 is no traffic. Cell synchronization is based on an error check code in the header. Figure 6.61 Physical layer of ATM. Data Communications addition to SDH the ATM cells can be carried in many other transmission systems such as DS-1 (1.544 Mbps), E-1 (2.048 Mbps), or SONET. Because the cell stream is continuous, there are no explicit indicators for the start and end of a cell. The synchronizing scheme relies on the check code (HEC) in the frame header. The receiving equipment calculates the code over four subsequent bytes as each byte arrives and checks if the next byte corre- sponds to the calculated result. If they are the same, it is likely that a cell header has just been received. If the codes of a few cells match, the synchroni- zation is accepted, and the equipment (ATM system) becomes operational. If more than a defined number of cells arrive with a wrong code, the equipment assumes that synchronization is lost and attempts to reestablish it. 6.8.4 Switching of ATM Cells When an ATM connection is established through the network, the cells of this connection carry a certain VPI that is changed for each link between switching nodes as explained in Section 6.2.4. The circuit identification is divided into two parts and ATM switches can operate at two levels, the virtual path level (a group of virtual channels) or virtual circuit level (see Figure 6.62). Virtual paths act as pipes for a collection of VCs. ATM switches may act at the VP level, the VC level, or both. A VP switch does not look at the VCs within a path, and end systems can freely establish and remove VCs without the network carrying VPs being involved. ATM switch VCI = 32 VCI = 42 VPI=1 VP|=1 VCI = 33 VCI = 43 VCI = 42 VCI = 32 VPI=2 VPI=2 VCI = 43 VCI = 33 VCI = 10 VCI = 10 VPI=3 VPI=3 VCI = 11 VCI = 11 VCI = 25 VPI=4 VCI = 41 VPI=4 VCI = 42 VCI = 27 VPI=5 Figure 6.62 Virtual paths and channels of ATM. Introduction to Telecommunications Network Engineering The routing at the VP level allows the network operator to provide a VPN for a corporation 