over the sig- naling link by means of signal units. There are three types of signal units used in SS No. 7: 1. Message signal unit (MSU) 2. Link status signal unit (LSSU) 3. Fill-in signal unit (FISU) These units are differentiated by means of the length indicator. MSUs are retransmitted in case of error; LSSUs and FISUs are not. The MSU carries signaling information; the LSSU provides link status information; and the FISU is used during the link idle state-it fills in. The signaling information field is variable in length and carries the signaling informa- tion generated by the user part. All other fields are fixed in length. Figure 14.5 illustrates the basic formats of the three types of signal units. As shown in the figure, the mes- sage transfer control information encompasses eight fixed-length fields in the signal unit that contains information for error control and message alignment. These eight fields are described in the following. In Figure 14.5 we start from right to left, which is the direction of transmission. The opening flag indicates the start of a signal unit. The opening flag of one signal unit is normally the closing flag of the previous signal unit. The flag bit pattern is 01111110. The forward sequence number (FSN) is the sequence number of the signal unit in which it is carried. The backward sequence number (BSN) is the sequence number of a signal unit being acknowledged. The value of the FSN is obtained by incrementing (modulo 128) the 14.6 THE SIGNALING LINK LAYER (LAYER 2) First bit 8n, n > 2 transmitted (a) Basic format of a message signal unit (MSU) First bit 8 or 16 transmitted (b) Format of a link status signal unit (LSSU) First bit transmitted (c) Format of a fill-in signal unit (FISU) Backward indicator bit Backward sequence number Check bits Forward indicator bit Forward sequence number Length indicator Number of octets in the SIF Status field Signalling information field Service information octet Figure 14.5 Signal unit formats. [From ITU-T Rec. Q.703, Figure 3/Q.703, page 5 (Ref.