When parallel transmission is used a group of bits are sent at the same time?

In data transmission, parallel communication is a method of conveying multiple binary digits (bits) simultaneously using multiple conductors. This contrasts with serial communication, which conveys only a single bit at a time; 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 electrical conductors used at the physical layer to convey bits. Parallel communication implies more than one such conductor. For example, an 8-bit parallel channel will convey eight bits (or a byte) simultaneously, whereas a serial channel would convey those same bits sequentially, one at a time. If both channels operated at the same clock speed, the parallel channel would be eight times faster. A parallel channel may have additional conductors for other signals, such as a clock signal to pace the flow of data, a signal to control the direction of data flow, and handshaking signals.

Parallel communication is and always has been widely used within integrated circuits, in peripheral buses, and in memory devices such as RAM. Computer system buses, on the other hand, have evolved over time: parallel communication was commonly used in earlier system buses, whereas serial communications are prevalent in modern computers.

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 and better performance of integrated circuits has led to serial links being used in favor of parallel links; for example, IEEE 1284 printer ports vs. USB, Parallel ATA vs. Serial ATA, and FireWire or Thunderbolt are now the most common connectors for transferring data from audiovisual (AV) devices such as digital cameras or professional-grade scanners that used to require purchasing a SCSI HBA years ago.

One huge advantage of having fewer wires/pins in a serial cable is the significant reduction in the size, the complexity of the connectors, and the associated costs. Designers of devices such as smartphones benefit from the development of connectors/ports that are small, durable, and still provide adequate performance.

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).

There are two ways to transfer data between computers: Serial Transmission and Parallel Transmission.

Serial Transmission

Data is sent bit by bit from one computer to another in two directions.  Each bit has a clock pulse rate.  Eight bits are transmitted at a time, with a start and stop bit known as a parity bit, which is 0 and 1, respectively.  Data cables are used when transmitting data over a longer distance.  The data cable has D-shaped 9 pin cable that connects the data in series.

Categories of Serial Transmission

Asynchronous transmission – an extra bit is added to each byte to alert the receiver to the arrival of new data.  0 is used as a start bit, while 1 is used as a stop bit.

Synchronous transmission – no extra bit is added to each byte.  Data is transferred in batches, each of which contains multiple bytes.

Parallel Transmission

Several bits are transmitted simultaneously with one clock pulse rate.  It transmits quickly as it utilises several input and output lines for sending the data.

It uses a 25-pin port with 17 signal lines and 8 ground lines.  The 17 signal lines are divided as

• 4 lines – initiate handshaking
• 5 lines – communicate and notify errors
• 8 lines – transfer data

Applications

Serial transmission occurs between two computers, or from a computer to an external device located far away.

Parallel transmission can take place within a computer system, through a computer bus, or to an external device located nearby.

Examples

One example of serial mode transmission is a connection established between a computer and a modem using the RS-232 protocol.  An RS-232 cable can accommodate 25 wires, but only two of these wires are for data transmission; the rest are designated for overhead control signalling.  The two data wires run using the simple serial transmission in either direction.

In this example, a computer may be far from the modem, making parallel transmission very expensive.  With this in mind, the speed of transmission is considered less important when compared to the economic advantage of serial transmission.

An example of parallel mode transmission is a connection established between a computer and a printer.  Most printers are within 6 meters (about 20 feet) from the transmitting computer, and the slight cost for extra wires is offset by the added speed gained through the parallel transmission of data.

Comparison between Serial and Parallel Transmission

Basis for ComparisonSerial TransmissionParallel TransmissionDefinitionData flows in 2 directions, bit by bitData flows in multiple directions, 8 bits (1 byte) at a timeCostEconomicalExpensiveNumber of bits transferred per clock pulse1 bit8 bits or 1 byteSpeedSlowFastApplicationsUsed for long-distance communicationUsed for short-distance communicationExampleComputer to computerComputer to printer

Differences between Serial and Parallel Transmission

• Serial transmission requires a single line to send data. Parallel transmission requires multiple lines to send data.
• There are fewer errors and less noise in serial transmission, since the transmission is done one bit at a time. There are more errors and noise in parallel transmission, since the transmission is done multiple bits at a time.
• Serial transmission is slower since data flows through a single line; conversely, parallel transmission is faster since data flows through multiple lines.
• Serial transmission is ‘full duplex’ since the sender can send and receive data at the same time. Parallel transmission is ‘half duplex’ since the data can be sent or received at any given time.
• The cables used in serial transmission are thinner, longer, and more economical compared to the cables used in parallel transmission.
• Serial transmission is reliable and straightforward. Parallel transmission is unreliable and complicated.

Both serial and parallel transmissions have advantages and disadvantages.  Parallel transmission is used for shorter distances and provides greater speed,  while serial transmission is reliable for transferring data over longer distances.  Both serial and parallel transmissions are individually essential for transferring data.

Which supports transmission of multiple bits of data at the same time?

How many bits are normally transmitted at any one time using parallel data transmission?

In which mode of data transmission are data bits sent on network simultaneously?

Where is parallel data transmission used?