Switched and Non-Switched Trunking

312 VOICE OVER ATM AND MPLS SSTED-PDU payload SSTED- UUI SSTED-PDU trailer C I L P Length CRC Reserved Figure 12.18 The SSTED-PDU trailer. • Congestion indicator CI : A 1-bit field provided for compatibility with the service of the CPCS of the AAL 5. It is transparently transported from user to user of the transmitter to the user of the receiver. • Loss priority LP : A 1-bit field provided for compatibility with the service of the CPCS of the AAL 5. It is transparently transported from user to user of the transmitter to the user of the receiver. • Length : A 2-byte field that gives the length of the SSTED-PDU payload field. • CRC : The following polynomial is used: x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1 If the CRC fails, then the management network is informed and the PDU is discarded.

12.6.3 SSADT

SSADT uses sequence numbers to detect missing PDUs that have been either lost or discarded by a lower SEG-SSCS layer because they were corrupted. Missing PDUs are recovered by selective retransmission. A flow control mechanism allows an SSADT receiver to control the rate at which the peer SSADT transmitter sends information.

12.7 VOICE OVER MPLS VoMPLS

MPLS can provide QoS on a per-connection basis as in ATM, and therefore is a suitable technology for voice over packet. The MPLS and Frame Relay Alliance have defined so far the following two specifications, known as implementation agreements: • TDM Transport over MPLS using AAL 1 • I.366.2 Voice Trunking Format over MPLS The first implementation agreement defines a service which emulates a point-to-point TDM circuit, such as fractional DS1E1 n × 64 Kbps, T1, E1, T3, and E3, over MPLS. It assumes that the TDM traffic to be carried over MPLS is already encapsulated in AAL 1 SAR-PDUs see Section 3.7.1, and it simply provides an efficient transport of the SAR-PDUs over an LSP. The second implementation agreement was defined to convey voice calls, voiceband data, such as facsimile and data transmitted over a modem, and fractional T1E1 circuit- mode data, and is similar to the voice over ATM specification described in Section 12.4. It assumes that the information to be carried over MPLS is the output of the AAL Type 2 SSCS for trunking, described above in Section 12.5. This convergence sublayer