What is SAS? SAS . Communication Standard Features

What is SAS?

Serial Attached SCSI (SAS)  is a point-to-point protocol used in computers to transfer data to and from computer storage devices such as hard drives and tape drives. SAS is the successor of Parallel SCSI. Like its parent, SAS also uses the standard SCSI instruction set. SAS also supports optional Serial ATA (SATA) version 2 or later. 

This allows the SATA drive to be connected to most slave devices or SAS controllers. SAS drives cannot be re-connected to a SATA-enabled device. Learning about  what SAS is , it is found that the current SAS standard is supporting 255 direct connections, and each of those ports can be used by storage devices, storage servers or extenders. 

The extender can also support up to 255 connections, resulting in a maximum capacity for a SAS device of 65,535 if any available direct connections are used for the extender. SAS is suitable for businesses with large backup and storage requirements. 

SAS is considered by many to be the most popular interface to Direct-Attached Storage and is used to support hard drive controllers in enterprise-class Server Farms.

What is the development process of SAS?

The INCITS/T10-SCSI Host Interface Committee develops the Serial-Attached SCSI (SAS) standards.

• SAS-1 technology is the first version of the SAS standard released in 2004 and supports a data bandwidth of 3 gigabits per second (Gbps). 
• SAS-2 is the second version available since February 2009 and supports 6 Gbps
• The SAS-3 standard is the third version available since  March 2013 and  supports 12Gbps
• The SAS-4 version, which was completed in 2017, supports 24 Gbps.
• SAS-5 is a standard currently under development and will support bandwidths greater than 24 Gbps.

What are the characteristics of the SAS standard?

Technical details

The SAS standard defines several layers (in descending order): application, port, transport, link, physical, PHY. SAS transport protocols fall into three categories: 

• Serial SCSI Protocol (SSP) – used to communicate with SCSI devices at the command level.
• Serial ATA Tunneling Protocol (STP) – used to communicate with SATA devices at the command level.
• Serial Management Protocol (SMP) is used to manage the SAS fabric.

Besides, when you learn about  what SAS is , you will see that the SAS communication standard defines its own protocols for the PHY and  Link layers.

The SAS standard defines connectors and voltage levels at the physical layer. The physical characteristics of SAS wiring and signals are compatible with those of SATA up to 6 Gbit/s, although SAS defines more stringent physical signal specifications as well. Wider allowable differential voltage dynamics to allow longer cables. 

Furthermore, SCSI Express uses the PCI Express infrastructure to connect SCSI devices directly over a more common interface.

Architecture

Have you ever wondered  what the architecture of SAS is  ? Its architecture is made up of six layers:

Physical layer:   Specify the electrical and physical characteristics or differential signal transmission.

Different connector types:

• SFF-8482 – compatible with SATA.
• SFF-8087, SFF-8484 and SFF-8643 are internal four-lane connectors.
• SFF-8088, SFF-8470 and SFF-8644 are external four-lane connectors.

PHY class: 

What are the characteristics of the PHY layer in SAS ? 

• Data encryption at 8B/10B (3, 6 and 12 Gbit/s)
• 128b/150b SPL packet encryption (22.5 Gbit/s). In it, 2-bit header, 128-bit payload, 20-bit Reed-Solomon forward error correction.
• Speed ​​negotiation, link initialization and re-sequencing.
• Negotiable Link (SAS-2 Technology onward)

If you have read through the specifications of the SAS communication standard, you will know  what the next layer of SAS is .

Connection class:

• Insert and delete the original bases to match the clock speed difference.
• Base encoding.
• The data is shuffled to make the EMI lower.
• Create and break root connections between SAS destinations and initiators.
• Tunnel connections between the SAS initiator and the SATA destination connected to the SAS extender must be established and dismantled.
• Power management (recommended for SAS-2.1 technology standard).

Port Layer:  Create wide ports by combining multiple PHYs with the same address.

Transport layer:  What is the important layer you need to know when learning about the architecture of  SAS ? There are three transport protocols including:

• SSP Protocol: This protocol is used for command level communication with SCSI devices.
• STP Protocol: This protocol is used for command-level communication with SATA devices.
• SMP Protocol: This protocol is used to manage the SAS fabric.

Application class

Topology

When considering what the characteristics of the PHY layer in  SAS are , it is easy to see that the initiator can directly connect to the target using one or more PHYs. Such a connection is called a Port whether it uses one or more PHYs, although the term Wide Port is sometimes used for a multiple PHY connection. 

