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SAS defines an external (box-to-box) connection scheme using InfiniBand 4x cables. Known as "Wide" links, these connections include 2 or 4 physical links and are used to aggregate bandwidth and provide a convenient way to attach multiple external end devices.

The SAS specification also introduces a Fan-out PHY architecture that utilizes Expander devices to enable one or more SAS host controller ports to connect to a large number of drives. Each Expander allows connectivity to 64 ports, which may include host connections, other Expanders, or hard disk drives.

Expanders have a profound effect on test and analysis because they have a unique ability to use different physical pathways across a wide link to complete a single SCSI operation. This means the SAS Initiator can open a connection to a SAS target device yet complete the transfer over a different physical connection. These transfers over wide links may change their pathways dynamically within the expander device.

This increases the complexity of SAS test and debug because it requires users to monitor multiple links concurrently to record all dwords associated with a single SCSI transaction. Engineers can't predict or control which physical pathway is used which means they must have the ability to monitor all relevant paths. Only by monitoring all 4 links in a wide connection can the user be assured they will record all frames associated with the transfer.

Other complexities introduced by wide links:

• Because expander devices influence which physical path is used, protocol validation requires the ability to set a single trigger definition across multiple or "wide" links.
• Once traffic is recorded, trace-viewing software must be able to recognize command/data/status transmitted across wide links. This includes logically grouping, at the Application layer, all frames and transactions that are part of a common operation, even if they are spread across multiple physical pathways within a wide link.

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