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Multiple Segment and Channel Support
In its simplest form, a host adapter provides support for a single SCSI chain: that is, a single set of devices that are all connected together on the same SCSI bus. This is the way that many older and low-end SCSI host adapters work. They are fine for simple implementations, but are too limiting for complex SCSI setups. Especially with modern systems that need to use both LVD and single-ended devices, an adapter with support for just a single segment is insufficient for maximum performance. To expand capabilities, host adapter manufacturers make cards that support multiple segments, multiple channels, or both.
A segment is an electrically-isolated "piece" of a SCSI bus; a single bus can be made up of one or more segments. Cards that implement multiple segments allow for more flexibility because the segments are electrically separate. Each segment can have a cable as long as the normal maximum cable length allowed for that particular type of SCSI, for example. One segment can use an internal cable within the PC and another an external cable. It's important to remember though that two segments on a single channel are logically considered to be part of the same SCSI bus even if they are electrically separate. This means all devices on all segments must have unique IDs, and that maximum bandwidth is shared between all devices on all segments that make up the bus.
The most expensive host adapters go beyond multiple segment support and actually have multiple channels. These are similar in concept to the way an IDE/ATA controller typically has two channels. Each channel is completely independent of the other, both electrically and logically. This means the two run in parallel with each other: you get support for twice as many devices, and twice as much throughput. In essence, a card with two channels is two host adapters in the same package. For example, an Ultra160 host adapter with dual channels will support 30 drives (16 per channel less one each per channel for the host adapter) and theoretical throughput of up to 320 MB/s (160 MB/s per channel). Note that each channel can itself have more than one electrical segment.
Host adapters that support multiple channels are not really needed for most applications, especially if already using high-performance SCSI like Ultra160; they are more common in servers than desktop PCs. Multiple segments, however, are commonly found even in desktop SCSI cards. One common use for multiple segments is to allow independent use of LVD and SE devices on the same host adapter without causing the LVD devices to degrade to SE operation. See this page for more on this important feature of Ultra2 and faster host adapters. Some cards use multiple channels to isolate LVD and SE, which is probably even better (though it may be more expensive.)
Next: RAID Support