Biblio5

Figure 17.16 Basic generalized SDH multiplexing structure. (From ITU-T Rec. G.709, Figure 1/ G.709 [Ref. 7].)

The information is suitably conditioned for serial transmission on selected media at a rate that is synchronized to the network. A basic STM (STM-1) is defined at 155,520 kbps. Higher-capacity STMs are formed at rates equivalent to N times multiples of this basic rate. STM capacities for N c 4 and N c 16 are defined, and higher values are under consideration by ITU-T. An STM comprises a single administrative unit group (AUG) together with the SOH. STM-N contains N AUGs together with SOH.

Figure 17.17 SDH multiplexing method directly from container-1 using AU-3. (From ITU-T Rec. G.708, Figure 2-3/ G.708 [Ref. 6].)

526 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS

Container, C-n (n c 1 to n c 4). This element is a defined unit of payload capacity, which is dimensioned to carry any of the bit rates currently defined in Table 17.2, and may also provide capacity for transport of broadband signals that are not yet defined by CCITT (ITU-T Organization) (Ref. 6).

Virtual Container-n (VC-n). A virtual container is the information structure used to support path layer connection in the SDH. It consists of information payload and POH information fields organized in a block frame that repeats every 125 ms or 500 ms. Alignment information to identify VC-n frame start is provided by the server network layer. Two types of virtual container have been identified:

1. Lower-Order Virtual Container-n, VC-n (n = 1, 2). This element comprises a single C-n (n c 1, 2), plus the basic virtual container POH appropriate to that level.

2. Higher-Order Virtual Container-n, to VC-n (n = 3, 4). This element comprises a single C-n (n c 3, 4), an assembly of tributary unit groups (TUG-2s), or an assembly of TU-3s, together with virtual container POH appropriate to that level.

Administrative Unit-n (AU-n). An administrative unit is the information structure that provides adaptation between the higher-order path layer and the multiplex section. It consists of an information payload (the higher-order virtual container) and an administrative unit pointer, which indicates the offset of the payload frame start relative to the multiplex section frame start. Two administrative units are defined. The AU-4 consists of a VC-4 plus an administrative unit pointer, which indicates the phase alignment of the VC-4 with respect to the STM-N frame. The AU-3 consists of a VC-3 plus an administrative unit pointer, which indicates the phase alignment of the VC-3 with respect to the STM-N frame. In each case the administrative unit pointer location is fixed with respect to the STM-N frame (Ref. 6). One or more administrative units occupying fixed, defined positions in a STM payload is termed an administrative unit group (AUG). An AUG consists of a homogeneous assembly of AU-3s or an AU-4.

Tributary Unit-n (TU-n). A tributary unit is an information structure that provides adaptation between the lower-order path layer and the higher-order path layer. It consists of an information payload (the lower-order virtual container) and a tributary unit pointer, which indicates the offset of the payload frame start relative to the higher-order virtual container frame start. The TU-n (n c 1, 2, 3) consists of a VC-n together with a tributary unit pointer. One or more tributary units occupying fixed, defined positions in a higher-order VC-n payload is termed a tributary unit group (TUG). TUGs are defined in such a way that mixed-capacity payloads made up of different-size tributary units can be constructed to increase flexibility of the transport network. A TUG-2 consists of a homogeneous assembly of identical TU-1s or a TU-2. A TUG-3 consists of a homogeneous assembly of TUG-2s or a TU-3 (Ref. 6).

Container-n (n c 1–4). A container is the information structure that forms the network synchronous information payload for a virtual container. For each of the defined virtual containers there is a corresponding container. Adaptation functions have been defined for many common network rates into a limited number of standard containers (Refs. 6 and 7). These include standard E-1/DS-1 rates defined in ITU-T Rec. G.702 (Ref. 8).

Figure 17.18 STM-N frame structure.

17.3.3.2 Frame Structure. The basic frame structure of SDH is illustrated in Figure 17.18. The three principal areas of the STM-1 frame are section overhead, AU pointers, and STM-1 payload.

17.3.3.2.1 Section Overhead. Section overhead is contained in rows 1–3 and 5–9 of columns 1 − 9 × N of the STM-N shown in Figure 17.18.

17.3.3.2.2 Administrative Unit (AU) Pointers. The AU-n pointer (like the SONET pointer) allows flexible and dynamic alignment of the VC-n within the AU-n frame. Dynamic alignment means that the VC-n floats within the AU-n frame. Thus the pointer is able to accommodate differences, not only in the phases of the VC-n and the SOH, but also in the frame rates.

Row 4 of columns 1 − 9 × N in Figure 17.18 is available for AU pointers. The AU-4 pointer is contained in octets H1, H2, and H3, as shown in Figure 17.19. The three individual AU-3 pointers are contained in three separate H1, H2, and H3 octets, as shown in Figure 17.20.

