Timing - Asynchronous vs. Synchronous Transmission

This webpage discusses two different methods of transmitting data through the media so that the destination station reliably receives the data. Both the source and destination must be configured for the same method of timing in order to work properly.

Timing refers to the method used by the data communication system so that the destination station recognizes the start of a data stream and reliably reads the information sent. Two major timing schemes are used: Asynchronous Transmission and Synchronous Transmission.

  1. Asynchronous Transmission sends only 1 character at a time. A character being a letter of the alphabet or number or control character. Preceding each character is a Start bit and ending each character is 1 or more Stop bits.

    Note: Conventional representation has asynchronous data flowing left to right and synchronous data flowing right to left.

  2. Synchronous Transmission sends blocks of data at a time. Each block is preceded by a Start Field which is used to tell the receiving station that a new block of data is arriving and to synchronize the receiving station's internal clock. The blocks also have an End Field to indicate the end of the block. The block can contain up to 64 Kbits (note: little k = 1000, big K = 1024). Both Start and End Fields have a special bit sequence that the receiving station recognizes to indicate the start and end of a block. The Start and End fields may be only 2 bytes each.

    Block of data

Asynchronous vs. Synchronous Transmission

Asynchronous transmission is simple and inexpensive to implement. It is used mainly with Serial Ports and dialup connections. Requires start and stop bits for each character - this adds a high overhead to transmission. For example: for every byte of data, add 1 Start Bit and 2 Stop Bits. 11 bits are required to send 8 bits! Asynchronous is used in slow transfer rates typically up to 56 kbps.

Synchronous transmission is more efficient as little as only 4 bytes (3 Start Framing bytes and 1 Stop Framing byte) are required to transmit up to 64 kbits. Synchronous transmission is more difficult and expensive to implement. It is used with all higher comunication transfer rates: Ethernet, Token Ring etc... Synchronous is used in fast transfer rates typically 56 kbps to 100 Mbps.

Example: Compare a 10K Byte data transmission using Asynchronous transmission and Synchronous Transmission. Determine the efficiency (10 kBytes = 80 kbits).

Asynchronous: Add 3 bits (1 Start and 2 Stop bits) for every byte transmitted.

80 kbits + 30 kbits = total of 110 kbits transmitted

Synchronous: Add 4 bytes (32 bits) for the complete 10K byte data packet.

80 kbits + 32 bits = total of 80.032 kbits transmitted

efficiency = data transmitted x 100 = 80 kbits x 100 = 99.9%

Comparison between asynchronous and synchronous data transmission:

Transmission Advantages Disadvantages
Asynchronous Simple and Inexpensive High Overhead
Synchronous Efficient Complex and Expensive

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Copyright July 2013 Eugene Blanchard