[Fibre Channel] The “Bridge” port within a bridge device used to extend a Fibre Channel inter-switch link; it connects only to an E_Port on a Fibre Channel switch.


[Data Recovery] Acronym for Backup to Disk.


[Data Recovery] Acronym for Backup to Tape.


1. [Data Recovery] A collection of data stored on (usually removable) non-volatile storage media for purposes of recovery in case the original copy of data is lost or becomes inaccessible; also called a backup copy.

To be useful for recovery, a backup must be made by copying the source data image when it is in a consistent state.

2. [Data Recovery] The act of creating a backup. See archive.

backup client

[Data Recovery] A computer system containing online data to be backed up.

backup copy

[Data Recovery] A recoverable copy of a given set of data.

See replica.

backup manager

[Data Recovery] An application program whose purpose is to schedule and manage backup operations.

backup policy

[Data Recovery] An IT installation's rules for how and when backup should be performed, such as which files or directories are to be backed up, the schedule on which backups should occur, which devices and media are eligible to receive the backups, how many copies are to be made, and actions to be performed if a backup does not succeed.

Backup to Disk (B2D)

[Data Recovery] Backup onto disk media.

Backup to Tape (B2T)

[Data Recovery] Backup onto tape media.

backup window

[Data Recovery] An interval of time during which a set of data can be backed up without seriously affecting applications that use the data.

For example, if an application accesses data from 8 AM until midnight, then the window between midnight and 8 AM is available for making backup copies. Offline backups require that applications not update data during the backup. Online backups typically use point in time copy technology to create consistent images of data for backup. If a backup uses different resources (storage devices, I/O paths, processing power) than the application, as is common with split mirror point-in-time copies, then the backup window is the time required to create the image. If the online backup shares resources with the applications using the data, as is common with copy-on-write point in time copies, the backup window may be increased due to resource contention.


1. [General] The numerical difference between the upper and lower frequencies of a band of electromagnetic radiation.

2. [Data Communication] A deprecated synonym for data transfer capacity that is often incorrectly used to refer to throughput.

bandwidth-length product

[Network] A figure of merit for optical fiber, usually expressed as MHz*kilometer.

As an example, a Fibre Channel link operating at 1 Gb/s using a fiber with a bandwidth-length product of 500 MHz*kilometer will support a link distance of approximately 500 meters.


[Network] The maximum rate of signal state changes per second on a communications circuit.

If each signal state change corresponds to a code bit, then the baud rate and the bit rate are the same. It is also possible for signal state changes to correspond to more than one code bit, so the baud rate may be lower than the code bit rate.


[Fibre Channel] Buffer-to-buffer credit; used to determine how many frames can be sent to a recipient when buffer to buffer flow control is in use.


[General] Acronym for Best Current Practice.

beginning running disparity

[Fibre Channel] For a data stream using 8B/10B encoding, the running disparity present at a transmitter or receiver when an ordered set is initiated.


[Network] [Storage System] Acronym for Bit Error Rate.

Berkeley RAID Levels

[Storage System] A classification of disk array data protection and mapping techniques developed by Garth Gibson, Randy Katz, and David Patterson in papers written while they were performing research into I/O subsystems at the University of California at Berkeley.

There are six Berkeley RAID Levels, usually referred to by the names RAID Level 1, etc., through RAID Level 6. See RAID 0, RAID 1, RAID 2, RAID 3, RAID 4, RAID 5, RAID 6. Many other levels such as RAID 10, RAID 50 and so on have since been proposed.

Best Current Practice (BCP)

[General] A recommendation for what is currently believed to be the best manner of proceeding.

best effort (class of service)

[Fibre Channel] [Network] A class of service that does not guarantee delivery of packets, frames, or datagrams, but for which the network, fabric, or interconnect makes every reasonable delivery effort.

bidirectional authentication

[Data Security] Synonym for mutual authentication.

big data

[Computer System] A characterization of datasets that are too large to be efficiently processed in their entirety by the most powerful standard computational platforms available.

big endian

[Computer System] A format for the storage and transmission of binary data in which the most significant bits are stored at the numerically lowest addresses, or are transmitted first on a serial link.

BIOS (Basic Input Output System)

[Computer System] A relatively small program that resides in programmable, non-volatile memory on a personal computer and that is responsible for booting that computer and performing certain operating system independent I/O operations.

Standard BIOS interrupts are defined to allow access to the computer's disk, video and other hardware components (for example, INT13 for disk access).

bit error rate (BER)

[Network] [Storage System] The probability that a transmitted bit will be erroneously received.

The BER is measured by counting the number of bits in error at the output of a receiver and dividing by the total number of bits in the transmission. BER is typically expressed as a negative power of 10.

bit synchronization

[Data Communication] The process by which the receiver of a serial communication establishes its clocking used to locate code bits in a received data stream.

blade system

[Computer System] A computer or storage system composed of a chassis that provides power, cooling and other common infrastructure, and one or more removable server or storage units, usually called blades.

