ISDN - Integrated Services Digital Network
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. In the mainframe context, ISDN primarily served as a digital transport mechanism for connecting remote devices, terminals, or smaller branch offices to the central mainframe system, often as an alternative or backup to dedicated leased lines.
Key Characteristics
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- Digital Transmission: Unlike traditional analog telephone lines, ISDN provides end-to-end digital connectivity, offering higher bandwidth and better signal quality for data.
- Circuit-Switched: ISDN establishes a dedicated circuit for the duration of a call, similar to a traditional phone call, which can be beneficial for consistent data rates but less efficient for bursty traffic.
- Bearer (B) and Delta (D) Channels: It typically comprises multiple B-channels (64 kbps each) for carrying user data (voice, video, data) and a D-channel (16 or 64 kbps) for signaling and control information.
- Basic Rate Interface (BRI) and Primary Rate Interface (PRI): BRI offers two B-channels and one D-channel (2B+D), commonly used for small offices. PRI provides 23 B-channels and one D-channel (23B+D) in North America, or 30 B-channels and one D-channel (30B+D) in Europe, suitable for larger connections.
- Dial-up Capability: ISDN lines can be dialed up on demand, providing flexible connectivity options compared to always-on leased lines, making them suitable for backup or intermittent connections.
Use Cases
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- Remote 3270 Terminal Connectivity: Historically, ISDN was used to connect remote IBM
3174or3274cluster controllers, allowing3270terminals at branch offices to access mainframe applications. - SNA Network Backup: Providing a dial-up backup link for critical System Network Architecture (SNA) connections to the mainframe in case of a primary leased line failure.
- Point-to-Point Data Transfer: Establishing temporary, high-speed digital links for batch file transfers or data synchronization between distributed systems and the mainframe.
- Voice/Data Integration for Call Centers: In environments where call centers interacted with mainframe applications, ISDN could facilitate both voice communication and data access to the mainframe over the same physical infrastructure.
- Remote 3270 Terminal Connectivity: Historically, ISDN was used to connect remote IBM
Related Concepts
ISDN functions primarily at the lower layers of the network stack, providing a digital transport medium. It often served as the underlying physical layer for higher-level protocols like SNA (System Network Architecture) or early TCP/IP implementations on the mainframe. It can be seen as an evolution from analog dial-up modems and a precursor to modern MPLS or IP VPN solutions, offering better performance than modems but lacking the flexibility and cost-effectiveness of pure IP-based networks. Mainframe Front-End Processors (FEPs) like the IBM 3745 or 3746 could be configured with ISDN interfaces to manage these connections.
- Evaluate Modern Alternatives: For new deployments or upgrades, prioritize modern IP-based solutions like dedicated fiber, MPLS, or secure VPNs over the internet, which generally offer superior performance, flexibility, and cost-efficiency compared to ISDN.
- Proper FEP/Network Controller Configuration: If ISDN is still in use, ensure that the mainframe's network controllers (e.g.,
VTAMdefinitions,NCPconfigurations on FEPs) are correctly configured for the specific ISDN interface and channels being utilized. - Security for Dial-up Access: Implement robust security measures for any dial-up ISDN connections, including strong authentication, callback mechanisms, and regular auditing, as dial-up points can be potential entry vectors.
- Monitor Link Status: Continuously monitor the status and performance of ISDN links, especially if they serve as critical backup paths, to ensure they are available and functioning correctly when needed.
- Cost Management: Be mindful of call charges associated with ISDN, as it is a circuit-switched technology, and prolonged connections can incur significant costs compared to flat-rate internet services.