A port multiplier offers the ability to cost-effectively increase the number of drives connected to a single SATA Host port. Without the availability of a port multiplier, a DVR that needs additional ports would have required a costly and time-consuming redesign and re-spin of the chip which contains the SATA Host to add more ports. By using a port multiplier, the SATA port can connect to multiple drives without changing the design.
A port multiplier enables up to 15 drives or devices to connect to each SATA Host port. Unfortunately, the bandwidth available to all of the drives is limited to the shared bandwidth of the SATA Host port at 1.5 Gbps, 3.0 Gbps or 6.0 Gbps. Even with the use of a port multiplier, each drive acts as if it is communicating directly to the Host, but the Host is aware that multiple drives are connected and must maintain the context for all of the drives. With each of the drives competing for the bandwidth and the Host context switching between the drives, it is critical to maximize the available bandwidth between the Host and the port multiplier. This is accomplished through the way the SATA Host “talks” to the port multiplier: command-based switching (CBS) or FIS (Frame Information Structure)-based switching (FBS) as shown in Figure 3.