IWF Functionality for 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 TDM TRANSPORT OVER MPLS USING AAL 1 313 was defined in ITU-T I.366.2 recommendation, which explains the name of this imple- mentation agreement. The voice trunking information formatted per the ITU-T I.366.2 recommendation, is subsequently encapsulated in CPS-packets, described in Section 3.7.2. The CPS-packets are not transported over CPS-PDUs, but they are transported directly over MPLS. This implementation agreement describes only how the CPS-packets are transported over MPLS, and in view of this, it is commonly referred to as AAL 2 over MPLS A2oMPLS . One method for implementing VoIP is to transport the IP packets containing the voice samples over MPLS. In this case, the voice samples are first encapsulated in RTP, UDP, and IP and then in MPLS. Compressed headers are used in some implementations. The encapsulated information is then conveyed by an MPLS transport arrangement, such as frame relay, ATM, PoS, and Ethernet. A2oMPLS by-passes the RTPUDPIP encapsula- tion, and therefore it provides a more efficient mechanism for voice over MPLS. We now proceed to describe the two implementation agreements in detail.

12.8 TDM TRANSPORT OVER MPLS USING AAL 1

The reference architecture for the TDM transport over MPLS using AAL 1, or TDM- MPLS for sort, is shown in Figure 12.19. Each TDM device is connected to a provider edge PE device over a PDH TDM link, such as T1, E1, T3, or E3. The PE is equivalent to the CES IWF see Section 12.3. It is connected to the destination PE over an MPLS network via a point-to-point bidirectional LSP, which has been created either by manual provisioning or by using an MPLS signaling protocol, such as CR-LDP or RSVP-TE. The PE provides multiple functions, including: • Transport of fractional T1E1 i.e. n × 64 Kbps or of an entire signal i.e. of a T1, E1, T3, or E3 over an LSP. • End-to-end preservation of the order of the TDM frames. • Transparent transfer of CAS bits. • A mechanism for the reconstruction of the TDM clocks. • Transport of standard alarms between the two TDM devices. The TDM traffic transmitted to a PE from the TDM device is first encapsulated using AAL 1. Recall from Section 3.7.1 that AAL 1 consists of a SAR sublayer and a con- vergence sublayer CS . The CS performs a variety of functions, such as handling of the cell delay variation, processing of the sequence count, structured and unstructured data transfers, and transfer of timing information. The SAR sublayer is responsible for the bit error detection, and possibly correction, of blocks of data received from CS. It accepts TDM device PE PE LSP TDM link TDM device TDM link MPLS network LSR LSR LSR Figure 12.19 The TDM-MPLS reference architecture.