Home > Articles > Networking > Network Design & Architecture

  • Print
  • + Share This
This chapter is from the book

3.5 SONET/SDH Transparency Services

SONET and SDH have the following notions of transparency built-in, as described in Chapter 2:

  1. Path transparency, as provided by the SONET line and SDH multiplex section layers. This was the original intent of SONET and SDH, that is, transport of path layer signals transparently between PTEs.

  2. SONET line and SDH multiplex section transparency, as provided by the SONET section and SDH regenerator section layers, respectively.

  3. SONET section and SDH regenerator section transparency, as provided by the physical layer.

Of these, only (1) was considered a “user service” within SONET and SDH. There are reasons now to consider (2) and (3) as services, in addition to newer transparency services.

Figure 3-8 shows a typical scenario where transparency services may be desired. Here, two SONET networks (labeled “Domain 1”) are separated by an intervening optical transport network of some type (labeled “Domain 2”). For instance, Domain 1 could consist of two metro networks under a single administration, separated by a core network (Domain 2) under a different administration. The two disjoint parts of Domain 1 are interconnected by provisioning a “link” between network elements NE1 and NE2, as shown. The characteristics of this link depend on the type of transparency desired. In general, transparency allows NE1 and NE2 to use the functionality provided by SONET overhead bytes in various layers. For instance, section transparency allows the signal from NE1 to NE2 to pass through Domain 2 without any overhead information being modified in transit. An all-optical network or a network with transparent regenerators can provide section layer transparency. This service is equivalent to having a dedicated wavelength (lambda) between NE1 and NE2. Thus, the service is often referred to as a lambda service, even if the signal is electrically regenerated within the network. Section transparency allows NE1 and NE2 to terminate the section layer and use the section (and higher layer) overhead bytes for their own purposes.

03fig08.gifFigure 3-8. Networking Scenario Used to Define SONET/SDH Transparency Services

If the OC-N to be transported between NE1 and NE2 is the same size (in terms of capacity) as those used within the optical network, then the section transparency service is a reasonable approach. If the optical network, however, deals with signals much larger than these OC-N signals, then there is the potential for inefficient resource utilization. For example, suppose the optical network is composed of DWDM links and switches that can effectively deal with OC-192 signals. A “lambda” in this network could indeed accommodate an OC-12 signal, but only 1/16th of the capacity of that lambda will be used. In such a case, the OC-12 signal has to be multiplexed in some way into an OC-192 signal. But SONET (SDH) multiplexing takes place at the line (multiplex section) layer. Hence, there is no standard way to convey the OC-12 overhead when multiplexing the constituent path signals into an OC-192 signal. This means that section and line overhead bytes presented by NE1 will be modified within Domain 2. How then to transfer the overhead bytes transparently across Domain 2? Before we examine the methods for accomplishing this, it is instructive to look at the functionality provided by overhead bytes and what it means to support transparency.

Tables 3-3 and 3-4 list the overhead bytes available at different layers, the functionality provided and when the bytes are updated (refer to Figures 2-4 and 2-5).

Table 3-3. SONET Section (SDH Regenerator Section) Overhead Bytes and Functionality

Overhead Bytes

Comments

A1 and A2 (Framing)

These are repeated in all STS-1 signals within an OC-N. No impact on transparency.

J0 (Trace)

Only conveyed in the 1st STS-1, and covers entire frame. J0 bytes in signals 2–N are reserved for growth, i.e., Z0. Used to identify entire section layer signal.

B1 (Section BIP-8)

Only conveyed in the 1st STS-1, and covers entire frame. B1 bytes in signals 2–N are undefined. B1 byte must be updated if section, line or path layer content changes.

E1 (Orderwire)

F1 (User)

Only conveyed in the 1st STS-1, and covers for entire frame. E1 and F1 in signals 2–N are undefined.

D1-D3 (Section DCC)

Only conveyed in the 1st STS-1, and covers the entire frame. D1-D3 bytes in signals 2–N are undefined.

Table 3-4. SONET Line (SDH Multiplex Section) Overhead Bytes and Functionality

Overhead Bytes

Comments

H1, H2, H3 (Pointer bytes)

These are repeated in all STS-1s within an STS-N.

B2 (Line BIP-8)

This is used for all STS-1s within an STS-N. Must be updated if line or path layer content changes. Used to determine signal degrade conditions.

K1, K2 (APS bytes)

Only conveyed in the 1st STS-1 signal, and covers entire line. This space in signals 2 – N are undefined. This is the line APS functionality.

D4-D12 (Line DCC)

Only conveyed in the 1st STS-1 for the entire line. D4–D12 bytes in signals 2 – N are undefined.

S1 (Synchronization byte)

Only conveyed in the 1st STS-1, and carries the synchronization status message for the entire line. S1 bytes in STS-1 signals 2 – N are reserved for growth (Z1 byte). Note that if a re-multiplexing operation were to take place, this byte cannot be carried through.

M0, M1, (Line, Remote Error indication)

M0 or M1 is conveyed in the Nth STS of the STS-N signal. If N > 1, this byte is called M1. If N = 1, this byte is called M0. When N > 1, the corresponding bytes in signals 1 to N – 1 are reserved for growth (Z2 byte).

E2 (Line order wire)

Only conveyed in the 1st STS-1, and covers the entire line. The E2 bytes in signals 2 – N are undefined.

