Book Excerpt: Storage Area Network Fundamentals

The following is an excerpt from Chapter 4 of "Storage Area Network Fundamentals" (ISBN 158705065X ) published courtesy of Cisco Press.


With the growing popularity of storage area networks (SANs), Fibre Channel technology has emerged to the forefront as an effective means of solving storage-related problems that have plagued corporate networks all over the world. A wealth of Fibre Channel products are available, including Host Bus Adapters (HBAs), connectors, switches, hubs, gateways, and Fibre Channel-to-Small Computer System Interface (SCSI) bridges. Along with optical cables, Fibre Channel products enable network administrators and designers to develop solutions to storage problems related to performance, distance, backups and restoration, bandwidth, and security. For example, Fibre Channel switches play an important role in enhancing the performance of database servers by switching data queries and their results much faster. Similarly, switched Fibre Channel hubs provide high-speed access to disk arrays, tape libraries, and Just a Bunch of Disks (JBODs).

To build a successful SAN that fulfills all or most of the requirements of a corporation, you must choose each device of a SAN with care and understanding. Understanding the purpose and the capabilities of each Fibre Channel device will help you make effective choices while designing a SAN.

With the infiltration of SAN and Fibre Channel technology in corporate storage solutions, many vendors have jumped into the field of Fibre Channel devices. You need not restrict yourself to the Fibre Channel products offered by one single vendor. As a SAN designer, an intelligent mix and match of compatible products will help you to implement a cost-effective and high-performance storage solution.


Similar to network interface cards (NICs) that are used in traditional Ethernets, HBAs provide the physical interface between the input/output (I/O) host bus of Fibre Channel devices (such as servers and storage devices) and the underlying Fibre Channel network. In other words, HBAs connect Fibre Channel devices to Fibre Channel links.

NOTE: Popularly used I/O host buses include IBM's PCI-MCA, HP's HSC, and Sun's SBus. The term PCI-MCA is a combination of two terms—PCI (Peripheral Component Interconnect) and MCA (Micro Channel Architecture). PCI-MCA is a 32-bit, high-speed interface between the processor of a computer and the attached peripheral devices and expansion cards. HP's High Speed Connect (HSC) is a high-speed proprietary interface that functions much like PCI. SBus is a 32-bit bus used in Sun's SPARC workstations. SBus facilitates the transactions between the processor and the attached peripheral devices. SBus can also help the processor in identifying the corresponding device drivers of the attached devices.

In addition to acting as the physical interface between the host bus and the underlying Fibre Channel link, other functions of HBAs include the following:

  • Initialization of Fibre Channel nodes and ports onto the underlying arbitrated loop or Fabric. Similar to NICs, HBAs also provide a hard-coded, 64-bit Node_Name or World-Wide Name (WWN) address and Port_Name or World-Wide Port Name (WWPN) address to the device and its ports. These addresses help the Fabric in identifying a node or a port before the node or port has been initialized by the loop or has attempted a Fabric logon.

  • Support to various upper-level protocols (ULPs), such as TCP/IP, SCSI, and so on.

  • Interpretation of incoming data streams by performing context switching. When context switching is done at the HBA level, a significant amount of switching overhead is reduced. SEPARATED BY RULES NOTE Context switching is the capability of an HBA to issue and process multiple commands to various SAN storage devices simultaneously to maximize efficiency when accessing data. The entire concept is similar to multitasking. However, the basic difference between multi-tasking and context switching is that in multi-tasking, inactive programs continue to run in the background. In contrast, in context switching, any inactive program is suspended until it becomes active again. END RULE SEPARATION

  • 8B/10B encoding of data

HBAs can differ on the basis of many criteria. These criteria include the following:

  • Physical links supported—Fibre Channel physical links include copper links, single-mode fiber-optic links, or multi-mode fiber-optic links.

  • Protocols supported—Commonly implemented Fibre Channel protocols include SCSI, IP, FCP-SCSI, IPI-3, and SB-2.

NOTE: IPI is a high-bandwidth interface between the host computer and its peripheral devices (hard drives, tape drives, optical libraries, and so on) that supports transactions ranging from 3 to 25 Mbps. IPI-3 is the latest version of IPI. Based on the Single Byte Command Code Set (SBCCS), Single Byte-2 (SB-2) is a signaling protocol that provides high-bandwidth and high-performance communication between the processor and I/O devices. In addition, SB-2 also facilitates long-distance data exchanges.

  • Operating systems supported—UNIX, Windows NT, AIX, and Macintosh are some of the operating systems that are commonly used in storage environment.

  • Topologies supported—Point-to-point, Fibre Channel-Arbitrated Loop (FC-AL), and switched Fabric are the three Fibre Channel topologies.

  • Number of connections supported—Connections can be a single node to Fibre Channel link connections, multiple connections, or multiple switched connections.

With an increase in the number of vendors manufacturing Fibre Channel products, SANs are growing heterogeneous in nature. This implies that SANs are using more and more varied hardware and software platforms. Therefore, of the many HBAs that are available today, it is very important to choose HBAs that can support a wide variety of platforms. Several vendors offer a variety of HBAs. IBM, JNI Corp., Qlogic, and HP are some of the more popular HBA vendors. The price of an HBA can range from $500 to $1500 per adapter. Figure 4-1 shows an HBA.


