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[~l Remote Operation - SNA/SDLC 29
Segmenting Inbound
The S41 supports inbound segmentation if the bind parameters specify an RU length greater than 256. All transmissions containing more than 256 data bytes will then be segmented. Each R U segment (except the first and the last) will contain 259 data bytes. The first RU segment will contain 256 data bytes.
Session Processing States
The processing of SN A commands and responses as well as the data transfers are controlled by a set of session states. These session states are maintained in both Communication Processor 4101 and Display Unit 4110 (only the data traffic state is maintained for printer units). The session states are described in the following sections. The diagrams in Figs. 9 and 10 show the various states and the transitions between them.
Data Traffic States
Data Traffic States for LV-LV Sessions
Bind
The data traffic reset state is entered when a Bind or Clear command is received from the PL U. The data traffic active state is entered when an SDT command is received. When the S41 is in the data traffic reset state, it cannot send any data or commands to the PL U; it also rejects any FMD RUs or DFC commands.
Data traffic active
Send-Data traffic
reset Contention '~-n-ot-~-ra-ns-m-it~---tr-an-sm~ii Receive ERP1
S41 receives a chain with CD
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Fig. 9. Session states for LV-LV session
30 Remote Operation - SNA/SDLC
Data Traffic States for SSCP-LU Sessions
The data traffic active state is entered upon the acceptance of the ACTL U command. The data traffic reset state exists before the receipt of the ACTLU command or after the DACTLU command is received.
Contention State
Send State
The contention state for the LU-LU session exists only between brackets.
The SL U can accept data from either the operator or the host. The first arrival triggers a change to the send or receive state. When data or a Bid command is received from the host, Display Unit 4110 is consulted, and if it is in the send state the PL U request will be rejected.
For the SSCP-SLU session, the contention state exists after the successful completion of the transmission of a chain (only single-element chains are used in an SSCP-SLU session).
In the send state, the SLU half-session resources are allocated for inbound operation. The send state is subdivided in two states called send-not-transmit and send-send-not-transmit. The send-not-send-not-transmit state is maintained only by Display Unit 4110 and not by Communication Processor 4101. It exists while the operator is entering data from the keyboard, the magnetic identification device or the selector pen.
The send-not-transmit state is entered from contention in response to the first keystroke capable of changing data on the screen. This state is changed to send-transmit by a send initiating action (usually depression of the ENTER key). The transition to send-transmit also causes transition to the in-bracket (lNB) state and causes the keyboard to be locked. When the in-bracket state prevails, the send-not-transmit state is entered from the receive state or the ERP1 state after an outbound chain carrying CD (but not EB) is successfully processed.
The keyboard is unlocked when the send state is entered from either the receive state or the ERPl state if a previous write control character (WCe) specified keyboard restore. The terminal operator can also depress the RESET key to unlock the keyboard after the send state has been entered.
Any normal outbound requests received in the send-not-transmit state will be discarded and a negative response (receiver in transmit mode) will be sent.
The data is sent from the display unit to the PLU while the send-transmit state prevails. All normal flow chains in the LU-LU session will carry the CD. Transition from the send-transmit state depends upon the response type called for by the request that sends data to the PL U. If a definite response is called for, a transition from send-transmit to receive takes place after the communication processor has received the response to the request. If an exception response is called for, a transition from send to receive takes place as soon as the communication processor has sent the complete chain.
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I~l Remote Operation - SNAisDLC 31
Display Unit 4110 will change its state to receive as soon as the last part of the message is transferred from the display unit to a buffer in the com-munication processor.
The SSCP-SLU session operates only in the definite response mode. After reception of a positive response, a transition from the send-transmit to the contention state takes place. After a negative response is received a transition to the receive state takes place.
Receive (ReV) State
When the S41 is in the RCV state it cannot send inbound normal flow requests. Only inbound responses and control commands (the latter using expedited flow) can be sent. The keyboard is locked while the display is in the receive state (except the SYREQ key and, in LV-LV sessions, the ATTN key).
In an LU-LU session, the S41 enters the receive state from the contention state when an outbound normal flow message is accepted for processing. It also enters the receive state from the send-transmit state after a message is sent. The transition occurs a) when the S41 receives the response if the inbound request specifies CD and definite response, or b) after the S41 has successfully transferred the chain to the data link if the request specifies CD and exception response.
In an LU-LU session, the receive state is changed to send-transmit after the S41 has successfully processed the last request in a chain if this last request specifies CD. The receive state is changed to the contention state after the S41 has successfully processed and responded to a chain specifying EB, or after the S41 has received a chain specifying EB and exception response. The S41 changes from the receive state to the ERP1 state if a negative response is returned to the outbound request.
Data traffic active
-SSCP data transferred to DU 4110 and
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any RSP returned to PLU- I
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I Temporary or permanent device error
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Fig. 10. Session states for SSCP-LU session
32 Remote Operation - SNA/SDLC
In an SSCP-SLU session, the S41 enters the receive state from the con-tention state when it accepts an outbound normal flow message and returns to the contention state after returning the response to the outbound request. The S41 enters the receive state from the send-transmit state if it receives a negative response to an inbound request.
ERPI State
ERP! is a special state used in connection with error recovery. Since the PL U is always responsible for error recovery, the SL U generally awaits an outbound request to correct the error condition. However, there are times when the SLU must first recover and notify the PLU of its recovery by means of a LUSTAT command before the PLU can take action.
The SLU ERPI state allows a form of contention within brackets. While in this state, the S41 is able to receive any request, but it can only send LUSTATs. The S41 remains in the ERP1 state while LUSTATs are being sent to the PL U. As a result, successive L USTATs can be sent without requiring the general exchange of CDs between each LUSTAT.
After the S41 has sent a negative response (except for negative responses containing bracket bid reject or receiver in transmit mode), it enters the ERP! state. If the negative response does not cause a changeover to the between brackets (BETB) state, the transition to ERPI takes place at end-of-chain.
The ERPI state is maintained only in the communication processor.