signal
1) In electronics, a signal is an electric current or electromagnetic field used to convey data from one
place to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is
the principle by which the early telegraph worked. More complex signals consist of an alternating-
current (AC) or electromagnetic carrier that contains one or more data streams.
Data is superimposed on a carrier current or wave by means of a process called modulation. Signal
modulation can be done in either of two main ways: analog and digital. In recent years, digital
modulation has been getting more common, while analog modulation methods have been used less
and less. There are still plenty of analog signals around, however, and they will probably never
become totally extinct.
Except for DC signals such as telegraph and baseband, all signal carriers have a definable frequency
or frequencies. Signals also have a property called wavelength, which is inversely proportional to the
frequency.
2) In some information technology contexts, a signal is simply "that which is sent or received," thus
including both the carrier (see 1) and the data together.
3) In telephony, a signal is special data that is used to set up or control communication. See signaling.
phase
In electronic signaling, phase is a definition of the position of a point in time (instant) on a
waveform cycle. A complete cycle is defined as 360 degrees of phase as shown in Illustration A
below. Phase can also be an expression of relative displacement between or among waves having
the same frequency.
Phase difference, also called phase angle, in degrees is conventionally defined as a number greater
than -180, and less than or equal to +180. Leading phase refers to a wave that occurs "ahead" of
another wave of the same frequency. Lagging phase refers to a wave that occurs "behind" another
wave of the same frequency. When two signals differ in phase by -90 or +90 degrees, they are said
to be in phase quadrature. When two waves differ in phase by 180 degrees (-180 is technically the
same as +180), the waves are said to be in phase opposition. Illustration B shows two waves that are
in phase quadrature. The wave depicted by the dashed line leads the wave represented by the solid
line by 90 degrees.
Phase is sometimes expressed in radians rather than in degrees. One radian of phase corresponds to
approximately 57.3 degrees. Engineers and technicians generally use degrees; physicists more often
use radians.
The time interval for one degree of phase is inversely proportional to the frequency. If the
frequency of a signal (in hertz) is given by f, then the time tdeg (in seconds) corresponding to one
degree of phase is:
tdeg = 1 / (360f)
The time trad (in seconds) corresponding to one radian of phase is approximately:
trad = 1 / (6.28f)
wavelength
Wavelength is the distance between identical points in the adjacent cycles of a waveform signal
propogated in space or along a wire, as shown in the illustration. In wireless systems, this length is
usually specified in meters, centimeters, or millimeters. In the case of infrared, visible light, ultraviolet,
-9
and gamma radiation, the wavelength is more often specified in nanometers (units of 10 meter) or
-10
Angstrom units (units of 10 meter).
Wavelength is inversely related to frequency. The higher the frequency of the signal, the shorter the
wavelength. If f is the frequency of the signal as measured in megahertz, and w is the wavelength as
measured in meters, then
w = 300/f
and conversely
f = 300/w
Wavelength is sometimes represented by the Greek letter lambda.
modulation
Modulation is the addition of information to an electronic or optical signal carrier. Modulation can be
applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals.
One can think of blanket waving as a form of modulation used in smoke signal transmission (the
carrier being a steady stream of smoke). Morse code, invented for telegraphy and still used in amateur
radio, uses a binary (two-state) digital code similar to the code used by modern computers. For most
of radio and telecommunication today, the carrier is alternating current (AC) in a given range of
frequencies. Common modulation methods include:
Amplitude modulation (AM), in which the voltage applied to the signal is varied over time
Frequency modulation (FM), in which the frequency of the carrier signal is transmitted is
varied in small but meaningful amounts
Phase modulation (PM), in which the natural flow of the alternating current waveform is
delayed temporarily
These are sometimes known as continuous wave modulation methods to distinguish them from pulse
code modulation (PCM), which is used to encode both digital and analog information in a binary way.
Radio and television broadcast stations typically use AM or FM. Most two-way radios use FM,
although some employ a mode known as single sideband (SSB).
More complex forms of modulation are Phase Shift Keying (PSK) and Quadrature Amplitude
Modulation (QAM). Optical signals are modulated by applying an electromagnetic current to vary the
intensity of the laser beam.
Modem Modulation and Demodulation
Any computer with an online or Internet connection includes a modem. This term is derived by
combining the first three letters of the words modulator and demodulator. In a modem, the modulation
process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states)
to analog audio-frequency (AF) tones. Digital highs are converted to a tone having a certain constant
pitch; digital lows are converted to a tone having a different constant pitch. These states alternate so
rapidly that, if you listen to the output of a computer modem, it sounds like a hiss or roar. The
demodulation process converts the audio tones back into digital signals that a computer can
understand. directly.
