3.5 Topologies

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3.5 Topologies

3.5.1 Point-to-Point

The first topology to discuss is the simplest. It is called a point-to-point topology. It is two nodes (devices) connected together. One node could be a computer system and the other node could be a disk array.

Figure 3-3. Point-to-Point Topology

This topology uses two nodes (each node must have at least one port), and the nodes are connected using one link or cable. The point-to-point connection guarantees instant access with no interference from any other node or application.

If a peripheral node, such as a disk array, has two N_Ports, access to the disk array could be shared between two computer systems. For example, the disk array could act as a repository of common software for two computer systems. This would be considered two point-to-point connections. See Figure 3-4.

Figure 3-4. Two Point-to-Point Connections

A point-to-point connection may also be considered a two-node loop. Hewlett-Packard s implementation of point-to-point is a two-node loop. This brings us to our next topology.

3.5.2 Arbitrated Loop

A loop, called Fibre Channel Arbitrated Loop (FC-AL), can have up to 127 ports connected in series (one right after the other) continuing around and back to the originator. For example, the node 1 transmitter is connected to the node 2 receiver, the node 2 transmitter is connected to the node 3 receiver, and so on, until the final node transmitter is connected to the node 1 receiver, thus completing the loop. Figure 3-5 illustrates this example.

Figure 3-5. Arbitrated Loop Topology

A node loop port (NL_Port) wins arbitration on the loop and establishes a connection with another NL_Port on the loop. At the time the connection is established it is considered to be a point-to-point connection or two-node loop.

In an arbitrated loop only the two connected ports can communicate at any given time. All the other ports act as repeaters. When the communication comes to an end between the two connected ports, the loop becomes available for arbitration and a new connection may be established. Fairness is provided for during arbitration to provide equal access to all ports. The FC-AL features, operations, and implementations will be discussed in further detail later in this chapter.

3.5.3 Switch Topology or Fabric

The switch topology uses the concept of fabric. The fabric is a mesh of connections. When attached to a fabric, a single N_Port can access all the rest, including members of loops .

Unlike the FC-AL topology, many connections may be established within the fabric. It can be compared to a telephone system where many phone calls may be occurring all at the same time.

Any node can be attached to a fabric through the N or NL_Ports by way of a link. The port in the fabric is called an F_Port. An N_Port attaches to an F_Port. If an NL_Port is attached to a fabric, then the fabric port is an FL_Port.

Figure 3-6. Fabric Topology
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Storage Area Networks: Designing and Implementing a Mass Storage System
ISBN: 0130279595
EAN: 2147483647
Year: 2000
Pages: 88