The pointer contained in H1 and H2 designates the location of the octet where the VC-n begins. The two octets (or bytes) allocated to the pointer function can be viewed as one word, as illustrated in Figure 17.21. The last ten bits (bits 7–16) of the pointer word carry the pointer value.

As shown in Figure 17.21, the AU-4 pointer value is a binary number with a range of 0–782, which indicates offset, in three-octet increments, between the pointer and the first octet of the VC-4 (see Figure 17.19). Figure 17.21 also indicates one additional valid point, the concatenation indication. This concatenation is given by the binary sequence 1001 in bit positions 1–4; bits 5 and 6 are unspecified and there are ten 1s in bit positions 7–16. The AU-4 pointer is set to the concatenation indications for AU-4 concatenation.

There are three AU-3s in an AUG, where each AU-3 has its own associated H1, H2, and H3 octets. As detailed in Figure 17.20, the H octets are shown in sequence. The first H1, H2, H3 set refers to the first AU-3, and the second set to the second AU-3, and so on. For the AU-3s, each pointer operates independently. In all cases the AU-n pointer octets are not counted in the offset. For example, in an AU-4, the pointer value of 0 indicates that the VC-4 starts in the octet location that immediately follows the last H3 octet, whereas an offset of 87 indicates that the VC-4 starts three octets after the K2 octet (byte) (Ref. 7). Note the similarity to SONET here.

528 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS

Figure 17.19 AU-4 pointer offset numbering. (From ITU-T Rec. G.709, Figure 3-1/ G.709 [Ref. 7].)

Figure 17.21 AU-n/ TU-3 pointer (H1, H2, H3) coding. (From ITU-T Rec. G.709, Figure 3-3/ G.709 [Ref. 7].)

17.3.3.2.3 Frequency Justification. If there is a frequency offset between the frame rate of the AUG and that of the VC-n, then the pointer value will be incremented or decremented as needed, accompanied by a corresponding positive or negative justification octet or octets. Consecutive pointer operations must be separated by at least three frames (i.e., every fourth frame) in which the pointer values remain constant.

If the frame rate of the VC-n is too slow with respect to that of the AUG, then the alignment of the VC-n must periodically slip back in time and the pointer value must be incremented by one. This operation is indicated by inverting bits 7, 9, 11, 13, and 15 (I-bits) of the pointer word to allow 5-bit majority voting at the receiver. Three positive justification octets appear immediately after the last H3 octet in the AU-4 frame containing the inverted I-bits. Subsequent pointers will contain the new offset.

For AU-3 frames, a positive justification octet appears immediately after the individual H3 octet of the AU-3 frame containing inverted I-bits. Subsequent pointers will contain the new offset.

If the frame rate of the VC-n is too fast with respect to that of the AUG, then the alignment of the VC-n must periodically be advanced in time and the pointer value must then be decremented by one. This operation is indicated by inverting bits 8, 10, 12, 14, and 16 (D-bits) of the pointer word to allow 5-bit majority voting at the receiver. Three negative justification octets appear in the H3 octets in the AU-4 frame containing inverted D-bits. Subsequent pointers will contain the new offset.

For AU-3 frames, a negative justification octet appears in the individual H3 octet of the AU-3 frame containing inverted D-bits. Subsequent pointers will contain the new offset.

The following summarizes the rules (Ref. 7) for interpreting the AU-n pointers:

530 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS

Figure 17.22 Administrative units in an STM-1 frame. (From ITU-T Rec. G.709, Figure 3-2/ G.708 [Ref. 6].)

1. During normal operation, the pointer locates the start of the VC-n within the AU-n frame.

2. Any variation from the current pointer value is ignored unless a consistent new value is received three times consecutively or it is preceded by one; see rules 3, 4, or 5 (below). Any consistent new value received three times consecutively overrides (i.e., takes priority over) rules 3 and 4.

3. If the majority of I-bits of the pointer word are inverted, a positive justification operation is indicated. Subsequent pointer values shall be incremented by one.

4. If the majority of D-bits of the pointer word are inverted, a negative justification operation is indicated. Subsequent pointer values shall be decremented by one.

5. If the NDF (new data flag) is set to 1001, then the coincident pointer value shall replace the current one at the offset indicated by the new pointer values unless the receiver is in a state that corresponds to a loss of pointer.

17.3.3.2.4 Administrative Units in the STM-N. The STM-N payload can support N

AUGs, where each AUG may consist of one AU-4 or three AU-3s. The VC-n associated with each AUG-n does not have a fixed phase with respect to the STM-N frame. The location of the first octet in the VC-n is indicated by the AU-n pointer. The AU-n pointer is in a fixed location in the STM-N frame. This is illustrated in Figures 17.17, 17.18, 17.22, and 17.23.

