OH and ENC functions directly on top of connection-less SCCP. How- ever, the CNL-SCCP service is not a transport layer service in the OSI sense, since it has a restricted SDU size. The maximum size SDU that can be sent over CNL- SCCP is 222 octets (restricted by the SDU size that MTP allows). Since TCAP does not include any segmentation function, the size of ASPs that can be sent over TCAP is even more restricted. As a matter of fact, TCAP is designed for sending multiple ASP instances in a single SCCP invocation, which implies that abstract protocols that run on TCAP are supposed to have very small ASPs (in terms of the number of bits that are created by TCAP). If the TCAP users tries to send more data than the CNL-SCCP allows, TCAP just refuses to send. TCAP applications that are defined by abstract protocols might therefore have to define some rather artificial operations and ASPs. This unfortunate situation could have been avoided by either running TCAP on connection-oriented SCCP (which is not restricted in terms of SDU size), or by extending TCAP with a TAH function. 5 Protocol Network Modelling Types of connectivity service constraints that might motivate a TAH in an inte- grator are: Restricted SDU-size (requires a segmentation function), SDUs out of sequence (CNL services, requires a sequencing function), Unreliable service (requires an error handling function), Lack of flow control (requires a flow control function). Figure 5.32 shows the possible functions of a TAH. In reality, a TAH plays the same role in a logical node as the OSI transport layer plays in the OSI RM. There- fore, TAH-messages include a set of overhead parameters that are similar to those found in the OSI link and transport layer protocols, see e.g. Tanenbaum (1996), Chaps. 3 and 6. Integrator ENCmess sendReq( TAH-SDU) Service properties: recRepCnf( TAH-SDU) Unrestricted SDU size Sequenced service Reliable Segmentation Flow controlled TAHmess Sequencing Error handling Flow control Service properties: sendCnlOrd(...,SDU) Restricted SDU 