Chapter 1. Disk attachment technology 5
Optical SFP modules are commonly available in several categories:
򐂰 850 nm 550m MMF (SX)
򐂰 1310 nm 10 km SMF (LX)
򐂰 1550 nm [40 km (XD)]
򐂰 80 km (ZX)
򐂰 120 km (EX or EZX)
SFP transceivers are also available with a copper cable interface, allowing a host device
designed primarily for optical fiber communications to also communicate over unshielded
twisted pair networking cable.
SFP transceivers are commercially available with capability for data rates up to 4.25 Gbit/s.
The standard is expanding to SFP+ which supports data rates up to 10.0 Gbit/s (that will
include the data rates for 8 gigabit Fibre Channel, 10 GbE, and OTU2).
FC World Wide Names (WWN)
A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier that
identifies a particular Fibre Channel, Advanced Technology Attachment (ATA) or Serial
Attached SCSI (SAS) target. Each WWN is an 8 byte number derived from an IEEE OUI and
vendor-supplied information.
There are two formats of WWN defined by the IEEE:
򐂰 Original format: addresses are assigned to manufacturers by the IEEE standards
committee, and are built into the device at build time, similar to an Ethernet MAC address.
The first 2 bytes are either hex 10:00 or 2x:xx (where the x's are vendor-specified),
followed by the 3-byte vendor identifier and 3 bytes for a vendor-specified serial number
򐂰 New addressing schema: first nibble is either hex 5 or 6 followed by a 3-byte vendor
identifier and 36 bits for a vendor-specified serial number
1.2 Serial Attached SCSI (SAS) disk attachment
SAS is a computer bus used to move data to and from computer storage devices such as
hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the
parallel SCSI bus technology, and uses the standard SCSI command set.
At the time of writing, typical SAS throughput is 6 Gbps full duplex. SAS has the capability to
reach 24 Gbps if the host can drive it at that speed. When the first 6 Gbps connection is full,
the next 6 Gbps connection is used, and so on, up to four connections.
Figure 1-3 on page 6 shows the SAS technical specifications.
6 IBM System Storage DS3500 Introduction and Implementation Guide
Figure 1-3 SAS Technical Specifications
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 can 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 be greatly increased. 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
phy) layer
This layer represents the hardware components, such as transceivers, that 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

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