e others will receive SSU/BITS. 6.6.1 Synchronization Status Message To achieve traceability, the physical interfaces, in addition to carrying timing information, also need to carry a clock quality level (QL) indicator from the master to the slave node. This is achieved via the synchronization status message (SSM) signals for the SDH/SONET and T1/E1 physical inter- faces. SDH/SONET frames have an overhead frame, which is not considered as part of regular data or protocol data unit (PDU), that carries extra information from one end to the other. This overhead frame is used to carry SSM information. These signals (sometimes called "messages," though in networking terminology these are not really messages) are defined in ANSI T1.105 and ITU-T G.781. They carry the quality level of the clock signal throughout the network to deliver end-to-end clock traceability. The exact mechanism for transporting of SSM is as follows: For optical lines, there are two S bytes in the overhead frame for both SDH and SONET, and the SSM is transported in the S1 byte of this overhead frame. If the SSM is carried over an electrical line: For SDH, the SSM is transported in the Sa bits of the E1. For SONET, the SSM is transported out-of-band in an extended super frame (ESF) data link, in addition to transmitting the in-band signal in the payload. It is important to note that ITU-T specifications classify SDH and packet networks by means of Op- tions I, II, and III processing. Option I applies to SDH networks optimized for the 2048-kbps hier- archy. Option II applies to SDH networks optimized for the 1544-kbps hierarchy that includes the rates 1544 kbps, 6312 kbps, and 44,736 kbps. Option III applies to SDH networks optimized for the 1544-kbps hierarchy that includes the rates 1544 kbps, 6312 kbps, 33,064 kbps, 44,736 kbps, and 97,728 kbps. For each of these different options, ITU-T G.781 also specifies the following clock source quality levels: Option I synchronization networking: The clock source quality levels that are defined under Optio