Mainframe Storage Networking: ESCON and FICON

The Bus-and-Tag parallel channel architecture was once the primary storage interface used by IBM mainframes. As mainframes evolved, the ESCON architecture broadly replaced Bus-and-Tag. ESCON is now being replaced by the FICON architecture to avail mainframes of the cost/performance benefits enabled by mainstream adoption of Fibre Channel.

ESCON

ESCON is a proprietary IBM technology introduced in 1990 to overcome the limitations of the Bus-and-Tag parallel channel architecture. ESCON converters were made available to preserve Bus-and-Tag investments by bridging between the two architectures. Today, very little Bus-and-Tag remains in the market. Whereas Bus-and-Tag employs copper cabling and parallel transmission, the ESCON architecture employs optical cabling and serial transmission. The ESCON architecture is roughly equivalent to Layers 1 and 2 of the Open Systems Interconnection (OSI) reference model published by the International Organization for Standardization. We discuss the OSI reference model in detail in Chapter 2, "OSI Reference Model Versus Other Network Models."

In addition to transporting channel protocol (SBCCS) frames, ESCON defines a new frame type at the link level for controlling and maintaining the transmission facilities. ESCON operates as half-duplex communication in a point-to-point topology and supports the optional use of switches (called ESCON directors) to create a mesh of point-to-point connections. ESCON is connection-oriented, meaning that the host channel adapter and CU must exchange link-level frames to reserve resources for a logical connection before exchanging channel protocol frames. The ESCON director circuit-switches all physical connections resulting in a 1:1 ratio of active logical connections to active physical connections. Consequently, each channel adapter supports only one active logical connection at a time. An ESCON director is limited to 256 physical ports (254 usable). No more than two ESCON directors may separate a host channel adapter from a CU. (Daisy-chaining switches in a linear manner is called cascading.)

ESCON originally provided for multi-mode and single-mode host channel adapters. However, the majority of installations use multi-mode channel adapters. The maximum supported distance per link is three kilometers (km) using multi-mode fiber (MMF) or 20 km using single-mode fiber (SMF). By using two ESCON directors and SMF, the distance from host channel adapter to CU theoretically could be extended up to 60 km. However, 43 km was the maximum distance ever officially supported between a host and a CU. Support for SMF interfaces in hosts and CUs was terminated years ago, so SMF is used only between directors today. SMF between directors is called extended distance facility (XDF). It yields a maximum distance of 26 km from host to CU. ESCON remote channel extenders also can be used (in place of directors) to extend the channel distance. In practice, the lack of SMF cable plants during the heyday of ESCON limited most installations to 9 km or less.

ESCON transmissions are encoded via 8b/10b signaling. The ESCON signaling rate is 200 megabits per second (Mbps), which provides a maximum of 20 megabytes per second (MBps) link-level throughput at distances up to 8 km. This equates to 17 MBps of data throughput. Throughput decreases as distance increases beyond 8 km (a condition known as droop). This droop effect results from lack of buffer capabilities and the chatty nature of SBCCS. FICON reduces droop by introducing buffers and reducing SBCCS chatter. ESCON is losing market share to FICON and likely will be deprecated by IBM in the not-too-distant future.

FICON

FICON was introduced in 1998 to overcome the limitations of the ESCON architecture. FICON is the term given to the pseudo-proprietary IBM SBCCS operating on a standard Fibre Channel infrastructure. The version of SBCCS used in FICON is less chatty than the version used in ESCON. The ANSI FC-SB specification series maps the newer version of SBCCS to Fibre Channel. All aspects of FICON infrastructure are based on ANSI FC standards. FICON operates in two modes: bridged (also known as FCV mode) and native (also known as FC mode). Some FICON hardware supports SCSI (instead of SBCCS) on Fibre Channel. This is sometimes called FICON Fibre Channel Protocol (FCP) mode, but there is nothing about it that warrants use of the term FICON. FICON FCP mode is just mainstream open-systems storage networking applied to mainframes.

In bridged mode, hosts use FICON channel adapters to connect to ESCON directors. A FICON bridge adapter is installed in an ESCON director to facilitate communication. A FICON bridge adapter time-division multiplexes up to eight ESCON signals onto a single FICON signal. Investments in late-model ESCON directors and CUs are preserved allowing a phased migration path to FICON over time. Early ESCON directors do not support the FICON bridge adapter. The ESCON SBCCS is used for storage I/O operations in bridged mode.

In native mode, FICON uses a modified version of the SBCCS and replaces the ESCON transmission facilities with the Fibre Channel transmission facilities. FICON native mode resembles ESCON in that it:

• Employs serial transmission on optical cabling

• Is roughly equivalent to Layers 1 and 2 of the OSI reference model

• Is connection-oriented (See Chapter 5, "OSI Physical and Data Link Layers," for a detailed discussion of connection-oriented communication)

• Transports channel protocol and link-level frames

• Supports the point-to-point topology

• Supports the optional use of switches called FICON directors to create a mesh of point-to-point connections (FICON directors are actually just Fibre Channel directors that support transport of SBCCS and management via CUP.)

Unlike ESCON, FICON native mode operates in full-duplex mode. Unlike ESCON directors, Fibre Channel switches employ packet switching to create connections between hosts and CUs. FICON native mode takes advantage of the packet-switching nature of Fibre Channel to allow up to 32 simultaneously active logical connections per physical channel adapter. Like ESCON, FICON is limited to 256 ports per director, but virtual fabrics can extend this scale limitation. FICON native mode retains the ESCON limit of two directors cascaded between a host channel adapter and a CU, though some mainframes support only a single intermediate director.

Fibre Channel transmissions are encoded via 8b/10b signaling. Operating at 1.0625 gigabits per second (Gbps), the maximum supported distance per Fibre Channel link is 500 meters using 50 micron MMF or 10 km using SMF, and the maximum link level throughput is 106.25 MBps. Operating at 2.125 Gbps, the maximum supported distance per Fibre Channel link is 300 meters using 50 micron MMF or 10 km using SMF, and the maximum link-level throughput is 212.5 MBps. By optionally using IBM's mode-conditioning patch (MCP) cable, the maximum distance using 50 micron MMF is extended to 550 meters operating at 1.0625 Gbps. The MCP cable transparently mates the transmit SMF strand to a MMF strand. Both ends of the link must use an MCP cable. This provides a migration path from multi-mode adapters to single-mode adapters prior to cable plant conversion from MMF to SMF. The MCP cable is not currently supported operating at 2.125 Gbps. By using a SMF cable plant end-to-end and two FICON directors, the maximum distance from host channel adapter to CU can be extended up to 30 km. Link-level buffering in FICON equipment (not found in ESCON equipment) enables maximum throughput over long distances. However, throughput decreases as distance increases unless additional link-level buffering is implemented. Increasing the end-to-end device level buffering is also required as distance increases. IBM FICON hosts and CUs currently support sufficient buffering to enable operation at distances up to 100 km (assuming that intermediate link-level buffering is also sufficient). The use of wavelength division multiplexing (WDM) equipment or FICON optical repeaters is required to extend the end-to-end channel to 100 km because no more than two Fibre Channel directors may be cascaded.

The Fibre Channel Protocol for SCSI (FCP) is a mapping for SCSI to be transported on Fibre Channel. In FICON FCP mode, Linux-based mainframes use SCSI in place of SBCCS for storage I/O operations. As mentioned previously, the term FICON really has no meaning in the context of FCP mode. All transmission parameters of FICON FCP mode are comparable to FICON native mode because both use Fibre Channel infrastructure. FICON FCP mode is not supported on OS/390 mainframes.