connector is very large. Real estate on the front of a switch is normally needed for other purposes, such as network ports or even as holes to allow front-to-back airflow. Many times, engineers will select other, smaller connectors than BNC to save real estate on the faceplate. In that case, some form of dongle or convertor cable will be required between the PRC and other equipment. For ex- ample, the 10-MHz input on most Cisco routers is known as a DIN 1.0/2.3 connector that has a push-pull snap on mechanism. Figure 3-18 shows the route switch processor (RSP) from a Cisco router with two golden DIN 1.0/2.3 connectors for 10 MHz, one for input and one for output (highlighted). The other two DIN 1.0/2.3 connectors are for phase; see the following sections "1PPS" and "Time of Day". 10MHZ 10MHZ CONSOLE A900-RSP3C400-S ASR 900 SERIES CISCO Figure 3-18 DIN 1.0/2.3 Connectors for 10-MHz Input and Output There are recommendations specifying the physical and electrical characteristics of the 10-MHz in- terface in clause 20 of the ITU-T Recommendation G.703. 2-MHz Frequency and E1, T1 Legacy Circuits Although the preceding section on 10-MHz frequency mentioned that a PRC/PRS may output a 2- MHz signal and the connector may be BNC, there is another form of this signal. Rather than using a 50-Ohm cable with BNC connectors, an alternative is to use RJ-45 connectors with 120-Ohm E1 cable, which takes up a lot less space on the faceplate. Many routers have a BITS port that will ac- cept this signal using a short run of twisted-pair cable (in Figure 3-18, see the RJ-45 port labeled BITS, second from right). So, the RSP in Figure 3-18 will accept both an input 10-MHz reference signal on the DIN port and/or a 2-MHz signal on the BITS port. It is possible to buy equipment, called a balun, that allows the conversion and impedance matching between the 120-Ohm twisted-pair and the 50-Ohm coaxial versions. Cabling and connector choices are always an issue when working with these signals. SDH/SONET also provides the capability to multi