d from an 8.448-Mbps (E2) digital link, the following applies: At the multiplex input/output interface, there should be compliance with CCITT Recs. G.703 for electrical characteristics and G.704 for functional characteristics-in particular, the frame structure. The signaling bit rate is 64 kbps. The standard time slots for use of a signaling data link are time slots 67-70 in descending order of priority. When these time slots are not available, any channel time slot available for 64-kbps user transmission rate may be used. No bit inversion is performed (Ref. 5). For North American applications of SS No. 7, Telcordia Notes on SS No. 7 and CCS Network Evolution (Ref. 6) states that data rates from 4.8 to 64 kbps may be used. 14.6 THE SIGNALING LINK LAYER (LAYER 2) This section deals with the transfer of signaling messages over one signaling link directly connecting two signaling points. Signaling messages delivered by upper hierarchical layers are transferred over the signaling link in variable-length signal units. The signal units include transfer control information for proper operation of the signaling link in addition to the signaling information. The signaling link (layer 2) functions include: Signaling unit delimitation Signal unit alignment Error detection Error correction Initial alignment Signal link error monitoring Flow control All of these functions are coordinated by the link state control as shown in Figure 14.4. 14.6.1 Signal Unit Delimitation and Alignment The beginning and end of a signal unit are indicated by a unique 8-bit pattern called the flag. Measures are taken to ensure that the pattern cannot be imitated elsewhere in the unit. Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (i.e., more than six consecutive Is) is received, or when a certain maximum length of signal unit is exceeded. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor. 14.6 THE SIGNALING LINK LAYER (LAYER 2) Signaling link control (layer 2