and other mediums speeds of around 2×10^8m/s are more realistic. In any case the speed of the wave is incredibly fast. At a speed of 2×10^8m/s, travelling the 20,000km around to the other side of the Earth takes one tenth of a second.
Bandwidth
The term bandwidth is often used incorrectly, people make statements such as “video requires much more bandwidth than text” or “my bandwidth decreases as more people use the Internet”. Statements such as these are incorrect; they are using bandwidth when they really mean speed or bps. Bandwidth is not a measure of speed at all; rather it is the range of frequencies used by a transmission channel. Presumably misunderstandings have occurred because the theoretical maximum speed does increase as the bandwidth of a channel increases. However, it is simply impossible for the bandwidth of most channels to change during transmission. Each channel is assigned a particular range of frequencies when it is first setup; unless you run a high-speed Internet company or are creating your own hardware transmitters and receivers, then altering bandwidth is really beyond your control. So what is bandwidth? It is the difference between the highest and the lowest frequencies used by a transmission channel. Frequency is measured in hertz(Hz), meaning cycles per second. Each cycle is a complete wavelength of an electromagnetic wave, so 20Hz means 20 complete wavelengths occur every second. As frequency is expressed in hertz then so to is bandwidth. For example, standard telephone equipment used for voice operates within a frequency range from about 200Hz to 3400Hz, so the available bandwidth is approximately 3200Hz. As high-speed connections routinely use bandwidths larger than 1,000Hz or even 1,000,000Hz, bandwidth is usually expressed using kilohertz (kHz) or megahertz (MHz). For example 3200Hz would be expressed as 3.2kHz.All signals need to be modulated in such a way that they remain within their allocated bandwidth. This places restrictions on the degree of frequency modulation that can be used. As a consequence most modulation systems rely on amplitude and phase modulation. For example, most current connections to the Internet use Quadrature Amplitude Modulation (QAM), this system represents different bit patterns by altering only the amplitude and phase of the wave. 16QAM uses 16 different symbols to represent 4 bits/symbol, 64QAM uses 64 different symbols to represent 6 bits/symbol and 256QAM uses 256 different symbols representing 8 bits/symbol. Amplitude, phase and frequency are related; altering one has an effect on each of the others. Increasing the available frequency range (bandwidth) results in a corresponding increase in the total number of unique amplitude and phase change combinations (symbols) that can accurately be represented and detected. In general, it is true that the speed of data transfer increases as the bandwidth is increased. It is difficult to discuss bandwidth without mentioning the related term ‘broadband’. Broadband, is a shortened form of the words broad and bandwidth. As is the case with numerous computer related terms there are various accepted meanings. In common usage broadband simply refers to a communication channel with a large bandwidth. However, the term is also used in reference to a physical transmission medium that carries more than one channel. In essence, the total bandwidth is split into separate channels that each use a distinct range of frequencies. Using either meaning, most long distance Internet connections and both ADSL (Asymmetrical Digital Subscriber Line) and cable are examples of broadband technologies. They all deliver high data rates (theoretically in excess of 5Mbps) by splitting the total bandwidth into separate communication channels. The opposite of broadband is baseband. Baseband connections include Ethernet, 56kbps modem links and 128kbps ISDN links where a single communication channel is used. The term “narrowband” refers to a single channel that occupies a small bandwidth, such as traditional voice telephone lines.