that can be multiplexed using the SONET frame with the data from the IP network. The disadvantage is the physical layer redundancy when WDM is used under the SONET layer. In this case, the POS advantages are paid with a larger and larger CAPEX while the number of SONET machines increases following the increase of traffic and WDM wavelengths. The current POS specifications [59] define the requirements that are needed to transport data packets through POS across a SONET network. These requirements are summarized as follows: High-order containment: POS frames must be placed in the required synchronous transport signals used in SONET. For example, an optical carrier-12 (OC-12) con- catenated POS interface requires a Synchronous Transport Signal-12 (STS-12) cir- cuit to contain the payload. The client-generated IP packet has to be encapsulated into a layer 2 format to be adapted to the SONET frame. The POS standards spec- ify PPP encapsulation for POS interfaces. The layer 2 PPP information is encap- sulated into a generic HDLC header and placed into the appropriate field of the SONET frame. This can be a confusing concept at first. Although HDLC and PPP are different, mutually exclusive layer 2 protocols, HDLC is used as delimiter in the SONET frame. The encapsulation process of an IP packet to a SONET frame is illustrated in Figure 3.47. Bytes alignment: This refers to the alignment of the data packet byte boundaries to the STS byte boundaries. Payload scrambling: The ANSI standard for T1 transmission requires a density of 1s of 12.5% with no more than 14 consecutive 0s for unframed signals and no more than 15 consecutive 0s for framed signals. The primary reason for enforcing a 1s density requirement is for timing recovery or network synchronization. However, other factors such as equalization and power usage are affected by 1s density. SONET has a default scrambler that was designed for voice transport. The 7 bit SONET scrambler is not well suited for data transport. Unlike voice signals, data transmissions Datagr