fig: PDUs in wireless transaction protocol:

WTP makes use of six types of PDUs. Each PDU begins with a fixed header portion as shown in above figure and may be followed by a variable header portion that contains supplementary control information. The supplementary information is in the form of one or more transaction protocol items (TPIs).

1) Invoke PDU: The Invoke PDU is used to convey a request from an initiator to a responder

It is four bytes long and includes the following fixed header fields:

• Continue Flag: If this flag is set, there are one or more TPIs following the fixed header. In turn, each TPI begins with a continue flag bit to indicate whether there are more TPIs to follow or this is the last TPI.

• PDU Type: Indicates that this is an Invoke PDU.

• Group Trailer Flag: Used when segmentation and reassembly are employed, as explained subsequently.

• Transmission Trailer Flag: Also used with segmentation and reassembly.

• Retransmission Indicator: Indicates whether this is a retransmission.The initiator will retransmit an Invoke PDU ifit does not receive an acknowledgment within a specified time.

• Transaction Identifier: Used to associate a PDU with a particular transaction.

• Version: Version of WTP.

• TIDnew Flag: Set when the initiator has "wrapped" the TID value; that is, the next TID will be lower then the previous one.

• UIP Flag: When set, it indicates that the initiator require a user acknowledg- ment from the server WTP user. This means the WTP user confirms every received message. When this flag is clear, the responding WTP entity may acknowledge an incoming PDU without a confirmation from its user.

• Transaction Class: Indicates the desired transaction class to be used in processing this Invoke PDU.

If the message to be sent by WTP (the block of data from WSP) is too large for the current bearer, WTP may segment that message and send it in multiple packets, one per Invoke PDU.

2) ACK PDU: ACK PDU is used to acknowledge an Invoke or Result PDU. It is three bytes long. The PDU includes a Tve/Tok flag, whose interpretation depends on the direction of the PDU In the direction from the responder to the initiator, this is a Tve flag. If the Tve flag is set, it has the interpretation.

3) Result PDU: The Result PDU is a 3-byte PDU used to convey the response of the server to the client.

4) Abort PDU: Abort PDU is used to abort a transaction. Two abort types are defined: user and provider. If the abort is generated by the WTP user, then the user's reason for the abort is conveyed in the body of the PDU and provided to the WTP user at the destination of the abort PDU If the abort is generated by the WTP provider, then the abort reason field in the PDU indicates one of the following reasons:

• Unknown: Unexplained error.

• Protocol error: The received PDU could not be interpreted.

• Invalid TID: Used by the initiator as a negative result to the TID verification.

• Not implemented Class 2: The respondent does not support class 2, which was requested.

• Not implemented SAR: The respondent does not support segmentation and reassembly.

• Not implemented user acknowledgment: The responder does not support user acknowledgment.

• WTP version 1: The initiator requested a version of WTP that is not supported; current version is l.

• Capacity temporarily exceeded: Due to an overload situation, transaction cannot be completed.

5) Segmented invoke PDU: The segmented invoke PDU and the segmented result PDU may be used for segmentation and reassembly. When they are used, each packet is numbered sequentially.

6) Negative acknowledgment PDU: Negative acknowledgment PDU is used to indicate that one or more packets in a sequence did not arrive.