SNA Overview 445
boundary node for the remote PU 2 is the PU 4 in the 3745 FEP. If there were not a 3745 FEP,
VTAM would assume the boundary function on the mainframe. When VTAM performs the
boundary function, the boundary function is a PU 5, which is shown in the figure where the PU
2 is directly connected to the mainframe by a channel.
Figure 12-11
Boundary Function
Path Control Layer
The path control layer operates at Layer 3 of the SNA model. The path control layer handles
data addressing and routing, controls data flow, and transmits data over specified links. The path
control layer performs the following functions:
·
Addressing--Translates local addresses into complete network addresses when message
units pass from peripheral nodes to subarea nodes. The network administrator assigns the
local address, and the system assigns the network address.
·
Sending messages--Builds a transmission header for each message.
·
Segmenting--Segments messages. Messages are broken into smaller segments if they are
too large or are combined to form larger blocks if they are too small.
·
Routing--Determines the next node through which the message will pass and over which
transmission groups it will flow.
·
Sequencing--Provides message unit sequencing because units can travel over different
links in the same transmission group.
·
Network control--Detects congestion and regulates flow control.
IBM 3745
IBM 3174
Cluster controller
Mainframe
IBM S/370
PU 5 performs boundary function for
cluster controller ---->
Channel attached
IBM 3X74
PU 2
<-----
Remote
PU 2
<---
PU 4 ---->
performs boundary
function
87200333.book Page 445 Wednesday, August 22, 2001 2:53 PM