IBM DS3000 Introduction And Implementation Manual page 38

7065DiskAttach.fm
SCSI IDs are only unique within their SCSI bus. SAS WWNs are globally unique
identifiers.
The SAS WWN is also referred to as
SAS expanders
Basically, SAS uses point-to-point serial links. Point-to-point topology essentially dictates that
only two devices can be connected; however, with the use of SAS expanders, number of
devices in a SAS domain can greatly increase. There are two types of expanders:
Fan-out expanders
A SAS domain can have only one fan-out expander. Devices attached to such a fan-out
expander can be initiators, targets and edge expanders. The fan-out expander can attach
to a maximum of 128 SAS devices.
Edge expanders
Edge expanders typically connect to the fan-out expander and to a mixture of SAS
initiators and targets. Up to 128 devices are allowed on a single edge expander.
The maximum number of devices in a SAS domain is 128 edge expanders multiplied by 128
devices minus 128 connections to the fan-out expander. This gives a total of up to 16256 SAS
devices.
In the current DS3000 implementation, up to 48 drives can be configured in a single DS3000
- using expansion units.
SAS protocol layers
The SAS protocol consists of four layers:
The physical (or
This layer represents the hardware components, such as transceivers, which send and
receive electrical signals on the wire.
The link layer
The link layer manages connections across phy interfaces.
The port layer
The port layer passes the SAS frames to the link layer. It also selects the most appropriate
physical layer for data transmission (when multiple layers are available).
The transport layer
This layer encapsulates information into SAS frames and passes the frames to the port
layer. It also disassembles the frames received from other SAS devices and relays the
information to the driver/application.
Benefits of SAS
SAS technology provides several benefits over SCSI:
SAS uses point-to-point technology, while SCSI uses multidrop cables. Because SAS
devices have dedicated connections to the initiators, there is no bus contention; this
provides more efficient throughput. On a SCSI bus, the bandwidth is shared between the
devices, so as the number of devices on the bus increases, the throughput efficiency
decreases.
SAS has lower signalling overhead than SCSI, which again improves efficiency.
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IBM System Storage DS3000: Introduction and Implementation Guide
Phy SAS address
phy
) layer
Draft Document for Review August 30, 2007 12:59 am
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