Packet over SONET

Chapter Description

This sample chapter covers Packet over SONET (PoS) operation, encapsulation, protection, and convergence.

Applications for PoS

PoS is a Layer 2 technology that uses PPP in HDLC encapsulation, using SONET framing. The PoS solution lowers the cost per megabyte when compared to other Wide Area Networking architectures. The PoS interface supports SONET level alarm processing, performance monitoring, synchronization, and protection switching. This support enables PoS systems to seamlessly interoperate with existing SONET infrastructures and provides the capability to migrate to IP+Optical networks without the need for legacy SONET infrastructures. PoS is used in a point-to-point environment, much like the legacy T-carrier architectures, but without the need for TDM.

PoS efficiently encapsulates IP traffic with a low-overhead PPP header. When encapsulated, the traffic is placed inside an HDLC-delimited SONET SPE and transported across SONET. Voice, video, and data can be carried within the IP packets using Layer 3 QoS mechanisms to control priority when bandwidth contention occurs.

PoS can be used in tandem with other technologies carried over SONET architectures. PoS is not compatible with these other technologies, but is not aware of them because they are being transported over different time slots. PoS, TDM voice, ATM, and Dynamic Packet Transport (DPT) can each use their required synchronous transport signals, not interacting with each other. PoS interfaces are available in concatenated and nonconcatenated (channelized) options. Channelized interfaces are more costly than concatenated interfaces.

ATM and PoS

ATM and PoS can be used within the same network. ATM technology provides an effective, flexible provisioning mechanism for low- to high-speed network access. ATM switches can be used to aggregate digital subscriber line (DSL), cable, and customer traffic by using permanent virtual circuits (PVCs) or switched virtual circuits (SVCs), which can then feed into compatible downstream routers. This traffic is then fed into higher-speed links attached to the Cisco 12000 series router for transport through the core through PoS interfaces.

An advantage of ATM is its innate support for QoS. PoS is fully capable of supporting the transport of time-sensitive data, using Layer 3 mechanisms. Technologies such as Resource Reservation Protocol (RSVP), committed access rate (CAR), and Weighted Random Early Detection (WRED) enable providers to offer QoS solutions in a more cost-effective manner than ATM. These technologies are Layer 3 implementations for QoS. This book does not focus on QoS, but other Cisco Press books covering QoS are available (such as IP Quality of Service; ISBN: 1-57870-116-3).

Figure 9-2 is an ATM aggregation design in which switches are placed on the ATM network edge and translate ATM traffic into PoS traffic. Notice the amount of PoS interfaces required to create a resilient network design with no single point of failure. PoS is a point-to-point technology, regardless of the distance traveled. A point of presence (POP) environment where equipment might be closely located would be perfect for Very Short Reach (VSR) optics using PoS technology. VSR optics are lower in price than normal PoS interfaces because they are not meant to travel long distances. They can be manufactured with lasers that are weaker in strength and photodiodes that are not as sensitive as photodiodes required for long spans.

Figure 2Figure 9-2 ATM Aggregation over PoS Networks

PoS Transport

PoS does not require SONET transport but works in tandem with such as a result of the SONET framing that PoS employs. Two PoS devices can be connected directly with duplex fiber. Because PoS interfaces are Layer 3 enabled, PoS interfaces are an example of an IP+Optical architecture.

Figure 9-3 displays three different ways in which PoS traffic can be transported. The three mechanisms are explained as follows:

  • Connectivity to SONET ADMs—SONET circuits are provisioned as point-to-point circuits over SONET rings. Routers with PoS interfaces can be attached to SONET add/drop multiplexers (ADMs). As long as the proper number of STS are provisioned, the PoS interfaces will have connectivity. The PoS traffic is multiplexed with the other traffic that the SONET ADMs are carrying.

  • Connectivity to transponders in a DWDM system—PoS traffic can be translated to a DWDM ITU-grid wavelength using a transponder. Most transponders support SONET framing. Through the DWDM system, 32 PoS circuits can be multiplexed onto one fiber.

  • Dark-fiber connectivity—PoS interface can be connected directly over dark fiber using PoS interfaces. Dark fiber is fiber that is leased from a service provider; the customer provides the source (Laser or LED)and destination (photodiode receiver). This process is normally referred to as lighting the fiber. Long spans can be accommodated through standard SONET regenerators that provide regeneration, reshaping, and retiming (3Rs) of the signal. The Cisco 15104 is an OC-48 SONET regenerator that fits this application.

Figure 3Figure 9-3 PoS Transport Options

Multiaccess Protocol over SONET

Multiaccess Protocol over SONET (MAPoS) is a high-speed, link-layer protocol that provides multiple access capability over SONET/SDH.

MAPoS is defined in RFCs 2171 and 2176. The MAPoS frame format is based on HDLC-like framing for PPP. MAPoS is a frame switch that allows multiple nodes to be connected in a star topology to form a LAN using MAPoS.


You can find all RFCs online at, where xxxx is the number of the RFC. If you do not know the number of the RFC, you can find it by doing a topic search at

MAPoS can be used to allow SONET connectivity directly to the desktop. MAPoS is much more costly than Ethernet connectivity and has not been deployed, albeit a little in European markets. Most ATM-to-the-desktop environments have migrated their infrastructures to Ethernet technologies. MAPoS will probably never gain the market acceptance that Ethernet has. With SPs looking for more ways to leverage the low cost of Ethernet, it would be unlikely that enterprise environments roll out expensive SONET interfaces to their desktops in place of the ubiquitous, cost-effective Ethernet interfaces they currently use.

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