SEARCH 

SNA/SDLC Protocol Support

Systems Network Architecture/Synchronous Data Link Control (SNA/SDLC) is IBM's version of bit-oriented protocol. SDLC is the link level protocol, and SNA provides the intelligence for the connection. SNA and SDLC use a series of commands to control the flow of data through the network. All of our products support most of the capabilities used by SNA devices.

SNA/SDLC Components

SNA consists of specialized hardware and control software.

Communications Controller (for example: 3745) provides the physical interface for the SNA device and runs the NCP/VS program.

 

NCP/VS (Netwrok Control Program/Virtual Storage) controls the network.

 
VTAM (Virtual Telecommunications Access Method) controls the access between the application program and the logical unit (terminal).

Connection Flow

A major advantage of SNA is the ability of terminals to switch between applications. The terminal is attached (bound) to an application by:
  1. NCP/VS issues an Activate Physical Unit (ACTPU) Command to the controller.
  2. NCP/VS issues an Activate Logical Unit (ACTLU) Command to the terminal.
  3. The terminal operator issues a request to VTAM for connection to an application (SSCP-LU session). VTAM can also be configured to automatically initiate this interchange.
  4. VTAM issues a Bind Session Command (BIND) to the terminal. The terminal is now connected to the application. The terminal operator can issue a command (System Request) to terminate the connection with the application (UNBIND) and reconnect to VTAM so that a different application can be selected.

SNA Terminal Support

There are many different terminal implementations of SNA/SDLC protocol. Each terminal uses different hardware and performs different functions. SNA/SDLC terminals are grouped into two major categories: Remote Job Entry (RJE) and interactive. The RJE terminals are characterized by large amounts of data sent in long bursts. An interactive terminal is characterized by an interchange of small messages between the host and the terminal. SNA/SDLC controls the flow of data more efficiently than bisync. This allows RJE and interactive terminals to share the same communications line.

Further information on the RJE terminals is available here. Information on the 3270 terminals is available here.


Space and Character Compression

Some RJE terminals support space and character compression. With space compression, the transmitting terminal replaces consecutive strings of spaces with a counter. The receiving device restores the spaces. Space compression is used to reduce the amount of data sent over the communications line. Character compression works in the same manner for consecutive occurances of the same character.


Data Formats

SNA devices can support either ASCII or EBCDIC data formats.


Error Reporting

SNA terminals use several levels of reporting error conditions. In addition to the same type of application messages as used in bisync, SNA devices can use protocol level messages to communicate directly with the network control programs.


SNA Advantages

SNA was developed to address many of the shortcomings encountered with bisync protocol. The structure of SNA/SDLC offers the following advantages:
Transparency - SNA/SDLC is inherently transparent to the different data formats. Information in the SDLC frame is not interpreted by the network devices.

Dissimilar Devices - SNA/SDLC restricts the size of each frame. NCP/VS controls the flow of data from each terminal on the communications line so that different stations can send data. With bisync, each terminal sends all of its data before the next terminal can send.

Line Efficiency - SNA/SDLC is multi-thread with several distinct communication paths existing in different states simultaneously. This provides better throughput than bisync, which is single-thread.

Diagnostics - SNA/SDLC provides more information on failures than bisync since temporary (soft) errors are communicated to the host. The information allows corrective action to be undertaken before hard failures occur.

JBM Electronics Implementation

The terminal emulation is PU1 or PU 2. The host emulation is a subset of PU4/5. The host emulation performs all of the host functions, including ACTPU, ACTLU, BIND and SSCP-LU. This emulation is designed to activate an SNA device and facilitate data transfer.

To view the available options that can be specified for this protocol, please review the worksheets. If you need further information on SNA/SDLC protocol or our implementation and support for this protocol, please e-mail us.