2. USAGE CONSIDERATIONS › 2.2 General Analysis Considerations › 2.2.1 Pre-SNA Communications

2.2.1 Pre-SNA Communications

SNA-based communications networks are made up of elements
that combine hardware and software to perform a single
logical function.  Before SNA development, IBM networks were
made up solely of hardware elements.  Because much of the
terminology of the pre-SNA environment (e.g., terminal, line
control unit, etc.) is still commonly (and often
preferentially) used to refer to SNA network elements
(e.g., logical units, links, physical units, etc.), confusion
exists as to the actual meaning of the SNA terms.  This
section briefly reviews the evolution of data communications
in the IBM environment in an attempt to relate SNA concepts
to common experience.

Unlike some other mainframe architectures, the System 360
architecture did not inherently provide for terminal
communication within its basic design.  Thus, when terminal
communications were desired, IBM engineers were faced with
the problem of interfacing the IBM channel, a high bandwidth
(data carrying capacity), low addressability, data transfer
mechanism, to a potentially large number of independent
terminals which randomly generated small bursts of traffic.

To accomplish this, IBM engineers developed a device, the
2701 Transmission Control Unit (TCU), consisting of three
hardware elements:

o Channel Interface -- communicates with the IBM channel

o Transmission Interface Converter -- handles internal
  communication between the Channel Interface and the
  Transmission Adapter.

o Transmission Adapter -- handles terminal-dependent and
  line activities


          +--------------------------------------------+  C
 --- I ---|   C    I    |   T   I   C   |    T    A    |  O
 --- B ---|   H    N    |   R   N   O   |    R    D    |  M
 --- M ---|   A    T    |   N   T   N   |    N    A    |---
 ---   ---|   N    R    |   S   R   V   |    S    P    |  /
 --- C ---|   N    F    |   M   F   E   |    M    T    |  ---
 --- H ---|   E    A    |   I   A   R   |    I    E    |  L
 --- A ---|   L    C    |   S   C   T   |    S    R    |  I
 --- N ---|        E    |   N   E   R   |    N         |  N
          +--------------------------------------------+  E


    Figure 2-1.  2701 Transmission Control Unit

The 2701 TCU was the forerunner of today's 37xx Communication
Control Units.  It performed five basic functions which are
still central to data communications in an SNA environment:

o Decoded IBM channel commands

o Performed conversion from the channel (parallel data
  stream) to the communications line (serial data
  stream)

o Recognized line control characters

o Provided transmission error checking

o Provided status and sense information to the IBM
  channel

The development of multiple terminal types, each with special
transmission control requirements, necessitated the design of
a number of specialized transmission adapters.  The desire to
reduce the required inventory of hardware and increase the
number of terminal types which could be supported resulted in
the design of the programmable 3705 Communications Control
Unit.

The 3705 performed essentially the same functions as its
predecessor, but the transmission adapter function was
implemented in software modules called Emulators because they
performed the function of the 270x transmission adapter
hardware.  The transmission interface converter function was
also implemented in software.  The software executed on a
specialized processor called the central control unit, which
accessed both a parallel data path (IBM channel) and a serial
data path (communications line) via peripheral devices.  The
addition of software components to the transmission control
unit function and the resultant flexibility prompted the
proliferation of the hardware-software element philosophy
evident in SNA.

          +--------------------------------------------+  C
 --- I ---|   C    A    |   C   C   U   |  C C  S      |  O
 --- B ---|   H    D    |   E   O   N   |  O A  C   +--|  M
 --- M ---|   A    A    |   N   N   I   |  M T  A   |L |---
 ---   ---|   N    P    |   T   T   T   |  M I  N   |I |  /
 --- C ---|   N    T    |   R   R       |  U O  N   |B |  ---
 --- H ---|   E    E    |   A   O       |  N N  E   +--|  L
 --- A ---|   L    R    |   L   L       |  I S  R      |  I
 --- N ---|             |  (PROCESSOR)  |  -           |  N
          +--------------------------------------------+  E

    Figure 2-2.  3705 Communications Control Unit

The programmable communications control unit reduced the
effort and cost of supporting new terminal types, but major
changes in terminal hardware still affected the entire
network.  The designers of SNA worked to reduce the scope of
a given change by isolating the logical functions of
communications into layers joined by well-defined interfaces.
These interfaces could only be maintained through the use of
combined hardware-software elements.

Thus, at its basic level, SNA can be viewed as simply the
application of structured design techniques to the task of
data communication.  The confusion between SNA and pre-SNA
terminology results from the lack of distinction between a
given piece of hardware and the logical SNA functions it
implements.  The following sections of this chapter discuss
these logical functions in detail.