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Appendix A: Introduction to SAS ● 59
How Do SAS Devices Communicate?
SAS devices communicate with each other through links. A link is a physical connection
between two phys.
As shown in the following figure, SAS devices contain ports which contain phys (see next
section), and each phy contains one transmitter and one receiver (one transceiver). A phy can
belong to one port only.
What’s a Phy?
Phys are part of the physical communication connection between SAS devices. Each phy
contains a transceiver that sends data back and forth between SAS devices.
When a connection is formed between two end devices, a link is established from a phy in one
port to a phy in the other port. As shown in the figure above, a wide port can support multiple
independent links simultaneously.
Phys are internal, within SAS connectors (see page 60).
SAS cables physically connect one or more phys on one SAS device to one or more phys on
another SAS device.
What’s a SAS Port?
Note: Because the physical link between SAS devices is from phy to phy, rather than port to port, a
port is more of a virtual concept, different from what is normally considered a port on other types of
RAID controllers and storage devices.
A port is one or more phys. A narrow port contains one phy. A wide port typically contains four
phys.
Each port has its own unique SAS address (see page 60), and all the phys in a port share that
same SAS address.
Wide
Port
Wide
Port
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Narrow
Port
Phy
Receiver
Transmitter
Narrow
Port
SAS DeviceSAS Device
SAS Device
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Phy
Receiver
Transmitter
Wide
Port
Wide
Port
SAS Device
link