Home > Articles > Networking > Network Design & Architecture

This chapter is from the book

3.2 All about Concatenation

Three types of concatenation schemes are possible under SONET and SDH. These are:

  • Standard contiguous concatenation

  • Arbitrary contiguous concatenation

  • Virtual concatenation

These concatenation schemes are described in detail next.

3.2.1 Standard Contiguous Concatenation in SONET and SDH

SONET and SDH networks support contiguous concatenation whereby a few standardized “concatenated” signals are defined, and each concatenated signal is transported as a single entity across the network [ANSI95a, ITU-T00a]. This was described briefly in the previous chapter.

The concatenated signals are obtained by “gluing” together the payloads of the constituent signals, and they come in fixed sizes. In SONET, these are called STS-Nc Synchronous Payload Envelopes (SPEs), where N = 3X and X is restricted to the values 1, 4, 16, 64, or 256. In SDH, these are called VC-4 (equivalent to STS-3c SPE), and VC-4-Xc where X is restricted to 1, 4, 16, 64, or 256.

The multiplexing procedures for SONET (SDH) introduce additional constraints on the location of component STS-1 SPEs (VC-4s) that comprise the STS-Nc SPE (VC-4-Xc). The rules for the placement of standard concatenated signals are [ANSI95a]:

  1. Concatenation of three STS-1s within an STS-3c: The bytes from concatenated STS-1s shall be contiguous at the STS-3 level but shall not be contiguous when interleaved to higher-level signals. When STS-3c signals are multiplexed to a higher rate, each STS-3c shall be wholly contained within an STS-3 (i.e., occur only on tributary input boundaries 1–3, 4–6, 7–9, etc.). This rule does not apply to SDH.

  2. Concatenation of STS-1s within an STS-Nc (N = 3X, where X = 1, 4, 16, 64, or 256). Such concatenation shall treat STS-Nc signals as a single entity. The bytes from concatenated STS-1s shall be contiguous at the STS-N level, but shall not be contiguous when multiplexed on to higher-level signals. This also applies to SDH, where the SDH term for an STS-Nc is an AU-4-Xc where X = N/3.

  3. When the STS-Nc signals are multiplexed to a higher rate, these signals shall be wholly contained within STS-M boundaries, where M could be 3, 12, 48, 192, or 768, and its value must be the closest to, but greater than or equal to N (e.g., if N = 12, then the STS-12c must occur only on boundaries 1–12, 13–24, 25–36, etc.). In addition to being contained within STS-M boundaries, all STS-Nc signals must begin on STS-3 boundaries.

The primary purpose of these rules is to ease the development burden for hardware designers, but they can seriously affect the bandwidth efficiency of SONET/SDH links.

In Figure 3-1(a), an STM-16 (OC-48) signal is represented as a set of 16 time slots, each of which can contain a VC-4 (STS-3c SPE). Let us examine the placement of VC-4 and VC-4-4c (STS-3c and STS-12c SPE) signals into this structure, in line with the rules above. In particular a VC-4-4c (STS-12c SPE) must start on boundaries of 4. Figure 3-1(b) depicts how the STM-16 has been filled with two VC-4-4c (STS-12c) and seven VC-4 signals. In Figure 3-1(c), three of the VC-4s have been removed, that is, are no longer in use. Due to the placement restrictions, however, a VC 4-4c cannot be accommodated in this space. In Figure 3-1(d), the STM-16 has been “regroomed,” that is, VC-4 #5 and VC-4 #7 have been moved to new timeslots. Figure 3-1(e) shows how the third VC-4-4c is accommodated.

03fig01.gifFigure 3-1. Timeslot Constraints and Regrooming with Contiguous (Standard) Concatenation

3.2.2 Arbitrary Concatenation

In the above example, a “regrooming” operation was performed to make room for a signal that could not be accommodated with the standard contiguous concatenation rules. The problem with regrooming is that it is service impacting, that is, service is lost while the regrooming operation is in progress. Because service impacts are extremely undesirable, regrooming is not frequently done, and the bandwidth is not utilized efficiently.

To get around these restrictions, some manufacturers of framers, that is, the hardware that processes the SDH multiplex section layer (SONET line layer), offer a capability known as “flexible” or arbitrary concatenation. With this capability, there are no restrictions on the size of an STS-Nc (VC-4-Xc) or the starting time slot used by the concatenated signal. Also, there are no constraints on adjacencies of the STS-1 (VC-4-Xc) time slots used to carry it, that is, the signals can use any combination of available time slots. Figure 3-2 depicts how the sequence of signals carried over the STM-16 of Figure 3-1 can be accommodated without any regrooming, when the arbitrary concatenation capability is available.

