 the case is with the synchronization status message (SSM) bits for E1/T1 circuits. For E1 to send the SSM bits to signal its quality level, it must be configured as using the framing type of Cyclic Redundancy Check 4 (CRC-4). Other framing will not support SSM. There is a similar situation for T1 circuits, because SSM signaling is only supported when con- figured to use the framing method known as the extended super frame (ESF). The last trap that is easy to fall into is where the devices processing QL information are configured to use incompatible sets of QL values: some using those from the SDH E1 world and others from the SONET T1 world. By default, Cisco routers interpret SSM and QL values according to the ITU- T SDH E1 scheme (Option I), SO engineers in North America should change it immediately to use the scheme appropriate for their own geography (Option II). If the PRC/PRS is putting out a set of SSM values that are aligned with the Option II markets and the router is still at the default of Option I, the node will misinterpret the values it receives and make wrong decisions about which sources it should use. This is a very common mistake-the whole network must be running the same option value for QL scheme (unless you are running inter- national TDM links between North America and other continents). Phase/Time Inputs/Outputs (1PPS, ToD) The phase input signals of 1 pulse per second (PPS) and time of day (ToD) are somewhat distinct and different from each other, although they both work together to transmit phase and time. In its simplest form, the 1PPS is just a simple pulse that cycles at a rate of once per second. On the other hand, the ToD is a method of transmission that carries a representation of the time of day from the PRTC to the router. See "1PPS" and "Time of Day" in Chapter 3 for more details. As seen in Chapter 3, the length of the positive pulse of the 1PPS signal must lie in a range between 100 ns and 0.5 S (according to clause 19.2.1 of ITU-T G.703). This is often configurable, SO the en-