Data Communications Networking Devices - 4th Ed.pdf

734 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTwhen access between the PC and the protocol converter is via a packet network. Theadditional cost results from the packet network forming packets containing singlecharacters. Since most packet networks have a charge component based upon thenumber of packets carried, the use of a character mode emulator can become veryexpensive. Concerning delay time, since the character has to transmit the packetnetworkto the protocol converter and then be echoed backto the personalcomputer, you can experience delays of up to 0.5 s from the time you type acharacter until it is displayed on your screen. This large delay results from eachpacket network node checking packets prior to forwarding them through thenetwork, a condition that now occurs in each direction on single character packets.In comparison, a blockmode emulator program used on your personal computerwould immediately echo characters to your screen and only transmit data to theprotocol converter when a PA, PF or Enter key was pressed. Therefore, you shouldalways attempt to use a blockmode emulator to access a protocol converter via apacket network.ApplicationsThe number of applications to which protocol converters can be employed islimited only by your imagination, such is the diversity of the devices. Figure 6.78illustrates two popular application areas where protocol converters are typicallyemployed.In the top part of Figure 6.78, a protocol converter functioning as an asynchronous/synchronousconverter is employed to permit an asynchronous terminal to beFigure 6.78 Typical protocol converter applications.Protocol converters can be used forasynchronous to synchronous conversion top) or to make asynchronous personalcomputers that transmit data on a line-by-line format compatible with a screen-orienteddisplay protocol bottom)