(--(
SSCP-PU Session
Before establishing the SSCP-PU (access method-3274 control unit) session, the physical transmission or channel connection to the host must be established. In locally attached systems, the Online/Offline switch must be placed in the Online position before communication can be established between the 3274 ModellA and the host.
The SSCP-PU session must be established before establishing the SSCP-SLU or LU-LU sessions. When the access method network operator activates a specific 3274, the access method issues the Activate Physical Unit (ACTPU) command to the control unit. A predefined start procedure for the access method may also request the activation of specific 3274 control units. The SSCP-PU session is the first session established between the host system and the 3274.
The SSCP-PU session is terminated when the access method network operator deactivates the 3274. When all SSCP-LU sessions for the control unit have been terminated, the access method issues the Deactivate PhYSical Unit (DACTPU) command. When the 3274 returns a positive response to the DACTPU command, the SSCP-PU session is terminated.
In locally attached systems, the Online/Offline switch may be placed in the Offline position when the host communication function is terminated.
SSCP-Secondary LU Session
LU-LU Session
Initiating an LU-LU Session
When the SSCP-PU session is established, an activate command may be issued to the access method to establish the SSCP-SLU session. The access method will issue an Activate Logical Unit (ACTLU) for the appropriate SLU or SLUs in the 3274. The SSCP-SLU session must be established before establishing the LU-LU session.
The SSCP-SLU session is terminated when the access method sends a Deactivate Logical Unit (DACTLU) command to the specified SLU. When the control unit returns a positive response to the DACTLU command, the SSCP-SLU session is terminated.
Three types of LU-LU sessions are supported by the 3274. Further description of these sessions is provided later in this section. The LU-LU session types are:
Type 1
Type 2
Type 3
-The device attached to the 3274 SLU is a printer, and the data stream is the SNA character string (SCS).
The device attached to the 3274 SLU is a keyboard/display, and the data stream is in the 3270 data stream compatibility (DSC) mode format.
The device attached to the 3274 SLU is a printer, and the data stream is in the 3270 DSC mode format.
The SNA Bind command is used to differentiate these types of sessions.
Chapter 5. SNA/SDLC Communication 5-3
Host System Host
Application Program
ACTLU (SSCP-PLU)
_ Logon Exit
m
Open SessionThe command flow sequence required to establish a session is summarized in Figure 5-1. The command flow nomenclature is generalized, and access method specific macro names are not used. The example assumes that no sessions are active between the host and the 3274. The access method sends the ACTPU command to establish the SSCP-PU session.. ACTLU commands
II
are then sent to establish SSCP-PLU andSSCP-SLU sessions. The SSCP-PLU session can be established by the host application any time prior to logon. The network is now ready for LU-LU sessions to be established.3274 SSCP
ACTPU (SSCP-PU)
(Access
D
Physical UnitMethod)
fJ fJ
ACTLU (SSCP-SLU) DisplayI Logical Unit
1 2 or
I Printer
1 1- _ _ _ _
- -
-...
Logon DisplayII
1 I Logon BindII
Logical UnitII
orII
I +Resp-
3 Printer1
IJ
Display1_- _ _ _ __ ...
Logical Unit
01-4 PI-i nter
~ ...
';~
,.~
Logical Unit Display
L. _ _ _ _ _ _ _ ....
n* or
Printer
"\
*The highest LU number for a 3274 is 33. (Note that LUl is reserved.)
Figure 5-1. Establisbing a Session witb a 3274
An LU-LU session is started by the host application program when it issues the Bind request. The LU-LU session may be initiated by the host application program (for example, acca:!!iring the terminal or by a simulated logon) or by the display terminal operator" (a character-coded logon). If a character coded logon is received by the access method, the access method translates the logon request and schedules a logon exit
II
for the PLU. Mter the PLU receives control at the logon exit, or when the PLU a~uires a terminal, the PLU passes an open session request to the access method. which results in an SNA BindII
being passed to the SLU. The 3274 LU examines the session parameters of the
Bind and, if they are acceptable, allows the session to be established by sending a
(1- '\
positive response. to the Bind command. If the session parameters are not
V
(
Terminating an LU-LU Session
(
Transmission Header
response, indicating that the session parameters are invalid (sense code X'0821 ').
