POS - Packet Over Sonet

The Provider's Backbone Protocol  -  RFC 1619, 2615

*** click Here for a PoS primer or Here for a Cisco PoS paper

Packet-over-SONET (POS) is a standardized way for mapping IP packets across a provider's network backbone, into SONET/SDH frames.  Cisco Systems helped pioneer PoS technology and has been at the forefront in delivering high-performance and cost-effective PoS solutions for use in service provider and enterprise networks.

IMPORTANT: POS is not generally used by customer access lines because they are rarely on a SONET ring !!!  It is primarily a backbone architecture !!

Without POS, backbone routers or switches also transmit IP packets over SONET rings, but they have to first encapsulate them within Frame Relay frames or ATM cells, and then encapsualte the frames/cells into SONET frames.  But Frame Relay has no QoS, and ATM has a large amount of overhead (the dreaded "cell tax").  PoS overhead, which averages about 3 percent, is significantly lower than the 15 percent average for the asynchronous transfer mode (ATM) cell tax.

POS is for "pure IP" backbones, and it allows encapsulation with either PPP, HDLC, or both (IP encapsulated into PPP, which is encapsulated into HDLC, which is encapsulated into SONET). 

IMPORTANT:  POS does not directly encapsulate IP packets into SONET frames !!!  As a comparison, with IP encapsulation into FR or ATM, the packets are encapsulated into the frames or cells, which are then encapsulated into SONET  .  .  .  POS also encapsulates the packets.  The only difference is that it uses PPP, and/or HDLC frames for the encapsulation.  The advantage is simplicity, and lower overhead.

PoS Hardware

The routers (almost always Cisco GSR 12000's) in a POS backbone must have POS line cards, and these PoS interfaces are connected to Add Drop Multiplexers (ADMs), or they can connect directly to another router via point-to-point SONET/SDH links over dark DWDM fiber links. PoS line cards are very popular on the Cisco gigabit switch router (GSR) series routers. Other Cisco products such as the Cisco 7500 and 7200 series routers also feature PoS interfaces. The Catalyst® 6500 and 8500 series enterprise switches are potential candidates for PoS interfaces in the future.

PoS Protocols

PoS can be viewed as three layers:

PPP provides a standard method for transporting multiprotocol datagrams over point-to-point links. RFC 2615 is the primary document that describes the use of PPP over SONET and SDH circuits. All the physical layer requirements, framing, and configuration details are outlined in the RFC.

MPLS extends the functionality of PoS by supporting ATM-like virtual circuits and QoS capabilities in a PoS environment.

RFC 2823 (PPP over Simple Data Link [SDL] Using SONET/SDH with ATM-like framing, May 2000) describes a new encapsulation for PPP called SDL (Simple Data Link). SDL provides a very-low-overhead alternative to HDLC-like encapsulation, and can also be used on SONET/SDH links. The IETF Point-to-Point Protocol Extensions (pppext) Working Group has a full list of PPP drafts and RFCs.