Multiplexing
More information can be conveyed in a given amount of time by dividing the bandwidth of a signal
carrier so that more than one modulated signal is sent on the same carrier. Known as multiplexing, the
carrier is referred to as a channel and each separate signal carried on it is called a subchannel. The
device that puts the separate signals on the carrier and takes them off of received transmissions is a
multiplexer. Common types of multiplexing include frequency-division multiplexing (FDM) and time-
division multiplexing (TDM). FDM is usually used for analog communication and divides the main
frequency of the carrier into separate subchannels, each with its own frequency band within the overall
bandwidth. TDM is used for digital communication and divides the main signal into time-slots, with
each time-slot carrying a separate signal.
amplitude modulation
Also see modulation.
Amplitude modulation (AM) is a method of impressing data onto an alternating-current (AC) carrier
waveform. The highest frequency of the modulating data is normally less than 10 percent of the
carrier frequency. The instantanous amplitude (overall signal power) varies depending on the
instantaneous amplitude of the modulating data.
In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in
the form of signal components at frequencies slightly higher and lower than that of the carrier.
These components are called sidebands. The lower sideband (LSB) appears at frequencies below the
carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency.
The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versus
frequency, as shown in the illustration. The sideband power accounts for the variations in the
overall amplitude of the signal.
When a carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33 percent) of the overall
signal power is contained in the sidebands. The other 2/3 of the signal power is contained in the
carrier, which does not contribute to the transfer of data. With a complex modulating signal such as
voice, video, or music, the sidebands generally contain 20 to 25 percent of the overall signal power;
thus the carrier consumes 75 to 80 percent of the power. This makes AM an inefficient mode. If an
attempt is made to increase the modulating data input amplitude beyond these limits, the signal
will become distorted, and will occupy a much greater bandwidth than it should. This is called
overmodulation, and can result in interference to signals on nearby frequencies.
phase-shift keying
See also frequency-shift keying (FSK).
Phase-shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase
of a transmitted signal is varied to convey information.
There are several schemes that can be used to accomplish PSK. The simplest method uses only two
signal phases: 0 degrees and 180 degrees. The digital signal is broken up timewise into individual
bits (binary digits). The state of each bit is determined according to the state of the preceding bit.
If the phase of the wave does not change, then the signal state stays the same (low or high). If the
phase of the wave changes by 180 degrees -- that is, if the phase reverses -- then the signal state
changes (from low to high, or from high to low). Because there are two possible wave phases, this
form of PSK is sometimes called biphase modulation.
More complex forms of PSK employ four or eight wave phases. This allows binary data to be
transmitted at a faster rate per phase change than is possible with biphase modulation. In four-
phase modulation, the possible phase angles are 0, +90, -90, and 180 degrees; each phase shift can
represent two signal elements. In eight-phase modulation, the possible phase angles are 0, +45, -45,
+90, -90, +135, -135, and 180 degrees; each phase shift can represent four signal elements.
frequency-shift keying
See also phase-shift keying (PSK).
Frequency-shift keying (FSK) is a method of transmitting digital signals. The two binary states, logic 0
(low) and 1 (high), are each represented by an analog waveform. Logic 0 is represented by a wave at a
specific frequency, and logic 1 is represented by a wave at a different frequency. A modem converts the
binary data from a computer to FSK for transmission over telephone lines, cables, optical fiber, or
wireless media. The modem also converts incoming FSK signals to digital low and high states, which
the computer can "understand."
The FSK mode was introduced for use with mechanical teleprinters in the mid-1900s. The standard
speed of those machines was 45 baud, equivalent to about 45 bits per second. When personal
computers became common and networks came into being, this signaling speed was tedious.
Transmission of large text documents and programs took hours; image transfer was unknown. During
the 1970s, engineers began to develop modems that ran at faster speeds, and the quest for ever-
greater bandwidth has continued ever since. Today, a standard telephone modem operates at
thousands of bits per second. Cable and wireless modems work at more than 1,000,000 bps (one
megabit per second or 1 Mbps), and optical fiber modems function at many Mbps. But the basic
principle of FSK has not changed in more than half a century.
bandwidth
Bandwidth (the width of a band of electromagnetic frequencies) is used to mean (1) how fast data flows
on a given transmission path, and (2), somewhat more technically, the width of the range of frequencies
that an electronic signal occupies on a given transmission medium. Any digital or analog signal has a
bandwidth.
