Sas (Serial Attached Scsi) Disk Attachment - IBM DS3000 Introduction And Implementation Manual

Draft Document for Review August 30, 2007 12:59 am
FC0 is the physical layer, which describes cabling, connectors, signalling etc. This layer
defines the physical media implementation.
FC1 is the data link layer. This layer contains the 8b/10b encoding and decoding of signals
for transmission across the physical media.
FC2 is the network layer and it defines the main FC protocols. This layer defines how the
frames are transferred.
FC3 is the common services layer.
FC4 is the application protocol mapping layer. In storage connectivity applications, FCP
protocol is used to encapsulate SCSI data into FC frames.
FC cable types
FC implementations can utilize either single-mode or multi-mode FC cables.
The name of multi-mode fiber indicates that multiple modes, or rays of light, can travel
through the cable core simultaneously. The multi-mode fiber cable uses a larger diameter
core, which makes it easier to couple than single-mode fibre. At throughput of 4 Gbps, the
length of cable can be up to 150 m.
Single-mode fibre transfers a single ray of light. The core diameter is much smaller than the
core of multi-mode cable. Therefore, coupling is much more demanding and tolerances for
single-mode connectors and splices are very low. However, single-mode fiber cables can be
much longer. Cable length can exceed 50 km.
Multi-mode cabling is much more common, as it is easier to work with and meets the
requirements of most customer scenarios. However, in situations where very long cable
lengths are needed, single-mode cabling will be required.
FC World Wide Names (WWN)
FC devices are presented with a unique identifier, called
WWNs are somewhat similar to the MAC addresses in Ethernet terms. For example, each FC
HBA has its own WWN, which is hard-coded (or burned-in) during manufacturing. The HBA
will be uniquely identified by the storage subsystem, using its WWN.

1.2 SAS (Serial Attached SCSI) disk attachment

SAS technology evolved from parallel SCSI. While traditional SCSI is a parallel interface, SAS
uses serial communication protocol. However, SAS does use the SCSI command set to
communicate with the attached devices. When parallel SCSI reached the transfer rate of 320
MB/s, it became apparent that further transfer rate increase would be difficult to achieve. Even
though the Ultra-640 parallel SCSI technology is available, it is not very practical due to very
limited cable lengths. Therefore, most manufacturers moved from Ultra-320 SCSI to SAS,
rather than adopting the Ultra-640 SCSI.
At the time of writing, typical SAS throughput is 3 Gbps full duplex. However, 6 Gbps capable
SAS devices are already appearing and will become more widely used in the near future.
A SAS domain is a set of SAS devices connected together with SAS cabling. Each SAS
device has a unique identifier, a World Wide Name (WWN). The role of this unique identifier
might look similar to a SCSI ID, however there are some differences:
On a SCSI bus, there can be up to 16 SCSI IDs, while a SAS domain can have up to
16256 devices.
7065DiskAttach.fm
World Wide Name
(WWN). The
Chapter 1. Disk attachment technology
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