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9-22 Audio Recording Systems

rates are likely to render current SCSI variants as bottlenecks in future systems built around fast
new microprocessors. These and other limitations of SCSI are creating demand for a better solu-
tion to the high-performance l/O needs of the computer systems market, and at least one solution
is a serial data interface called Fibre Channel Arbitrated Loop. FC-AL is a subset of the box-to-
box standard created by original members of the Fibre Channel Association.

1

 Like its Fibre

Channel superset, FC-AL is an industry-standard interface endorsed by the American National
Standards Institute (ANSI). Fibre Channel is usually thought of as a system-to-system or system-
to-subsystem interconnection standard that uses optical cable in a point-to-point or switch con-
figuration. This is what it was originally designed to do, in fact, since HIPPI and the Internet
Protocol were among the protocols defined for it. One of the goals in the development of the
Fibre Channel interface was to improve or eliminate SCSI shortcomings, particularly in the areas
of connectivity, performance, and physical robustness. 

In 1991 the Fibre Channel box-to-box interconnect standard was enhanced to include support

for copper (nonoptical) media and multidrop configurations, both of which enable the low-cost
connection of many devices to a host port. This subset of the Fibre Channel standard is called
Fibre Channel-Arbitrated Loop, and is what made it possible for Fibre Channel to be used as a
direct-disk-attachment interface (SCSI-3 has been defined as the disk interface protocol, specifi-
cally SCSI-FCP). The implications of that capability are enormous in terms of the cost savings
and ease with which users can migrate to standard systems with performance capabilities hereto-
fore only found in expensive proprietary systems at the workstation or mainframe level.

The basic features of Fibre Channel are listed in Table 9.1.5. The interface features of FC-AL

are given in Table 9.1.6.

FC-AL Topology

The Fibre Channel-Arbitrated Loop interface is a loop topology, not a bus in the conventional
sense like SCSI [3]. It can have any combination of hosts and peripherals, up to a loop maximum
of 126 devices.

Using a connector based on the 80-pin parallel SCSI single connector attachment (SCA),

Fibre Channel disk drives attach directly to a backplane. This not only eliminates cable conges-

1.  Hewlett-Packard, IBM, and Sun Microsystems

Table 9.1.5 Fibre Channel Features and Capabilities

Parameter

Range of Capabilities

Line rate

266, 531, or 1062.5 Mbits/s

Data transfer rate (maximum)

640–720 Mbits/s @ 1062.5 line rate

Frame size

2112 byte payload

Protocol

SCSI, IP, ATM, SDI, HIPPI, 802.3, 802.5

Topology

Loop, switch

Data integrity

10E–12 BER

Distance

Local and campus; up to 10 km

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Audio/Video Server Systems 9-23

tion, it makes hot drive insertions practical and simplifies mechanical designs. Fewer cables and
components translate to lower-cost systems and higher reliability. 

Connectivity Considerations

As discussed previously, applications such as video and image processing have pushed the
demand for huge increases in disk capacity per system [3]. In some cases, the capacity require-
ments for these types of applications are such that it is difficult to configure a sufficient number
of SCSI buses on a system so that enough drive addresses are available to attach the necessary
disk storage. Moreover, simply increasing the addressability of SCSI—making it possible to have
more than 15 devices per Wide SCSI bus, for example—would not be a solution, because more
bus bandwidth is needed to support the additional drives. Besides, protocol overhead is already
rather high. FC-AL can address up to 126 devices, but practical usage is another matter. A loop
can practically support about 60 drives in a UNIX (with 8 KB I/Os) environment, as an example.
With an 18 GB FC-AL interface disk drive, a loop of 50 drives would make more than 900 GB
available on a single FC-AL host adapter. This makes it possible for any workstation or system
with a Fibre Channel port to become an incredibly large file server.

Bandwidth

A Fibre Channel loop, as stated previously, supports data rates up to 100 MB/s in single-port
applications and up to 200 MB/s in dual-port configurations [3]. Applications such as digital
video data storage and retrieval, computer modeling, and image processing are growing in popu-
larity and demanding ever-increasing improvements to disk data transfer performance. More-
over, file servers are increasingly looked upon as replacements for mainframe computers. In
order to fulfill that promise, they will need to deliver higher transaction rates to provide a main-
frame level of service. 

Magnetic disk drive areal densities are known to be increasing at about 60 percent per year in

production products. Because the number of bits per inch—one of the two components of disk
areal density—must increase at about 30 percent per year, the data rate performance must also
increase proportionately. In addition, drive spindle speeds (or spin rates) continue to increase,
which directly contribute to the need for improved data rate performance.

Table 9.1.6 Basic Parameters of FC-AL

 (

After [3].)

Parameter

Range of Capabilities

Number of devices

126

Data rate

100 MB/s (1.062 GHz using an 8B/10B code)

Cable distance

30 m between each device using copper (longer, with other cabling options)

Cable types

Backplane, twinaxial, coaxial, optical

Fault tolerance

Dual porting, hot plugging

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9-24 Audio Recording Systems

Remote Online Storage

Because the FC-AL interface is part of, and fully compatible with the Fibre Channel standard,
optical cabling can be used in any portion of a subsystem (with the exception that optical signals,
of course, cannot be used on a backplane) [3]. This makes it possible to have a disk subsystem a
significant distance from the computer system to which it is attached. For example, using single-
mode fibre optics, on-line disk storage could be as far as 10 km away from its host. A computer
system could have disks within the system connected via a Fibre Channel loop. That loop can be
extended by using an electronic-to-optic signal adapter and lengths of fibre optic cable. On the
same loop running the internal disks, remote disks would appear to the system to be directly
attached exactly like the local disks, even though they might be five miles away. This can be an
advantage in many ways, including the shadowing the local disks in the event a disaster destroys
the on-site data banks. That capability, in particular, offers an attractive means for having a
remote and secure on-line copy of critical data that could be used to continue operations should
anything happen to the facility housing the primary computer system.

