Data Communications Networking Devices - 4th Ed.pdf

440 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.47 Rationale for ¯ow control. When the modem achieves a 2:1 compressionratio, data entering the device at 57 600 bps is immediately placed on the line at 28 800 bps.When the compression ratio is under 2:1 data cannot exit the modem as fast as it enters themodem and is placed into a buffer area. To prevent the buffer from over¯owing, the modemrequires a mechanism to stop the terminal from sending additional dataÐa process knownas ¯ow controlAs long as the modem can compress two characters into one, the terminal devicecan keep on transferring data to the modem at 57 600 bps. Suppose, however, aportion of the data entering the modem cannot be compressed or is compressible ata ratio less than 2:1. Either situation would cause data ¯owing into the modem to belost. To prevent a potential data loss, modems that compress data or perform otherfunctions, including error detection and correction, will include data buffers astemporary storage locations to compensate for the difference between the data ¯owinto the modem and the rate at which the modem can process and transmit data.Since data buffers represent a ®nite amount of storage, a modem that accepts dataat one rate and transfers it at a different rate must be able to control the ¯ow of datainto its buffers. Known as ¯ow control, this technique is used to prevent modembuffers from over¯owing and losing data.To illustrate how modem ¯ow control operates, consider Figure 4.48 whichillustrates a modem's buffer storage area. As data enters the modem from anattached terminal device at a faster rate than it can be placed on the line, the bufferbegins to ®ll. When the occupancy of the buffer reaches a prede®ned high levelH L ) the modem initiates ¯ow control to inhibit additional data from ¯owing intoits buffer. Otherwise, the buffer could continue to ®ll until data eventually cannotbe stored and is then lost. As data ¯ows out of the buffer and is modulated andplaced on the line, the point of low occupancy is reached L L ). When this level ofoccupancy is reached the modem uses ¯ow control to enable transmission to resumeto the modem. Now that we have reviewed why ¯ow control is used in modems, letus examine the methods by which it is implemented.Figure 4.48 Flow control and buffer utilization. H L = high level to disable data ¯ow, L L =low level to enable data ¯ow