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19. Timing |
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19. Timing
Timing refers to how the receiving system knows that it received the start of a group of bits and the end of a group of bits. Two major timing schemes are used: Asynchronous and Synchronous Transmission.
- 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.
- Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End frames may be only 2 bytes each.
Packet
Conventional representation has asynchronous data flowing left to right and synchronous data flowing right to left.
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.
Historically, synchronous communications were operating over 2400/4800 baud modems on point-to-point communications, for example: IBM2770/IBM2780/IBM3780 (historical information courtesy of Jacques Sincennes, University of Ottawa)
19a. Asynchronous vs. Synchronous Transmission