03fig02.gifFigure 3-2. Timeslot Usage with Arbitrary Concatenation

3.2.3 Virtual Concatenation

As we saw earlier, arbitrary concatenation overcomes the bandwidth inefficiencies of standard contiguous concatenation by removing the restrictions on the number of components and their placement within a larger concatenated signal. Standard and arbitrary contiguous concatenation are services offered by the network, that is, the network equipment must support these capabilities. The ITU-T and the ANSI T1 committee have standardized an alternative, called virtual concatenation. With virtual concatenation, SONET and SDH PTEs can “glue” together the VCs or SPEs of separately transported fundamental signals. This is in contrast to requiring the network to carry signals as a single concatenated unit. HIGHER-ORDER VIRTUAL CONCATENATION (HOVC)

HOVC is realized under SONET and SDH by the PTEs, which combine either multiple STS-1/STS-3c SPEs (SONET), or VC-3/VC-4 (SDH). Recall that the VC-3 and STS-1 SPE signals are nearly identical except that a VC-3 does not contain the fixed stuff bytes found in columns 30 and 59 of an STS-1 SPE. A SONET STS-3c SPE is equivalent to a SDH VC-4.

These component signals, VC-3s or VC-4s (STS-1 SPEs or STS-3c SPEs), are transported separately through the network to an end system and must be reassembled. Since these signals can take different paths through the network, they may experience different propagation delays. In addition to this fixed differential delay between the component signals, there can also be a variable delay component that arises due to the different types of equipment processing the signals and the dynamics of the fiber itself. Note that heating and cooling effects can affect the propagation speed of light in a fiber, leading to actual measurable differences in propagation delay.

The process of mapping a concatenated container signal, that is, the raw data to be transported, into a virtually concatenated signal is shown in Figure 3-3. Specifically, at the transmitting side, the payload gets packed in X VC-4s just as if these were going to be contiguously concatenated. Now the question is, How do we identify the component signals and line them up appropriately given that delays for the components could be different?

03fig03.gifFigure 3-3. Mapping a Higher Rate Payload in a Virtually Concatenated Signal (from [ITU-T00a])

The method used to align the components is based on the multiframe techniques described in Chapter 2. A jumbo (very long) multiframe is created by overloading the multiframe byte H4 in the path overhead. Bits 5–8 of the H4 byte are incremented in each 125µs frame to produce a multiframe consisting of 16 frames. In this case, bits 5–8 of H4 are known as the multiframe indicator 1 (MFI1). This multiframe will form the first stage of a two-stage multiframe. In particular, bits 1–4 of the H4 byte are used in a way that depends on the position in the first stage of the multiframe. This is shown in Table 3-1.

Within the 16-frame first stage multiframe, a second stage multiframe indicator (MFI2) is defined utilizing bits 1–4 of H4 in frames 0 and 1, giving a total of 8 bits per frame. It is instructive to examine the following:

  1. How long in terms of the number of 125µs frames is the complete HOVC multiframe structure? Answer: The base frame (MFI1) is 16 frames long, and the second stage is 28 = 256 frames long. Since this is a two-stage process, the lengths multiply giving a multiframe that is 16 × 256 = 4096 frames long.

  2. What is the longest differential delay, that is, delay between components that can be compensated? Answer: The differential delay must be within the duration of the overall multiframe structure, that is, 125µS × 4096 = 512mS, that is, a little over half a second.

  3. Suppose that an STS-1-2v is set up for carrying Ethernet traffic between San Francisco and New York such that one STS-1 goes via a satellite link and the other via conventional terrestrial fiber. Will this work? Answer: Assuming that a geo-synchronous satellite is used, then the satellite's altitude would be about 35775 km. Given that the speed of light is 2.99792 × 108 m/sec, this leads to a round trip delay of about 239 ms. If the delay for the fiber route is 20 ms, then the differential delay is 209 ms, which is within the virtual concatenation range. Also, since the average circumference of the earth is only 40,000 km, this frame length should be adequate for the longest fiber routes.

