DEFINE LINK command: Horizon for VTAM

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System manager, User 0
Defines a connection from Model 204 to the network, specifying a node name to be used when Model 204 connects to the SNA network and indicating the transport type and protocol




name refers to the link entity and is used on DEFINE PROCESSGROUP commands to refer to this connection to the network. The length of name must be eight characters or fewer. ALL is a reserved keyword and cannot be used.
LIKE gives the current link the attributes of the link entity referred to by previousname. If you use LIKE, you must also specify WITH SCOPE=SYSTEM. If you also do not specify a new LOCALID value, the DEFINE LINK command will be valid but you cannot open both links.
SCOPE=SYSTEM indicates that the definition is available to all users of the Model 204 system for the entire run.
TRANSPORT is the physical communication transmission method. For Horizon, the transport must be SNA Communications Server.
PROTOCOL identifies the conversation protocol. LU62 specifies the SNA LU 6.2 protocol. This option supports basic LU 6.2 and also supports parallel sessions, namely CNOS.
SESSIONS specifies the maximum number of concurrent sessions that can be activated for the link. A session is activated when a conversation is initiated. The valid range of values is 1 to 32767.

For CNOS, the SESSIONS parameter value should include two extra control sessions per remote. These sessions are not used for application conversations, and they are defined automatically by Model 204. Since a single Horizon link can support both CNOS and non-CNOS conversations, you are not required to specify an additional DEFINE LINK and DEFINE PROCESS command for CNOS if you also have non-CNOS sessions.

INBUFSIZE specifies the size of the Horizon receiving buffer, the buffer into which each unit of data transferred over the session is received. In SNA, data is sent and received in units called Request Units (RUs).

You should set INBUFSIZE to the value of the SNA Communications Server MODETAB table parameter RUSIZES, the maximum RU a session partner is allowed to send. It is recommended that both RUSIZES values be set to 2K. For a discussion about setting RUSIZES to a value other than 2K, see SNA Communications Server tuning for Horizon.

Set INBUFSIZE to the value of the SNA Communications Server MODETAB table parameter RUSIZES (2K recommended). The valid range of INBUFSIZE values is 256 to 32767.

INBUFSIZE and the DEFINE PROCESS command parameter DATALEN are independent; they refer to different buffers.


The following standard Horizon link definitions identify Model 204 as an LU to the network (setting LOCALID to a SNA Communications Server applid), specify the TRANSPORT and PROTOCOL options, indicate the maximum number of available sessions, and specify the size of the receive buffer (INBUFSIZE):


Usage notes

The names for the link and processgroup entities for a given link are system-specific and are not pointed to by other systems in the network, unlike the LOCALID value of DEFINE LINK and the REMOTEID value of DEFINE PROCESSGROUP. Thus, you may assign the link and processgroup entities any names you choose, even names that are used on other systems. Also, they need not be changed if, for example, the LOCALID value of DEFINE LINK or the REMOTEID value of DEFINE PROCESSGROUP is changed.

The DEFINE LINK command can be issued only if the named link is not open. Model 204 rejects a DEFINE LINK command for an open link. If the link is open, the CLOSE LINK command must be issued for the DEFINE LINK to complete successfully.

A single link may be shared by:

  • Conversations having different destinations, such as a client that converses with more than one server

  • Client and server processes defined on the same copy of Model 204.

    You can issue multiple DEFINE LINK commands. Multiple links can be used to segregate conversations that are different in type or destination. When defining multiple links:

  • Each link must have a different unique LOCALID value. Specifying more than one link with the same LOCALID results in an error when OPEN LINK is executed.

  • TRANSPORT=VTAM must be specified for each link. SNA Communications Server is required to avoid an error when OPEN LINK is executed. If SNA Communications Server is not specified, the OPEN LINK command fails.

If you are to have multiple links open simultaneously, you must make sure the value of the Model 204 User 0 parameter NSUBTKS is sufficiently large. Each open link requires an increment to NSUBTKS: 2 for z/OS, 4 for CMS.

For more information about a Horizon network and entities, see Horizon network management.