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Fibre
Channel is technology standard that, at the simplest level,
is specifically designed to marry together network and
data storage technologies to encompass a full physical,
network, communications, and protocol layer common medium.
Combining a widely understood logical network layout with
the high throughput data transfer systems of data storage
technologies, Fibre Channel is perhaps the first step
down the road into a world where systems, data, and communications
are fused together into a seamless single environment.
Fibre Channel Arbitrated Loop (FC-AL) is an ANSI-standard
serial connectivity technology designed for data and communication
intensive storage applications such as data warehousing,
data mining, on-line transaction processing (OLTP), Internet/intranet
access, and film/video/broadcast implementations.
Fibre
Channel transfers data at 200 MB/sec in a dual-loop configuration
or at 100 MB/sec in redundant mode, with future (&
prototype) products expected to push performance to 400
MB/sec. Bus lengths reach 30 metres using copper cables
and 10 kilometres with fibre-optic cabling.
Number of Devices
The maximum number of devices per loop
using FC-AL is 126. The address space for Fibre Channel
switch fabric however allows for millions of devices,
more than adequate for large, enterprise networks.
Switch
products are typically built from 8 to 16 port configurations,
but can integrate many more devices via Fabric Loop ports
attached to Arbitrated Loop hubs or extensions to additional
switches. Fibre Channel standards include specifications
for mapping SCSI, HIPPI, IP, ATM, and other channel and
network protocols.
Fibre
Channel systems can be built with few restrictions, and
through this flexibility they expand the ability of IT
organisations to continue the use of legacy equipment,
together with modern state-of-the-art equipment.
Fibre
Channel is intended to weld these two worlds - the legacy
and the new - into a single dynamic network of scalable
storage. With the inherent ability to run SCSI and IP
protocols on the same network, Fibre Channel systems bring
new levels of capability and performance.
The present separation of system to system communication
and data accessibility through standard network &
data infrastructures is not only inefficient, but is severely
lacks the high performance and data throughput rates required
in modern data intensive environments.
Any
network is only as fast as its slowest component, and
by combining a number of disparate technologies, in the
real world many high performance data centres are usually
forced to communicate with the furthest client at abysmally
slow transfer rates. It is in this transfer of raw data
across networks from system to system that conventional
networking technologies cannot offer the bandwidth, redundancy,
or compatibility required.
Virtually
any topology that an IT organisation requires through
specification, legacy equipment demands, or future growth
calculations, is possible. The basic building blocks are
point-to-point dedicated bandwidth, loop shared bandwidth,
and switched scaled bandwidth. Switches and hubs are stackable,
interchangeable, and above all, compatible with legacy
& future developments.
The
basic connection to a Fibre Channel device is made by
two serial cables, one carrying in-bound data and the
other carrying out-bound data. These cables can be fibre
optic or twin-axial copper. How the devices are to be
connected together is a question of system topology. Several
topologies are defined for Fibre Channel including:
Of
all the available options, Arbitrated Loop using twin-axial
copper offers the lowest cost per device and is thus the
logical choice for use as a disk drive interface. The
standard abbreviation for this is FC-AL (Fibre
Channel Arbitrated Loop).
In
a typical FC-AL environment, with a single initiator and
several target devices, the out-bound cable from each
device becomes the in-bound cable for the next device
in the loop. This efficient use of cabling (and the associated
receivers and transmitters) is part of what makes FC-AL
the lowest cost topology. Maximum bandwidth of a FC-AL
implementation is 100MB/sec in a single loop and 200MB/sec
in a dual loop. This throughput remember, is network bandwidth
not simply device capability.
Although
the first inroads by FC-AL were in high-end point-to-point
RAID and other mass-storage subsystems, it is now broadening
into other areas such as on-line transaction processing
(OLTP), video/ graphics networks, and visual imaging systems.
The requirements of these applications are diverse.
For
example, OLTP requires fast transfer of small blocks of
data, while video-editing systems and sophisticated Internet
applications need to access and move large image files
quickly. The inclusions of SCSI in the Fibre Channel specifications
means that even servers that need to access and output
multiple channels of high-quality video, usually at 20
MB/sec per channel for example, can easily achieve this
through a combination of FC-AL connections and legacy
SCSI peripherals.
This
versatility combined with the ability to utilise existing
SCSI devices - and even increase their performance
- makes Fibre Channel a de facto requirement for environments
where enterprise scaled storage capacity is required together
with high bandwidth performance. Fibre Channel also addresses
the need to access storage from various locations around
company facilities, from floor to floor or between buildings.
Fibre Channel makes the concept of a Storage Area Network
become a reality.
Four
kinds of copper cables are defined in the Fibre Channel
standard. The most popular implementations are twin-coax
using DB-9 or HSSD connectors. Point-to-point is used
in simple scenarios such as attaching a RAID array or
other storage device to a single system. Point-to-point
does not require a hub or loop as only two devices are
involved.
