Packet over SONET

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 7, 2003.

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.

3. Packet over SONET Operation and Specifications | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive:

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020