SNA Data Formats
Network elements use several different message-unit formats to exchange data among themselves.
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These formats are often referred to in the following sections:
Basic Information Unit (BIU)
Network addressable units (NAUs) communicate via basic information units (BIUs). BIUs are either requests or responses. Requests initiate an exchange; responses acknowledge the receipt of a request. Request BIUs are made up of a request header (RH) and a request unit (RU). Response BIUs are made up of a response header (RH) and a response unit (RU).
Request and response headers are three-byte fields which identify the type of data in the associated request or response unit. The request/response indicator bit distinguishes a response header from a request header.
Request units are variable length fields which contain either end-user data or an SNA command.
Response units are either positive or negative and contain information about the request. Positive acknowledgements to commands generally identify the command request. Positive acknowledgements to data requests contain no RU. Negative response units are four to seven bytes long and indicate why the request was not accepted.
Path Information Unit (PIU)
Path control elements exchange information via path information units (PIUs). PIUs are basic information units with a transmission header (TH) prefixing them. The transmission header contains the origin and destination NAU addresses, the explicit route number, and the virtual route number.
Transmission header formats vary depending on the type of nodes to which data is routed. The differences are indicated by the format identifier (FID) type.
- FID 0 is used to route data between adjacent subarea nodes for non-SNA devices. This format is not now commonly used.
- FID 1 is used to route data between adjacent subarea nodes if one or both do not support explicit and virtual route protocols.
- FID 2 is used to route data between a subarea node and an adjacent type 2.0 or 2.1 peripheral node or between directly connected type 2.1 nodes.
- FID 3 was used to route data between a subarea node and a type 1 node.
- FID 4 is used to route data between adjacent subarea nodes which support explicit and virtual route protocols.
Basic Link Unit
Data link control uses basic link units (BLUs) to transmit data across links. A BLU consists of a PIU with a link header (LH) as a prefix and a link trailer (LT) as a suffix.
The various prefixes and suffixes are added and deleted as necessary as the message passes through the various SNA layers. The SNA Data Formats diagram shows the relationship between basic information units, path information units, and basic link units.
SNA Data Formats
+----+----------------------------+ | RH | RU | +----+----------------------------+ |---- BASIC TRANSMISSION UNIT ----| +----+----+----------------------------+ | TH | RH | RU | +----+----+----------------------------+ |-------- PATH INFORMATION UNIT -------| +----+----+----+----------------------------+----+ | LH | TH | RH | RU | LT | +----+----+----+----------------------------+----+ |--------------- BASIC LINK UNIT ----------------|
It is these data formats that are exchanged within the SNA seven-layer hierarchy:
- Presentation services (SNA Layer 6) in one node communicates with presentation services in another node by exchanging basic information units.
- Path control services (SNA Layer 3) in the originating node communicates with path control services in the destination node by exchanging transmission headers which have been appended to the BIU to form a path information unit.
- Data link control services (SNA Layer 2) in both nodes communicate via link headers and link trailers with which they frame the PIU to form a basic link unit.