Data Communications Networking Devices - 4th Ed.pdf

150 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSNote that since full-duplex transmission is permissible under HDLC,theprimary station continues to transmit information I) frames while the secondarystation is responding to the primary's polls. If an 8-bit control ®eld is used,themaximum frame number that can be outstanding is limited to seven since 3 bitpositions are used for NS) frame numbering. Thus,after frame number seven hasbeen transmitted,the primary station then begins frame numbering again at NS)equal to zero. Notice that when the primary station sets its poll bit when transmittingframe seven the secondary station responds,indicating that it expects toreceive frame zero. This indicates to the primary station that frames ®ve to sevenwere received correctly,since the previous secondary response acknowledge frameszero to four.In the transmission sequence indicated on the right-hand side of Figure 1.86,assume a line hit occurs during the transmission of frame two. Note that incomparison to BISYNC,under HDLC the transmitting station does not have towait for an acknowledgement of the previously transmitted data block; and it canTable 1.35 Protocol characteristics comparisonFeature BISYNC DDCMP SDLC HDLCFull duplex No Yes Yes YesHalf duplex Yes Yes Yes YesMessage format Variable Fixed Fixed FixedLink control Control character, Header Control ®eld Control ®eldcharacter ®xed) 8 bits) 8/16 bits)sequences,optional headerStation addressing Header Header Address ®eld Address ®eldError checking Information ®eld Header Entire frame Entire frameonlyinformation®eldError detection VRC/LRC-8 CRC-16 CRC-ITU CRC-ITUVRC/CRC-16Request for Stop and wait Go back N Go back N Go back N,retransmissionselectedrejectMaximum frames 1 255 7 127outstandingFramingÐstart 2 SYNs 2SYNs Flag FlagÐend Terminating Count Flag FlagcharactersInformationtransparency Transparent Inherent Inherent Inherentmode count) zero zeroinsertion/deletion)Control character Numerous SOH, DLE, None NoneENQCharacter codes ASCII ASCII Any AnyEBCDICTranscode SBT)controlcharacteronly)insertion/deletion