An Operating Systems Vade Mecum

Device hardware 153tracksdirections.read/write headsFigure 5.3 Structure of a drumHardware characteristics.Whether the communication line is connectedto a terminal or to another computer, certain characteristics are important. The first is thespeed at which information can be transferred. Speed is usually measured in baud,which, strictly speaking, is the number of electrical transitions per second. These may betransitions between voltage levels or waveforms. Usually the baud rate is equivalent tobits per second. Commonly available baud rates for terminals are 110 (10 cps), 300 (30cps), 1200 (120 cps), 9600 (960 cps), and 19200 (1920 cps). The abbreviation ‘‘cps’’stands for ‘‘characters per second.’’ In the standard ASCII character code, a characterrequires 7 bits for data. Additional bits are used to make sure that the sender and thereceiver agree which bits are the first in a character. Inter-computer communication linescan be much faster. High-quality telephone lines can transmit data at 56K baud. TheEthernet, a multiple-access communication line, runs at 3M to 10M baud, and someexperimental lines run at 100M baud. We will discuss computers linked by such linesinto networks in Chapter 9.No matter what form of transmission is used, all parties to the conversation mustagree on conventions for formatting information, acquiring the rights to use the transmissionmedium, and interpreting messages. These conventions are called a protocol.There are thousands of protocols serving hundreds of purposes. We will glance at only afew.Synchronous transmission.‘‘Synchronous’’ comes from Greek rootsmeaning ‘‘at the same time.’’ The essential feature of synchronous transmission is thatboth the sender and the receiver share a common clock. Bits are converted by the senderinto voltages, which are sent across the communication line and are sampled at theappropriate rate by the receiver. If they both use the same clock, the receiver knowswhen to sample the voltage — namely, at the center of the time during which each bit istransmitted. (If it samples too near the end, it may be confused by the transition between