VC4 Into a Synchronous Transport Module

Although a VC4 can have its payload constructed in a variety of ways, its POH conforms to the same principles as those of a VC12 i.e. it is generated and attached at the point where the C4 is loaded, and remains unchanged until the C4 is unloaded at its destination. As with the VC12, the VC4 POH is capable of indicating that errors have been introduced into the VC4 payload during its journey, hut it is not capable of identifying which network element was respon­sible. This problem is solved by the addition of yet another set of overhead bytes to the VC4, known as the Section Overhead bytes (SOH). The combination of the SOH plus the VC4 is termed a Synchronous Transport Module (STM), but another way of looking at this structure is to regard the VC4 as fitting neatly into the payload area of the STM. (See Figure 42.12.) Like the VC4, the STM repeals every 125μs. In order to appreciate the structure of an STM, and particularly the SOH, it is best to revert to using the two dimensional representation of a block of bytes. Figure 42.12 shows the standard representation of the smallest STM, known as an STM-1, which has 9 rows of 270 bytes each. This produces a transmission rate of 155.520Mbit/s.

The significance of the STM SOH is that, unlike the VC4 POH, is it generated afresh by every network element that handles the VC4. This handling of the VC4 includes the operations of creating, multiplexing or routing within the network, even if such multiplex­ing or routing happens to be completely inflexible (e.g. hardwired). When a network element (i.e. line terminal, multiplexer or cross connect) receives an STM, it immediately examines the relevant bytes of the SOH to determine whether any errors have been introduced into the payload i.e. the VC4. Unless this particular network element happens to contain the VC4 path termination point, it subsequently calculates a new, replacement set of SOH bytes which are then attached to the VC4 for onward transmission to the next network element. (See Figure 42.16.)

This section by section monitoring gives the PTO a powerful tool for locating the source of any poor performance within his network, and compliments the capabilities of the VC POHs, which are solely concerned with end to end, rather than section by section (i.e. network element to network element) issues. It could he argued that the POH monitoring is redundant, because the end to end path performance could be synthesised from the individual section indi­cations. However, not only would this he difficult to do, especially if the VC4 journey happened to traverse the networks of more than one PTO, hut it would not necessarily detect all the errors in the VC4, because it is quite possible for errors to be generated within the confines of a transited network element, i.e. within that portion of a network element between where the old SOH has been removed, and a new one has been added, e.g. between X and Y of the Mux at site C in Figure 42.16.

Although both the VC POHs and STM SOHs have other duties in addition to the performance monitoring described above, the SOHs shoulder by far the larger part of the burden, hence the reason for the much larger number of bytes in the SOH than, for instance, in a VC4 POH. (See Figure 42.12.) These additional duties include STM alignment function, the carriage of the network management channels, Engineer Orderwire channels, data channels reserved for the PTO and synchronisation signalling channels. Even when these have been accommodated, there is substantial unallocated capacity which is being kept in reserve, to service future network control requirements that have not yet been identified.

 

Figure 42.14 SDH multiplexing structure

 

 

Figure 42.15 Partially filled VC4 with TU structured payload

Figure 42.16 VC12 path between sites A and D, showing the VC4 and STM sections involved

 

Further use of Pointers

The use of STMs and their associated SOHs entails a few additional complications beyond those discussed above. An STM may well be generated by a network element which did not have the privilege of also generating the particular VC4 that it is attempting to load. This immediately introduces the possibility of a slight asynchronism between STM and VC4, and, as with the VC12, this problem is also solved by a slip mechanism plus pointer. The pointer bytes occupy defined positions within the SOH and indicate the offset, in bytes, between themselves and the first byte of the VC4 POH. The main difference between this and the VC12 pointer is that when a VC4 slips its phase relative to the STM SOH, it does so by three bytes at a time, rather than the single byte phase change experienced by the VC12.

A second difference between this case, and that of a VC12, is that the combination of a VC4 plus its pointer is known as an Adminis­trative Unit 4 (AU4), rather than a TU4, when the pointer is located in an STM. The AU4 is used in all CEPTcountries as the size oftraffic block on which networks are planned and operated. There is also an AU3, which is used mainly in North America. This refers to an alternative construction of an STM, whereby the payload con­sists not of a single VC4, but instead, of a group of three VC3s, together with their associated pointers. A further difference between the STM SOH and a VC POH is that the SOH can be divided into two parts, known as the Multiplexer Section OverHead (MS OH) and the Regenerator Section OverHead (RSOH). (See Figure 42.12.). The reason for this is that on long line transmission systems the regenerators do not need to perform the rather costly, and in this case, unnecessary operation of generating and destroying the com­plete SOU. Instead, only a subset of a SOH is processed, leading to a reduction in gate count, power consumption etc., but still preserv­ing the ability to detect traffic errors and access some management channels. On the other hand, the line terminal equipments process both the RSOHand MSOH. (See Figure 42.17.)

