Mainframes still use Systems Network Architecture (SNA) internally and there are still a great many applications in use that are SNA-based. Conversion to non-SNA formats isn’t always an option for time and cost reasons, but convergence of all network traffic onto one infrastructure—Internet Protocol (IP)—is attractive because of significant cost savings and because it’s a conventional standard. Most new z/OS solutions use IP, and with the 3745 Communications Controller heading toward the end of its lifespan, the drive toward such convergence is strong.
To facilitate convergence, IBM introduced Enterprise Extender (EE ), a method of encapsulating SNA transmission units in User Datagram Protocol (UDP) packets (while still in z/OS ). Packets are carried across the IP infrastructure, removing the need to provide SNA support in the physical network, but allowing the SNA applications to persist unchanged, and end-user SNA facilities to continue.
Not every site can use EE and not every protocol can be used over EE, but where possible, it’s by far the best solution available.
EE was introduced in the late ’90s after the advent of Advanced Peer-to- Peer Networking/High-Performance Routing (APP N/HPR). It remains IBM’s primary and preferred solution for connectivity among systems with APP N support and where the Wide Area Network (WAN) infrastructure is IPbased.
These systems include:
- z/OS (and z/Linux) with Communications Server for z/OS
- Microsoft Windows with Communications Server for Windows
- Microsoft Windows with Microsoft HIS Server 2004
- OS /2 with Communications Server for OS/2
- AIX with Communications Server for AIX
- Linux with Communications Server for Linux
- Cisco Systems with SNA support.
The wide choice of supporting systems makes EE ideally suited as a common medium for connecting the APP N-enabled enterprise. APP N/HPR is comprised of two components: Rapid Transport Protocol (RTP) and Automatic Network Routing (ANR). ANR provides the routing, while RTP handles the route control, recovery, and is responsible for non-destructive routing around failures.
EE architecture carries APP N/HPR over an IP backbone in Network Layer Packets (NLPs), allowing APP N to see the IP network as a single hop connection. UDP is the chosen protocol because it provides the best performance.
Unlike EE, alternatives such as Channel Interface Processor (CIP) routers, protocol converters, and Data-Link Switching (DLSw) devices have never brought the IP endpoints so close to the SNA host, and none has been able to maintain SNA traffic priorities in the IP network. The alternatives require that conversion to IP be performed away from the z/OS TCP /IP stack, which means that although the physical network is shared, the stack knows nothing about this SNA traffic. This results in more problematic monitoring and control functions.
As a standard part of Communications Server for z/OS , EE passes all encapsulated SNA traffic through the local stack. Especially in a Sysplex, this means all the latest highspeed hardware and software improvements (e.g., OS A Express and HiperSockets) can be exploited. It accomplishes this while still supporting channel-attached routers and other existing hardware, while taking advantage of IP network routing and failure protection and implementing some of its own failure protection via HPR.
With the z/OS Communications Server performing the conversion, full control of the sessions remains in the mainframe, eliminating any need to rewrite SNA code. Implementing EE requires only the APP N user to code an IP address (VIPA) and a device (usually IUTS AMEH ) into the TCP /IP stack, and then configure two VTAM members (an XCA and a SWNET major node); there are no product installation or license costs.