There are three different types of information that the calling ISDN user may specify during call set-up to identify low layer capabilities needed in the network and by the destination terminal:
a) type I information is information about the calling terminal which is only used at the destination end to allow a decision regarding terminal compatibility. An example would be modem type. This information is encoded in octets 5 to 7 of the Low layer compatibility information element.
b) type II information is the selection of bearer service from the choices of bearer services offered by the network to which the calling user is connected. This type of information is present even if no interworking occurs. An example is unrestricted digital information (UDI). This information is coded in
i) octets 3 and 4 of the Bearer capability information element when the transfer mode required by the calling user is circuit mode;
ii) octets 3, 4, 6 and 7 of the Bearer capability information element when the transfer mode required by the calling user is packet mode.
c) type III information is information about the terminal or intended call which is used to decide destination terminal compatibility and possibly to facilitate interworking with other ISDNs or other dedicated networks. An example is A-law encoding.This information is encoded in octet 5 of the Bearer capability information element.