4.1 Point-to-Point

A point-to-point topology is a simple, direct connection between two N_Ports. As shown in Figure 4-1, the transmit lead of one N_Port is connected via copper or optical cabling to the receive lead of its partner. The partner's transmit, in turn, is cabled to the other N_Port's receive. This cabling scheme creates dedicated bandwidth between the pair, with throughput of up to 100MBps in each direction for 1Gbps Fibre Channel, and 200MBps in each direction for 2Gbps Fibre Channel.

Figure 4-1. A point-to-point Fibre Channel link between a server and a disk

Before data transactions can occur, the two N_Ports must perform an N_Port login to assign N_Port addresses or N_Port IDs. Thereafter, a persistent connection is maintained, with utilization of the dedicated link determined by the application.

Although it is conceivable to have an application requiring simultaneous full duplex transfers for example, 200MBps total throughput at 1Gbps in practice only one side of the link sees any real traffic at a given moment. Depending on the application, a server and disk in a point-to-point configuration may be performing either reads or writes of data, or both concurrently. From the server's standpoint, an ongoing read operation would initiate incoming frames on the server's receiver, with only ACKs (for Class 1 or 2 service) leaving the server's transmitter. Even then, the 100MBps available bandwidth on the server's receive link is not likely to saturate. Link utilization in point-to-point configurations is determined by the performance of the Fibre Channel controllers at either end, along with the buffering available to queue up data to be transmitted or received.

The original point-to-point configurations were based on quarter-speed, 266Mbps bandwidth with an effective throughput of 25MBps in each direction. Distributed primarily by Sun Microsystems, quarter-speed implemen tations used fiber-optic GLMs as the transceiver interface to the host bus adapter and disk controller logic. Tens of thousands of these systems were shipped to customers during the mid-1990s, creating a large but unadvertised base of Fibre Channel products in production environments long before SANs became popular. This legacy base has provided valuable experience for improving both the physical transport and protocol support.

As server and disk performance has increased, Fibre Channel through put has kept pace with 1Gbps and 2Gbps implementations. At the same time, advances in fabric and arbitrated loop technology have enabled more flexibility and functionality than point-to-point provides. A point-to-point configuration is still viable for simple configurations, but for growth of the SAN it is important to select the proper HBA and controller components. If the vendor includes device drivers for point-to-point protocol, and arbitrated loop and fabric attachment, you can accommodate additional devices on the SAN with minimal pain.