gtpn1m07 | Non-SNA Data Communications Reference |
Messages will be transmitted in variable length blocks, with maximum block length subject to agreement by link. The total message length is restricted by the maximum number of blocks used for one message (sixteen).
A single block will contain characters of only one message; parts of different messages will not be packed into the same block. This applies also to messages from and to indirect terminals attached to the TPF system via the SITA High Level Network.
Error detection based on character and block parity checking is applied to all blocks. Multi-block message protection is by message label and block chaining. The message block identifier, MBI and the link characteristics indicator, LCI are used to make possible correct reassembly of multi-block messages on receipt.
The link will be controlled by means of fixed length link control messages. These Link Control Blocks (LCBs) will indicate correct or incorrect reception of information blocks and provide information about the status of the link. Two types of data messages will be recognized:
A subset called service messages will be exchanged to enable the TPF processor to stop and start the polling of indirect terminal interchanges and to inform the TPF processor of a failure on the High Level Network.
The following order of precedence will be observed when transmitting message blocks between the processors:
The security of Type B messages sent to an SLC link may be achieved by one of two ways.
The application informs SLC which type of security is being used by setting an indicator in the message status bytes.
Keypoint Record D (CTKD) is used exclusively by the TPF Synchronous Link Control Package (18-SLC) and contains two tables, the Link Routing Table and the Link Pool Availability Directory.
The header of CTKD is 16 bytes long. The header contains a record ID, a byte count indicating the length of the record, a four-byte record name, a four-byte file address, and a four-byte indexing factor.
The ID of CTKD is 'CK', indicating it is a Control Program keypoint record.
The byte count indicates the length of CTKD in bytes.
The program name updating the keypoint is placed in the record name field.
CTKD contains the file address of Keypoint CTK1.
The indexing factor is used by the load sequence to define the displacement of the record relative to a base register.
This table is used by the TPF Synchronous Link Control programs to route Type A messages to or from a high level communications network (e.g., SITA HLN) attached to the system via a synchronous link. There are two types of eight-byte entries in the link routing table:
Each entry consists of the following fields:
If on, this shows that a poll initiation service message has been sent from the Pseudo Line Start/Stop And Display program (03-CMP) to the HLN controlling the line to which this TI is attached.
If off, this shows that a stop polling service has been sent to the HLN controlling this line.
If on, the initiate poll request from CMP was for polling at the normal rate.
If off, the request from CMP was for polling at a reduced rate.
This bit controls the forward routing of type A messages associated with this entry by the TPF Synchronous Link Control component of the ECB Communications Source Program.
If on, CILL will route the message to the common processing routines of the ECB Communications Source Program (03-CIAA).
If off, CILL will route the message to either the Message Router (03-ROUT) or an application program specified by the user. The former is activated only when the Message Router function exists in the system.
These bits are used to control the line code in which messages associated with this LRT entry will be transmitted. The available codes are:
This table is used by the Input Message Handler (03-CMR) to control the allocation and use of file addresses in a fixed file area defined by FACE. Each bit corresponds to a file address:
1 = file address available.
0 = file address in use.
The bit number within the field is used as a FACE ordinal number and the record type is #LKIBR.
TPF Synchronous Link Control Component of Communications Source (03-CIAA) uses the Link Routing Table to determine the correct processing of input messages from a high level network.
TPF Synchronous Link Control Input Message Handler (03-CMR) uses the Link Pool Availability Directory to dispense file addresses to incoming message blocks for storing on file until a complete message is received.
TPF Synchronous Link Control CCP I/O Routines (03-CIOAI) are used. Send Intercept Routine CXKAII in CLQK uses the Link Routing Table to determine the additional routing characters for messages destined for an indirect TI.
TPF Synchronous Link Control Link Controller (03-CMC) segments CMCC/CMCD, CMR Restart, correct the Link Pool Availability Directory for any discrepancies caused by retrieval errors and the loss of chains of input message blocks.
TPF Synchronous Link Control Pseudo Line Start/Stop/Display program (03-CMP) uses the Link Routing Table to generate the appropriate polling message when a link is restarted and to maintain the polling status indicators according to commands.
The address of both tables is to be found in the SLC Common Area (CX#EV). The DSECT defining the fields of the Link Routing Table is in CAIEQ.
CK#LRT
The file copy of CTKD is stored in a 4K block. The size of the tables contained in CTKD are configuration-dependent. Care should be exercised to ensure that each table within CTKD is equal in length to the corresponding area reserved in the core-resident Control Program Segment CLQA.
CTKD has two backup copies on the file system. Due to the fixed nature of the Control Program keypoints, a special disk duplication exists. This duplication has a cylinder and head displacement of zero and, as such, may differ from the normal system duplication displacement (reference, MSP-PLM, label CK1DUP).
