m the process control system: when a logical node in a TDMA/136 network works in conjunction with an SGSN in GPRS, the TDMA/136-node must con- vert events of the ANSI 41 protocol (the TDMA/136 correspondence to MAP in GSM) to MAP events (and the vice versa). Another example, related to the resource system, is analogue-digital and A/u-law conversion, discussed in Sect. 3.4.2. In addition, protocol conversion is almost always needed in protocol ele- ments that operate between network interfaces belonging to different physical network levels (level 1-2, 2-3, etc.) Protocol conversion functions may be quite complex. The complexity is often because protocols for similar purposes are developed by different organizations and at different points in time. Protocol conversion is discussed in Sect. 5.6.3 5.6.2 Relaying Chains In a relaying chain, protocol elements are configured to be part of a chain. Fig- ure 5.39 shows intuitively how a relaying chain works. Protocol elements that are part of a relaying chain must all include at least a relay- unction (R). Some or all of the elements include functions that can terminate (T) and originate (O) messages that carry communication events2. Integrators exchange messages that (in some way or another) identify where in the chain the event shall be terminated. A proto- col element can contain a number of service elements, which can be individually addressed over the chain by a general functional identifier, here called Fid. Service elements may communicate by events, abstract data or messages, depending on the In AML, a communication event is an instance of an ASP. It defines the action taken by the ini- tiator of the event, or to be taken by the detector of the event. Events in a chain are explicit events if the integrator supports operation handling (OPH). Otherwise they are implicit, mean- ing that terminating and originating units must themselves provide operation handling. 5.6 Relaying in Protocol Networks type of protocol that the integrator supports. If they communicate by e