5.10 Message Composition
| In the list of octets (p. 143), only the group of parameters that are enabled or identified in SPar(1) field are sent, depending on the command or message type being sent. Table 5.4 below shows required fields for inclusion in each of the defined message types. Note the messages requiring transmission of the parameter octets are CL, CLR, MS, and MP. The remaining messages do not pass any information parameter octets. 5.11.1 Transaction TypesIn the preceding discussion, we looked at the data structure of the transmitted message types. For proper use of the above message types and structures, G.994.1 defines permitted transaction types for use in the preactivation (handshake) session. In this section, we provide a general description of the permitted transaction types defined in G.994.1; complete details are provided in Recommendation G.994.1 [1]. There are two types of transactions defined in G.994.1, namely, basic and extended transactions. Table 5.4. Required Field per Each Message Type
5.11.1 Basic TransactionsBasic transactions are used to:
The CP unit always initiates a basic transaction. Table 5.5 summarizes the permitted basic transactions as defined in G.994.1, and each transaction ends with an ACK(1). Transactions A, B, and D are all stand-alone transactions; at the successful end of any of these transaction types, the modems generally exit the handshake session. Otherwise the modems reenter the initial transaction state. Successful execution of Transaction C requires follow-up with either transaction type A, B, or D. In transaction A, the CP unit selects a mode of operation and requests that the CO unit transition to this mode of operation. If the CO unit accepts this request, it responds with an ACK(1) message, and then the modems exit handshake and transition to the selected mode of operation. In transaction B, the CP unit requests that the CO unit select the mode of operation. The CO unit then responds with the desired mode of operation; if the CP unit agrees, then it responds with an ACK(1). The modems then exit handshake and transition to the selected mode of operation. Table 5.5. G.994.1 Basic Transaction Types
In transaction C, the CP unit sends a list of capabilities to the CO unit who then responds with a capabilities list of the CO unit, which also serves as an acknowledgment. The CP unit responds with an ACK(1) upon successful reception of the CL message. In this transaction, both stations have exchanged and negotiated capabilities. For selection to be complete, follow-up with a transaction of type A, B, or D is needed. In transaction D, the CP unit proposes a mode of operation to the CO unit and requests that the CO unit select the mode of operation. The CO unit responds with a MS message to select the mode of operation. The CP unit then responds with an ACK(1) message, and then the modems exit handshake and transition to the selected mode of operation. In any of the above transaction types, if either the CP or CO units cannot determine an agreed common mode of operation (standard or nonstandard), then the corresponding unit sends a MS message to the other end with the nonstandard field bit and all information parameter and standard information parameter bits set to zero. When the other end receives the MS message, it responds with an ACK(1), and then the modems execute the clear-down procedure. 5.11.2 Extended TransactionsExtended transactions are essentially a concatenation of two basic transactions arranged in such a way that the CO unit is positioned to control the negotiation procedures. Recall that the basic transactions are initiated by the CP unit and so are the extended transactions. However, after the initial message of MS, MR, or MP is sent by the CP unit, the CO unit responds with a REQ-MR, REQ-MS, or REQ-CLR message, which positions the CO unit to control negotiation of operating mode. At the end of an extended transaction, the modems either exit handshake or go to the initial transaction state. There are five extended transaction types. Table 5.6 below provides a list of the extended transactions, and a brief description of each is provided. In transaction A:B, the CP unit selects a mode of operation, but instead of the CO unit responding with an ACK(1) as in basic transaction A, it responds with a request for mode request (REQ-MR), which requests that the CP unit directly transition to transaction B where the CO unit initiates a mode select request. In transaction B:A, the CP unit requests that the CO unit select the mode of operation by issuing a mode request (MR) message, but instead of responding with a mode select (MS) as in basic transaction B, the CO unit sends a REQ-MS message requesting the CP unit to make the mode selection. The CP unit then responds with the MS message as in basic transaction B. Table 5.6. G.994.1 Extended Transaction Types
In transaction A:C, the CP unit requests a specific mode of operation by issuing a MS message, but instead of responding with ACK(1) as in basic transaction A, the CO unit requests that the capabilities list be exchanged. Once the two units exchange capabilities and negotiate mode of operation, a follow-up transaction is needed for mode selection. In transaction B:C, the CP unit requests that the CO unit select the mode of operation via a MR message, but instead of the expect MS response of basic transaction B, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection. In transaction D:C, the CP unit proposes a mode of operation via the MP message and requests that the CO unit select the operating mode. Instead of the expected MS message from the CO unit of basic transaction D, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection. 5.11.3 Message SegmentationNote that in the above transactions, some messages can become rather large when passing messages that contain the identification and standard information parameter fields. Excluding the two FCS octets and any octets inserted for transparency, the maximum message length in a frame is 64 octets. If a message is longer than 64 octets, then it must be segmented into two or more messages. The message types that can be segmented are those that contain the parameter octets, namely, CL, CLR, MP, and MS. When a receiving station is parsing a segmented message, the receiving station sends an ACK(2), which indicates to the sending station that it is ready to receive the remainder of the message. Once the complete message is received, the receiving station responds with an ACK(1) or other appropriate response. 5.11.4 Example TransactionsIn this section, we provide some example G.994.1 sessions to demonstrate the handshake process. Example 1 : Table 5.7 shows the use of a transaction sequence that combines basic transaction C with A. The CP unit first initiates a capabilities list request where the two units exchange and negotiate capabilities via the CLR and CL messages. The CP unit then selects the mode of operation via the MS command (basic transaction A). Table 5.7. Example 1 ”Basic Transaction C Followed by Basic Transaction A
Table 5.8. Example 2 “ Extended Transaction A:C Followed by Basic Transaction A
Example 2 : Table 5.8 shows an example transaction that combines extended transaction A:C with basic transaction A. First the CP unit selects a mode of operation and requests that the CO select this mode. Instead the CO unit requests the CP unit for a capabilities list request. The two modems then exchange capabilities lists and negotiate operating modes. Once the exchange and negotiation are complete, the CP unit selects the mode of operation via the MS message. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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