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what synchronous system
Synchronous means "agreed timing for the sending of ones and zeroes (bits)"--that is, the transmit and receive sides of the communications circuit have bothered to coordinate (synchronize) their signal and have agreed just what a digital bit encoded into the signal looks like. All communications paths have carrier signals, the signals have a frequency, and encoding bits into the signal involves spacing them out at regular intervals, and carving out just how long it takes to transmit a bit.

Synchronous systems negotiate the communication parameters at the data link layer before communication begins. Basic synchronous systems will synchronize the signal clocks on both sides before transmission begins, reset their numeric counters and take other steps. More advanced systems may negotiate things like error correction and compression.

It is possible to have both sides try to synchronize the connection at the same time. Usually, there is a process to decide which end should be in control. Both sides in synchronous communication can go through a lengthy negotiation cycle where they exchange communications parameters and status information. With a lengthy connection establishment process, a synchronous system using an unreliable physical connection will spend a great deal of time in negotiating, but not in actual data transfer. Once a connection is established, the transmitter sends out a signal, and the receiver sends back data regarding that transmission, and what it received. This connection negotiation process takes longer on low error-rate lines, but is highly efficient in systems where the transmission medium itself .


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