The ISDN system includes three distinct interface types.

1. For the ISDN dial-up connection a B-channel is provided for an ISDN service through the dial-up procedure to any ISDN subscriber. In the same way, access can be obtained to other networks (an analogue telephone system, mobile cellular phones, X.25).
2. Digital fixed lines provide permanently switched digital 64-kbit/s channels. A maximum number of 30 B channels (PRI) are available per connection. These dedicated fixed lines can be used both with and without a control channel.X.25 via ISDN
3. A service implemented at the beginning of 1995 in Euro-ISDN has opened the possibility of transporting X.25 packets in ISDN. The integration of X.25 into ISDN is standardised in the CCIT X.31 guideline case D and case B. Both channel D and channel B access is provided for base connections. In contrast, with the primary rate interface only channel D access is supported.
   Packets are transmitted in X.25 format. ISDN only provides the functions strictly related to transportation, in other words data packets are not switched. The latter occurs in the switching nodes of the corresponding X.25 network. The transition from ISDN to the X.25 network takes place in the so-called ISDN switching centres, which are equipped with special packet handlers (PH) and frame handlers (FH). The particular X.25 network used and the functions to be provided are negotiated with the X.25 operating company and stored as an individual user profile in the packet handler under a specific ISDN dial-up number.

The physical connection is established via an SP interface (special interface for packet data). This interface, operating in accordance with the X.75 standard, is able to transmit data in both directions (full duplex mode) at 64 kbits/s. Each SP interface can manage up to 350 virtual connections simultaneously. Connections between two Euro-ISDN interfaces with corresponding performance characteristics and connections from and to X.25 interfaces can be established via the SP interface.

Data is transferred in Channel D at a rate of 9.6 kbits/s and in channel B at a rate of 64 kbits/s. According to the difference in bandwidth, the costs for the B channel are correspondingly higher.

This service can only be used if corresponding end devices are able to perform the function of an X.25-PAD ( P acket A ssembler D isassembler). A PAD is a device, which adapts ISDN hardware or asynchronous end devices, such as PCs with Modems, to the X.25 network operating synchronously switching packets. Nowadays, this function is built into most universal remote bridge/router. The benefit of the X.25 service is that according to the application, costs are saved and the ISDN interface can also be used for other applications. In addition, communication is also possible with locations in countries in which ISDN is not yet widely available and only an X.25 network is provided.