FDDI & HDLC: Legacy Network Protocols
Fiber Distributed Data Interface (FDDI)
Overview
FDDI is a high-speed networking standard primarily used for creating LANs. Developed in the 1980s by ANSI, it offered improvements over existing LAN technologies like Ethernet, especially in speed, reliability, and distance capabilities.
Key Aspects of FDDI
- Topology: FDDI networks typically use a dual-ring topology for redundancy. One ring is primary; the other is a backup.
- Media: FDDI operates over optical fiber, providing high bandwidth and resistance to EMI and RFI.
- Speed: FDDI supports data rates up to 100 Mbps.
- Token Passing: FDDI uses token passing to regulate network access, ensuring fair access and preventing collisions.
- Fault Tolerance: FDDI is designed for high reliability. In case of failure, the network reroutes traffic through the redundant ring.
- Standards: FDDI is standardized by ISO and ANSI under ANSI X3T9.5.
- Applications: FDDI was commonly used in the 1990s for backbone networks, but has become less common with the rise of Ethernet.
High-Level Data Link Control (HDLC)
Overview
HDLC is a bit-oriented synchronous protocol operating at the data link layer (Layer 2) of the OSI model. Developed by ISO, it’s used in both WANs and LANs.
Key Aspects of HDLC
- Frame Structure: HDLC frames have header, data, and trailer fields. The trailer contains an FCS for error checking.
- Modes of Operation:
- Normal Response Mode (NRM): One device is the primary sender, the other the receiver.
- Asynchronous Balanced Mode (ABM): Both devices can send and receive interchangeably.
- Asynchronous Response Mode (ARM): Similar to NRM, but the secondary can initiate transmissions after receiving permission.
- Flow Control: HDLC supports flow control to manage data flow and prevent data loss.
- Error Detection and Correction: HDLC uses error detection (FCS) to ensure data integrity.
- Variants: Several HDLC variants exist, including Cisco HDLC and LAPB.
- Applications: HDLC is used in PPP, Frame Relay, and ISDN.