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VOICE OVER ATM AND MPLS
1 2 3 4 5 6 7 8
Figure 12.15 Dialed digits packet.
1 2 3 4 5 6 7 8
together into the same payload, and N is the number of time slots in the N x 64 Kbps stream. Sequence numbers are also used for circuit mode data.
• Dialed digits packet: The common facilities for Type 3 packet is used to transport multifrequency tones for the DTMF signaling system, and is also used for the signaling systems R1 (MF-R1) and R2 (NF-R2) across an AAL 2 connection. Dialed digits can be used to convey the destination address, either during the call setup or in the middle of a call. The format of the dialed digits packet is shown in Figure 12.15. The message-dependent information consists of a 5-bit signal level field, a 3-bit digit type field, a 5-bit digit code field, and a 3-bit reserved field (RES). The signal level field gives the total power level. The digit type field indicates the type of multi-frequency tone used, such as DTMF, MF-R1, and MF-R2. Finally, the digit code indicates the actual character transmitted (such as a 0 to 9 digit) and special characters.
• CAS bits packet: The common facility for Type 3 packets is used to transport the ABCD CAS bits. The packet format is given in Figure 12.16.
12.6 THE SEGMENTATION AND REASSEMBLY SSCS FOR AAL 2 (SEG-SSCS)
This standard specifies a convergence sublayer for the transport of packets with a size bigger than the maximum length of 45 bytes permitted in the payload of the CPS packet.
SEG-SSCS is subdivided into the following sublayers: service-specific segmentation and reassembly (SSSAR), service-specific transmission error detection (SSTED), and service-specific assured data transfer (SSADT). These sublayers are shown in Figure 12.17.
Redundancy Time stamp
RES A B C D
Figure 12.16 CAS bits packet.
Redundancy Time stamp
RES Signal level
Digit type Digit code
THE SEGMENTATION AND REASSEMBLY SSCS FOR AAL 2 (SEG-SSCS)
Service specific assured data transfer (SSADT)
Service specific transmission error detection (SSTED)
Service specific segementation and reassembly (SSSAR)
Common part sublayer (CPS)
Figure 12.17 The SEG-SSCSC layers.
SSSAR is concerned with the transfer of PDUs which are larger than the maximum 45-byte payload permitted in the CPS-packet. SSTED is used to detect corrupted PDUs, and SSADT is used to recover corrupted PDUs by retransmission. The minimum service provided by SEG-SSCS is the SSSAR function. The other two sublayers are optional.
It transfers PDUs over CPS which can be up to 65,568 bytes. In this sublayer, there is no mechanism to detect corrupted PDUs or to recover corrupted PDUs by retransmission; that is, the transfers are non-assured. The receiving SSSAR can detect some errors during the reassembly of a PDU, such as when the received PDU exceeds a pre-defined maximum length. PDUs discovered to be in error are discarded. SSSAR cannot detect partial or complete loss of a PDU, bit errors, and data merged together from separate PDUs. Hence, it might deliver corrupted data. Also, SSSAR does not have any mechanism to correct lost or corrupted PDUs by retransmission.
SSSAR uses the CPS-packet format shown in Figure 3.20. When the SSAR sublayer receives a PDU, it segments it into a number of segments and submits them to CPS. At the SSAR receiver, the CPS-packet payloads are reassembled into the original PDU. SSSAR uses the UUI field in the CPS-packet header to implement “more data.” Specifically, UUI = 27 means more data is required to complete the reassembly of a PDU at the receiving SSSAR. Any other value between 0 and 26 indicates the receipt of the final data for a PDU.