Introducing ISDN

ISDN stands for Integrated Services Digital Network, meaning a public network composed of digital telephony and data-transport services; these services are offered by regional telephone carriers.

The main difference between PSTN and ISDN involves the totally digital approach to the telephone network, which allows voice, data, text, graphics, music, video, and other source material to be transmitted over the already existing telephone wires. ISDN applications include high-speed image applications (e.g., the well-known Group IV facsimile), additional telephone lines in homes to serve the telecommunication industry, high-speed file transfer, and videoconferencing.

From the penetration tester's point of view, ISDN is pretty nice since it allows incredibly fast phone scanning (less than one second), in order to find which telephone numbers are active or not, avoiding having to wardial many phone numbers and then discovering that 50 percent of them are not active.

Also, value-added services (VAS) such as toll-free numbers offer the penetration tester clues. To give you a very nice example, an 800 phone number (e.g., 800-123-4567) does not really exist in the telephone network; instead it's an alias: Every 800 corresponds to a real phone number, such as 212-123-4567, which is assigned a flag in the phone carrier's database, specifying the phone number itself as a toll-free number to avoid billing the caller, and assigning the bill instead to the receiving party.

This means that—depending on the toll-free number configuration and the number of PRI lines—you could obtain the real phone number and then scan around it in order to locate answering modems, like Enrique did in the story at the beginning of this chapter.

The ISDN world offers two different types of services:

• ISDN BRI Service ISDN Basic Rate Interface (BRI) Service is the ISDN wall-plugged adapter in homes or small offices. This service offers two B channels and one D channel (2B+D). The BRI B-channel service operates at 64 kbps and is meant to carry user data; the BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances (X.25 over D-channel; see "RFC 1356—Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode"). The D-channel signaling protocol comprises Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps. The BRI physical layer specification is the International Telecommunication Union-Telecommunications Standards Section (ITU-T) I.430 (the ITU was formerly the Consultative Committee for International Telegraph and Telephone [CCITT]).

• ISDN PRI Service ISDN Primary Rate Interface (PRI) Service offers 23 B channels and 1 D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI-D channel runs at 64 kbps). In Europe, Australia, and other countries, ISDN PRI provides 30 B channels plus 1 (or 2) 64-kbps

D channel, with a total interface rate of 2.048 Mbps (a 2-Mbits line). The PRI physical layer specification is ITU-T I.431.

In the corporate world, ISDN is mainly used for two focused and specific assets:

• PBXs ISDN PRI lines are generally connected to the company's PBX in order to manage the incoming and outgoing voice communications easily.

• Backup ISDN lines When referring to backup ISDN lines, we mean ISDN BRI lines, usually connected to Cisco boxes and properly configured to set up an ISDN data connection to the ISP, should the main Internet link fail. In this last case, the penetration tester can discover previously unknown ISDN-related information by examining the ISDN configuration and logs of the Cisco box itself.

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