of fact, due to ATM segmentation of the layer 3 packets Networks Fundamentals and Present Architectures TABLE 3.5 Packet over SONET Frame Efficiency for "Good" Packet Sources Client Packet POS Efficiency (% of SPE Size (Bytes) Carrying Client Data) TABLE 3.6 Packet over SONET Frame Efficiency for "Bad" Packet Sources SONET Effective Bandwidth Data Bit Rate Payload Rate Efficiency (%) 10 Mbit/s Ethernet STS-1 ~48.4Mbit/s 100 Mbit/s Fast Ethernet STS-3c 150 Mbit/s 1 Gbit/s GbE Ethernet STS-48c -2.4Gbit/s in ATM cells and the need of adapting ATM cells to the SONET frame, the bandwidth efficiency of this solution is quite insufficient. POS does much better, especially when the client rate is well matched with the used concatenated SONET frame. As an example, the POS framing efficiency is reported in Table 3.5 [64] for different "good" dimensions of the client packets. The situation can be completely different in case of "bad" dimensions of client packets, as shown in Table 3.6 [65] where the POS framing efficiency is shown in case of Ethernet encapsulation. It is to be noted that the situation of Table 3.6 is bad, but not uncommon, especially in the metropolitan area. The situation gets better with the encapsulation of MPLS or Ethernet frames into the OTN frame-the so-called digital wrapper (compare Figure 3.37) when the client signal is high speed-starting from 2.5 Gbit/s. On the contrary, the OTN framing is quite inefficient for a large number of low-speed client signals. This behavior is due to the way in which the OTN was designed: it was conceived as a physical layer that is able to transport huge traffic quantities, while processing of low- speed tributaries is left to the upper layer. This effectively happens in the Ethernet over OTN scheme and even more in MPLS over OTN architectures, where switches and LERs, respectively, can offer to the OTN high- speed tributaries (e.g. 10 GbitE or 10 Gbit/s MPLS packets) for which the OTN frame effi- ciency is very high. This qualifies the OTN as a core technology that c