High-Speed Serial Interface (HSSI) is a short-distance communications interface that is commonly used to interconnect routing and switching devices on local area networks (LANs) with the higher-speed lines of a wide area network (WAN). HSSI is used between devices that are within fifty feet of each other and achieves data rates up to 52 Mbps. Typically, HSSI is used to connect a LAN router to a T-3 line. HSSI can be used to interconnect devices on token ring and Ethernet LANs with devices that operate at Synchronous Optical Network (SONET) OC-1 speeds or on T-3 lines. HSSI is also used for host-to-host linking, image processing, and disaster recovery applications.
Like ISDN and DSL, HSSI operates at the physical layer of a network, using the standard Open Systems Interconnection (OSI) model. The electrical connection uses a 50-pin connector. The HSSI transmission technology uses differential emitter-coupled logic (ECL). (ECL is a circuit design in which two transistor emitters are connected to a resistor that is switched between the emitters, producing high bit rates.) HSSI uses gapped timing. Gapped timing allows a Data Communications Equipment (DCE) device to control the flow of data being transmitted from a Data Terminating Equipment (DTE) device such as a terminal or computer by adjusting the clock speed or deleting clock impulses.
For diagnosing problems, HSSI offers four loopback tests. The first loopback tests the cable by looping the signal back after it reaches the DTE port. The second and third loopbacks test the line ports of the local DCE and the remote DTE. The fourth tests the DTE's DCE port. HSSI requires two control signals ("DTE available" and "DCE available") before the data circuit is valid.
The HSSI cable uses the same number of pins and wires as a SCSI-2 cable, but uses the HSSI electrical interface. It is not recommended to use a SCSI-2 cable with an HSSI interface.
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- Bay Networks' Overview of WAN Protocols summarizes HSSI and compares it to E-carrier, Integrated Services Digital Network, T-carrier system, and other WAN protocols.