Parallel communications
Encyclopedia
In telecommunication
and computer science
, parallel communication is a method of sending several data signals simultaneously over several parallel channels. It contrasts with serial communication
; this distinction is one way of characterizing a communications link.
The basic difference between a parallel and a serial communication channel is the number of distinct wires or strands at the physical layer
used for simultaneous transmission from a device. Parallel communication implies more than one such wire/strand, in addition to a ground connection. An 8-bit parallel channel transmits eight bits (or a byte
) simultaneously. A serial channel would transmit those bits one at a time. If both operated at the same clock speed, the parallel channel would be eight times faster. A parallel channel will generally have additional control signals such as a clock, to indicate that the data is valid, and possibly other signals for handshaking and directional control of data transmission.
The decreasing cost of integrated circuits, combined with greater consumer demand for speed and cable length, has led to parallel communication links becoming deprecated in favor of serial links; for example, IEEE 1284
printer ports vs. USB, Parallel ATA vs. Serial ATA
, and SCSI
vs. FireWire.
On the other hand, there has been a resurgence of parallel data links in RF
communication. Rather than transmitting one bit at a time (as in Morse code
and BPSK), well-known techniques such as PSM
, PAM
, and Multiple-input multiple-output communication send a few bits in parallel. (Each such group of bits is called a "symbol"). Such techniques can be extended to send an entire byte at once (256-QAM). More recently techniques such as OFDM have been used in Asymmetric Digital Subscriber Line
to transmit over 224 bits in parallel, and in DVB-T
to transmit over 6048 bits in parallel.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
and computer science
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, parallel communication is a method of sending several data signals simultaneously over several parallel channels. It contrasts with serial communication
Serial communication
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
; this distinction is one way of characterizing a communications link.
The basic difference between a parallel and a serial communication channel is the number of distinct wires or strands at the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
used for simultaneous transmission from a device. Parallel communication implies more than one such wire/strand, in addition to a ground connection. An 8-bit parallel channel transmits eight bits (or a byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
) simultaneously. A serial channel would transmit those bits one at a time. If both operated at the same clock speed, the parallel channel would be eight times faster. A parallel channel will generally have additional control signals such as a clock, to indicate that the data is valid, and possibly other signals for handshaking and directional control of data transmission.
Examples of parallel communication systems
- Computer peripheral buses: ISAIndustry Standard ArchitectureIndustry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
, ATAAT AttachmentParallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...
, SCSISCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, PCIPeripheral Component InterconnectConventional PCI is a computer bus for attaching hardware devices in a computer...
and Front side busFront side busA front-side bus is a computer communication interface often used in computers during the 1990s and 2000s.It typically carries data between the central processing unit and a memory controller hub, known as the northbridge....
, and the once-ubiquitous IEEE-1284 / CentronicsCentronicsCentronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered primarily for the parallel interface that bears its name.-The beginning:Centronics began as a division of Wang Laboratories...
"printer port"Parallel portA parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
- Laboratory Instrumentation bus IEEE-488IEEE-488IEEE-488 is a short-range digital communications bus specification. It was created for use with automated test equipment in the late 1960s, and is still in use for that purpose. IEEE-488 was created as HP-IB , and is commonly called GPIB...
- (see more examples at Computer busComputer busIn computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
)
Comparison with serial links
Before the development of high-speed serial technologies, the choice of parallel links over serial links was driven by these factors:- Speed: Superficially, the speed of a parallel data link is equal to the number of bits sent at one time times the bit rateBit rateIn telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
of each individual path; doubling the number of bits sent at once doubles the data rate. In practice, clock skewClock skew-In circuit design:In circuit designs, clock skew is a phenomenon in synchronous circuits in which the clock signal arrives at different components at different times...
reduces the speed of every link to the slowest of all of the links. - Cable length: CrosstalkCrosstalk (electronics)In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel...
creates interference between the parallel lines, and the effect worsens with the length of the communication link. This places an upper limit on the length of a parallel data connection that is usually shorter than a serial connection. - Complexity: Parallel data links are easily implemented in hardware, making them a logical choice. Creating a parallel portParallel portA parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
in a computer system is relatively simple, requiring only a latch to copy data onto a data bus. In contrast, most serial communication must first be converted back into parallel form by a universal asynchronous receiver/transmitterUniversal asynchronous receiver/transmitterA universal asynchronous receiver/transmitter, abbreviated UART , is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial forms. UARTs are commonly used in conjunction with communication standards such as EIA RS-232, RS-422 or...
(UART) before they may be directly connected to a data bus.
The decreasing cost of integrated circuits, combined with greater consumer demand for speed and cable length, has led to parallel communication links becoming deprecated in favor of serial links; for example, IEEE 1284
IEEE 1284
IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices.-History:In the 1970s, Centronics developed the now-familiar printer parallel port that soon became a de facto standard...
printer ports vs. USB, Parallel ATA vs. Serial ATA
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
, and SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
vs. FireWire.
On the other hand, there has been a resurgence of parallel data links in RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
communication. Rather than transmitting one bit at a time (as in Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
and BPSK), well-known techniques such as PSM
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
, PAM
Pulse-amplitude modulation
Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses....
, and Multiple-input multiple-output communication send a few bits in parallel. (Each such group of bits is called a "symbol"). Such techniques can be extended to send an entire byte at once (256-QAM). More recently techniques such as OFDM have been used in Asymmetric Digital Subscriber Line
Asymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
to transmit over 224 bits in parallel, and in DVB-T
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
to transmit over 6048 bits in parallel.
See also
- Data transmissionData transmissionData transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
- serial portSerial portIn computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
- Bit-level parallelismBit-level parallelismBit-level parallelism is a form of parallel computing based on increasing processor word size. From the advent of very-large-scale integration computer chip fabrication technology in the 1970s until about 1986, advancements in computer architecture were done by increasing bit-level...