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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


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


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.

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