User Functions THE AAL 2 SERVICE-SPECIFIC CONVERGENCE SUBLAYER SSCS

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. 314 VOICE OVER ATM AND MPLS TDM-MPLS header 48-Byte sub-frame 48-Byte sub-frame . . . L R Reserved Reserved Length Seq. number 0-3 4 5 6-9 10-15 16-31 Figure 12.20 The TDM-MPLS frame. blocks of 47 bytes from the convergence sublayer, and adds a 1-byte header to form the SAR-PDU. The resulting 48-byte SAR-PDUs are then transported over MPLS to the des- tination PE, which extracts the TDM traffic and transmits it to its TDM device over the TDM link. The same occurs in the opposite direction. Unlike in ATM where each SAR-PDU is transported in a single ATM cell, many SAR- PDUs can be transported together over MPLS in a single frame. The format of this frame, referred to as the TDM-MPLS frame, is shown in Figure 12.20. As we can see, it consists of a TDM-MPLS header and multiple SAR-PDUs referred to as 48-byte subframes. The TDM-MPLS frame is further encapsulated with a label stack if the underlying network of the MPLS is packet over SONET PoS or Ethernet. The following fields have been defined in the TDM-MPLS header: • L bit : This is the fourth bit in the header and is used to indicate physical layer loss of signal. • R bit : Occupies the fifth bit in the header and is used to indicate that the source is not receiving packets at its TDM-MPLS receive port. • Length : This is a 6-bit field that indicates the length of the TDM-MPLS frame header and payload in case padding is employed to meet minimum transmission unit require- ments of layer 2 of the MPLS network. It must be used if the TDM-MPLS frame length plus the layer 2 overhead is less than 64 bytes, and it must be 0 if this length exceeds 64 bytes. • Sequence number : The 16 bits sequence number is used to guarantee ordered frame delivery. The payload of a TDM-MPLS frame consists of one to thirty subframes. As mentioned above, each subframe is a 48-byte SAR-PDU. The number of subframes in the payload can be inferred by the receiving side from the length indicated in the TDM-MPLS header. It is pre-configured and is typically chosen to trade-off the delay to fill in a TDM- MPLS frame against overheads. For instance, using one subframe per TDM-MPLS frame reduces delay to minimum, but incurs the highest overhead. Using eight subframes per TDM-MPLS frame reduces the overhead, but increases the delay by a factor of eight. Each subframe is a 48-byte SAR-PDU, of which 47 bytes comes from the AAL 1 convergence sublayer. The structure of these 47 bytes follows the AAL 1 definition. That is, it could be an unstructured data transfer or a structured data transfer see Section 3.7.1. It could also be a structured data transfer with CAS, which is a structured data transfer with additional provisioning to carry the CAS bits. TDM-MPLS assumes that the QoS guarantee is provided by the MPLS network. Specif- ically, it is assumed that sufficient bandwidth has been allocated to the LSP carrying the I.366.2 VOICE TRUNKING FORMAT OVER MPLS 315 TDM-MPLS frames, so that to provide a low end-to-end transfer delay and a low packet loss probability.

12.9 I.366.2 VOICE TRUNKING FORMAT OVER MPLS

As we have seen, AAL 2 can be used to multiplex many voice calls over the same ATM connection. To that effect, the AAL 2 SSCS for trunking described in Section 12.5 is needed in order to convert the voice traffic and signals into packets at the transmitter, and extract the voice traffic and signals from the packets at the receiver. These packets are in the form of CPS-packets, which are transmitted over ATM. This implementation agreement, assumes the presence of an AAL 2 service-specific convergence sublayer for trunking, but instead of carrying the CPS-packets over ATM, they are carried over MPLS. The implementation agreement, therefore, is only concerned with the transport of AAL 2 CPS-packets over MPLS, and in view of this, it is commonly referred to as AAL 2 over MPLS A2oMPLS . The reference architecture for this implementation agreement is shown in Figure 12.21. The A2oMPLS functionality is implemented in a gateway GW, which can be a line card in a device that implements the AAL 2 SSCS for trunking. The device is attached to one or more LSRs, and the gateways are interconnected over the MPLS network via bidirectional point-to-point LSPs. In the AAL 2 CPS, the CPS-packets are packed into CPS-PDUs, and each CPS-PDU is carried in a separate ATM cell. In the A2oMPLS architecture, multiple CPS-packets can be placed onto the same frame, known as the A2oMPLS frame, and transported over an LSP. The structure of the A2oMPLS frame is shown in Figure 12.22. The following fields have been defined: GW GW GW MPLS Network LSR LSR LSR LSR PSTN PSTN Figure 12.21 The A2oMPLS reference architecture. Outer label CPS-packet CPS-packet . . . Reserved Length Seq. number 10-15 16-31 Inner label A2oMPLS header 0-9 Figure 12.22 The A2oMPLS frame structure.