Editing Systems Network Architecture (section)

==Network Addressable Units==
''Network Addressable Units'' in a SNA network are any components that can be assigned an address and can send and receive information. They are distinguished further as follows:<ref>{{citation |title=Basic SNA terms and concepts |url=https://www.ibm.com/docs/en/ztpf/2023?topic=communications-basic-sna-terms-concepts}}</ref>
* a ''System Services Control Point'' (SSCP) provides resource management and other session services (such as directory services) for users in a subarea network;<ref name="IBMSSCP">{{cite web |publisher=IBM Corporation |title=z/OS Communications Server: SNA Network Implementation Guide (6) |website=IBM Knowledge Center |url=http://www-01.ibm.com/support/knowledgecenter/#!/SSLTBW_2.1.0/com.ibm.zos.v2r1.istimp0/snnig6.htm |access-date=October 3, 2015}}</ref>
* a ''Physical Unit'' is a combination of hardware and software components that control the links to other nodes.<ref name="IBMPU">{{cite web |publisher=IBM Corporation |title=z/OS Communications Server: SNA Network Implementation Guide (11) |website=IBM Knowledge Center |date=September 11, 2014 |url=http://www-01.ibm.com/support/knowledgecenter/SSLTBW_2.1.0/com.ibm.zos.v2r1.istimp0/snnig11.htm?lang=en |access-date=October 3, 2015}}</ref>
* a ''Logical Unit'' acts as the intermediary between the user and the network.<ref name="IBMLU">{{cite web |publisher=IBM Corporation |title=z/OS Communications Server: SNA Network Implementation Guide (12) |website=IBM Knowledge Center |date=September 11, 2014 |url=http://www-01.ibm.com/support/knowledgecenter/SSLTBW_2.1.0/com.ibm.zos.v2r1.istimp0/snnig12.htm?lang=en |access-date=October 3, 2015}}</ref>

===Logical Unit (LU)===
SNA essentially offers transparent communication: equipment specifics that do not impose any constraints onto LU-LU communication. But eventually it serves a purpose to make a distinction between LU types, as the application must take the functionality of the terminal equipment into account (e.g. screen sizes and layout).

Within SNA there are three types of data stream to connect local display terminals and printers; there is SNA Character String (SCS), used for LU1 terminals and for logging on to an SNA network with Unformatted System Services (USS), there is the [[IBM 3270|3270]] [[IBM 3270#Data stream|data stream]] mainly used by mainframes such as the [[System/370]] and successors, including the [[zSeries]] family, and the [[IBM 5250|5250]] data stream mainly used by minicomputers/servers such as the [[System/34]], [[System/36]], [[System/38]], and [[AS/400]] and its successors, including System i and [[IBM Power Systems]] running [[IBM i]].

SNA defines several kinds of devices, called Logical Unit types:<ref>{{harv|Schatt|1991|p=229}}.</ref>
*LU0 provides for undefined devices, or build your own protocol. This is also used for non-SNA 3270 devices supported by [[Telecommunications Access Method|TCAM]] or VTAM.
*LU1 devices are printers or combinations of keyboards and printers.
*LU2 devices are IBM 3270 display terminals.
*LU3 devices are printers using 3270 protocols.
*LU4 devices are batch terminals.
*LU5 has never been defined.
*LU6 provides for protocols between two applications.
*LU7 provides for sessions with IBM 5250 terminals.
The primary ones in use are LU1, LU2, and [[LU6.2]] (an advanced protocol for application to application conversations).

===Physical Unit (PU)===<!-- IBM Network Control Program redirects here -->
*PU1 nodes are terminal controllers such as [[IBM 6670]] or [[IBM 3767]]
*PU2 nodes are cluster controllers running configuration support programs such as [[IBM 3270|IBM 3174]], [[IBM 3270|IBM 3274]], or the [[IBM 4701]] or [[IBM 4702]] Branch Controller
*PU2.1 nodes are [[peer-to-peer]] [[IBM Advanced Peer-to-Peer Networking|(APPN)]] nodes
*PU3 was never defined
*PU4 nodes are front-end processors running the [[IBM Network Control Program|Network Control Program]] (NCP) such as the [[IBM 37xx]] series
*PU5 nodes are host computer systems<ref>{{cite web |last=Microsoft |title=Physical Unit (PU) |url=http://msdn.microsoft.com/en-us/library/ee253218%28v=bts.10%29.aspx |access-date=September 7, 2012}}</ref>

The term [[37xx]] refers to IBM's family of SNA communications controllers. The 3745 supports up to eight high-speed [[Digital Signal 1|T1]] circuits, the 3725 is a large-scale node and [[front end processor|front-end processor]] for a host, and the [[IBM 3720|3720]] is a remote node that functions as a [[concentrator]] and [[router (computing)|router]].