Subject: Storage Network Management and Retrieval

Difficulty: Medium.

Marks: 10M

FC Ports

A port has one input and one output channel. The connection between two ports is called a link. The link in the point to-point topology is bidirectional. The arbitrated loop topology has a unidirectional link. Arbitrated loop can also be realized by a hub. Both fabric and arbitrated loop topologies are realized by different, incompatible protocols.

The following are the different types of ports with different capabilities:

N-Port (Node Port): N-Port is a node port. A node describes an end device (server and storage device), and device it is connected to any end device. The end device can be a part of the fabric topology or the point-to-point topology.

F-Port (Fabric Port): F-Ports are equivalent to N-Ports in the FC switch. The F-Port sends the passing frames sent by an end device with N-Port to the intended end device.

L-Port (Loop Port): Arbitrated loop uses different protocols than the fabric for data exchange. An L-Port connected to the arbitrated loop indicates that the device connected to the L-Port can be treated as an end device (server and storage device), and is a part of the arbitrated loop topology.

NL-Port (Node Loop Port): An NL Port can be used as both an N-Port and an L-Port. Hence, it can be connected both in a fabric and in an arbitrated loop; Most modern HBA cards are equipped with NL-Ports.

FL-Port (Fabric Loop Port): An FL- Port allows a fabric to connect to an arbitrated loop. The FL-port will be present of FC switch when it is connected with arbitrated loop.

E-Port (Expansion Port): Two FC switches are connected using E-Port. An E-Port is responsible for transmission of the data between the end devices that are connected to two different FC switches. E-Port facilitates smooth communication over the FC network.

G-Port (Generic Port): G-ports can be used as generic ports, meaning that they are configured automatically as per their usage in FC switches.

B-Port (Bridge Port): Connecting two FC switches together via asynchronous transfer mode (ATM), synchronous optical networking (SONET) synchronous digital hierarchy as well as Ethernet and IP is facilitated using B-Port. Thus, FC SANs that are long distance apart can be connected together using classical widearea network (WAN) techniques.

Login types

The communication between two ports could be established only if the ports know each other before application processes can exchange data over them. For this purpose, there is a three stage-login mechanism.

Fabric login (FLOGI): Establishes a session between an N-Port and F-Port. Fabric login is an essential pre-requisite for further exchange of frames. The link is established by exchanging buffer-to-buffer credits and a dynamic address is assigned by F-Port to N-Port. The fabric login is very much important for the point-to-point and fabric topology. The response received to $N$ -Port conveys the information from the corresponding port whether it is a fabric topology or a point-to-point topology. Fabric login is optional in arbitrated loop topology.

N-Port login (PLOGI): Establishes a session between two N-Ports. To perform the data exchange at FC-4 layer, first the N-Port login takes place and later the fabric login is an essential pre-requisite. The N-Port login exchanges various service parameters such as end-to-end credit. For Class 3 communication, N-Port login is optional and it is compulsory for all other service classes.

Process login (PRLI): Establishes a session between two FC-4 processes that are based upon two different N Ports. After N-Port login, process login takes place.