Packet Over SONET/SDH (PoS) maps IP datagrams into the SONET frame payload using Point-to-Point Protocol (PPP). Packet over SONET (PoS) is a highly scalable protocol that overcomes many of the inefficiencies of ATM, while providing legacy support to internetworks with existing SONET architectures. PoS provides a mechanism to carry packets directly within the SONET synchronous payload envelope (SPE) using a small amount of High-Level Data Link Control (HDLC) or PPP framing. PoS can be viewed as three layers:
- Top layer: IP encapsulation into PPPÂ
- Mid layer: Framing of PPP with HDLC
- Bottom layer: Mapping into SONETÂ Â
The bottom layer, mapping into SONET, is the unique part defined by IETF in RFC 2615 for PoS. It contains the following characters:
- High-order containmentPoS frames must be placed in the required synchronous transport signals used in SONET.
- Octet alignmentThis refers to the alignment of the data packet octet boundaries to the STS octet boundaries.
Payload scramblingScrambling is the process of encoding digital 1s and 0s onto a line in such a way that provides an adequate number for a 1s density requirement. The ANSI standard for T1 transmission requires an average density of 1s of 12.5 percent 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 automatic-line-build-out (ALBO), equalization, and power usage are affected by 1s density.
Packet over SONET: Encapsulating IP into a SONET frame
PoS: Packet over SONET
Related Terms: ATM, IP, SONET, PPP, HDLC
Reference Links:
http://www.ciscopress.com/articles/article.asp?p=101735: Packet Over SONET/SDH (POS)
http://www.javvin.com/rfc1661.pdf: The Point-to-Point Protocol, July 1994
http://www.javvin.com/rfc1662.pdf: PPP in HDLC-like Framing, July 1994)
http://www.javvin.com/rfc2615.pdf: PPP over SONET/SDH, June 1999