In the FC [Fibre Channel] protocol, there are three different fabric topologies: Point-To-Point, Arbitrated Loop, and Fabric.
PP or PTP [Point-To-Point] topology is a direct connection between two N_Ports, with at least one of the ports being a server (initiator). The topology requires no arbitration (contention-sharing) for the storage media because of separate links/circuits for transmission and reception. However, the topology is limited to two nodes, making it not scalable.
There are two types of FC AL [Arbitrated Loop]: the hub to interconnect the ports and daisy-chaining the devices. AL combines the advantages of the fabric topology (support for multiple devices) with the ease-of-operation of point-to-point topology. In a FC-AL topology, devices are connected to a central hub. Like Ethernet LAN [Local Area Network] hubs, the Hub arbitrates (shares) access to devices and offers no additional functionality or intelligence to the connected devices beyond serving as a centralized connection point.
In an AL hub, devices must seize control of the loop (carrier sense) and then establish a point-to-point connection with the receiving device. Once the point-to-point connection is established, data is transferred. Once the transmissions have ended, devices connected by the hub can once again “arbitrate” (share-contend) to gain control of the loop for establishing subsequent point-to-point connections. In hub topology, there can be up to 126 nodes connected to a single link though with contention access the more the devices on network, the lower the performance. This makes is highly uncommon to attach the 126-node maximum to a single link.
In an FC-AL daisy-chain, the devices are connected in a series and the transmit (send port) of one device is connected to the receive port of the next device in the chain. Daisy-chain networks are ideal for small networks but not very scalable since all devices on the loop must be operational (on) at the same time for the loop to be up and running. The failure of a single device will cause a break to the loop, and cause the entire loop to cease operation.
Fabric topology consists of one or more fibre channel switches interconnected through one or more ports. The fabric ports are X ports that connect to either another X port on a different switch or to a Y port on a node. Each switch typically contains 8, 16, 32 or 64 ports; the two largest types are sometimes called director switches.