Also, if power is not on at the device, a negative sense code X'080A' or X'0845' is returned to the Bind. Bind is discussed later in this chapter.
After the Bind command has been accepted with a positive response, the host program can issue the Start Data Traffic command to allow data traffic to flow for the session.
The manner in which an LU-LU session may be initiated depends on the type of session being started. A type 1 or type 3 session must be initiated by the PLU. A type 2 session may be initiated by either the PLU or SLU.
The PLU can terminate an LU-LU session by requesting that the SSCP close the session. The SSCP then sends the Unbind command to the secondary LU, and the LU-LU session is terminated.
Type 2 sessions can also be terminated by the display operator in either of two ways. The first method is to notify the PLU (where supported), on the LU-LU session, that termination is desired; the PLU then terminates the session. In the second method, the display operator changes from an LU-LU session to an SSCP-SLU session by using the System Request key (SYS REQ) and enters a logoff message. The SSCP then passes the logoff request to the PLU, if the logoff message is conditional, or issues the Unbind for the PLU if the logoff message is unconditional.
A PLU may close the session in an orderly fashion by issuing a Shutdown command. When the host program issues the Shutdown command, the 3274 returns the Shutdown Complete command after completing any outstanding operation and entering the between b~acket state. Note that the PLU must close a bracket with end bracket before the Shutdown command is effective.
The 3274 terminals support FID2 transmission headers (TH). The transmission header consists of 6 bytes:
THO:
Destination Address Field (See Figure 5-2 and "Device Addressing" in Chapter 1) Origin Address Field
Sequence Number on Normal, ID Number on expedited flow requests and responses
The 3274 handles transmission headers received on outbound requests as follows:
1. All reserved parameters are ignored on requests.
2. MPF-The 3274 supports outbound segmenting for FM data.
3. EFI-The expedited flow indicator identifies normal (0) or expedited (1) flow requests.
Chapter 5. SNA/SDLC Communication 5-5
EFI=l
Device Device Address Field
Number Bits: 0 1 2 3 4 5 6 7 PU
..
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 1 0
1 0 0 0 0 0 0 1 1
2 0 0 0 0 0 1 0 0
3 0 0 0 0 0 1 0 1
4 0 0 0 0 0 1 1 0
5 0 0 0 0 0 1 1 1
6 0 0 0 0 1 0 0 0
7 0 0 0 0 1 0 0 1
8 0 0 0 0 1 0 1 0
9 0 0 0 0 1 0 1 1
10 0 0 0 0 1 1 0 0
11 0 0 0 0 1 1 0 1
12 0 0 0 0 1 1 1 0
13 0 0 0 0 1 1 1 1
14 0 0 0 1 0 0 0 0
15 0 0 0 1 0 0 0 1
16 0 0 0 1 0 0 1 0
17 0 0 0 1 0 0 1 1
18 0 0 0 1 0 1 0 0
19 0 0 0 1 0 1 0 1
20 0 0 0 1 0 1 1 0
21 0 0 0 1 0 1 1 1
22 0 0 0 1 1 0 0 0
23 0 0 0 1 1 0 0 1
24 0 0 0 1 1 0 1 0
25 0 0 0 1 1 0 1 1
26 0 0 0 1 1 1 0 0
27 0 0 0 1 1 1 0 1
28 0 0 0 1 1 1 1 0
29 0 0 0 1 1 1 1 1
30 0 0 1 0 0 0 0 0
31 0 0 1 0 0 0 0 1
• * Address reserved.
Figure 5-2. Device Addressing for SNA Terminals
The 3274 supports the following requests as outbound expedited flow requests:
RU Category SC
NC DFC FMD
Request
ACTPU, DACTPU, ACTLU, DACTLU, BIND, UNBIND, CLEAR, SDT
Not supported
SIGNAL, SHUTDOWN Not supported
When the 3274 receives any requests listed above with correct categories and EFI= 1, they will be passed through for further processing.