Generally speaking, bandwidth is directly proportional to the amount of data transmitted or received per
unit time. In a qualitative sense, bandwidth is proportional to the complexity of the data for a given level
of system performance. For example, it takes more bandwidth to download a photograph in one second
than it takes to download a page of text in one second. Large sound files, computer programs, and
animated videos require still more bandwidth for acceptable system performance. Virtual reality (VR)
and full-length three-dimensional audio/visual presentations require the most bandwidth of all.
In digital systems, bandwidth is expressed as data speed in bits per second (bps). Thus, a modem that
works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps. In analog systems,
bandwidth is expressed in terms of the difference between the highest-frequency signal component and
the lowest-frequency signal component. frequency is measured in the number of cycles of change per
second, or hertz. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an
analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -- some 2,000
times as wide as the voice signal.
Communications engineers once strove to minimize the bandwidths of all signals, while maintaining a
minimum acceptable level of system performance. This was done for at least two reasons: (1) low-
bandwidth signals are less susceptible to noise interference than high-bandwidth signals; and (2) low-
bandwidth signals allow for a greater number of communications exchanges to take place within a
specified band of frequencies. However, this simple rule no longer applies in general. For example, in
spread spectrum communications, the bandwidths of signals are deliberately expanded. In digital cable
and fiber optic systems, the demand for ever-increasing data speeds outweighs the need for bandwidth
conservation. In the electromagnetic radiation spectrum, there is only so much available bandwidth to
go around, but in hard-wired systems, available bandwidth can literally be constructed without limit by
installing more and more cables.
The speed of...
This table shows the stated data rates for the most important end-user and backbone transmission
technologies.
Technology Speed Physical Medium Application
GSM mobile telephone Mobile telephone for business and
9.6 to 14.4 Kbps RF in space (wireless)
service personal use
High-Speed Circuit-
Mobile telephone for business and
Switched Data service Up to 56 Kbps RF in space (wireless)
personal use
(HSCSD)
Regular telephone
Up to 56 Kbps twisted pair Home and small business access
service (POTS)
Dedicated 56Kbps on Business e-mail with fairly large
56 Kbps Various
frame relay file attachments
The base signal on a channel in the
DS0 64 Kbps All
set of Digital Signal levels
General Packet Radio Mobile telephone for business and
56 to 114 Kbps RF in space (wireless)
System (GPRS) personal use
BRI: 64 Kbps to 128 Kbps
PRI: 23 (T-1) or 30 (E1) BRI: Faster home and small
assignable 64-Kbps BRI: Twisted-pair business access
ISDN
channels plus control PRI: T-1 or E1 line PRI: Medium and large enterprise
channel; up to 1.544 access
Mbps (T-1) or 2.048 (E1)
Faster home and small business
IDSL 128 Kbps Twisted-pair
access
Local area network for Apple
devices; several networks can be
AppleTalk 230.4 Kbps Twisted pair
bridged; non-Apple devices can
also be connected
Enhanced Data GSM Mobile telephone for business and
384 Kbps RF in space (wireless)
Environment (EDGE) personal use
400 Kbps (DirecPC and Faster home and small enterprise
satellite RF in space (wireless)
others) access
Large company backbone for
Twisted-pair or
frame relay 56 Kbps to 1.544 Mbps LANs to ISP
coaxial cable
ISP to Internet infrastructure
Twisted-pair, coaxial
Large company to ISP
DS1/T-1 1.544 Mbps cable, or optical
ISP to Internet infrastructure
fiber
Universal Mobile Mobile telephone for business and
Telecommunications Up to 2 Mbps RF in space (wireless) personal use (available in 2002 or
Service (UMTS) later)
Twisted-pair, coaxial
32-channel European equivalent of
E-carrier 2.048 Mbps cable, or optical
T-1
fiber
Twisted-pair, coaxial Large company to ISP
T-1C (DS1C) 3.152 Mbps
cable, or optical fiber ISP to Internet infrastructure
Twisted-pair, coaxial Second most commonly-used local
IBM token ring/802.5 4 Mbps (also 16 Mbps)
cable, or optical fiber area network after Ethernet
Twisted-pair, coaxial Large company to ISP
DS2/T-2 6.312 Mbps
cable, or optical fiber ISP to Internet infrastructure
Twisted-pair (used as Home, small business, and
Digital Subscriber Line
