SAN - good cabling, bad cabling

Published 28 August 2002

Authors: Jon Tate


This Tip discusses SAN disciplines - cabling and good cabling practices.


There are a number of different types of cable that can be used when designing a SAN. Consider the type of cable and the route it will take. The following section details various types of cable and issues related to the cable route.

The Fibre Channel cabling environment has many similarities to telecommunications or open systems environments. The major difference between a Fibre Channel and an open systems LAN/WAN environment is the reduced cable distance between devices and associated attenuation loss. The increase in flexibility and adaptability in the placement of the electronic network components is similar to the LAN/WAN environment, and a significant improvement over previous data center storage solutions.

Shortwave or longwave
Every data communications fiber falls into one of two categories:

  • Single-mode
  • Multi-mode

In most cases, it is impossible to distinguish between single-mode and multi-mode fiber with the naked eye (unless the manufacturer follows the color coding schemes specified by the Fibre Channel physical layer working subcommittee (orange for multi-mode and yellow for single-mode). There may not be a difference in outward appearance, only in core size. Both fiber types act as a transmission medium for light, but they operate in different ways, have different characteristics, and serve different applications.

Single-mode (SM) fiber allows for only one pathway, or mode, of light to travel within the fiber. The core size is typically 8.3 µm. Single-mode fibers are used in applications where low signal loss and high data rates are required, such as on long spans between two system or network devices, where repeater/amplifier spacing needs to be maximized.

Multi-mode (MM) fiber allows more than one mode of light. Common MM core sizes are 50 µm and 62.5 µm. Multi-mode fiber is better suited for shorter distance applications. Where costly electronics are heavily concentrated, the primary cost of the system does not lie with the cable. In such a case, MM fiber is more economical because it can be used with inexpensive connectors and laser devices, thereby reducing the total system cost. This makes multi-mode fiber the ideal choice for short distance under 500meters from transmitter to receiver (or
the reverse).

50/125 micrometers or 62.5/125 micrometers
Optical fiber for telecommunications consists of three components:
  • Core
  • Cladding
  • Coating
We show the structure of a fiber optic cable below.
Structure of a fiber optic cable

The core is the central region of an optical fiber through which light is transmitted. In general, the telecommunications industry uses sizes from 8.3 micrometers (µm) to 62.5 micrometers. The standard telecommunications core sizes in use today are 8.3 µm (single-mode), 50 µm (multi-mode), and 62.5 µm (multi-mode).

The diameter of the cladding surrounding each of these cores is 125 µm. Core sizes of 85 µm and 100 µm have been used in early applications, but are not typically used today. The core and cladding are manufactured together as a single piece of silica glass with slightly different compositions, and cannot be separated from one another.

The third section of an optical fiber is the outer protective coating. This coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber. During the installation process, this coating is stripped away from the cladding to allow proper termination to an optical transmission system. The coating size can vary, but the standard sizes are 250 µm or 900 µm. The 250 µm coating takes less space in larger outdoor cables. The 900 µm coating is larger and more suitable for smaller indoor cables. Most enterprises today use the 62.5 micron core fiber due to its high proliferation in the local area networks (LAN). The Fibre Channel SAN standard is based on the 50 micron core fiber and is required to achieve distances specified in the ANSI Fibre Channel standards. Customers should not use the 62.5 micron fiber for use in SAN applications. It is wise to check with any SAN component vendor to see if 62.5 is supported.

Single mode and multi-mode distances
Typical supported combinations are:
  • 50 Micron Multimode Shortwave <= 500 meters
  • 62.5 Micron Multimode Shortwave <= 175 meters
  • 9 Micron Singlemode Longwave =< 10 Km
We show the differences between single mode and multi-mode below.
Differences between single mode and multi-mode

Cabling practices
Some of the most common mistakes made in SAN setup and management are related to cabling and include:
  • Bad cabling practice: Mark or tag all the cables and have a clear picture of how the devices are connected.
  • Inconsistent naming practice: Employ naming conventions and stick to them.
  • Broken cables, because of bad cabling practices. For example:
    • Pulling cable ties too tight
    • Having too small a radius of curvature
    • Leaving cables hanging from connections with no support
    • Lack of strain relief on cables
    • Not using dust caps
    • Not keeping connectors clean
    • Leaving them dragging on the floor for people to kick and trip over

Business continuance
Metropolitan Area Network (MAN) cables are typically used for business continuance between two sites. MANs used for business continuance normally consist of a diverse route of a primary and alternate cable. The alternate route is normally only used when the primary route is not available. To ensure we introduce no single points of failure it is critical these cables enter and leave the building at separate locations and at no point share the same cable run or equipment. It is important to have a detailed intersite cable route plan to highlight any single points of failure and to determine the exact distance of both routes. If the primary site is several kilometers shorter than the secondary route, there may be latency issues to consider when using the secondary route.
As the MAN cables enter the buildings, the routes of the primary and alternate cables should be clearly marked on the floor plan.

Special Notices

This material has not been submitted to any formal IBM test and is published AS IS. It has not been the subject of rigorous review. IBM assumes no responsibility for its accuracy or completeness. The use of this information or the implementation of any of these techniques is a client responsibility and depends upon the client's ability to evaluate and integrate them into the client's operational environment.

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