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The RESYNCHRONIZATION (RS) PDU is used to request the resynchronization of buffers and state variables, and the RESYNCHRONIZATION ACKNOWLEDGMENT (RSAK) is used to acknowledge the resynchronization request. The ERROR RECOVERY (ER) PDU is used to recover from errors and the ERROR RECOVERY ACKNOWLEDGMENT (ERAK) to acknowledge an error recovery request.
The assured transfer of data is implemented using the PDUs: SEQUENCED DATA (SD), STATUS REQUEST (POLL), SOLICITED STATUS RESPONSE (STAT), and UNSOLICITED STATUS RESPONSE (USTAT). The assured data transfer scheme is described below.
The UNNUMBERED USER DATA (UD) PDU is used for non-assured data transfer between two SSCOP users. The UD PDU does not carry a sequence number as the SD PDU, and consequently it cannot be recovered by retransmission in the case where it gets lost or it arrives erroneously. UD PDUs are used by SSCOP to send information to its peer without affecting the SSCOP states or variables.
THE SIGNALING ATM ADAPTATION LAYER (SAAL)
Table 5.1 The SSCOP SD PDUs.
Function SSCOP PDU name Description
Establishment BEGIN (BGN) Used to request the establishment
an SSCOP connection.
BEGIN Used to acknowledge acceptance
ACKNOWLEDGEMENT of an SSCOP connection request
(BGAK) by the peer SSCOP.
Release BEGIN REJECT (BGREJ) Used to reject the establishment
END of a connection requested by the
Used to release an SSCOP
connection between two peer
END ACKNOWLEDGEMENT Used to confirm the release of an
(ENDAK) SSCOP connection requested by
the peer SSCOP.
Resynchronize RESYNCHRONIZATION (RS) Used to resynchronize buffers and
the data transfer state variables.
RESYNCHRONIZE Used to acknowledge the
ACKNOWLEDGEMENT acceptance of a resynchronization
Recovery ERROR RECOVERY (ER) Recovery command.
ERROR RECOVERY ACK Recovery acknowledgment.
Assured transfer SEQUENCED DATA (SD) Used to transfer user data.
STATUS REQUEST (POLL) Used by transmitting SSCOP to
request status information from
the receiving SSCOP.
SOLICITED STATUS Used to respond to a POLL. It
RESPONSE (STAT) contains the sequence numbers of
outstanding SD PDUs and credit
information for the sliding
UNSOLICITED STATUS Similar to STAT message, but
RESPONSE (USTAT) issued by the transmitter when a
missing or erroneous SD PDU is
Unacknowledged UNNUMBERED USER DATA Used to transfer data in an
data transfer (UD) non-assured manner.
Management UNNUMBERED Used to transfer management
data transfer MANAGEMENT DATA (MD) data in a non-assured manner.
SIGNALING IN ATM NETWORKS
Finally the UNNUMBERED MANAGEMENT DATA (MD) PDU is used to transmit management information. It does not include a sequence number and it can be lost without notification.
The assured data transfer scheme
This scheme provides recovery of lost or erroneously received data by retransmission. It also provides flow control using an adjustable sliding window. The data is carried in SEQUENCED DATA (SD) PDUs. An SD PDU is sequentially numbered and it can carry a variable length payload of up to 65,535 bytes. Sequence numbers range between 0 and n — 1 (where n = 224 — 1).
The PDUs STATUS REQUEST (POLL), SOLICITED STATUS RESPONSE (STAT), and UNSOLICITED STATUS RESPONSE (USTAT) are used to implement a retransmission scheme for erroneously received or lost SD PDUs. Specifically, the transmitter periodically sends a POLL PDU to request the status of the receiver. This POLL is either triggered when the timer, Timer POLL, expires or after a certain number of SD PDUs has been sent. The POLL contains the sequence number of the next SD PDU to be sent by the transmitter. It is sequentially numbered with a poll sequence number which essentially functions as a time-stamp. Poll sequence numbers are between 0 and n — 1 (where n = 224 — 1), and are independent of the SD PDU sequence numbers. The receiver, upon receipt of a POLL, responds with a STAT PDU. The STAT PDU contains the following information: an SD sequence number (which sets the maximum window at which the transmitter can transmit), the number of the next expected SD PDU, the echoed poll sequence number, and a list of all SD PDUs that are currently missing or have been received erroneously. The receiver can determine the missing SD PDUs by checking for gaps in the sequence numbers of the received SD PDUs and by examining the SD sequence number contained in the POLL. Based on the received STAT message, the transmitter retransmits the outstanding SD PDUs and advances the transmit window.