 discrimination based on SAPI is rather easy, since it does not require any extra protocol functions between logical nodes. There are, however, considerable drawbacks. The method might be costly, since each SAP is normally related to an account for which the user (i.e. the logical node owner) is charged. Within a logical network, logical nodes may prefer to call each other by names instead of SAPIs. For example, using email addresses is considerably more user- friendly than using an IP address. More important, since discrimination is based on SAPs, reallocating a logical node to another SAP requires that other logical nodes in the same logical net- work must be made aware of the new SAPIs of their former communication partner. The method cannot support temporary existence of service elements, since there must always be somebody answering on every SAP. A logical node that handles network transactions, e.g. call control of hundreds of simultaneous calls, cannot use SAPI for discrimination. It must implement discrimination between calls as part of the logical node protocol instead. In the model to the right in Fig. 5.15, discrimination is based on using a CON- CSP. Level 1 discrimination is based on the CEI value that is delivered by the CSP. This method eliminates most of the drawbacks of SAPI discrimination. The service elements have a temporary existence, since a service element only exists in the DC-plane if it has at least one connection to another service element. This tech- niques does, however, introduce other drawbacks: service elements must be instances of the same type, unless a special association mechanism (see Sect. 5.3.4) is used when the connection is established. We saw example of that in the ISDN service establishment scenario, see Sect. 3.3.1 and Fig. 3.30. The connection setup procedure that is required also introduces an extra delay. The last type of discrimination qualifier is the message type. Open, standardized CSPs do not support discrimination based on message types. If message type dis- cr