Frame Relay

Enhanced Definition

Frame Relay is a high-performance, connection-oriented packet-switching wide area network (WAN) protocol that operates at the data link layer (Layer 2) of the OSI model. In the mainframe context, it served as a cost-effective and efficient method for connecting remote sites, branch offices, or other data centers to a central z/OS system, primarily for transporting SNA (Systems Network Architecture) and later TCP/IP traffic.

Key Characteristics

- Packet-Switching Technology: Data is segmented into variable-length frames (packets) and routed across a shared network infrastructure, allowing multiple logical connections to share physical lines.
- Connection-Oriented (Logical Circuits): It establishes virtual circuits, either Permanent Virtual Circuits (PVCs) or Switched Virtual Circuits (SVCs), identified by Data Link Connection Identifiers (DLCIs), over which data flows. PVCs were far more common in mainframe environments.
- High Efficiency and Low Overhead: Compared to its predecessor X.25, Frame Relay has less error checking and flow control at the network edge, relying on higher-layer protocols (like SNA or TCP/IP) for end-to-end error recovery, leading to higher throughput.
- Shared Bandwidth: Multiple customers and multiple DLCIs can share the same physical access line to the Frame Relay network, making it a cost-effective solution for many-to-one or many-to-many connectivity.
- Congestion Control: It includes mechanisms like Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) to inform devices of network congestion, allowing them to reduce transmission rates.
- Committed Information Rate (CIR): Service providers guarantee a minimum bandwidth (CIR) for each DLCI, with the possibility of bursting above the CIR up to an Excess Information Rate (EIR) if network capacity allows.

Use Cases

- Remote Branch Office Connectivity: Connecting numerous remote offices to a central z/OS mainframe for applications like order entry, inventory management, or financial transactions using VTAM and SNA protocols.
- Data Center Interconnect: Providing WAN connectivity between primary and disaster recovery data centers for data replication, backup, and failover scenarios involving z/OS systems.
- Legacy SNA Traffic Transport: Efficiently carrying SNA traffic from remote 3270 terminals or LU 6.2 applications to VTAM on the mainframe over a shared network infrastructure.
- TCP/IP over WAN: As TCP/IP adoption grew on z/OS, Frame Relay was used to extend IP connectivity to remote sites, allowing distributed applications to communicate with mainframe-based DB2 or CICS services.
- Consolidation of Leased Lines: Replacing multiple point-to-point leased lines with a single Frame Relay connection, reducing network complexity and costs for organizations with many remote locations.

Related Concepts

Frame Relay served as a crucial underlying transport for SNA (Systems Network Architecture) traffic, allowing VTAM (Virtual Telecommunications Access Method) to establish sessions between mainframe applications and remote devices or other mainframes. It largely superseded X.25 due to its higher speed and lower overhead. While primarily associated with SNA, it also supported TCP/IP traffic as TCP/IP became dominant on z/OS. Later, Frame Relay itself was largely superseded by MPLS (Multi-Protocol Label Switching) and IP VPNs (Virtual Private Networks) which offered greater flexibility, scalability, and quality of service guarantees for modern enterprise networks connecting to z/OS.

Best Practices:

Proper DLCI Assignment and Management: Ensure a clear and consistent DLCI numbering scheme across the network to simplify configuration and troubleshooting.
CIR and EIR Sizing: Accurately assess application bandwidth requirements to provision appropriate CIR and EIR values for each DLCI to avoid congestion and ensure performance.
Congestion Monitoring: Implement network monitoring tools to track FECN/BECN flags and DE (Discard Eligibility) bits to proactively identify and address potential network congestion issues.
Security Considerations: While Frame Relay itself doesn't provide strong encryption, ensure that sensitive data transported over it is protected by higher-layer security mechanisms, such as IPSec VPNs if carrying IP traffic, or SNA security features.
Migration Planning: For organizations still using Frame Relay, develop a strategic plan for migrating to modern WAN technologies like MPLS or SD-WAN to leverage improved performance, cost -efficiency, and advanced features.