Connectors:
SC
Connector: The
SC connector is the standard connector for Fibre Channel
fibre optic cables. It is a push-pull connector and
is favoured over the ST connector in many implementations.
If the cable is pulled, the tip of the cable in the
connector does not move out, resulting in loss of signal
quality.
ST
Connectors: ST
connectorsare
vaguely similar in design to quarter inch audio jacks,
with a simple plug-in male/female connection that relies
on the friction between jack and plug to hold into place
and has no additional securing device. ST connectors
however, do have the advantage of being simpler and
cheaper to install.
Fibre
Multimode cable is dominant for short distances
of 2 Km or less. Multimode has an inner diameter of
62.5 or 50 microns, allowing light to enter the cable
in multiple modes, including straight and at different
angles. The many light beams tend to lose shape as they
move down the cable. This loss of shape is called dispersion
and limits the distance for multimode cable. Cable quality
is measured by the product of bandwidth and distance.
Existing 62.5 micron FDDI cable is usually rated at
100 or 200 MHz /Km, providing gigabit communications
up to 100 or 200 meters.
Fibre Single
Mode cable is used
for long distance cable runs. Its distance is limited
by the power of the laser at the transmitter and by
the sensitivity of the receiver. Single mode cable has
an inner diameter of 7 or 9 microns and only allows
a single ray of light to enter the cable. Therefore,
with single mode cables there is no dispersion. Single
Mode fibre optic cabling naturally carries a price premium
for this added functionality.
Proprietary
Implementations of FC-AL
Sun and HP have produced Fibre Channel
products in the past including Sun’s RSM 1xx/2xx SPARCStorage
Arrays and HP’s HP-FL storage devices that use a proprietary
implementation. These products are based upon point-to-point,
quarter-speed, Multi-Mode Fibre (MMF) and do not follow
the established Fibre Channel-Arbitrated Loop (FC-AL)
standard. Thus they do not operate with standard full-speed
adapters, storage devices, hubs or switches. For example,
Sun's Fibre Channel adapter (1057A) and optical module
(595A) is (was?) a point-to-point, quarter-speed Fibre
Channel adapter that will not work with FC-AL.
It is important to understand what you are getting when
a vendor talks about Fibre Channel and whether it is compatible
with industry standard that will give you the most flexibility
in the future. At Discovery, we endeavour to support industry-wide
specifications rather than any proprietary implementations.
The
Future of SCSI & SSA
Contrary to some popular belief, the advent
and introduction of FC-AL may well give the sale of SCSI
peripherals and devices a slight boost in the short to
medium term. The Fibre Channel specifications includes
a full subset of the SCSI standard (embodied as the SCSI-3
command set - see below), meaning that all legacy SCSI
devices can operate within a FC environment with no additional
infrastructure or changes to existing systems. The high
throughput and bandwidth offered by FC enables SCSI to
operate at the maximum level available.
SCSI-3 Command
Set & Fibre Channel
The SCSI-3 Command Set was actually introduced
to facilitate the integration of Fibre-Channel devices
into the SCSI storage realm. Fibre-Channel drives - most
commonly defined by either using an arbitrated loop topology
and denoted as FC-AL , or using simple Point-to-Point
topologies - still implement the SCSI protocol. That is,
they are commanded to read and write using the same SCSI
commands that 'normal' SCSI devices have been using for
years.
Therefore
FC-AL is actually part of the wider SCSI family. It simply
uses a different physical/electrical interface, in this
case a serial connection. The SCSI Fibre Channel Protocol
(FCP) is a mapping protocol (FC-4) for applying the SCSI
command set to the Fibre Channel. The FCP defines the
Fibre Channel information units in accordance with the
SCSI Architecture Model (X3.270-199X).
Strictly speaking, because of this dual usage, the bare
term 'SCSI' should really only be used when referring
to the Command Set structure or protocol specification.
When referring to the electrical parallel interface, the
term SCSI-2 for example should be referenced as SCSI-2
Parallel Interface. The SCSI Command Sets may be applicable
to devices not using a SCSI parallel interface.
In summary the FC specification incorporates SCSI-3 as
part of its own standard and SCSI devices include the
SCSI-3 command set into their architecture to facilitate
their inclusion into a FC environment.
SSA
and Fibre Channel, including Fibre Channel-Arbitrated
Loop (FC-AL), share many common characteristics:
- Both
are serial architectures sending a stream of bits over
a communication path
- Both
use fibre optics to digitally transmit high-speed serial
bit streams
- Both
support more devices over longer distances than parallel
SCSI
- Both
are open standards although SSA is perceived as an IBM
proprietary protocol
- Both
provide improved bandwidth compared to traditional parallel
SCSI with FC being faster than SSA
However,
the one main point to remember is this:
Used correctly, Fibre Channel
can increase the life-span of your existing legacy devices.
Next
Section - Fibre Channel Product Availability
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