As currently there are still 4 free bits in TP-PI, it can be expected that the extension bit will be zero even in the future, which helps to distinguish TP-PI field from TP-FCS field when information whether TPDU is part of positive or negative response is not available : if the most significant bit of the second octet of TPDU is 1, the second octet is TP-FCS ( in a negative response ), otherwise it is TP-PI ( in a positive response ).
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