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PoS: Packet over SONET/SDH
Packet Over SONET/SDH (PoS), also known as PPP over SONET/SDH, refers to the scheme of using PPP
encapsulation to map IP datagrams into the SONET/SDH frame payload. Since SONET/SDH is a point-to-point circuit, PPP is well suited to use over these links. PoS is a highly scalable protocol that overcomes many of the inefficiencies of ATM, while providing legacy support to internetworks with existing SONET/SDH architectures.
PoS provides a mechanism to carry packets
directly within the SONET/SDH 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 PPP
Mid layer: Framing of PPP with HDLC
Bottom layer: Mapping into SONET/SDH
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
containment - PoS frames must be placed in the required
synchronous transport signals used in SONET. PPP in HDLC-like framing
presents an octet interface to the physical layer.
·
Octet alignment
- This refers to the alignment of the data packet octet boundaries to
the STS octet boundaries. The octet stream is mapped into the SONET
STS-SPE/SDH Higher Order VC, with the octet boundaries aligned with
the SONET STS-SPE/SDH Higher Order VC octet boundaries.
·
Payload
scrambling - Scrambling 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. Scrambling is performed during insertion
into the SONET STS-SPE/SDH Higher Order VC to provide adequate
transparency and protect against potential security threats. For PPP
over SONET/SDH, the entire SONET/SDH payload (SONET STS-SPE/SDH Higher
Order VC minus the path overhead and any fixed stuff) is scrambled
using a self-synchronous scrambler of polynomial X**43 + 1.
The proper order of operation of PoS is:
When transmitting:
IP -> PPP -> FCS generation -> Byte stuffing -> Scrambling ->
SONET/SDH framing
When receiving:
SONET/SDH framing -> Descrambling -> Byte destuffing ->
FCS detection -> PPP -> IP
Protocol Structure - PoS: Packet over
SONET/SDH
Related Protocols
ATM,
IP, PPP,
HDLC, SONET/SDH
Sponsor Source
PoS protocols are defined by
IETF.
Reference
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
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