With standard SONET/SDH path layer multiplexing, the H1–H3 (pointer) bytes must be modified when the clocks are different for the streams to be multiplexed. The B2 byte must be updated when any of the line layer bytes are changed. Also related to timing is the S1 byte, which reports on the synchronization status of the line. This byte has to be regenerated if multiplexing is performed. Thus, it is not possible to preserve all the overhead bytes when the signal from NE1 is multiplexed with other signals within Domain 2. The additional procedures that must be performed to achieve transparency are discussed next.

3.5.1 Methods for Overhead Transparency

We can group the transport overhead bytes into five categories as follows:

  1. Framing bytes A1 and A2, which are always terminated and regenerated

  2. Pointer bytes H1, H2 and H3, which must be adjusted for multiplexing, and the S1 byte

  3. General overhead bytes: J0, E1, F1, D1-D3, K1, K2, D4-D12, M0/M1, E2

  4. BIP-8 error monitoring bytes B1 and B2

  5. An assortment of currently unused growth bytes

With regard to the network shown in Figure 3-8, the following are different strategies for transparently transporting the general overhead bytes:

  • Information forwarding: The overhead bytes originating from NE1 are placed into the OC-N signal and remain unmodified in Domain 2.

  • Information tunneling: Tunneling generally refers to the encapsulation of information to be transported at the ingress of a network in some manner and restoring it at the egress. With respect to Figure 3-8, the overhead bytes originating from NE1 are placed in unused overhead byte locations of the signal transported within Domain 2. These overhead bytes are restored before the signal is delivered to NE2.

As an example of forwarding and tunneling, consider Figure 3-9, which depicts four STS-12 signals being multiplexed into an STS-48 signal within Domain 2. Suppose that the J0 byte of each of these four signals has to be transported transparently. Referring to Table 3-1, it can be noted that the J0 space in signals 2–4 of the STS-48 are reserved, that is, no specific purpose for these bytes is defined within Domain 2. Thus, referring to the structure of the multiplexed overhead information shown in Figure 2-5, the J0 bytes from the second, third, and fourth STS-12 signals can be forwarded unmodified through the intermediate network. This is not true for the J0 byte of the first STS-12, however, since the intermediate network uses the J0 byte in the first STS-1 to cover the entire STS-48 signal (Table 3-1). Hence, the J0 byte of the first STS-12 has to be tunneled by placing it in some unused overhead byte in the STS-48 signal at the ingress and recovering it at the egress.

03fig09.jpgFigure 3-9. Transparency Example to lllustrate Forwarding and Tunneling

Now, consider the error monitoring bytes, B1 and B2. Their usage is described in detail in section 3.6. Briefly, taking SONET as an example, B1 and B2 bytes contain the parity codes for the section and line portion of the frame, respectively. A node receiving these bytes in a frame uses them to detect errors in the appropriate portions of the frame. According to the SONET specification, B1 and B2 are terminated and regenerated by each STE or LTE, respectively. With regard to the network of Figure 3-8, the following options may be considered for their transport across Domain 2:

  • Error regeneration: B1 and B2 are simply regenerated at every network hop.

  • Error forwarding: As before, the B1 and B2 bytes are regenerated at each hop. But instead of simply sending these regenerated bytes in the transmitted frame (as in the previous case), the bytes are XOR'd (i.e., bit wise summed) with the corresponding bytes received. With this process, the B1 or B2 bytes will accumulate all the errors (at the appropriate layer) for the transparently transported signal. The only drawback of this method is that the error counts within Domain 2 would appear artificially high, and to sort out the true error counts, correlation of the errors reported along the transparent signal's path would be required.

  • Error tunneling: In this case, the incoming parity bytes (B1 and/or B2) are carried in unused overhead locations within the transport signal in Domain 2. In addition, at each network hop where the bytes are required to be regenerated, the tunneled parity bytes are regenerated and then XOR'd (bit wise binary summation) with the error result that was obtained (by comparing the difference between the received and calculated BIP-8s). In this way, the tunneled parity bytes are kept up to date with respect to errors, and the standard SONET/SDH B1 and B2 bytes are used within Domain 2 without any special error correlation/compensation being performed.

3.5.2 Transparency Service Packages

We have so far looked at the mechanisms for providing transparent transport. From the perspective of a network operator, a more important issue is the determination of the types of transparency services that may be offered. A transparency service package defines which overhead functionality will be transparently carried across the network offering the service. As an example, let us consider the network shown in Figure 3-9 again. The following is a list of individual services that could be offered by Domain 2. These may be grouped in various combinations to create different transparency service packages:

  1. J0 transparency: Allows signal identification across Domain 2.

  2. Section DCC (D1–D3) transparency: Allows STE to STE data communication across Domain 2.

  3. B2 and M0/M1 transparency: Allows line layer error monitoring and indication across Domain 2.

  4. K1 and K2 byte transparency: Allow line layer APS across Domain 2. This service will most likely be used with (3) so that signal degrade conditions can be accurately detected and acted upon.

  5. Line DCC (D4-D12) transparency: Allows LTE to LTE data communication across Domain 2.

  6. E2 transparency: Allows LTE to LTE order wire communication across Domain 2.

  7. Miscellaneous section overhead transparency, that is, E1 and F1.

Whether overhead/error forwarding or tunneling is used is an internal decision made by the domain offering the transparency service, based on equipment capabilities and overhead usage. Note that to make use of equipment capable of transparent services, a service provider must know the overhead usage, termination, and forwarding capabilities of equipment used in the network. For example, the latest release of G.707 [ITU-T00a] allows the use of some of the unused overhead bytes for physical layer forward error correction (FEC). Hence, a link utilizing such a “feature” would have additional restrictions on which bytes could be used for forwarding or tunneling.

  • + Share This
  • 🔖 Save To Your Account

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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