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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 communications implies more than one such wire/strand, in addition to a ground connection.


EXAMPLES OF PARALLEL COMMUNICATIONS SYSTEMS




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 (see Parallel Transmission ). In practice, 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 communications must first be converted back into parallel form by a Universal Asynchronous Receiver Transmitter 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 , Advanced Technology Attachment vs. Serial ATA , 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 Communications send a few bits in parallel. (Each such group of bits is called a " Symbol (data) "). 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.