Home > Articles > Networking

Packet over SONET

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

This chapter covers the following topics:

  • Evolution of voice and data networks

  • Packet over SONET applications

  • Packet over SONET operation and specification

  • Packet over SONET efficiencies

  • Packet over SONET network designs

  • Packet over SONET protection schemes

SONET is a time-division multiplexing (TDM) architecture that was designed to carry voice traffic. All traffic in SONET is broken down into slots of 64-kbps DS0 increments. A DS0 is the voice line that is typically hard-wired into homes. TDM architectures are not ideal solutions for transporting data. Cable and DSL providers have shown this with their high-data-throughput broadband offerings that do not incur the same costs as comparable TDM services would incur.

When it was discovered that computer data could be transported over telephone circuits, service providers (SPs) leveraged their existing SONET rings. SONET rings were designed and deployed to transport voice but could transport voice by breaking down the data needs into manageable pieces and transporting in 64-kbps increments. When anything less than 100 percent of a TDM circuit was used, the remainder is stuffed with arbitrary data and therefore wasted from both the customer's and SP's perspective. Frame Relay technology offered statistical multiplexing, which offered a solution to the inefficiencies of SONET-based services. Unfortunately, the designers of Frame Relay did not have quality of service (QoS) in mind with the design of the technology. Many carriers also offered zero committed information rate (CIR) services only, which guaranteed the end user absolutely no class of service (CoS). Customers found this unacceptable.

ATM offered a solution to the QoS issues of Frame Relay and offered scalability in the optical carrier (OC-n) domain. ATM relied on a fixed-size cell that is not compatible with the Ethernet technologies that most LANs employ. ATM-designed hardware includes a segmentation and reassembly (SAR) layer to translate Layer 2 frames (Ethernet, Token Ring, FDDI, and so on) into ATM cells. The SAR functionality introduced slight delays in the network, but it is prohibitively expensive and complex to design. Because of the issues associated with ATM, many vendors have not deployed OC-192 ATM interfaces at this time. There is also a concept known as cell tax with ATM deployments. ATM introduces extra overhead into each transmission because of its fixed size of 53 bytes. If a Layer 2 (Ethernet) frame does not fall on a cell boundary, the rest of the cell is padded to meet the 53-byte cell requirement. ATM cells might be efficiently multiplexed into a SONET frame, but the architecture has delays and inefficiencies that must be accounted for.

Packet over SONET (PoS) is a highly scalable protocol that overcomes many of the inefficiencies of ATM, while providing legacy support to internetworks with existing SONET architectures. PoS provides a mechanism to carry packets directly within the SONET synchronous payload envelope (SPE) using a small amount of High-Level Data Link Control (HDLC) or PPP framing.

Evolution of Voice and Data Networks

Voice and data networking is constantly evolving as the technology evolves. After the telegraph, telecommunications networks evolved to transport the spoken word. The next evolutionary step, data networks, occurred in the mid-1900s with the advent of computers. Although data networking started out small because only the largest corporations could afford computers, computers have fallen to such a low entry-level price that most people can afford to have a computer now and to be connected to the Internet. Data networks have evolved to the point that the benefits of converging voice and data networks into the same data infrastructure can no longer be ignored. SPs that have legacy SONET infrastructures can still offer customers high-speed alternatives with technologies such as PoS.

The first communications systems were mainframe computers linked to dumb terminals. The Synchronous Data Link Control (SDLC) protocol, developed by IBM, made this system possible by allowing communication between a mainframe and a remote workstation over long distances. This protocol evolved into the High-Level Data Link Control (HDLC) protocol, which provided reliable communications and the ability to manage the flow of traffic between devices. HDLC is an open industry standard protocol, whereas SDLC is an IBM proprietary protocol that must be licensed by IBM. Industry standard protocols such as TCP/IP drive the adoption and low costs of telecommunications equipment.

Cisco offered an enhanced multiprotocol version of the HDLC protocol to enable various protocols over the HDLC (High-Level Data Link Control) Layer 2 framing. This Cisco HDLC protocol is proprietary and exclusive to Cisco. The HDLC standard was loose at the time of Cisco's creation and left too much room for interpretation. When this was standardized in RFC 1619 with PPP in HDLC-like framing, Cisco's HDLC protocol was not compliant. Point-to-Point Protocol (PPP) evolved from HDLC; it offers an industry- standard way to provide multiprotocol networking abilities, as well as many enhancements such as authentication, multilink, and compression. PPP is used for many other technologies, including ISDN. HDLC and PPP are scalable to architectures with fast speeds. Figure 9-1 shows this networking evolution.

Figure 1Figure 9-1 Voice and Data Network Evolution

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT 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 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 information@informit.com.

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 customer-service@informit.com 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 InformIT. 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: www.informit.com/u.aspx.

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 NevadaDesignatedRequest@pearson.com.

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