ported or produced by the equipment manufacturers. However, transitioning their customer base across to a different transport technology would be a disruptive and expensive migration project. Not only that, but much of the equipment in their customer sites may only support E1/T1 type TDM circuits. Examples are voice circuits on a PABX, trunked digital radios for emergency services, and signaling equipment on a railway track. Changing to non-TDM data circuits would require customers to migrate their equipment as well. The solution is to emulate the traditional TDM circuit using packet technology and IP/MPLS. From the view of an end user, the legacy edge equipment remains in service, although it is now connected across the core IP/MPLS network. The underlying E1/T1 circuit is terminated on a special router and the transport is replaced with a packet-based solution, as shown in Figure 2-9. This technique is known as circuit emulation (CEM). TDM attachment circuits IP/MPLS E1/T1 packet network E1/T1 Same frequency Figure 2-9 Circuit Emulation Replacing TDM Circuits Circuit emulation uses a packet router with specialized TDM interface modules and a dedicated pro- cessor on those cards to perform the packetization and depacketization steps between TDM and packet. The legacy connection, known as an attachment circuit (AC), is connected to the TDM in- terface module and the TDM circuit comes up normally. The onboard processor reads the incoming data stream and every few milliseconds sends that data off in an IP/MPLS packet to the remote CEM router. At the other end, the data from the IP/MPLS packet is pushed into a framer and played out down the AC at that end. Of course, this is a duplex process, SO transmission and reception both happen simultaneously at each end of the circuit. From the preceding discussion, you know that TDM circuits need frequency synchronization to en- sure that the receiver and transmitter are processing bits at the same rate. Now that we emulate the formerly synchronous circuit, the packet networ