Packet Transfer Mode

In August 2001, consensus was reached in ITU-T Study Group 15 to add a transmission convergence layer for a Packet Transfer Mode to the VDSL Foundation Document (ITU-T Recommendation G.993.1). With this addition, the tradition of having only ATM and STM in xDSL standards ended, and it is no coincidence that this happened in the context of the VDSL project: ^[8] given the relatively high bit rate supported by VDSL, there is no justification for ATM in terms of Quality of Service or packetization jitter. Moreover, certain prestandard implementations of Ethernet-over-VDSL were already available on the market at that time.

The Packet Transfer Transmission Convergence (PTM-TC) is a sublayer bounded by the functional ?( ?)-interface and the ?-interface, as shown in Figure 3.3. Its main function is to preserve packet boundaries during transmission, in order to provide a packet pipe transparent to the higher layers (collectively referred to as the packet entity or PTM entity), and unaware of the protocols used. To this end, the PTM-TC uses high level data link control (HDLC) ^[9] encapsulation in synchronous (or octet-based) mode. HDLC encapsulation was chosen for its simplicity, and for the fact that it was already part of the VDSL standard as the protocol for the embedded overhead channel (eoc).

Figure 3.3: Ethernet-over-xDSLreference model (shaded portions are optional).

The HDLC frame structure is illustrated in Figure 3.4. HDLC encapsulation prepends an opening flag (0x7E), an address...