Parallel communication

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.

Examples of parallel communication systems

• Computer peripheral buses: ISA, ATA, SCSI, PCI and Front side bus, and the once-ubiquitous IEEE-1284 / Centronics "printer port"

• Laboratory Instrumentation bus IEEE-488

• (see more examples at Computer bus)

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 rate of each individual path; doubling the number of bits sent at once doubles the data rate. In practice, clock skew reduces the speed of every link to the slowest of all of the links.
• Cable length: Crosstalk 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 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/transmitter (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 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.

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

See also

• Data transmission
• serial port
• Bit-level parallelism