Data Communications Networking Devices - 4th Ed.pdf

148 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSTable 1.34Unnumbered command/responseDesignation Abbreviation Command ResponseDisconnect DISC Disconnect modeDMFrame rejectFRMRRequest initialization RIM Reset RSET Set asynchronous balanced mode SABM Set asynchronous balanced mode extended SABME Set asynchronous response mode SARM Set asynchronous response mode extended SARME Set initialization modeSIMSet normal response mode SNRM Set normal response mode extended SNRME Test TEST Unnumbered acknowledgement UA Unnumbered information UI Unnumbered poll UP Exchange identi®cation XID Operational modesHDLC support three operational modesÐnormal response,asynchronous response,andasynchronous response balanced. In a normal response mode,a secondarystation can only initiate transmission after receiving explicit permission from acontrolling primary station. This operational mode is best suited for multipointoperations.The asynchronous response mode is only applicable when there is one secondarystation under primary control. In this operational mode,the secondary station caninitiate transmission without having to receive explicit permission from a primarystation.The third operational mode supported by HDLC is asynchronous balanced.This operational mode enables the symmetrical transfer of data between two`combined' stations on a point-to-point circuit. Here,each station has the ability toinitialize and disconnect the circuit and is responsible for both controlling its owndata ¯ow and for recovering from error conditions. This operational mode iscommonly used in packet switching. Figure 1.85 illustrates the difference betweenbalanced and unbalanced operational modes.To illustrate the advantages of HDLC over BISYNC transmission,consider thefull-duplex data transfer illustrated in Figure 1.86. For each frame transmitted,this ®gure shows the type of frame, NS), NR) and poll/®nal P/F) bit status.In the transmission sequence illustrated in the left part of Figure 1.86,theprimary station has transmitted ®ve frames,numbered zero through four,when itspoll bit is set in frame four. This poll bit is interpreted by the secondary station as arequest for it to transmit its status and it responds by transmitting a receiver readyRR) response,indicating that it expects to receive frame ®ve next. This serves asan indicator to the primary station that frames zero through four were receivedcorrectly. The secondary station sets its poll/®nal bit as a ®nal bit to indicate to theprimary station that its transmission is completed.