Mainframes and networks
Hardware Connectivity on the Mainframe
The OSA-Express and OSA-Express2 cards provide redundancy capability, as well as
throughput improvements when running in QDIO mode. QDIO mode allows direct access to
central memory. QDIO mode can be emulated within a CPC by allowing memory to memory
data transfer among LPARs running z/VM, Linux, or z/OS.
Under z/OS, OSPF is implemented by running an application (started task) called
OMPROUTE.
Hardware Channels
There are effectively three ways that network traffic can travel between an external network
and a z/OS host: through a channel-command word channel, a coupling channel, or a QDIO
channel.
CCW-based channels include parallel, ESCON, and FICON channels. A CCW can also be
used to talk to an OSA card. The CCW is the original I/O operation used for communications
with the channel subsystem. The CCW contains a channel command, such as read, write, or
control, along with the data address of the data area involved. The data is passed to the
channel subsystem. The channel subsystem communicates status of the I/O back to the issuing
application. When a channel communicates with an application in an asynchronous fashion, it
is referred to as a channel interrupt.
Communication among LPARs can be facilitated by Coupling Facility (CF) links. Coupling
Facility links are used to support the cross-system Coupling Facility, or XCF. The XCF
component in turn can be used to support the IP protocol.
The Open Systems Adapter is actually a network controller that you can install in a
mainframe I/O cage. The adapter integrates several hardware features and supports many
networking transport protocols. The OSA card is the strategic communications device for the
mainframe architecture. It has several key features that distinguish it from CCW-based
communications. Effectively, the OSA integrates the control unit and device into the same
hardware. It does so by placing it on a single card that directly connects to the central
processor complex I/O bus.
o QDIO is a highly efficient data transfer architecture, which dramatically improves
data transfer speed and efficiency for TCP/IP traffic. QDIO mode is referred to as
OSD because the CHPID type coded in the IOCDS is OSD.
o When the CHPID type is set to OSE, the OSA-Express card is functioning in non-
QDIO mode.
o The OSC CHPID type is available on newer mainframes running an OSA-Express2
card or an OSA-Express card with the Gigabit Ethernet feature. The OSC is a special
channel type that eliminates the need for an external console controller. The end
effect is that access to the HMC and to the z/OS system console is made easier. The
OSC CHPID can also be used to connect TN3270 sessions (with some limitations).
HiperSocket
Mainframe HiperSockets is a technology that provides high-speed TCP/IP connectivity within
a central processor complex. It eliminates the need for any physical cabling or external
networking connection between servers running in different LPARs.
The communication is through the system memory of the processor, so servers are connected
to form a "internal LAN."
The HiperSockets implementation is based on the OSA-Express Queued Direct I/O (QDIO)
protocol, hence HiperSockets is also called internal QDIO, or IQDIO. The microcode emulates
the link control layer of an OSA-Express QDIO interface.
The CHPID type used for a HiperSockets connection is called IQD.
z/OS network capability includes a fully-featured communications server with integration of
SNA and TCP/IP protocols.
Overview of mainframe network capabilities:
IP communication among independent operating systems running in logical partitions
(LPARs) on the same machine
IP communications among a tightly coupled cluster of mainframe LPARs (called a Parallel
Sysplex)
Communications via the TCP/IP suite of protocols, applications, and equipment (for
example, the Internet, intranets, and extranets)
System Network Architecture (SNA) suite of protocols and equipment, including subarea
and Advanced Peer-to-Peer Networking with high performance routing (APPN/HPR)
Integration of SNA into IP networks using Enterprise Extender (EE) technology
z/OS Communications Server
The z/OS operating system includes a software component called z/OS Communications
Server. z/OS Communications Server implements the SNA and TCP/IP protocols.
SNA applications and transaction servers (like CICS) can use SNA or TCP/IP to send and
receive data. Industry standard internet applications can use TCP/IP to send and receive data. For
example, a z/OS server may run FTP, telnet, web servers (HTTP), and mail programs (Simple
Mail Transfer protocol, or SMTP).
Shown in the figure, z/OS Communications Server includes three major components, which
are:
The TCP/IP protocol stack.
The SNA protocol stack contained in Virtual Telecommunications Access Method
(VTAM).
The Communications Storage Manager (CSM), which provides a shared I/O buffer area for
both TCP/IP and VTAM data flow. The CSM function allows authorized host applications
to share data without having to physically move the data.
TCP/IP
TCP/IP on z/OS supports all of the well-known server and client applications.
The TCP/IP started task is the engine that drives all IP-based activity on z/OS.
The TCP/IP profile data set controls the configuration of the TCP/IP environment.
The FTP server implements the FTP standard and can communicate with any FTP clients
on the network.
The TCP/IP started task is started as a program using Job Control Language (JCL).
The TCP/IP profile is read by TCP/IP when it is started.
If a change needs to be made to the TCP/IP configuration after it has been started, TCP/IP
can be made to re-read the profile dynamically (or read a new profile altogether). This is
accomplished by using the Obeyfile command.
