Serial communication devices and interfaces transmit signals over a point-to-point data link. Bits are transmitted one after the other in a serial stream. Serial links are generally used to connect computers and modems. The stream of bits travels over the serial link to the modem, where it is modulated into an analog signal that is transmitted over a telephone line.
Serial lines are usually bi-directional, with one line transmitting and the other receiving. This is a full-duplex link. A half-duplex link uses one line and only one end can transmit at a given time. The most common Internet protocol that uses serial links is PPP, a link layer protocol designed for simple links such as transmitting packets between equal units. Such links provide full-duplex operation and pass packets in sequence. It is mainly used to send data over a dial-up or leased line to establish a point-to-point connection, making it a **** through solution for simple connections between various hosts, bridges, and routers.
Serial communication is characterized by the transmission of bits of data, the transmission is carried out in bit order, and can be accomplished with a minimum of one transmission line, which is low-cost but slow. The distance of serial communication can be from a few meters to several thousand meters.
A parallel link (such as a parallel printer port on a PC rack) sends data over multiple lines at the same time, and bit transfers occur over eight or more lines simultaneously. Parallel connections have the advantage of high throughput, but distance is limited to avoid bit synchronization problems.
Since data is binary, it can be represented by two states on the line, a positive voltage (usually +5V) representing a binary 1, and a negative voltage (-5V) representing a binary 0. In order to read the bits correctly, some kind of synchronization and timing scheme must be used. Asynchronous communication sets the start and stop bits at the beginning and end of the character being sent, so there is no need to use the same clock for both sides of the communication. the characters are sent frame by frame, with the start bit as the beginning of each frame and the stop bit as the end of the frame. Receiver start bit as a reference, the use of the same frequency with the transmitter of the local local clock can be achieved in a character within the synchronization (character and character is asynchronous), asynchronous communication generally requires the sender and receiver of the same baud rate. Transmission of the start? Stop mode means that the transmission is restarted for each new character, which eliminates any timing differences that may have occurred during the last transmission.
Although asynchronous techniques are simple to use, the start and stop bits are extra overhead and waste bandwidth. The synchronous communication technique can send large blocks of data, say from 1500 to 4096 bytes. It adds a synchronization character at the beginning of each frame. Both communicating parties must use the same clock and divide each N bits into a time slice when transmitting, in which one party sends and one party receives within the same time slice. The receiver treats the digits after the detected synchronization character as actual information. When no information is transmitted, the digits after the synchronization character are filled with empty characters, because synchronous transmission does not allow gaps. Therefore, synchronous communication is synchronous both from character to character and from bit to bit within a character. Ethernet uses the Manchester signal encoding scheme to send clock pulses along with the data.
Other serial technologies include USB (Universal Serial Bus), Firewire ( IEEE 1394), SSA (Serial Storage Architecture), and HSSI (High Speed Serial Interface) HSSI (High Speed Serial Interface). Because of their high data transfer rates, these technologies are being used to connect disks or build small local networks.