 7).] last assigned value by 1. The FSN value uniquely identifies a message signal unit until its delivery is accepted without errors and in correct sequence by the receiving terminal. The FSN of a signal unit other than an MSU assumes the value of the FSN of the last transmitted MSU. The maximum capacity of sequence numbers is 127 message units before reset (modulo 128) (i.e., a 7-bit binary sequence, 27 = 128 - 1). Positive acknowledgment is accomplished when a receiving terminal acknowledges the acceptance of one or more MSUs by assigning an FSN value of the latest accepted MSU to the BSN of the next signal unit sent in the opposite direction. The BSNs of subsequent signal units retain this value until a further MSU is acknowledged, which will cause a change in the BSN sent. The acknowledgment to an accepted MSU also represents an acknowledgment to all, if any, previously accepted, though not yet acknowledged, MSUs. Negative acknowledgment is accomplished by inverting the backward indicator bit (BIB) value of the signal unit transmitted. The BIB value is maintained in subsequently sent signal units until a new negative acknowledgment is to be sent. The BSN assumes the value of the FSN of the last accepted signal unit. As we can now discern, the forward indicator bit (FIB) and the backward-indicator bit together with the FSN and BSN are used in the basic error-control method to perform signal unit sequence control and acknowledgment functions. The length indicator (LI) is used to indicate the number of octets following the length indicator octet and preceding the check bits and is a binary number in the range of 0-63. The length indicator differentiates between three types of signal units as follows: Length indicator = 0 Fill-in signal unit Length indicator = 1 or 2 Link status signal unit Length indicator > 2 Message signal unit CCITT SIGNALING SYSTEM NO. 7 Table 14.1 Three-Bit Link Status Indications Status Indication Meaning Out of alignment Normal alignment Emergency alignment Out of service Processor outag