Data Communications Networking Devices - 4th Ed.pdf

6.5 PACKET ASSEMBLER/DISASSEMBLER __________________________________________ 691ENQ; 32Ðno echo of characters ACK, NAK, STX, SOA, EOT, ETB, and ETX;64Ðno echo of characters de®ned by parameters 16, 17, and 18; 128Ðno echo of allother characters in columns 0 and 1 of International Alphabet Number 5 and thecharacter DEL.In addition to supporting asynchronous terminals a variety of PADs aremarketed to support vendor-speci®c protocols. Most PADs in this category supportIBM bisynchronous or SNA/SDLC terminals, although a few vendors manufactureequipment to support other vendor-speci®c protocols.The physical location of the PAD can be in one of three places. First, the PADcan reside on an X.25 networknode, with terminals required to access the node viathe PSTN or by leased lines. This is the most common PAD location, since themajority of persons accessing packet networks use asynchronous terminals and diala node via the PSTN. Secondly, a stand-alone PAD can be installed at the enduser'sterminal or host computer location as previously illustrated in Figure 6.43.The use of this type of PAD permits the multiplexed X.25 protocol to be routed onone circuit between the packet node and the end-user location, in effect serving tominimize the communications cost of the end-user. The third PAD location isinside a computer or terminal device. This type of PAD is normally a specialadapter card that can be installed within a personal computer or on the channeladapter of a front-end processor. Most PADs built on an adapter card permit thepersonal computer user to establish simultaneous communications with two ormore remote computers on one X.25 connection as illustrated in Figure 6.44. Thiscapability permits the user to switch from one host session to another withoutrequiring the log-off from one computer system prior to signing onto a secondsystem.As previously discussed when we examined packet switching networks earlier inthis book, the use of X.25 networks results in error checking at each node in thenetwork. This results in cumulative delays or latency when data is routed throughmultiple nodes, and makes X.25 performance detrimental for linking LANs.Although X.25 networks remain viable for transporting email, mainframe, andFigure 6.44PC based PAD utilization