- •42 The Synchronous Digital Hierarchy (sdh)
- •42.1 Introduction
- •42.2 Pdh deficiencies
- •42.3 The basis of sdh
- •42.3.1 The concept of pointers
- •42.4 The sdh standards
- •42.4.1 Path OverHead information
- •42.4.2 Multiplexing of Virtual Containers
- •42.4.3 Channels and Tributary Unit Groups
- •42.4.4 Vc4 Into a Synchronous Transport Module
- •42.4.5 Further use of Pointers
- •42.4.6 Other sizes of vCs and payloads
- •42.4.7 Sonet and sdh
- •42.4.8 Nni Optical Interface standardisation
- •42.4.9 Sdh network elements
- •42.5 Control and management
- •42.6 Sdh based networks
- •42.6.1 Sdh network topologies
- •42.6.2 Deployment strategies
- •42.7 Impact of broadband standards
- •42.7.1 Frame Relay
- •42.7.2 Switched Multimegabit Data Service (smds)
- •42.7.3 Fibre Distributed Data Interface (fddi)
- •42.8 Future technologies
- •42.8.1 Integrated circuits
- •42.8.2 Optical interfaces
- •42.8.3 Optical amplifiers
- •42.8.4 Optical switching
- •42.8.5 Memory and processing power
- •42.9 Conclusion
42 The Synchronous Digital Hierarchy (sdh)
Part 1 (42.1 –42.3)
1. Introduction
2. PDH deficiencies
3. The basis of SDH
42.1 Introduction
The Synchronous Digital Hierarchy (SDH) is a new standard for multiplexing together many low rate digital traffic channels into higher rate channels, in order that these low rate channels may be more efficiently transmitted around a telecommunications network. Viewed in this way, the SDH is merely a better alternative to the existing Plesiochronous Digital Hierarchy (PDH), which attempts to achieve more or less the same results. However, as we shall see later in this chapter, SDH has evolved to become much more than just the latest standard for combining and separating traffic channels. Due to the business pressures confronting the world's Public Telecommunications Operators (PTOs), SDH has developed into a comprehensive set of standards which address all aspects of a telecommunications transport network. The most important of these additional aspects concern the performance of traffic paths across the network, together with the automation of their management. The result of implementing the SDH standards is that a PTO now has a consistent mechanism for partitioning, monitoring and controlling the raw transport capacity of the whole network.
Nevertheless, without some sound business drivers, most PTOs would view a move from the current, well proven, PDH to the more advanced SDH as, at best, an interesting academic exercise, and at worst, a scandalous extravagance. To see why there is such frenetic activity to changeover to SDH it is necessary to briefly examine the business environment in which the world's more advanced PTOs are now operating.
The source of the problem is the PTOs' business customers. These are the customers from whom the PTOs derive a disproportionately large percentage of their profits, and who are becoming increasingly dependent on telecomms services for their very survival. Not surprisingly, business customers are looking to their use of telecomms to give them a competitive edge and thus, in addition to a straight reduction in tariffs, they are now demanding other things such as lower error rates and higher availability on their existing services, together with the bandwidth flexibility (i.e. channel capacity and routing capability) that enables the introduction of completely new services. In the past most PTOs operated as monopoly suppliers, and to some extent they could afford to resist these demands. Now, however, liberalisation and deregulation have introduced significant competition into several of these former monopoly markets, especially in the USA, UK and Japan. This competition to supply high quality telecom services to business customers is forcing PTOs to re-examine the cost effectiveness of their existing transmission networks. In particular, they are looking to balance the quality of service delivered to their customers against the capital and operating costs of their networks. The objective of this balance is, of course, to maintain or increase profits
The disturbing conclusion of most PTOs is that it is not possible to deliver the quality of service demanded by business customers, at the right price, if they continue to operate transmission networks based on the current PDH. Even more disturbing is the fact that this conclusion was reached with full awareness of the falling costs of raw bandwidth i.e. the effects of reduced costs for optical fibres, electro-optic devices and the Application Specific Integrated Circuits (ASICs), which are the heart of any piece of transmission hardware.