SAS . Expansion Kit

Serial Attached SCSI Expanders (SAS Expanders) are components that allow communication between a large number of SAS devices. External expansion ports are found on expanders. Each extender includes at least one SAS management protocol destination port for management and can include SAS devices. 

See the following example to see what the specification of the SAS extender  is :

For example, an extender might include a serial SCSI protocol destination port for connection to a peripheral device. The extender is not needed to connect the SAS initiator and the target, but it does allow a single initiator to communicate with multiple SAS/SATA targets. 

An extender can be compared to a network switch in a network, which connects multiple systems using a single switch port. SAS-1 technology has defined two types of extenders. However, the SAS-2.0 standard eliminated the distinction because it introduced unnecessary topology constraints with no practical benefit:

Edge Expander

Learn about the characteristics of the SAS expander  , you should not ignore Edge Expander. Edge Expander supports up to 255 SAS addresses, allowing the SAS initiator to communicate with these additional devices. Edge Expander can perform both direct table routing and exclusion routing. You can only use up to two Edge Expanders in the distribution subsystem without the Fanout Expander.

This congestion is alleviated by the Fanout Expander.

Fanout Expander

One Fanout Expander can connect up to 255 Edge Expander devices, allowing even more SAS devices to be handled. Each Edge Expander excluded routing port connects to the physical Fanout Expander. Fanout Expander can only forward exclusion routing requests to the connected Edge Expander. It cannot perform exclusion routing on its own. So if you've never learned what a  SAS extender is , you won't know this.

A device can identify devices directly connected to it using direct routing. The routing table determines which devices are allowed to connect to the extender associated with the device's own PHY. When devices in your sub-branch cannot be found, exclusion routing is used. 

This redirects the request completely to another branch. Extenders are available to enable more complex topologies. Who has learned about what is SAS extender  ? will see that the extender supports end devices in link switching instead of packet switching. 

They can find the terminal directly through the routing table, or if those methods fail, through minus routing: the only extender connected to a minus routing gateway will be the link. link is forwarded. The terminal cannot be connected if no extender is connected to the slave port.

The expander without PHY is configured as an exclusion function like the Fanout Expander, connecting to any number of other expanders. An extender with a minus PHY can only connect to two other extenders at a time, and they must connect to one extender through a minus port and the other through a non-minus port.

Compatibility of SAS

The rear panel of a SAS system can be connected to dual ports, high-performance SAS drives, or low-cost, high-capacity SATA drives. SAS system compatibility allows IT staff to use hard disks with a variety of interfaces to meet the capacity or performance requirements of a wide range of applications. 

Knowing that by learning  what SAS is , helps that as your storage system grows, you have more options to maximize the return on investment of your storage devices. SCSI Serial Attached technology also has the advantage of making it easier to design internal connections.

Components of the SAS . system

Initializer

A device that sends device service and task management requests to a target device to process and receive responses from other target devices. The initiator can be integrated into the motherboard as an onboard component (like many server-oriented motherboards) or as an additional Server Bus integrator.

Destination

If you learn what the components of a SAS system  are, you must know that the target is a device with logic units and destination ports that receive device service and task management requests to process and respond to. initialization devices. The hard drive or disk array system can be the target device.

Service delivery subsystem

A component of an I/O system that sends data between the initiator and the destination. The cables that will connect the initiator and the destination, with or without an extender, and back end typically constitute a subsystem that provides services.

Expansion Kit

Devices that form part of a subsystem that allows SAS devices to communicate with each other and provide services. The extender allows multiple SAS terminals to be connected to a single initiating port. 

What are the advantages and disadvantages of SAS technology?

Advantage

SAS can handle large data files with up to 32,768 variables, and the number of records is determined by the hard disk size. Because data is contained in only one file, this advantage can make it easier to process, organize, and analyze large amounts of data.

In addition to storage, SAS is also very strong in the field of data management, allowing users to easily manipulate data. The power of SAS also allows it to work with multiple data files at once, reducing the complexity of data preparation for analytical tasks that require working with multiple data files. 

Defect

Due to the high cost of the product, the number of users of the SAS interface is still limited compared to the SATA interface. Furthermore, to fully benefit from the SAS communication standard, users must first learn and understand how to manage SAS data and many other complex management tasks.

How does SAS work?

SAS is a point-to-point protocol used between storage devices and the computer on which they store data. All data transmitted over SAS is sent directly between two communicating entities – a storage device and a computer – that are connected by a physical cable. Serial means that all data sent over SAS is sent bit by bit, in sequence.

The SAS protocol is used on a computer to establish a dedicated link between the computer and the disk drives, tape drives, and other SCSI storage devices connected to the computer's Host Bus adapters (HBAs) through the serial interface.