    Table 3-1. Use of Bits 1–4 in H4 Byte for First Stage Multiframe Indication (MFI1)

    Multi-Frame Indicator 1 (MFI1)

    Meaning of Bits 1–4 in H4


    2nd multiframe indicator MFI2 MSB (bits 1–4)


    2nd multiframe indicator MFI2 LSB (bits 5–8)


    Reserved (0000)


    Sequence indicator SQ MSB (bits 1–4)


    Sequence indicator SQ LSB (bits 5–8)

Now, the receiver must be able to distinguish the different components of a virtually concatenated signal. This is accomplished as follows. In frames 14 and 15 of the first stage multiframe, bits 1–4 of H4 are used to give a sequence indicator (SQ). This is used to indicate the components (and not the position in the multiframe). Due to this 8-bit sequence indicator, up to 256 components can be accommodated in HOVC. Note that it is the receiver's job to compensate for the differential delay and to put the pieces back together in the proper order. The details of how this is done are dependent on the specific implementation. LOWER-ORDER VIRTUAL CONCATENATION (LOVC)

The virtual concatenation of lower-order signals such as VT1.5s (VC-11), VT2 (VC-12), and so on are based on the same principles as described earlier. That is, a sequence number is needed to label the various components that make up the virtually concatenated signal, and a large multiframe structure is required for differential delay compensation. In the lower-order case, however, there are fewer overhead bits and bytes to spare so the implementation may seem a bit complex. Let us therefore start with the capabilities obtained.

LOVC Capabilities and Limitations

Table 3-2 lists the LOVC signals for SONET/SDH, the signals they can be contained in and the limits on the number of components that can be concatenated. The last two columns are really the most interesting since they show the range of capacities and the incremental steps of bandwidth.

LOVC Implementation

Let us first examine how the differential delay compensating multiframe is put together. This is done in three stages. Recall that the SONET VT overhead (lower-order SDH VC overhead) is defined in a 500 µs multiframe, as indicated in the path layer multiframe indicator H4. This makes available the four VT overhead bytes V5, J2, Z6, and Z7, from one SONET/SDH frame byte. Since a number of bits in these bytes are used for other purposes, an additional second stage of multiframe structure is used to define extended VT signal labels.

This works as follows (note that SDH calls the Z7 byte as K4 but uses it the same way): First of all, the V5 byte indicates if the extended signal label is being used. Bits 5 through 7 of V5 provide a VT signal label. The signal label value of 101 indicates that a VT mapping is given by the extended signal label in the Z7 byte. If this is the case, then a 1-bit frame alignment signal “0111 1111 110” is sent in bit 1 of Z7, called the extended signal label bit. The length of this second stage VT level multiframe (which is inside the 500 µs VT multiframe) is 32 frames. The extended signal label is contained in bits 12–19 of the multiframe. Multiframe position 20 contains “0.” The remaining 12 bits are reserved for future standardization.

Table 3-2. Standardized LOVC Combinations and Limits


Carried in SONET/SDH


Capacity (kbit/s)

In steps of (kbit/s)

VT1.5-Xv SPE/VC-11-Xv


1 to 28

1600 to 44800


VT2-Xv SPE/VC-12-Xv


1 to 21

2176 to 45696




1 to 14

3328 to 46592


VT6-Xv SPE/VC-2-Xv


1 to 7

6784 to 47448




1 to 64

1600 to 102400




1 to 63

2176 to 137088




1 to 42

3328 to 139776


VT6-Xv SPE/VC-2-Xv


1 to 21

6784 to 142464




1 to 64

1600 to 102400




1 to 64

2176 to 139264




1 to 64

3328 to 212992




1 to 64

6784 to 434176


Note: X is limited to 64 due the sequence indicator having 6 bits.

Bit 2 of the Z7 byte is used to convey the third stage of the multistage multiframe in the form of a serial string of 32 bits (over 32 four-frame multi-frames and defined by the extended signal label). This is shown in Figure 3-4. This string is repeated every 16 ms (32 bits × 500 µs/bit) or every 128 frames.

03fig04.gifFigure 3-4. Third Stage of LOVC Multiframe Defined by Bit 2 of the Z7 Byte over the 32 Frame Second Stage Multiframe

The third stage string consists of the following fields: The third stage virtual concatenation frame count is contained in bits 1 to 5. The LOVC sequence indicator is contained in bits 6 to 11. The remaining 21 bits are reserved for future standardization.

Let us now consider a concrete example. Suppose that there are three stages of multiframes with the last stage having 5 bits dedicated to frame counting. What is the longest differential delay that can be compensated and in what increments? The first stage was given by the H4 byte and is of length 4, resulting in 4 × 125 µs = 500 µs. The second stage was given by the extended signal label (bit 1 of Z7) and it is of length 32. Since this is inside the first stage, the lengths multiply, resulting in 32 × 500 µs = 16 ms. The third stage, which is within the 32-bit Z7 string, has a length of 25 = 32 and is contained inside the second stage. Hence, the lengths multiply, resulting in 32 × 16 ms = 512 ms. This is the same compensation we showed with HOVC. Since the sequence indicator of the third stage is used to line up the components, the delay compensation is in 16 ms increments.

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