Blade systems are designed as a scalable solution to efficiently package and operate multiple processing or storage units in a single enclosure, and are designed for technicians to be able to easily add or replace hot-swappable boards in the field.

blanking plate

[General] [Energy] A solid plate that mounts over unused data center rack slots to maintain efficient air flow through components in the rack.

Blanking plates are also used for aesthetic reasons.

blind mating

[Hardware] The ability of pairs of components to be connected without the electrical or optical connection points being visible.

Blind mating is usually accomplished by mechanical guides (e.g., slots and rails) on the components.


1. [Storage System] The unit in which data is stored and retrieved on disk and tape devices; the atomic unit of data recognition (through a preamble and block header) and protection (through a CRC or ECC).

2. [Fibre Channel] A unit of application data from a single information category that is transferred within a single sequence.

block addressing

[Storage System] An algorithm for uniquely identifying blocks of data stored on disk or tape media by number, and then translating these numbers into physical locations on the media.

See C-H-S addressing.

block cipher

[Data Security] A symmetric encryption algorithm that operates on a block of plaintext, i.e., a string of bits of a defined length, to yield a block of ciphertext. [ISO/IEC 10116]

block services

[Storage System] A subsystem that provides block level access to storage for other systems or other layers of the same system. See block.

block virtualization

[Storage System] The act of applying virtualization to one or more block based (storage) services for the purpose of providing a new aggregated, higher level—e.g., richer, simpler, more secure—block service to clients.

Block virtualization functions can be nested. A disk drive, RAID system or volume manager all perform some form of block address to (different) block address mapping or aggregation. See file virtualization.

BNC (Bayonet Neill–Concelman) connector

[Network] A type of coaxial cable connector formerly used in Ethernet applications; the specification is contained in EIA/TIA 403-A and MIL-C-39012.




[Computer System] The process of loading Operating System code from a disk or other storage device into a computer's memory and preparing it to run.

Bootstrapping is an appropriate term since a code load typically occurs in steps, starting with a very simple program (BIOS) that initializes the computer's hardware and reads a sequence of data blocks from a fixed location on a pre-determined disk, into a fixed memory location. The data thus read is the code for the next stage of bootstrapping—usually an operating system loader. The loader completes the hardware setup and results in an executing operating system, in memory.

break mirror

[Storage System] Remove a mirror component from the mirror and make it a standalone volume in the system, voiding its synchronization relationship with the other mirror components.


1. [Fibre Channel] A Fibre Channel technology that provides a transparent fabric extension between two switch E_Ports through the use of 2 B_Ports tunneling through some alternative technology, resulting in an Inter-Switch Link (ISL) that “appears” to be a direct link between switches.

For example, a bridge pair can take an incoming Fibre Channel frame from one B_Port on a Bridge, encapsulate that frame using FCIP (Fibre Channel over IP) and transmit the frame as payload over an IP network to the remote Bridge where the original frame is forwarded to the remote Fibre Channel Fabric switch E_Port through the remote Bridge’s B_Port.

2. [Fibre Channel] A Fibre Channel technology that enables traffic carried along part of the path from a source device by Fibre Channel, (for example commands, blocks, status and control between a SCSI initiator or target source device) to be extended to the destination device using an alternative physical transport network technology (for example iSCSI or SCSI Interconnect).

In some cases this “Bridge” is also referred to as a physical transport gateway, or storage router.

3. [Network] A device that connects multiple LAN segments at the physical address layer.

As opposed to a hub, which indiscriminately rebroadcasts everything from one segment to the other, a bridge only retransmits traffic from one segment to another when the traffic is intended for the destination segment.


[Fibre Channel] The simultaneous transmission of a message to all receivers (ports) connected to a communication facility.

Broadcast can be contrasted with unicast (sending a message to a specific receiver) and multicast (sending a message to select subset of receivers). In a Fibre Channel context, broadcast specifically refers to the sending of a message to all N_Ports connected to a fabric. See multicast, unicast.


[Computer System] A solid state memory device or programming construct, used to hold data momentarily as it moves along an I/O path or between software components.

buffer to buffer flow control

[Fibre Channel] Flow control that occurs between two directly connected Fibre Channel ports, e.g., an N_Port and its associated F_Port.

A port indicates the number of frames that can be sent to it (its buffer credit) before the sender is required to stop transmitting and wait for the receipt of a "ready" indication. Buffer to buffer flow control is used only when an NL_Port is logged into another NL_Port on an Arbitrated Loop or when an Nx_Port is logged into an FX_Port." or "Flow control that occurs between two directly connected Fibre Channel ports, e.g., an N_Port and its associated F_Port or between two E_Ports. A port indicates the number of frames that can be sent to it (its buffer credit), before the sender is required to stop transmitting and wait for the receipt of additional credit.

business continuity

[Data Security] Processes and/or procedures for ensuring continued business operations. [ISO/IEC 27000]

bypass circuit

[Fibre Channel] A circuit that removes a device from a data path (such as a Fibre Channel arbitrated loop) when valid signaling is lost, or a controller directs the removal of the device for any reason.


[Computer System] An 8 bit unit of data.

Byte and bit ordering and meaning vary depending on context. It is necessary to consult the standards that apply in a given context to determine ordering and meaning.