Arbitrated loop
Encyclopedia
Arbitrated loop, also known as FC-AL, is a Fibre Channel
Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
topology in which devices are connected in a one-way loop fashion in a ring topology
Ring network
A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring...
. Historically it was a lower-cost alternative to a fabric topology
Switched fabric
Switched fabric, switching fabric, or just fabric, is a network topology where network nodes connect with each other via one or more network switches . The term is popular in telecommunication, Fibre Channel storage area networks and other high-speed networks, including InfiniBand...
. It allowed connection of many servers
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
and computer storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....
devices without using then very costly Fibre Channel switch
Fibre Channel switch
In the computer storage field, a Fibre Channel switch is a network switch compatible with the Fibre Channel protocol. It allows the creation of a Fibre Channel fabric, that is currently the core component of most storage area networks . The fabric is a network of Fibre Channel devices which...
es. As of 2007 the cost of the switches dropped considerably, so FC-AL is rarely used for a server-to-storage communication. It is however still commonly utilized on backend of some disk array controller
Disk array controller
A disk array controller is a device which manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache.A disk array controller name is...
s.
- It is a serialSerial communicationsIn telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
architecture that is compatible with SCSISCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, handling up to 127 ports (devices). One port may optionally connect a loop to fabric switch port. - The bandwidth on the loop is shared among all ports.
- Only two ports may communicate at a time on the loop. One port wins arbitration and may open one other port in either half or full duplex mode.
- Arbitrated loop with only 2 ports is a valid one, and while having the same physical topology as point-to-point still acts as a loop protocol-wise.
- Fibre Channel ports capable of arbitrated loop communication are NL_port (node loop port) and FL_port (fabric loop port), collectively referred to as the L_ports. Physical connectors on the hub are not considered ports in terms of the protocol.
- An arbitrated loop with no fabric port (with only NL_ports) is a private loop.
- An arbitrated loop connected to a fabric through FL_port, is a public loop.
- NL_Port must provide fabric logon (FLOGI) and name registration facilities to initiate communication with other node through the fabric (to be an initiator).
Arbitrated loop can be physically cabled in a ring fashion or using a hub. The physical ring ceases to work if one of the devices in the chain fails. The hub on the other hand, while maintaining a logical ring, allows a star topology on the cable level. Each receive port on the hub is simply passed to next active transmit port, bypassing any inactive or failed ports.
Fibre Channel hubs therefore have one other very useful function: They provide bypass circuits that
prevent the loop from breaking if one device fails or is removed. If a device is removed
from a loop (for example, by pulling its interconnect plug), the hub’s bypass circuit
detects the absence of signal and immediately begins to route incoming data directly
to the loop’s next port, bypassing the missing device entirely. This gives loops at least
a measure of resiliency—failure of one device in a loop doesn’t cause the entire loop
to become inoperable.
See also
- Storage area networkStorage area networkA storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
- Fibre ChannelFibre ChannelFibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
- Switched fabricSwitched fabricSwitched fabric, switching fabric, or just fabric, is a network topology where network nodes connect with each other via one or more network switches . The term is popular in telecommunication, Fibre Channel storage area networks and other high-speed networks, including InfiniBand...
- List of Fibre Channel standards