Fibre Channels Multiprotocol advantages

Posted on October 01, 1999

Fibre Channel`s Multiprotocol advantages

Running multiple protocols--such as SCSI and IP--saves slots and cabling, and opens up new network storage management opportunities.

Michael Eckley

As storage area networks (SANs) begin to take center stage in many IT operations, the multi-protocol capabilities of Fibre Channel will become increasingly important. Fibre Channel`s ability to simultaneously communicate SCSI or IP data over the same medium sets it apart from other interfaces and makes it more than just the next-generation I/O interface with fast data throughput. Although it can be thought of as a protocol, Fibre Channel is more accurately a serial transport mechanism that is capable of handling data encoded in just about any protocol, including SCSI, IP, HIPPI, and ATM.

Fibre Channel is able to establish a multi-protocol environment because it has a hierarchical architecture, which essentially isolates protocol types from each other as well as from the transmission medium. To accomplish this, Fibre Channel hardware and software must be designed to take advantage of Fibre Channel`s full potential. In the case of host-bus adapters (HBAs), that means a full-featured host interface must be designed into the hardware to make the most of Fibre Channel`s software capabilities.

A multi-protocol Fibre Channel SAN is based on an ordered structure, beginning at the level of character encoding (the FC-1 level), which makes use of an 8B/10B coding scheme that provides superior transmission characteristics. On top of this is the transport mechanism (FC-2), where a link or channel is established between pairs of ports on nodes. These ports can be originators, responders, or both.

Once a connection has been established, Fibre Channel makes use of several hierarchical protocol constructs to transmit and receive data (see table). The base level consists of ordered sets. These sets are made up of four 10-bit characters that provide low-level link functions such as frame demarcation and other signaling between two ends of a link.

The next level is the Fibre Channel frame. The frame is the smallest indivisible packet of data that is sent across a link. Typical of many protocols, the Fibre Channel frame consists of a start-of-frame delimiter (an ordered set), frame header, optional headers, a data payload area containing up to 2,112 bytes of data, a 32-bit cyclical-redundancy-checking (CRC) code for error detection, and an end-of-frame delimiter (another ordered set).

The header and optional header fields in a frame are used to transmit addressing and routing information, as well as information describing the type of data contained in the frame`s payload area.

Above the Fibre Channel frame level is a construct called a sequence, which is the basic transfer unit of Fibre Channel. There are no limits on the size of data transfers between applications because a sequence can comprise more than one frame flowing in the same direction. The sequence is also the recovery boundary in Fibre Channel--that is, when an error is detected, the sequence in error is identified so that it and subsequent sequences can be retransmitted.

The next layer in a Fibre Channel hierarchy is the exchange. An exchange gathers together multiple non-concurrent sequences that relate to one operation or process. An exchange may consist of several phases, which are encapsulated in sequences. For example, an exchange process may consist of a command sequence to read data, followed by a data sequence and a completion status sequence. Within an exchange, only one sequence can be active at any one time, although sequences from different exchanges can be active simultaneously. This is one of the ways Fibre Channel accomplishes multiplexing on the same line.

Multiplexing IP and SCSI

The Fibre Channel community has mapped several channel and networking protocols onto this architecture in order to implement multiplexed protocol environments. IP, which is a networking protocol, and SCSI, which is an I/O channel protocol, are two of the most prevalent at this time.

Standardizing the mapping process for multiple protocols began when the American National Standards Institute (ANSI) X3T11 Group developed a Fibre Channel IP specification (FC-IP). Building on ANSI`s efforts, the Fibre Channel Association (FCA) has issued an IP Profile. The IP Profile is not a standard or a specification, but it provides recommended implementation parameters for accommodating IP over Fibre Channel.

To date, the Fibre Channel community`s efforts at providing interoperability have met with considerable success. A recent multi-vendor interoperability plugfest involving over 35 vendors and run by the Fibre Channel Community (FCC) and the University of New Hampshire`s Interoperability Center, demonstrated that many Fibre Channel HBAs, hubs, switches, and other storage assets can effectively communicating with each other in a heterogeneous vendor environment.

To assure a high level of interoperability, the Internet Engineering Task Force (IETF) has recently undertaken the task of codifying a standard for IP communi- cation on Fibre Channel. In addition, a Management Information Base (MIB) standard is also being created that will allow the development of Fibre Channel network management tools based on the Simple Network Management Protocol (SNMP).

Benefits of multiplexing

The implications of communicating IP and SCSI data concurrently on the same transmission medium are far reaching. As more SANs are deployed, IT professionals will realize that dual infrastructures--one for networking and one for storage--is wasteful and unnecessary because Fibre Channel can accommodate both at a very high performance level.

For example, because of the limitations of the PCI bus, the number of card slots available in many PCI-based servers is limited to three or four in most cases. These slots can be filled quickly, especially when one slot must be devoted to a networking card and another to a storage controller card. But with a Fibre Channel HBA capable of multiplexing IP and SCSI, as well as other protocols, only one slot is needed to perform both networking and storage tasks.

Saving server slots is just the tip of the iceberg. Transmitting networking and storage data over one medium eliminates the need for a dual-cabling infrastructure. With multi-protocol Fibre Channel HBAs, one infrastructure carries all data, whether it`s intended for storage on a disk drive or for transmission over the organization`s intranet (see figure).

Another important consideration for IT professionals is SAN management. A Fibre Channel HBA capable of simultaneously transmitting SCSI and IP data allows simple in-band management by using its IP capabilities.

Enterprises are deploying SANs because effective and efficient access to mission-critical data can often provide a competitive advantage. As more SANs are implemented, IT managers, administrators, and network designers will seek simpler and more elegant solutions. Fibre Channel, with its inherent multi-protocol capabilities, has tremendous potential to satisfy these needs, but the full potential of Fibre Channel must be enabled in the HBAs, hubs, switches, and software used in the SANs.

Click here to enlarge image

Networking and storage data can be simultaneously transmitted over Fiber Channel, eliminating the need for dual cabling.

Michael Eckley is a member of the Fibre Channel Team at Interphase Corp., in Dallas, TX (www.iphase.com).