The AU-4 may be used to carry, via the VC-4, a number of TU-n’s (n c 1, 2, 3) forming a two-stage multiplex. An example of this arrangement is shown in Figure 17.23. The VC-n associated with each TU-n does not have a fixed phase relationship with respect to the start of the VC-4. The TU-n pointer is in a fixed location in the VC-4 and the location of the first octet of the VC-n is indicated by the TU-n pointer.

The AU-3 may be used to carry, via the VC-3, a number of TU-n’s (n c 1, 2) forming a two-stage multiplex. An example of this arrangement is shown in Figure 17.22 and 17.23b. The VC-n associated with each TU-n does not have a fixed phase relationship with respect to the start of the VC-3. The TU-n pointer is in a fixed location in the VC-3 and the location of the first octet of the VC-n is indicated by the TU-n pointer (Ref. 7).

17.3.3.3 Interconnection of STM-1s. SDH has been designed to be universal, allowing transport of a large variety of signals, including those specified in ITU-T Rec.

Figure 17.23 Two-stage multiplex. (From ITU-T G.708, Figure 3-3/ G.708 [Ref. 6].)

G.702 (Ref. 8), such as the North American DS1 hierarchy and the European E-1 hierarchy. However, different structures can be used for the transport of virtual containers. The following interconnection rules are used:

1. The rule for interconnecting two AUGs based on two different types of administrative unit, namely, AU-4 and AU-3, is to use the AU-4 structure. Therefore the AUG based on AU-3 is demultiplexed to the TUG-2 or VC-3 level according to the type of payload and is remultiplexed within an AUG via the TUG-3/ VC-4/ AU-4 route.

2. The rule for interconnecting VC-11s transported via different types of tributary unit, namely, TU-11 and TU-12, is to use the TU-11 structure. VC-11, TU-11, and TU-12 are described in ITU-T Rec. G.709 (Ref. 7).

REVIEW EXERCISES

1. What was the principal driving force for the development of SONET?

2. Being that the frame period of SONET/ SDH is 125 msec, what service are these formats optimized for?

3. Give at least five examples of digital services that can be transported on SONET.

4. Describe in words the basic SONET frame. Show its apparent payload and then its actual payload and explain the differences.

5. How can we calculate the bit rate of STS-3 if we know that the bit rate of STS-1 is 51.84 Mbps?

6. Explain how STS-Nc concatenation operates and its purpose.

7. What are the two primary uses of the payload pointer?

8. What network impairment is introduced by the payload pointer? What is its cause?

9. What are the three overhead levels of SONET?

10. Name at least three specific functions of the POH.

532 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS

11. What is payload mapping?

12. If the SONET line rate were OC-48, how could one calculate this bit rate?

13. What is the function of an add–drop multiplexer in the add–drop mode?

14. The SDH STM-1 bit rate equals what SONET line rate?

15. How do we derive higher line rates from the SDH STM-1?

16. What are the two types of pointers used in SDH?

17. When the frame rate of a VC-n is too slow with respect to that of the AUG, what happens to the alignment of the VC-n? What happens to the pointer value in this example?

18. We change the value of an AU pointer. How many frames must go by before changing it again? Why can we not change it right away, right on the next frame?

19. How many AU-3s or AU-4s may be contained in an AUG?

20. SDH was designed to carry the E1 hierarchy. Can it also carry the DS1 hierarchy?

REFERENCES

1. Synchronous Optical Network (SONET), Transport Systems, Common Generic Criteria, Bellcore, GR-253-CORE, Issue 1, Bellcore, Piscataway, NJ, 1994.

2. Introduction to SONET, Seminar, Hewlett-Packard Co., Burlington, MA, Nov. 1993. 3. C. A. Siller and M. Shafi, eds., SONET/ SDH, IEEE Press, New York, 1996.

4. SONET Add–Drop Multiplex Equipment (SONET ADM) Generic Criteria, Bellcore TR-TSY- 000496, Issue 2, Bellcore, Piscataway, NJ, 1989.

5. Synchronous Digital Hierarchy Bit Rates, ITU-T Rec. G.707, ITU, Helsinki, 1993.

6. Network Node Interface for the Synchronous Digital Hierarchy, ITU-T Rec. G.708, ITU, Helsinki, 1993.

7. Synchronous Multiplexing Structure, ITU-T Rec. G.709, ITU, Geneva, 1993.

8. Digital Hierarchy Bit Rates, CCITT Rec. G.702, Fascicle III.4, IXth Plenary Assembly, Melbourne, 1988.

9. G. Held, Dictionary of Communications Technology, Wiley, Chichester, UK, 1995.