Storage Area Networks (Sans) were designed to ease the burden of managing exploding storage demands. By placing all storage on its own network, there becomes a single point of management. In addition, the technology allow multiple servers to access the same storage.
This is helpful for fail-over and clustering systems, as building a network for storage previously required using a new network protocol. Existing standards, such as Ethernet, are designed for short transfers of packets and don't scale well to the requirements of storage.
Servers will often have to stream large amounts of data, so a new protocol was developed, Fibre Channel. And unlike other international standards, Fibre Channel has the 'correct' spelling, as the editor of the standard was British.
What's in Fibre Channel?
Fibre Channel is actually a combination of other standards, including Fibre Distributed Data Interface (FDDI), SCSI, High Performance Parallel Interface (HIPPI) and Intelligent Parallel Interface (IPI). We'll cover how these fit in as we walk through the Fibre Channel standard.
Topology
Fibre Channel offers the best parts of channels (point-to-point) and networks (an aggregation of hosts) through the Fabric. This is an intelligent interconnection scheme. Hosts only have to monitor their own point-to-point connection to the Fabric. This reduces overhead placed on the host and allows transfer speeds of up to 2Gbps.
As hosts only have to connect into the Fabric, the exact topology is irrelevant and can be point-to-point, arbitrated loop, or fully switched. Exact topologies will depend on management and cost decisions during the implementation of the San.
Fibre Channel layers
Fibre Channel offers a reliable and fast transmission platform. Using single-mode fibre optic media and a longwave light source allows data to be transmitted 10km at 1Gbps. The protocol is defined in a similar way to the OSI 7-Layer model, although Fibre Channel only has five layers.
FC-0 Layer
As with the physical layer of the OSI model, the FC-0 layer is concerned with providing the physical link. This includes defining the fibre, connectors, optical and electrical parameters for all the available data rates.
This layer has been designed to use a wide range of different link technologies. A San may consist of many different link technologies to give the most efficient performance and cost.
FC-1 Layer
FC-1 defines the transmission protocol. It's responsible for serial transmission and encoding and decoding data. The system used is similar in operation to FDDI, but Fibre Channel uses a 10-bit transmission code opposed to the 5-bit code used by FDDI. Data is encoded 8-bits at a time into the transmission character. This system introduces some redundancy into the system and allows some recovery in the event of an error.
FC-2 Layer
This layer defines the frame layout and header formats and serves as the transport mechanism. The layer is made up of a set of building blocks used to transfer data across a link. These are:
Ordered Set
Frame
Sequence
Exchange
Protocol
Ordered Set
Ordered Sets are 4-byte transmission words that have a special meaning. For example, the start-of-frame and end-of-frame delimiters. Other commands include 'Idle' for declaring a node ready for transmission and reception, and 'Receiver Ready' indicating that the interface buffer can accept more frames.
Frame
Fibre Channel frames are used to contain the data transmitted and have a maximum length of 2048
bytes. In addition, frames can also be Link Control frames, which are used to send messages including acknowledgements and rejections.
The Fabric has to accept frames from the source port and route them to the destination port. This layer is responsible for splitting data into frames and reassembling them at the other end.
Sequence
A sequence is a collection of related frames in transmission. Each frame is labelled with a unique sequence number so that they can be reassembled in the correct order at the destination end. Sequence numbers are also used for error control to make sure that all of the data has arrived.
Exchange
An exchange is one or more sequences for a single operation. They can be uni- or bi-directional, but only one sequence can be in operation at a time.
Protocol
The protocol defines the service offered by Fibre Channel. It may also be specific to high-layer services. Fibre Channel services include Fabric logon, and data transfer.
Flow Control
Part of the job of this layer is to provide adequate flow control. This prevents buffers from being overrun at the receiver's end. Flow control is provided by sending control frames.
When a receiver's buffer is full, a 'Busy' frame is sent over the network. This causes the transmitter to back off until the receiver is ready.
Service Classes
This layer supports the three service classes that Fibre Channel offers. The first class is circuit switching with guaranteed delivery in order, which is used by data channels. The second class is packet switching with guaranteed delivery. The third class is packet switching without guaranteed delivery.
FC-3 Layer
This layer provides common services required for advanced features. These include:
- Striping - this multiplies bandwidth by using multiple ports in a large pipe.
- Hunt groups - similar to hunt groups on telephones, this service allows multiple ports to respond to the same alias address.
- Multicast - this service lets one transmission be delivered to multiple addresses.
FC-4 Layer
This is the highest layer in the Fibre Channel structure. It defines the application interfaces that can be run. Upper layer protocols are mapped onto the Fibre Channel levels below. Fibre Channel can allow both network and channel information to be transmitted at the same time. The list of supported protocols is:
- HIPPI
- IPI
- SCSI
- ATM
- IP
How does it fit into the network?
For hosts to connect into the Fibre Channel network, it is essential that a Fibre Channel host adapter is installed. This is a network card that gives a host a physical address and allows them to hook into the fabric.
Hosts include servers, clients, tape libraries and disk drive arrays. Once a client machine connects into the Fabric, it can access storage on any other machine subject to permissions.
This is useful beyond just having a single point of management. It also makes it easier to deploy clusters, as all of the host machines can see the shared storage. Using standard SCSI soon becomes complex when more than two machines are involved.
Sans have the advantage of range. Backup servers and tape libraries can be placed many kilometres from the main storage, effectively making them offsite. The speed of the network ensures that this isn't a noticeable problem.
Sans don't ignore existing network hardware either. Old SCSI boxes can be added into a San through the use of a SCSI to Fibre Channel bridge.
While not as good as pure Fibre Channel, it at least allows existing equipment to be integrated into a new storage domain.
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