512 Kbps to 8 Mbps a digital, broadband enterprise access using existing
(DSL)
medium) copper lines
Twisted-pair, coaxial Carries four multiplexed E-1
E-2 8.448 Mbps
cable, or optical fiber signals
Coaxial cable (usually
512 Kbps to 52 Mbps uses Ethernet); in
cable modem (see "Key and some systems, Home, business, school access
explanation" below) telephone used for
upstream requests
10BASE-T (twisted-
pair); 10BASE-2 or -5
Most popular business local area
Ethernet 10 Mbps (coaxial cable);
network (LAN)
10BASE-F (optical
fiber)
Twisted-pair, coaxial Second most commonly-used local
IBM token ring/802.5 16 Mbps (also 4 Mbps)
cable, or optical fiber area network after Ethernet
Twisted-pair or optical
E-3 34.368 Mbps Carries 16 E-l signals
fiber
ISP to Internet infrastructure
DS3/T-3 44.736 Mbps Coaxial cable Smaller links within Internet
infrastructure
ISP to Internet infrastructure
OC-1 51.84 Mbps Optical fiber Smaller links within Internet
infrastructure
Between router hardware and
WAN lines
High-Speed Serial Short-range (50 feet)
Up to 53 Mbps HSSI cable
Interface (HSSI) interconnection between slower
LAN devices and faster WAN
lines
100BASE-T (twisted
pair); 100BASE-T Workstations with 10 Mbps
Fast Ethernet 100 Mbps (twisted pair); Ethernet cards can plug into a Fast
100BASE-T (optical Ethernet LAN
fiber)
Fiber Distributed-Data Large, wide-range LAN usually in
100 Mbps Optical fiber
Interface (FDDI) a large company or a larger ISP
ISP to Internet infrastructure
T-3D (DS3D) 135 Mbps Optical fiber Smaller links within Internet
infrastructure
Carries 4 E3 channels
E-4 139.264 Mbps Optical fiber Up to 1,920 simultaneous voice
conversations
Large company backbone
OC-3/SDH 155.52 Mbps Optical fiber
Internet backbone
Carries 4 E4 channels
E-5 565.148 Mbps Optical fiber Up to 7,680 simultaneous voice
conversations
OC-12/STM-4 622.08 Mbps Optical fiber Internet backbone
Optical fiber (and Workstations/networks with
Gigabit Ethernet 1 Gbps "copper" up to 100 10/100 Mbps Ethernet plug into
meters) Gigabit Ethernet switches
OC-24 1.244 Gbps Optical fiber Internet backbone
2.325 Gbps (15 OC-3
SciNet Optical fiber Part of the vBNS backbone
lines)
OC-48/STM-16 2.488 Gbps Optical fiber Internet backbone
OC-192/STM-64 10 Gbps Optical fiber Backbone
OC-256 13.271 Gbps Optical fiber Backbone
Key and Explanation
We use the U.S. English "Kbps" as the abbreviation for "thousands of bits per second." In
international English outside the U.S., the equivalent usage is "kbits s-1" or "kbits/s".
Engineers use data rate rather than speed, but speed (as in "Why isn't my Web page getting here
faster?") seems more meaningful for the less technically inclined. Many of us tend to think that
the number of bits getting somewhere over a period of time is their speed of travel.
Relative to data transmission, a related term, bandwidth or "capacity," means how wide the pipe
is and how quickly the bits can be sent down the channels in the pipe. (The analogy of multiple
lanes on a superhighway with cars containing speed governors may help. One reason why digital
traffic flows faster than voice traffic on the same copper line is because digital has managed to
convert a one-lane or narrowband highway into a many-lane or broadband highway.)
These "speeds" are aggregate speeds. That is, the data on the multiple signal channels within the
carrier is usually allocated by channel for different uses or among different users.
Key: "T" = T-carrier system in U.S., Canada, and Japan...."DS"= digital signal (that travels on
the T-carrier or E-carrier)..."E" = Equivalent of "T" that uses all 8 bits per channel; used in
countries other than U.S. Canada, and Japan...."OC" = optical carrier (Synchronous Optical
Network)...."STM" = Synchronous Transport Modules (see Synchronous Digital Hierarchy)
Only the most common technologies are shown. "Physical medium" is stated generally and
doesn't specify the classes or numbers of pairs of twisted pair or whether optical fiber is single-
mode or multimode. The effective distance of a technology is not shown. There are published
standards for many of these technologies. Some of these are indicated on pages linked to from
the table.
Cable modem note:The upper limit of 52 Mbps on a cable is to an ISP, not currently to an
individual PC. Most of today's PCs are limited to an internal design that can accomodate no more
than 10 Mbps (although the PCI bus itself carries data at a faster speed). The 52 Mbps cable
channel is subdivided among individual users. Obviously, the faster the channel, the fewer
channels an ISP will require and the lower the cost to support an individual user.