IBM System Storage DS3500 Introduction And Implementation Manual page 30

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A SAS Domain, an I/O system, consists of a set of SAS devices that communicate with one
another by means of a service delivery subsystem. Each SAS device in a SAS domain has a
globally unique identifier called a World Wide Name (WWN or SAS address). The WWN
uniquely identifies the device in the SAS domain just as a SCSI ID identifies a device in a
parallel SCSI bus. A SAS domain may contain up to a total of 65,535 devices.
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, the number of
devices in a SAS domain can greatly increase. There are two types of expanders:
Fan-out expanders
A fanout expander can connect up to 255 sets of edge expanders, known as an edge
expander device set, allowing for even more SAS devices to be addressed. A fanout
expander cannot do subtractive routing, it can only forward subtractive routing requests to
the connected edge expanders.
Edge expanders
An edge expander allows for communication with up to 255 SAS addresses, allowing the
SAS initiator to communicate with these additional devices. Edge expanders can do direct
table routing and subtractive routing.
In the current DS3500 implementation, up to 96 drives can be configured in a single DS3500
using three EXP3500 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
Serial Attached SCSI comprises three transport protocols:
Serial SCSI Protocol (SSP) — supporting SAS disk drives.
Serial ATA Tunneling Protocol (STP) — supporting SATA disks.
Serial Management Protocol (SMP) — for managing SAS Expanders.
At the physical layer, the SAS standard defines connectors and voltage levels. The physical
characteristics of the SAS wiring and signaling are compatible with and have loosely tracked
that of SATA up to the present 6 Gbit/s rate, although SAS defines more rigorous physical
signaling specifications as well as a wider allowable differential voltage swing intended to
support longer cabling. While SAS-1.0/SAS-1.1 adopted the physical signaling characteristics
of SATA at the 1.5 Gbit/s and 3 Gbit/s rates, SAS-2.0 development of a 6 Gbit/s physical rate
led the development of an equivalent SATA speed. According to the SCSI Trade Association,
12 Gbit/s is slated to follow 6 Gbit/s in a future SAS-3.0 specification.
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IBM System Storage DS3500: Introduction and Implementation Guide
phy
) layer
Draft Document for Review March 28, 2011 12:24 pm