Host adapter
From Free net encyclopedia
Image:QLA 2200F.jpg Image:Controller SCSI.JPG In computer hardware, a host adapter or host bus adapter (HBA) connects a host system (the computer) to other network and storage devices. The terms are primarily used to refer to devices for connecting Fibre Channel and SCSI devices (see SCSI host adapter), but devices for connecting to ESCON, Ethernet, and other systems may also be called host adapters. Recently, the advent of iSCSI has brought about Ethernet HBAs, some including TCP Offload Engines.
SCSI
Template:Main A SCSI host adapter connects a SCSI bus to a computer. The host adapter bridges the physical and logical chasm that separates the SCSI bus from the host computer's internal bus. Modern host adapters contain all the electronics and firmware required to execute SCSI transactions, and often include a BIOS that not only allows the host system to boot from a SCSI device, but also facilitates configuration of the host adapter. Typically a device driver, linked to the operating system, controls the host adapter itself.
In a typical SCSI subsystem, each device has assigned to it a unique numerical ID. As a rule, the host adapter appears as SCSI ID 7, which gives it the highest priority on the SCSI bus (priority descends as the SCSI ID descends; on a 16-bit or "wide" bus, ID 8 has the lowest priority, a feature that maintains compatibility with the priority scheme of the 8-bit or "narrow" bus).
The host adapter usually assumes the role of "initiator," in that it issues commands to other SCSI devices.
A computer can contain more than one host adapter, which can greatly increase the number of SCSI devices available.
Fibre Channel
Today, the term host bus adapter is most often used to refer to a Fibre Channel interface card. Fibre Channel HBAs are available for all major open systems, computer architectures, and buses, including PCI and SBus (obsolete today). Each HBA has a unique World Wide Name (WWN), which is similar to an Ethernet MAC address in that it uses an OUI assigned by the IEEE. However, WWNs are longer (16 bytes). There are two types of WWNs on a HBA; a node WWN, which is shared by all ports on a host bus adapter, and a port WWN, which is unique to each port. Major HBA manufacturers are Emulex, Qlogic and JNI.