IBM Midrange System DS4000 Series Hardware Manual page 31

FC supports a much higher number of devices and much longer cable lengths than SCSI. It
has become the preferred disk attachment technology in midrange and large scale data
center solutions.
At the time of writing, the IBM Midrange System Storage series' maximum FC throughput is
8 Gbps; 10 Gbps links can be used today, but only for SAN switch interconnection.
Host servers contain one or more FC Host Bus Adapters (HBA). The HBAs provide
connectivity to the storage devices using FC cabling and SAN Switch.
For more information about Fibre Channel and SANs, see Introduction to Storage Area
Networks, SG24-5470.
FC topologies
FC supports several connectivity topologies:
Point-to-point
This is the simplest topology and provides a direct link between an FC HBA inside a host
server and a storage device.
Arbitrated loop
This topology can be used to interconnect several FC devices. A typical example is to
attach a certain number of host servers to an FC storage subsystem. A loop can consist of
up to 126 devices.
Devices on the loop use one-way ring communication. In any given moment, only two
devices on the loop can communicate. This means the devices share bandwidth, so the
arbitrated loop topology is not suitable for high performance requirements.
Arbitrated loops were commonly implemented with the use of an FC hub. Even though this
is physically a star topology, logically it will be a loop. Alternatively, devices can be
connected in a daisy chain manner. Arbitrated loops are rarely seen these days, as
switched fabrics have become the norm.
Switched fabric
The most commonly used topology in a typical SAN today is switched fabric. SAN
switches are used to provide FC connectivity between the host servers and storage
devices. Switched fabrics can become very complex in large scenarios, connecting
hundreds of host servers to a very large number of storage subsystems.
SAN switches provide optimized traffic flow and increased performance by allowing
concurrent data transfers between many connected hosts and storage devices. Switched
fabrics can provide dedicated bandwidth, as opposed to arbitrated loop technology, where
the bandwidth is shared among all the devices in the loop.
FC protocol layers
The FC protocol is split into five layers, named FC0 through FC4. Let us look briefly at them:
FC0 is the physical layer, which describes cabling, connectors, signalling, and so on. This
layer defines the physical media implementation.
FC1 is the data link layer. This layer contains the 8b/10b encoding and decoding of signals
for transmission across the physical media.
FC2 is the network layer and defines the main FC protocols. This layer defines how the
frames are transferred.
FC3 is the common services layer. This layer provides services such as multi-casting and
striping.
Chapter 1. Introduction to IBM Midrange System Storage storage subsystems
13