42.3.1 The concept of pointers

The solution adopted by SDH is to associate a pointer with each VC so that when it is multiplexed, along with others, into a larger VC, its phase offset in bytes can he identified relative to some reference point in this larger VC. (See Figure 42.7.) Furthermore, there is also a mechanism for allowing the value of this pointer to change if, for some reason, there is a loss of synchronisation, and the smaller capacity VC is running either slightly slower, or slightly faster than the larger VC. In fact, each of the smaller capacity VCs has its own pointer, which can change independently of any of the others. Although the use of these pointers still entails some input buffers, these are very much smaller than would be required if there were no mechanism for changing the phase of a small capacity VC within a larger one, hence the problem of excessive delays can be contained. We now have a picture of an SDH network as one in which the majority of VCs, both large and small, are well synchronised to each other, but, at the same time, there are a few which are not so well synchronised, and every so often, the increasing strain of their asynchronism has to be relieved by a byte sized slip relative to the majority of the other VCs in the network. Nevertheless, we retain our ability to locale the management and control information in the VCs POH bytes, because the pointer value associated with VC is recalculated whenever a slip occurs.

The pointer mechanism described above is at the very heart of the SDH standard. It is this mechanism that enables us to construct networks that are nearly, hut not completely synchronous, and yet still allows us to easily locate each traffic channel (VC), together with its associated management and control information i.e. POH, but without incurring large penalties in transmission delay. It could be argued that SDH networks are not really synchronous at all, but are actually very tightly controlled asynchronous networks. However, the fact that we have quantised the slips due to this asynchronism means that it is now possible, at any time, to locate and route any of the traffic paths within an SDH network. This, together with network management software, gives us the traffic routing flexibility that was very difficult to achieve using PDH based equipment. In terms of actual network hardware, it opens the way to the production of economically viable drop and insert multiplexers and cross connects.

Figure 42.5 Difference in synchronisation between PDH and SDH: (a) PDH; (b) SDH

Figure 42.6 General representation of a 3 layer synchronous multiplexer structure

Figure 42.7 Pointers allow small VC to have arbitrary phase with respect to large VC

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

42.1

1.	transport network	транспортная сеть
2.	transport capacity	пропускная способность
3.	bandwidth flexibility	гибкость полосы пропускания

42.2

4.	bit interleaving	чередование битов
5.	cross-connection	перекрестная связь
6.	information capacity	информационная емкость
7.	management information	управленческая информация, административная информация
8.	aggregate port	порт составных (множества) значений
9.	line terminal	линейный вывод
10.	to tempt	соблазнять, искушать
11.	echo cancellor	эхо-компенсатор, эхо-подавитель
12.	transmission media	(среда) канал передачи информации

42.3

13.	ISO – International Standard Organization	международная организация по стандартам
14.	non-deterministic delay	недетерминированная задержка
15.	OSI – open system interconnection	взаимодействие открытых систем
16.	voice circuit	тональная цепь, телефонная цепь
17.	rigit synchronisation	жесткая синхронизация

Exercise 2 Read the text 42.1- 42.3

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

1.	the raw transport capacity
2.	frenetic activity
3.	they could afford to resist these demands
4.	liberalization and deregulation
5.	to obscure knowledge of the individual byte boundaries
6.	large and combersome
7.	a line transmission terminal
8.	the aggregate port
9.	a patchwork of smaller operations
10.	transmission media
11.	was formulated on the premise that…
12.	overheads
13.	North American counterparts
14.	VC payload
15.	path overheads
16.	rigit synchronization
17.	the nesting hierarchy

Exercise 4

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

1.	низкоскоростные цифровые каналы
2.	тракты трафика
3.	предоставлять высококачественные телекоммуникационные услуги
4.	техническое обеспечение
5.	пространственная коммутация
6.	структура цикла
7.	битовый поток
8.	исследование стандартов SDH
9.	изохронные каналы трафика
10.	циклы фиксированной длины
11.	пакетирование передаваемой информации
12.	чувствительные речевые каналы
13.	оптические волокна
14.	непроизводительные затраты мультиплексирования
15.	полезная нагрузка трафика
16.	синхронная структура цикла
17.	границы байта

Exercise 5

Answer the following questions:

1. What does the Synchronous Digital Hierarchy mean?

2. What has the SDH developed into due to the business pressures confronting the world’s Public Telecommunications Operators (PTOs)?

3.What is the result of implementing the SDH standards?

4.Why is there such a frenetic activity to changeover from PDH to SDH?

5.Is it possible for PTOs to deliver the quality of service demanded by business customers if they continue to operate transmission networks based on the current PDH?

6.What are the three main limitations of the PDH?

7.Which of the three main problem areas of PDH could have been solved and in what way?

8.Does PDH have currently any agreed management channels or protocol stacks?

9.Why is it difficult to overcome the problem of traffic path inflexibility?

10. What is SDH designed for and what is it based on?

11. What is the basic SDH frame structure?

12. How can a Synchronous Container be viewed? What does it consist of?

13.What is the plesiochronous channel synchronized to?

14.What does the Virtual Container allow PTOs to monitor?

15.What does the SDH perform?

Exercise 6

a) Translate into Russian in writing part 42.3 paragraphs 4,5 ( starting with “Before examining the questions… “)

b) Translate into Russian in writing part 42.3 paragraphs 6,7 ( starting with “This process of loading… “)

Exercise 7

Make a short report on the basis of SDH.

Part 2 (42.3.1-42.4.2)

1. The concept of pointers

2. The SDH standards

3. Path OverHead information

4. Multiplexing of Virtual Containers