Protocols are key components of communications architectures. Architectures are guidelines on how environments connecting two or more devices can be constructed, so most components of a given architecture in a network will be found on each communicating computer in that network. Protocols provide the rules for communications between counterpart components on different devices. However, there is one aspect of protocols that also applies to hardware: whether they are synchronous or asynchronous.
Transmission Protocols (Synch vs. Asynch)
All lower-level data communications protocols fall into one of the two following categories: synchronous or asynchronous. The words themselves are based on Latin roots indicating that they either are in or with time synchronous) or out of or separated from time (asynchronous). The underlying meanings are quite accurate, so long as one understands to what they must be applied.
All data communications depend on precise timing, or clocking.But how does the equipment determine precisely when the middle of a bit time occurs? The answer is clocking; equipment at both ends of a circuit must be synchronized during transmission in order that the receiver and the sender agree regarding beginnings, middles, and ends of bits during transmissions. There are two fundamentally different ways to do this clocking: asynchronously and synchronously.
Simply put, asynchronous transmissions are clocked (or synchronized) one byte at a time. Synchronous transmissions are clocked in groups of bytes. But the differences in how these two approaches work go beyond the differences between individual bytes and groups of bytes. Asynchronous communications is also called start/stop communications and has the following characteristics:
Every byte has added to it one bit signaling the beginning of the byte (the start bit) and at least one bit added at the end of the byte (the stop bits). Bytes with seven data bits typically also include a parity bit, whereas eight-data-bit bytes usually do not. Thus, generally speaking, the total bits actually transmitted for every asynchronous byte equals 10 or 11. To get seven usable data bits, we must transmit approximately 10 to 11, or strictly speaking, we use a 30-35% overhead. This was a special concern when data communications were initially used in the late 50s and early 60s. Then the cost per minute of a dial-up line was $0.60 to $0.65. Using that value, 30 cents of every dollar were spent just to provide the timing for the line. This amount of waste concerned everyone.
This also makes nominal speed calculations for such connections easy: dividing the rated speed of the circuit (e.g., 9600 bits/s) by 10 bits per byte gives a transmission speed in characters per second (e.g., 960 cps). As a rule, we divide the bits/s by 10 to get the nominal speed of an asynchronous circuit. (Nominal here means best case; in the real world, circuits rarely deliver 100% of their nominal capacity. But it’s a starting point for capacity calculations.)
The bytes are sent out without regard to timing of previous and succeeding bytes. That means that none of the components in a circuit ever assume that just because one byte just went by another will follow in any particular period of time. Think of a person banging away on a keyboard. The speed and number of characters sent in a given period does not indicate in any way how many or how quickly characters can be sent in the succeeding similar period.
Clocking is controlled by data terminal equipment (DTE). For example, when a personal computer is used to dial into CompuServe, clocking on bytes going toward the service is generated by the sending PC. That first start bit reaching the modem begins the sequence, with all succeeding bits in the same byte arriving in lockstep at the agreed-upon rate until the stop bit is received. Then clocking stops until the beginning of the next byte arrives. Any intervening devices (especially modems) between the communicating DTEs take the clocking from the data sent by the originating DTE for any given byte.
Most PC and minicomputer terminal communications employ asynchronous techniques. The default communications ports on PCs (the serial or COM ports) only support asynchronous communications. To use synchronous communications on a PC, a special circuit board is required. Synchronous communications have the following characteristics:
Synchronous communications transmit blocks of data rather than individual bytes (characters).
Individual bytes do not have any additional bits added to them on a byte-by-byte basis, except for parity.
However, bytes are sent and clocked in contiguous groups of one or more bytes. Each group is immediately (with no intervening time) preceded by a minimum of two consecutive synchronization bytes (a special character de-eci by the specific synchronous protocol, of which there are many) thatbegin the clocking. A11 succeeding bits in the group are sent in lockstep until the bit of thelast byte is sent, followed (still in lockstep) by an end of block byte.
Clocking is controlled by data communications equipment (DCE). Specifically, on any given circuit one specific DCE component is optioned (i.e., configured) at installation time as the master device. When the circuit is otherwise idle, the master generates the same synchronization character mentioned above on a periodic basis to all other DCE devices in order that all DCE clocks on the circuit are maintained in continuous synchronization.
Except in cases where smaller numbers of bytes (fewer than about 20) are sent at a time, synchronous communications make more efficient use of a circuit. Generally speaking, all circuits running at greater than 2400 bits/s actually operate in synchronous mode over the wire. This is done simply because building modems to reliably operate asynchronously at higher speeds over analog circuits is much more difficult than taking this approach. Asynchronous modems that run faster than 2400 bits/s actually incorporate asynchronous to synchronous converters; they communicate asynchronously to their respective DTEs. This doesn’t normally impact performance: When smaller groups of characters are sent, there is time to include the additional overhead for synchronous transmission. When larger groups are sent, the reduced overhead of synchronous transmission comes into play.
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