2. USAGE CONSIDERATIONS › 2.2 General Analysis Considerations › 2.2.2 SNA Overview › 2.2.2.1 SNA Terminology › 2.2.2.1.2 SNA Data Formats
2.2.2.1.2 SNA Data Formats
Network elements use several different message-unit formats
to exchange data among themselves. These formats are often
referred to in discussions of communications and are outlined
below.
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.
o FID 0 is used to route data between adjacent subarea
nodes for non-SNA devices. This format is not now
commonly used.
o FID 1 is used to route data between adjacent subarea
nodes if one or both do not support explicit and
virtual route protocols.
o 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.
o FID 3 was used to route data between a subarea node and
a type 1 node.
o 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. Figure 2-6 shows the relationship between basic
information units, path information units, and basic link
units.
+----+----------------------------+
| RH | RU |
+----+----------------------------+
|---- BASIC TRANSMISSION UNIT ----|
+----+----+----------------------------+
| TH | RH | RU |
+----+----+----------------------------+
|-------- PATH INFORMATION UNIT -------|
+----+----+----+----------------------------+----+
| LH | TH | RH | RU | LT |
+----+----+----+----------------------------+----+
|--------------- BASIC LINK UNIT ----------------|
Figure 2-6. SNA Data Formats
It is these data formats that are exchanged within the SNA
seven-layer hierarchy:
o Presentation services (SNA Layer 6) in one node
communicates with presentation services in another node
by exchanging basic information units.
o 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.
o 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.
