 network layer and some of the functions set of certification or conformance tests in order to gain per- specified in the OSI transport layer. The network layer functions mission to connect to their network. This can take a consider- which are not provided are routing and switching. This is because able period of time and money. This problem will be resolved X.25 is a network interface specification and not an internal network to a certain degree in EC countries as the NETs attachment specification as mentioned previously. The transport layer function testing regime becomes law. provided for in the X.25 packet layer is end to end data acknow- ledgement. 40.6.2 Using X.25 40.6.1 The benefits of X.25 X.25 can be used in many different way for communication. Figure 40.12 shows how an X.25 based network can operate with The advantages of using X.25 are as follows: different components such as: 1. Flexibility. A user can make and break communication sessions Asynchronous terminals and hosts communicating via PADs. with many destinations. 2. X.25 based devices connected directly to the network. 2. Economical. A user only pays for the resources used. It is 3. Synchronous devices communicating via protocol converts. possible for a user to establish many concurrent communication 4. Gateways to non-X.25 based networks. sessions with the same or different destinations over one physi- 5. Connection to international networks using the X.75 protocol. cal connection to the network. A network administration can produce a cost reduced system because not every user wishes Note that all devices can communicate through using X.25 as the to communicate at the same time, or continually. common carrier. Reliability. The probability of a fault occurring on a relatively X.25 networks can be bolted on to other X.25 networks simply by short leased line is far less than for a long distance line. If an making one look like an X.25 terminal or by using the X.75 protocol inter-node link fails within the network the communication (a protocol si