Array Implementations

Array controllers have traditionally been designed with multiple SCSI interfaces for drive attach-
ment [3]. This enables the controller to supply data and I/O rates equal to several times those
achievable from a single interface. This is sometimes referred to as an orthogonal array, because
the disks comprising the arrays elements, are across, or orthogonal to, the SCSI controllers.
Unfortunately, the decision to design a specific number of drive interfaces into a given controller
forces on the customer the parity amortization, granularity, and controller cost associated with
that decision. This limits the customer’s choices for configuring the optimal combination of
economy—that is, maximizing the granularity, or number of data drives per parity drive, the total
capacity per array, and overall performance. With Fibre Channel, it is possible to configure an
array along a single interface instead of across many. Because the drives constituting the array
unit are organized along an interface, it is sometimes described as a longitudinal array.

9.1.4c

Server-Based Audio/Video Editing

Audio/video editing is mostly a cutting and pasting process, and as such, lends itself to synergis-
tic applications involving the server [6]. The digital recorder enabled users to re-record the same
video several times with minimal generation loss. Digital recording also offered the ability to
read-before-write. With tape, this required judicious care, because the underlying video track
was erased by the subsequent one. Disk-based systems, either optical or magnetic hard-disk drive
designs, also offer this capability, but allow nondestructive read-out.

Server-based editing resembles digital disk-based editing, except that files from one user can

be instantly available to another. Facilities need only to endure the time penalty of transfer and
digitizing (as necessary) the input source material once. Thereafter; all potential users can access
the material simultaneously. For example, assume an important piece for the five o’clock news is
being produced edit room “A.” Via the server, the producer in edit room “B” can start putting
together the same story for the six o’clock news, accessing the same digitized elements.

Not every facility puts out back-to-back newscasts. Many that do, reuse stories with minimal

updating. There are, however, a number of facilities that not only produce multiple newscasts,
but provide separate news programming to cable channels or that sell news to other stations. As

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Audio/Video Server Systems 9-25

Internet broadcasting increases in importance, it will begin to consume editing resources as well.
With this many hands fighting for a field tape, conflict is inevitable. A unique economic advan-
tage of the server-based facility is that a multitude of users can cherry pick off the main storage
system, without disrupting the work flow of other operation.

9.1.4d

Perspective on Storage Options

The professional audio/video industry has become accustomed to an ever increasing number of
tape formats, each targeting a particular market segment. Disk storage of audio and video has
been an effective refuge from such “format wars.” This is not to say that disk stores do not vary
greatly. They do. The only standardization from vendor to vendor is their interface: SCSI, SCSI-
1, SCSI Wide, Fiber Channel, IDE, IEEE 1394, and so on. Because audio/video disk interchange
is not a major requirement today, there is no specific need to define the internal format. With no
standards of measurement except the ultimate performance of the system to fall back upon, it is
important to carefully examine the system architecture to be certain that it addresses the require-
ments for performance and reliability for a given application.

9.1.5

References

1.

Robin, Michael, and Michel Poulin: “Multimedia and Television,” in Digital Television
Fundamentals, McGraw-Hill, New York, N.Y., pp. 455–488, 1997.

2.

McConathy, Charles F.: “A Digital Video Disk Array Primer,” SMPTE Journal, SMPTE,
New York, N.Y., pp. 220–223, April 1998.

3.

Whitaker, Jerry C.: “Data Storage Systems,” in The Electronics Handbook, Jerry C. Whi-
taker (ed.), CRC Press, Boca Raton, Fla., pp. 1445–1459, 1996.

4.

Portions of this section based on background information provided by Micropolis, Chat-
sworth, Calif., 1996.

5.

Tyson, H: “Barracuda and Elite: Disk Drive Storage for Professional Audio/Video,”
Seagate Technology Paper #SV-25, Seagate, Scotts Valley, Calif., 1995.

6.

Lehtinen, Rick, “Editing Systems,” Broadcast Engineering, Intertec Publishing, Overland
Park. Kan., pp. 26–36, May 1996.

9.1.6

Bibliography

Anderson, D: “Fibre Channel-Arbitrated Loop: The Preferred Path to Higher I/O Performance,

Flexibility in Design,” Seagate Technology Paper #MN-24, Seagate, Scotts Valley, Calif.,
1995.

Goldberg, Thomas: “New Storage Technology,” Proceedings of the Advanced Television Summit,

Intertec Publishing, Overland Park, Kan., 1996.

Heyn, T.: “The RAID Advantage,” Seagate Technology Paper, Seagate, Scotts Valley, Calif.,

1995.

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9-26 Audio Recording Systems

Plank, Bob: “Video Disk and Server Operation,” International Broadcast Engineer, September

1995.

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