This record should be retrieved only by program MSP-PLM. No facility is available to update the file copy of this data record outside the keypoint update mechanism.
The next keypoint address in the keypoint chain is obtained from this record.
In this section, the formats of Link Control Message Blocks and Data Message Blocks will be described.
Operational control of the link is through the use of fixed format fixed length link control blocks. These blocks are used to indicate correct or incorrect reception of data message blocks, to provide information regarding the status of the link and for fault recovery. The link control block format is:
SYN SYN DLE LSI XXX ETB BCC
Message text is exchanged between centers in variable length data message blocks. Within each block, the text is preceded by a sequence of control characters. The format of a data block, whether Type A or Type B, will be one of the following:
SYN SYN DLE TSI MBI LCI ACI Text ETB BCC SYN SYN DLE TSI MBI LCI Text ETB BCC
SYN characters must precede each message block to establish and maintain character framing. The number of SYN characters required to establish character framing is a function of the hardware but each message block on a line will be preceded by at least two SYN characters.
The IBM 3705EP transmit side automatically generates two SYN characters to precede each message block.
The receive side does not pass incoming SYNs to the CPU. The first non-SYN character signals the start of an incoming message block. At least two SYN characters are required to establish character framing, but the software can provide up to 12 additional SYN characters on output in multiples of 4. SYN is a reserved character and may not be used elsewhere within a message block.
(The above described function should not be confused with bit synchronization which is solely a function of the communication hardware (e.g., modem) and has no relevance to the procedure described in this document).
The DLE character should be the first non-SYN character and indicates the start of a message block. It is a reserved character and may not be used elsewhere in a message block.
b7b6 Function Type Precedence 00* None - - 01 LSI LCB 1 10 TSI Type A 2 11 TSI Type B 3
In a link control block, the five bits b5 through b1 identify the type of LCB as described in Section "Link Control Messages".
In a non-LCB, the five bits b5 through b1 are used as a Transmission Sequence Identifier. 31 combinations are used sequentially to serially number data message blocks on transmission, and to verify proper reception.
The MBI is used to permit chaining of Data Message Blocks.
The MBI combination 1/15 is used to indicate a single block message.
MBI bits b7b6b5 provide seven combinations for individual multi-block message labels.
The combination 001 is reserved to indicate a single block message.
MBI bits b4b3b2b1 provide sixteen combinations for sequential block numbering per message label. For a single block message, these bits will be coded 1111 to avoid any possible coincidence with the reserved character SYN, DLE, ETB.
The LCI is mandatory in data message blocks. The bits of the LCI are used as follows:
The ACI is optional in the TPF System
The ETB character indicates the end of the message block. It is a reserved character and may not be used elsewhere in the message block.
The BCC provides longitudinal even parity for each bit column (b7 to b1) excluding parity bits.
The BCC extends from the DLE to the ETB and includes both.
The BCC itself in common with all other characters has odd character parity.
On outgoing messages the BCC is generated by the IBM 3705EP. On incoming messages it is not passed to the CPU but is replaced by an Error Indicator Byte.
Positive Acknowledgment - ACK
Where:
Negative Acknowledgment - NAK - IVB
Where:
Negative Acknowledgment - NAK - OST
Where:
Enquiry - ENQ
Where:
Acknowledge Message Label - AML
Where:
b7b6b5 indicate the cleared message label
b4b3b2 are coded 000
b1 is coded 0 for a Type A message
b1 is coded 1 for a Type B message
Stop Sending - STP
Where:
Stop Sending on a specific channel 1 to 7, 3/1 to 3/7.
Resume Sending - RSM
Where:
Resume Sending on a specific channel 1 to 7, 3/9 to 3/15.
Idle Line - ILB
Where:
The format of a data block, whether Type A or Type B, is either:
SYN SYN DLE TSI MBI LCI ACI Text ETB BCC or,
SYN SYN DLE TSI MBI LCI Text ETB BCC
The maximum length of the message block (excluding SYN characters) agreed by link governs the length of the text portion of the message.
Messages to and from a High Level Network (e.g., SITA) require two additional control fields to specify the HLN Entry and Exit Centers. The format of such a message will be:
SYN SYN DLE TSI MBI LCI HEX HEN ACI Text ETB BCC
or if the optional ACI character is not a component of the control character set:
SYN SYN DLE TSI MBI LCI HEX HEN Text ETB BCC
Where:
HEX and HEN comprise two characters each.
To identify the functional area to which input Type A messages should be routed, the first text character will be used as a Circuit Identity, and will not be translated.