¹³. Link Access Protocol on the D-channel (LAPD) is an ISDN-standard protocol, which defines the link operation for multiple ”dumb” terminals sharing a common physical link to a ”master” ISDN device. This standard was deemed ideal for the purposes of the CDPD channels with the following extensions: longer and variable-length IDs for the mobiles, Selective REJect capability for more efficient use of the airlink, no parity word because of the strong Reed-Solomon encoding, sleep mode for mobile’s battery life conservation, multicast capability for groups of mobiles.