Figure 4-1: HBA

Fibre Channel Connectors

Data transfer rates over the Fibre Channel infrastructure are measured in gigabits. As a result, the data transported over Fibre Channel links is sometimes referred to as gigabit transport. Fibre Channel connectors play an important role in facilitating the gigabit transport between two communicating ends. The connectors provide an interface that converts any type of communication transport into gigabit transport.

Four types of Fibre Channel connectors are used to interconnect Fibre Channel devices:

  • Gigabit Interface Converters (GBICs)

  • Gigabit Link Modules (GLMs)

  • Transceivers

  • Media Interface Adapters (MIAs)

The following sections discuss each of these connectors.


Fibre Channel devices use electronic signals known as differential serial data signals. These signals are encoded according to the 8B/10B encoding method. However, the link between two communicating devices is either copper-based or fiber-optic. Copper and fiber-optic links are not capable of transmitting differential serial data signals. Using GBICs solves this problem.

GBICs are hot-pluggable and easily replaceable interface modules that are responsible for converting differential serial data signals into corresponding optical or copper signals that can be transported to the destination. At the sender end, a GBIC takes differential serial data signals as input and converts them according to the Fibre Channel link on which the data is transmitted in the form of signals. At the recipient end, the GBIC receives the optical or copper signal and delivers it to the device as differential serial data signals.

On optical links, GBICs support two modes of optical operations depending on the wavelength of the laser being used. The two optical modes of a GBIC are ShortWave (SW) mode and LongWave (LW) mode.

ShortWave GBICs provide connectivity for comparatively short distances—up to 500 meters. The 50-micron fiber-optic cables offer SW mode at a distance of 2 to 500 meters. The 62.5-micron fiber-optic cables offer the SW mode at a distance of 2 to 175 meters. The 9-micron fiber-optic cables do not support the SW mode.

As the name suggests, GBICs that function in LW mode offer long connectivity distances of up to 10 kilometers. The 9-micron fiber-optic cables offer LW mode up to a distance of 10 kilometers. Both 50-micron and 62.5-micron fiber-optic cables offer the LW mode up to a distance of 550 meters.

GBICs are external pull-push type connectors that need to be attached to the HBA as shown in Figure 4-2. GBICs plug into the Fibre Channel device. Fibre Channel cables, or links, are then attached to the GBIC. They are commonly used with HBAs, switches, and gateways and support transfer rates of 1063 Mbps and above. Prices of GBICs range from $250 to $3000.

NOTE: Apart from Fibre Channel networks (such as SANs), GBICs can also be used in Gigabit Ethernets.


Figure 4-3 GBIC


Also referred to as Gigabaud Link Modules, GLMs are the low-cost predecessor of GBICs. GLMs facilitate full-duplex communication between Fibre Channel devices. They convert differential serial data signals to optical or copper signals so that the signals can be transmitted over the Fibre Channel link. At the recipient end, GLMs reconvert the optical or copper signals to differential serial data signals that the recipient can understand and process.

Although the GLMs function similarly to the GBICs, there are differences between the two.

Table 4-1 enumerates the differences between GLMs and GBICs.

The Differences Between GBICs and GLMs
GBICs offer transfer rates of 1063 Mbps and above. GLMs offer transfer rates of 266 Mbps and 1063 Mbps.
GBICs are hot-pluggable. This means that the GBICs can be attached to or removed from the Fibre Channel device while the device is still operational. GLMs are not hot-pluggable. The Fibre Channel device needs to be shut down before GLMs can be attached, replaced, or repaired.
GBICs are easy to configure and use. GLMs are difficult to configure and use.
GBICs are more expensive than GLMs. GLMs are comparatively cheaper than GBICs.

Similar to GBICs, GLMs also use two types of lasers for transportation of data over fiberoptic links. These include the following:

  • SW

  • LW

You can attach GLMs to the Fibre Channel devices as external connectors, or you can build GLMs into the HBA. Figure 4-3 shows an external GLM. Figure 4-4 shows a GLM that is a part of the HBA. The price of GLMs ranges from $900 to $2500.


Figure 4-3 External GLM


Figure 4-4 GLM as a Part of the HBA


Transceivers are hot-pluggable Fibre Channel connectors that are generally used in switch implementations. They facilitate high-speed, bidirectional, point-to-point communication between Fibre Channel devices. Transceivers support communication speeds of 2 Gbps and above (3.25 Gbps to 10 Gbps). Some transceivers can also provide transaction speeds up to 10 Gbps.

Commonly used Fibre Channel transceivers include the following:

  • 1 u 9 transceivers

  • Small Form Factor (SFF) transceivers

  • 1u 28 transceivers

1u9 optical transceivers are the most commonly used transceivers. Transceivers are preferred over GBICs because they are roughly two times faster and are much easier to maintain than GBICs. However, transceivers are also more expensive than GBICs and GLMs. The price of transceivers ranges from $350 to $2000.

Figure 4-5 shows a 1u9 transceiver.


Figure 4-5 1X9 Transceiver

SFF transceivers are small, laser-based optical connectors that are the same size as RJ-45 connectors. These transceivers offer high data transfer rates (1063 Mbps) and are well-suited for networking applications that require a high-speed serial interface. Being small in size, SFF transceivers are highly recommended for environments where devices are squeezed in less space. The performance of SFF connectors is directly related to the precision of fiber alignment. Therefore, take special care while connecting these transceivers to Fibre Channel devices. The price of SFF transceivers ranges from $450 to $3000.

Figure 4-6 depicts an SFF transceiver.


Figure 4-6 SFF Transceiver

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