Editing Packet over SONET/SDH

{{Short description|Communications protocol for transferring packets over fiber networks}}
{{refimprove|date=February 2011}}

'''Packet over SONET/SDH''', abbreviated '''POS''',{{efn|POS (packet over SONET) is a double-nested abbreviation. The S represents "SONET/SDH", which itself stands for "Synchronous Optical Network/Synchronous Digital Hierarchy". Given this, POS technically stands for "Packet over Synchronous Optical Network/Synchronous Digital Hierarchy".}} is a [[communications protocol]] for transmitting [[Packet (information technology)|packets]] in the form of the [[Point to Point Protocol]] (PPP) over [[Synchronous optical networking|SDH]] or [[SONET]], which are both standard protocols for communicating digital information using lasers or [[light emitting diodes]] (LEDs) over optical fibre at high line rates.

POS is defined by RFC 2615 as PPP over SONET/SDH.<ref>{{cite book |title=Cisco Self-Study: Building Cisco Metro Optical Networks (METRO) |date=Nov 7, 2003 |publisher=Cisco Press |url=https://www.ciscopress.com/articles/article.asp?p=101735&seqNum=3 |access-date=7 March 2025 |chapter=Packet over SONET}}</ref> PPP was designed as a standard method of communicating over point-to-point links and is thus suited to SONET/SDH links because these use point-to-point circuits.

==History==
[[Cisco Systems, Inc.|Cisco]] was involved in making POS an important wide area network protocol.  [[PMC-Sierra]] produced an important series of early semiconductor devices which
implemented POS.

==Applications==
The most important application of POS is to support sending of [[Internet Protocol|IP]] packets across [[wide area network]]s.  Large amounts of traffic on the [[Internet]] are carried over POS links. POS is also one of the link layers used by the [[Resilient Packet Ring]] standard known as [[IEEE]] 802.17.

== Security ==
[[Scrambler|Scrambling]] is performed during insertion of the PPP packets into the SONET/SDH frame to avoid various security attacks including [[denial-of-service attack]]s and the imitation of SONET/SDH alarms. This modification was justified as cost-effective because the scrambling algorithm was already used by the standard used to transport [[Asynchronous Transfer Mode|ATM]] cells over SONET/SDH. However, scrambling can optionally be disabled to allow a node to be compatible with another node that uses the now obsoleted RFC 1619 version of Packet over SONET/SDH which lacks the scrambler.

==Complementary Interfaces==
The [[System Packet Interface]] series of standards from the [[Optical Internetworking Forum]] including [[SPI-4.2]] and [[SPI-3]] and their predecessors [[PL-4]] and [[PL-3]] are commonly used as the interface between packet processing devices and framer devices that implement POS.  The acronym PL-4 means POS-PHY Layer 4.

== See also ==
* [[Ethernet over SONET]]

==Notes==
{{notelist}}

==References==
{{Reflist}}


{{DEFAULTSORT:Packet over SONET SDH}}
[[Category:Network protocols]]
[[Category:Synchronous optical networking]]