asynchronous link vs synchronous link

Discussion in 'Cisco' started by syuga2012, Mar 24, 2009.

  1. syuga2012

    syuga2012 Guest

    Hi Folks,

    Can somebody please explain the basic differences between a
    synchronous/asynchronous link?

    syuga2012, Mar 24, 2009
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  2. syuga2012

    Thrill5 Guest

    Serial connections use two "states" to transmit and receive traffic (1's and
    0's), usually sent as two different voltage levels on the line (a high and
    low). The receiver samples the line during a "clock cycle" and reads the
    voltage level to determine if it received a high or a low voltage. The
    clock is simply the bit rate. So at 9600 baud, the line is sampled every
    1/9600 of a second. The clock of the receiver must be synchonized with the
    transmitted signal in order to acurately determine if a high or low was
    sent. Why, because if the clock is off by half a cycle and a transition
    occurs between two consecutive states (a high to low) , the receiver can
    misinterpret the received signal as either high or low. The sample during
    the "clock" is not at the "high" level, and its not at the "low" level

    On a synchonous serial link, both sides transmit basically transmit a signal
    all the time and one end of the link provides clock, and the other side
    recovers it from the received signal. On an asynchronous link, each side
    provides their own clock and each side only transmits when it's sending
    data. Because of this, asynchronous links must first send some bits before
    the actual data so that the receive side can "synch" up with the transmitted
    signal. On low speed serial connections it is only a single bit (start
    bit), followed by the data, followed by a "stop bit". On high speed
    asynchronous serial connections (like Ethernet), the "start of frame" is 9
    bits (110101010). Synchonous serial connections require a much more
    sophisticated circuit to receive the data, generally use more sophisticated
    encoding schemes to transmit data, and are more expensive to build and
    implement. Asychronous circuits are simpler to build and cheaper.
    Synchronous lines make better use of the line because start and stop bits
    are not needed. The serial port on a PC is asynchronous and must send 10
    bits on the line for each 8 bits of data, so on a 9600 bit/s asynchronous
    connection only 7,680 of those bits are data (only 80% efficient). On the
    other hand a 9600 bit/s synchonous connection is 100% efficient.
    Thrill5, Mar 24, 2009
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