Finally, as with VCs, STMs come in various sizes. As mentioned earlier, the smallest size is termed an STM-1, and can accommodate a single VC4. However, larger sizes exist whose hit rates are integer multiples of the basic STM-1 rate. CCITT G.707 currently recog­nises the STM-4 and STM-16, but STM-12 based network elements are also being designed, and it is likely that STM-64 will become a de facto standard in the near future. For all these higher rate STMs, the construction mechanism is the same: The payload is produced by straight byte interleaving of the tributary VC4s, while the SOH is constructed in a more complicated way, particularly in relation to the way the error checking bytes are calculated.

 

 

 

Figure 42.17 Line transmission system showing the multiplexer section overshea (MSOH) operating between LTEs, while only the regenerator section overhead (RSOH) is recalculated between each pair of regenerators

 

 

Figure 42.18 Loading of a hypothetical i6Mbit/s signal into 3 concatenated VC-2s (VC2 -3c) within a VC4

 

1. Learn the following technical words and word-combinations:

42.4.3

1.	tributary	второстепенный, подчиненный
2.	to serialize	преобразовывать (из параллельной) в последовательную форму
3.	contiguous	смежный
4.	overflow position	разряд переполнения
5.	stuffing	вставка, подстановка
6.	to wander	отклоняться, смещаться

42.4.4

7.	two-dimensional representation	двумерное представление
8.	to traverse	пересекать, проходить, переходить

42.4.5

9.

integer multiples

целые кратные

 

Exercise 2 Read the text 42.4.3- 42.4.5

 

Exercise 3 Find the Russian equivalents for the following English technical word- combinations:

 

1.	to serialize by scanning left to right
2.	contiguous block of bytes
3.	overflow byte positions
4.	VC12 slips phase
5.	Section Overhead bytes
6.	Synchronous Transport Module alignment function
7.	Engineer Oderwire channels
8.	to entail a few additional complications
9.	Multiplexer Section Overhead
10.	Regenerator Section Overhead
11.	the basic STM-1 rate
12.	the error checking bytes
13.	substantial unallocated capacity
14.	to service future network control requirements
15.	the payload capacity
16.	the payload bandwidth of a VC4

 

Exercise 4

Find the English equivalents for the following Russian technical word- combinations:

 

1. двумерный блок битов
2. разрушать данные в другом канале
3. отклоняться по фазе
4. довольно гибкий механизм
5. замена двумерного представления
6. смесь виртуальных контейнеров различных размеров
7. точка окончания тракта
8. ограничения транзитных сетевых элементов
9. синхронизированные каналы сигнализации
10.размер блока трафика
11.обнаружить ошибки трафика

 

Exercise 5

Answer the following questions:

1.In what way is it helpful to consider the internal structure of the VC4?

2.What is known as a Tributary Unit Group?

3.Is VC 12 the only size of Synchronous Container which has been defined?

4.Do Tributary Unit Groups constitute an extremely flexible mechanism for partitioning the payload bandwidth of a VC4?

5.What is VC 4 POH capable of?

6.How is the combination of the SOH (Section Overhead bytes) plus the VC 4 termed?

7.How can you appreciate the structure of a STM (Synchronous Transport Module)?

8.What is the standard representation of the smallest STM?

9.What is the significance of the STM SOH? What does the handling of the VC 4 include?

10.What happens after a network element receives an STM?

11.What is a powerful tool for locating the source of any poor performance within a network?

12.What can a slight asynchronism between STM and VC 4 introduce? How can this problem be solved?

13.What is the main difference between VC4 POH and VC 12 pointer?

 

Exercise 6

Make a short report on differences between VC POH and VC 12 pointer (part 42.4.5).

 

Exercise 7

a) Translate into Russian in writing part 42.4.3 paragraph 1.

b) Translate into Russian in writing part 42.4.3 paragraphs 2,3 (up to “A TUG3 is one third…”).

 

Part 4(42.4.6-42.4.9)

1. Other sizes of VCs and payloads

2. SONET and SDH

3. NNI Optical Interface standardization

4. SDH network elements