CommScope® Enterprise Data Center Design Guide - Public ...

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Skew can be caused by 3 basic reasons: timing differences between the optical transmitters,
differences in speed that the light signal travels down one fiber compared to other fibers in that
cable or differences in the lengths of each fiber within the cable. The manufacturing process of
today’s fibers and cables are uniform enough that skew will not be an issue. Research models
[developed by CommScope and Nortel, and accepted by the IEEE 802.3ba Task Force
committee] have shown that for 10 GbE, as an example, there would have to be at least 15
meters of difference in fiber length over a 100 meter nominal cable length to cause any issues.
For MPO trunk cables, the actual difference in fiber length within the same cable and 12-fiber
bundle would be insignificant, only be around a tenth of a meter over a 100 meter length.
For more information on skew, review the CommScope white paper What to do About Fiber
Skew, 2008.
Other Protocols
iSCSI (Internet Small Computer System Interface) is an Internet Protocol (IP)-based storage
networking standard for linking data storage facilities. By carrying SCSI commands over IP
networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over
long distances. iSCSI can be used to transmit data over local area networks (LANs), wide area
networks (WANs) or the Internet and can enable location-independent data storage and
retrieval. The protocol allows clients (called initiators) to send SCSI commands (CDBs) to SCSI
storage devices (targets) on remote servers. It is a popular storage area network (SAN)
protocol, allowing organizations to consolidate storage into data center storage arrays while
providing hosts (such as database and web servers) with the illusion of locally-attached disks.
iSCSI uses TCP/IP (typically TCP ports 860 and 3260). In essence, iSCSI simply allows two
hosts to negotiate and then exchange SCSI commands using IP networks. By doing this, iSCSI
takes a popular high-performance local storage bus and emulates it over wide-area networks,
creating a storage area network (SAN). Unlike some SAN protocols, iSCSI requires no
dedicated cabling; it can be run over existing switching and IP infrastructure.
So iSCSI is an alternative SAN transport protocol, and at this time, 10GbE iSCSI appears to be
a very inexpensive alternative to 8G FC, which requires a dedicated infrastructure. Applications
for iSCSI are limited, however, because it is IP based and carries higher overhead than FC.
Although Fibre Channel is the leader, iSCSI is playing an important role in today’s systems.
About 55% of the new server virtualization environments out there are being attached to Fibre
Channel and about 30% are being attached to iSCSI. (Nexsan Technology, July 2009)
ESCON (Enterprise Systems Connection) is a data connection created by IBM, and is
commonly used to connect their mainframe computers to peripheral devices such as disk
storage and tape drives. ESCON is an optical fiber, half-duplex, serial interface. Although SC
connectors are also utilized, an ESCON optical fiber connector is the interface developed
specifically for this application. This 2-fiber connector did not provide higher density, but it did
control the fiber positioning well, as the duplex patch cords were used to connect equipment
with ESCON ports to the cabling with ST, SC or other connector styles.
FICON (Fibre Connectivity) is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte
Command Code Sets-3 Mapping Protocol for Fibre Channel (FC) protocol. It is a FC layer four
protocol used to map both IBM’s antecedent (either ESCON or parallel) channel-to-control-unit
cabling infrastructure and protocol onto standard FC services and infrastructure. The topology is
fabric utilizing FC switches or directors. Valid data rates include 1, 2 and 4 Gigabit/s. Today’s
FICON has become similar to Fibre Channel and there are many ways it can interoperate with
Fibre Channel devices and switches. FICON utilizes SC and LC optical connections.
Other protocols exist for WAN and MAN applications, such as ATM and SONET. These
applications typically do not penetrate far into the Data Center. They would likely be translated
at the Core routing into an Ethernet Protocol before continuing into the data center.