Protocol Analysis

Fibre Channel is rich in capability and complexity. The trend towards heterogeneous storage area networks makes testing all the more critical. The difficulty of the validation task is further burdened by Fibre Channel&squot;s high data rate and the many reliability features inherent in it&squot;s design. Simply identifying the presence of errors is often masked by built-in error recovery features and redundancy at the ordered set level.

LeCroy&squot;s family of storage analysis products combine a powerful triggering and decoding engine with an easy to use graphical interface that removes complexity and trains the eye to understand more information faster. The intuitive use of color and the logical grouping of Sequence and Exchange events help communicate the meaning of fibre channel traffic. The LeCroy Tracer software detects and alerts the user to problems at all functional levels of the Fibre Channel layering including:

• FC-1: Invalid CRC; running disparity error, invalid 10bit codes
• FC-2: frame, sequence and exchange violations
• FC-4: FCP mapping and SCSI errors

The data rate generated by 2Gbps fibre channel can exceed 400MB per second (full duplex). This underscores the importance of filtering to isolate the most important traffic. Excluding channels from the recording and truncating the data portion of the payload are frequently used to preserve memory. But FCTracer goes well beyond these typical strategies with context sensitive filtering.

FCTracer&squot;s filtering engine leverages LeCroy&squot;s new sequencer capable of monitoring two unrelated series of events in parallel. It can filter different fields based on traffic detected on the link. This powerful triggering and filtering model is particularly useful for trapping intermittent problems on live SANs.

The FCTracer protocol analyzer offers special features for system level validation. It combines real-time monitoring with extensive performance metrics that provide visual and statistical information about the nature of the Fibre Channel traffic. These summary reports allow FCTracer to automatically track SCSI Command completion rates over extended periods allowing users to identify errant or overloaded devices.

Fibre Channel&squot;s complexity requires advanced testing capability. By selecting tools appropriate to the task, engineers can minimize the learning curve and reduce time to market for Fibre Channel products.

Learn more about Fibre Channel Technology

Fibre Channel Overview

Fibre Channel is a networking standard that is designed to move data through specific devices at specific speeds. Initially, Fibre Channel was used primarily to attach servers to a storage device, such as a RAID array or a tape backup device. More recently, Fibre Channel has evolved as the architecture of choice for many storage area networks. Fibre Channel is ideally suited to this role because it is very reliable, very scalable, and very flexible. It&squot;s possible to add more storage without disrupting operations, use a number of backup schemes that don&squot;t load the main network, and manage very large data collections that span multiple disks.

Fibre Channel is a high speed (100 to 800 Mbps) medium used for data transfer and storage. Fibre Channel provides a logical bi-directional, point-to-point connection between a host and a device. It is essentially a serial data channel created over fiber optic cabling which makes it possible to transfer data at much greater distances than SCSI and ATA interfaces.

Fibre Channel is designed to work with either copper wires or with fiber. Although copper is used only for shorter distances, a variety of copper mediums can be used, including telephone wire or coaxial cable. If fiber optic cable is used, distances of about six miles can be achieved with minimal loss of speed. Another advantage of dedicated channel based communication is its ability to provide increased bandwidth than network based links.

Features and Benefits

Storage Area Networks are not new ideas in data storage, however they are increasing in popularity due to high demand by users who need to store large volumes of data. A SAN is a group of storage devices connected via a network of connections to host computer or server. The primary advantages of SANs are:

• Storage resource pooling/sharing
• LAN and server-free backup
• Centralized storage resource management
• Data sharing - Large number of users
• Performance - High Speed, Low Latency
• Distance (>10KM)

Architecture

Channels and networks are the two primary ways that data is transferred between devices. Fibre Channel uses channels and frames instead of packets to move data. Each frame is approximately 2KB in size. Of this amount, 1.5 percent is header information. Channels transfer data through switched or direct point-to-point connections and they work by creating a fixed connection between the source and destination devices until the transfer is complete.

Connections are made on Fibre Channel systems through "interconnection components" including switches, hubs, and bridges. The ability of Fibre Channel to use different interconnect devices makes it scalable depending on user needs. For small Fibre Channel networks, hubs and bridges may be used for connecting devices in a topology called Fiber Channel arbitrated Loop (FC- AL). As Fibre Channel networks get larger and network demands increase, switching may be implemented. A switched Fibre Channel network is called a fabric.

Fabric topology permits multiple paths between two ports on the Fabric. Loop topology, on the other hand, there can only be one active circuit at a time. Loop and Fabric topologies can be combined to provide both connectivity and performance. If a link in a point-to-point topology fails, communication between that pair of ports stops. Communication between other point-to-point connected Ports continues.

Fiber optics are excellent for transmitting data across a network because they are reliable. They do not have the same problems that are associated with copper cabling such as attenuation (loss of signal strength) and noise. It is also more secure than copper because crosstalk does not occur with Fiber Optic cables. Fibre Channel traffic can also be transmitted over other cable types such as copper, coaxial cables or Unshielded twisted pair (UTP) wires.

Class of Service - Fibre Channel technology makes use of classes of service to define communication between devices.

• Class 1 a dedicated channel between two connection devices. In this configuration, if a host and a device are connected, no other host can use that connection. The advantage of using service class 1 is speed and reliability.
• Class 2 is known as a "connectionless" service. It a frame-switched link that guarantees delivery of packets from device to device and packet receipt acknowledgments.
• Class 3 is called unacknowledged connectionless service and is good for broadcasts. This configuration allows multiple transmissions to be sent across the Fibre Channel fabric to multiple devices.
• Class 4 is called "intermix", which creates a dedicated connection but it also allows class 2 traffic to access the link. This method is very efficient and it allows for greater bandwidth because more than one connection can access the system at any time.

The Fibre Channel architecture is illustrated in Figure 1. The layers in the table represent different functions that exists within a Fibre channel system. As with the other communcation standards, protocol level analysis is often focused at the link layer (FC-2) and above.

Figure 1