December 14th, 2004

System Network Architecture


The Past

Once upon a time there was no communication between people, due to the lack of transportation means. Information was like mouth Herpes virus: orally transmitted. Still in the beginning of the Industrial Era if you were a Bombay, India based Her Majesty Officer, it took six months to send a letter to London. Then another six months to get an answer. So any problem needed one year to get discussed.

Then around the mid 60s they began to come out the first serious computer languages, ideas and inventions. But still there was a lack of communication, there was not yet a clear idea how to join two or more computers. Till we reached the 1974. When a company named [Those Darn Expensive] Industrial Business Machines, later shorten in IBM, came out and said: So you need to communicate? We can help but well SNAtch your money. And so was invented the System Network Architecture.

It is a communication format, used on local-area networks, to allow multiple systems access to centralized data. As a protocol suite, SNA offers functionality similar to the TCP/IP and OSI protocols. Like these protocols, SNA offers a layered approach to communications. Although there is not a one-to-one correspondence, in the table below there is an idea.
SNA ISO Correspondence
But in a short an other problem rose: in the mid 70s it was a world mainframe ruled. The networks were pyramidal, or upside-down tree, with the mainframe at the top. The stations and the terminals, rather than to use own resources, used the mainframes, which was a huge waste of resources. With the comimg of the PCs, networks grew to hundreds of terminals, and companies couldnt rely on SNA. So in 1982 IBM added a protocol called Logical Unit (LU) 6.2 to SNA. LU6.2 makes all computers peers on an SNA network, including hosts. But that changed the structure of the layers.
SNA ISO Correspondence
In practice, SNA and LU6.2 use only layers 4 through 6.
Over the Logical Unit there is the Physical Unit too. Basically the LU and the PU are the two primary entities in a SNA network. In IBM jargon PU are also known as Node Types (NT). IBM defined five node types, including hosts (PU5) and terminals (PU2), and left a sixth available for future definition. So a node on a SNA network can be either a PU (hardware) or a LU, like a logical session connection.

So LU6.2 works as an interface between a user and the SNA. Strictly associated with LU6.2 is PU2.1, that was an enhancement to PU2.0 for cluster controllers. PU2.0 is also known as 3270 terminal emulation. Here we go. The 3270 emulation, with its twin 5250, made a large part of the IT history of the corporate computing.

Humbly yours personally installed and transformed both PS/2 and nameless PCc in stream 3270 and 5250 with a IBM System/36 mini. Yeah, lot of fun, above all the configuration of the keyboards was a pain in the neck.
However, one of the most used implementation of LU6.2 was the Advanced Program-to-Program Communication (APPC). Which brings us almost to the


Here Ill skip the specs of the APPC, saying only it was sold to the users in the form of developer's toolkit, allowing them to create transaction programs, using entities called APPC verbs. IBM has made APPC its preferred LU6.2 implementation in a variety of its systems, including its PCs, System/38/36, AS/400, and 9370 mainframe [1].

However the APPC consumed 164 KB of RAM, and we are still talking of DOS based machine with 640KB of RAM. But by now was on the scene the OS/2, that could address a whopping 16MB.

In the meantime IBM made an other implementation of the LU6.2 : the Advanced Peer-to-Peer Networking (APPN). APPN adds network management capabilities to LU6.2 peer-to-peer communications services. APPN also includes a routing capability that can create new routes between nodes on a network dynamically[1].

In a short other competitors came out. SNA Server is a software gateway that lets PCs on a LAN connect to IBM mainframes, such as the ES/9000, and IBM midrange systems, such as the AS/400. IBM mainframes and AS/400s both use IBM's SNA protocol as their primary method of communications. Since PC networks do not use SNA, gateways such as SNA Server provide the necessary SNA communications link required for PC-to-IBM-host connectivity. SNA Server is built on a flexible architecture that enables it to work with NT Server networks as well as with TCP/IP, IBM LAN Server, and Novell NetWare networks . In the first versions SNA Server needed just a 486 system with 12MB of memory and 20MB of hard disk storage.

In the meantime Internet was progressing, so came out articles like the Network Magazines Web-Enabling Your Mainframe, that is at [3].
Then SNA evolved through corporate WANs, and Ive read of some implementations through ATM and Sonet.

The Future

SNA is alive and well. At least in a short it appears SNA has a good future, prolly through GigaEthernet. And surely VPN and voice/IP will help in the communication process. Although I heard of dumb terminals using Java to connect to SNA networks, Java looks headed to the place where it belongs: washing machines and cellphones programming. As it appears that the IBM new born WebSphere is a good step in integrating the legacy data. As usual it depends how much it will cost to the final user.


Vincenzo Maggio

References:; jsessionid=F4YFNL1LIPXXAQSNDBCCKH0CJUMEKJVN?articleId=9000011& pgno=2

Other sources: