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Figure 12.19 The TDM-MPLS reference architecture.
VOICE OVER ATM AND MPLS
TDM-MPLS 48-Byte 48-Byte
header sub-frame sub-frame
4 5 6-9 10-15 ÈÉÒ"-..
Reserved L R Reserved Length Seq. number
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 destination 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 requirements 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. Specifically, it is assumed that sufficient bandwidth has been allocated to the LSP carrying the
I.366.2 VOICE TRUNKING FORMAT OVER MPLS
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:
Figure 12.21 The A2oMPLS reference architecture.
Outer Inner A2oMPLS CPS-packet CPS-packet
label label header
^ 0-9 10-15 16-31 "v
Reserved Length Seq. number
Figure 12.22 The A2oMPLS frame structure.
VOICE OVER ATM AND MPLS