 operation specifications, message specifications and encoding algorithms. However, this is rarely the case. Most standardized pro- tocols are described without reference to any logical node model. The protocol is described in terms of bits and message formats only. In this section we will look at an existing radio network interface protocol and take a reverse engineering approach, analysing it primarily with respect to the message-handling model. The purpose of this exercise is to show that all the functions of the message-handling model exist in every protocol, which could be specified accordingly. A secondary purpose is to exemplify the specification method. The selected protocol is EIA/TIA-553 (which we will refer to as EIA553). Today, EIA553 is a part of the ANSI standard IS-136 (revision A and later). EIA553 is a rather old protocol that describes how MS and networks communicate in TDMA/136 systems over an analogue radio access in the 800-MHz band. Proto- cols in radio network interfaces are very complex, which is why it is important to apply some type of structure to the protocol. The EIA553 specification makes no effort in that direction, which makes the specification difficult to understand. No logical network structure model was used to guide the structure of the proto- Neither was any attempt made to identify protocol networks and to separate integrator functions from service elements in the interface. Messages and procedures are described on the bit level only, i.e. no distinction is made between abstract and message protocol levels. No formal modelling and specification methods are used, i.e. everything is described in natural language and intuitive pictures. No standardized encoding is used. Instead, each ASP is individually encoded, SO that as few bits as possible must be transported over the interface (considering 5 Protocol Network Modelling that the total capacity for service and mobility control message exchange within a cell is less than 2 kbps, there was a reason for this approach). 5.5.2 The