ATM BASICS - Chapter 1 pps

1 Chapter 1 What Is ATM The intent of this chapter is to provide the reader with the crucial concepts of ATM. It describes the historic background that influenced the way ATM devices work even until today. The chapter also positions ATM as the tech- nology that belongs to the world of telecommunication standards for more about fourteen years already. It covers the basic idea that is actually pre- sent behind the term ‘ATM’ without focusing on detailed and technical descriptions. How to describe ATM using a short definition? In very brief, ATM can be addressed as an integrated multiplexing and packet switching technique that supports large variety of applications and provides for incomparable Quality of Service. In the end of the 80’s it became obvious to anyone that a newly defined stan- dard such as ISDN would not be able satisfy requirements from applications envisaged for the next decade. The new wide area service called B-ISDN (Broadband ISDN) has been proposed by the ITU-T. B-ISDN was intended to offer video on demand, live television from many sources, CD-quality music, LAN inter-connections, as well as high speed data transport for sci- ence and industrial applications. As a part of B-ISDN development process, a number of underlying technologies, capable of fulfilling these require- ments were analyzed and tested. Finally, the ATM has been chosen as the underlying technology that was going to make B-ISDN possible. 1.1 Why Asynchronous? What does the term “asynchronous transmission” mean with the relation to ATM? Most of the telecommunication systems such as fixed and mobile tele- phony networks are based on synchronous transmission. With a synchro- nous system the most common method for sharing network resources among users is to grant them the access in time domain. Hence, in traditional tele- phony and ISDN as well as in many mobile telephony systems the Time Division Multiplexing (TDM) is the most popular method. In synchronous systems, which are optimized for streaming applications such as uncom- pressed voice transmission, every user is provided with a channel by means of time slots allocated within a frame structure. On the other hand, packet transmissions require different approach. The data is transmitted in bursts, which appear depending on the source and the actual network load. Using synchronous systems for packet transmission is not optimal since some resources become unused. The asynchronous approach is more adequate whenever data transmission is concerned. ATM is designed to support various types of services including voice, video and data. Therefore, to make the new technology capable of efficient delivery of such differentiated services, the asynchronous transmission method was chosen. Due to the requirements imposed by real-time streaming applica- tions (voice, video), ATM transmits information in very tiny portions, which are called cells. Their size is the compromise between efficiency for data and other services. Unlike TDM in traditional telecommunication systems, ATM does not assign any time slots to a given user and it is not tied to a master clock. Instead of this it dynamically allocates ATM cells when user equip- ment wishes to transmit information. Cells representing traffic generated by different users are multiplexed, which is shown in the Fig. 1-1) over the physical medium. However, ATM does not run directly over the physical medium, so the physical layer technology must be used to provide ATM with a set of necessary functions. In result, ATM cells are often transmitted in the asynchronous mode but using the synchronous physical layer. ATM Basics 2 1.2 Is ATM a packet switched or circuit switched technology? After some initial research had been completed, it became clear that none of the existing technologies would fit well into the B-ISDN model. A number of different concepts have been taken into consideration but the final agree- ment introduced a hybrid solution. ATM represents a combination of circuit and packet switching. On one hand, ATM transmission relies on very small packets – cells, which benefits in large efficiency for data transmission. Resources are used in an efficient way due to the gain resulting from multi- plexing of cells. On the other hand, the opposite requirements from voice and video services request for guaranteed bandwidth and quality of service. Hence, some form of circuit switching is also necessary, even though the con- nections (circuits) are purely logical. ATM is a technology that behaves like a mixture of packet and circuit switching. In result some of the ATM fea- tures are typical to packet switched networks (efficiency for data transmis- sion) but other features are similar to those of circuit switched networks (low and predictable delay, guaranteed bandwidth). Chapter 1 3 Fig. 1-1, The idea of asynchronous transmission 1.3 Advantages of ATM 1.3.1 Support for variety of applications ATM has been designed to handle practically any type of transmission. In result, ATM is capable of simultaneous transmission of voice, video and data representing different applications. Such a broad support for different ser- vices highly influenced the large number of ATM mechanisms. Both voice and video are real-time or near real-time applications that can hardly toler- ate delays and delay variations. Therefore, they imply the need for control- ling of some QoS parameters as long as the service should observe the qual- ity comparable to the one present in classical networks such as PSTN, ISDN. Data applications primarily require the opposite treatment. They can tolerate both the delay and delay variation, but they strongly rely on reli- able transmission. The great advantage of ATM over other technologies is ATM Basics 4 Fig. 1-2, The origins of ATM concept. related to its unique ability of using the consolidated network infrastruc- ture, while independently provisioning quality of service for particular applications. Within last decade ATM networks have become the underlying infrastructure capable of handling voice services (Voice over ATM), video services (e.g. Video Dial Tone) and data (IP over ATM). 1.3.2 Scalability ATM is highly scalable. The scalability of ATM as the technological solution can be described at two levels: •The interface level. ATM was initially developed to operate over optical fiber physical media. With the time passing by, it soon became clear, that ATM should be also defined for other physical media. Today ATM can operate not only over various types of optical fiber (single mode and multi- mode, glass and plastic fiber) but it can be also deployed using copper twist- ed pairs and wireless interfaces. ATM has been adapted to the transmission systems and technologies that are widely used both in telecommunication networks as well as in computer networks. Therefore, in many cases ATM can be run over existing transmission networks without the need for costly re-wiring process. The variety of interfaces allows for different transmission speeds ranging from 1.5 Mbps (DS 1) up to 10 Gbps (the latter is available for single-mode, multi-mode fiber as well as twisted pair copper wire) •The network level. Scalability with regards to the network level refers to ATM’s ability to grow in physical size and speed reflecting the num- ber of users and volume of traffic generated. The only limiting factors are the characteristics of the physical medium and the switching equipment capacity. Thanks to a number of advanced traffic management mechanisms ATM networks work and scale well in the LAN environment, campus net- works, MANs and WANs. Therefore, a network operator can use ATM in almost any part of its network as the uniform method of transport that can be used on almost any physical infrastructure. Chapter 1 5 1.3.3 Interworking with other technologies ATM can be used to interconnect systems. Although ATM was developed to offer the unified solution, capable of replacing existing solutions and tech- nologies (i.e. classical telephony systems), due a number of factors it was not able to replace them completely. Most service providers/operators actually upgraded their backbone infrastructure to ATM yet there remain a number of customers who demand connectivity at the level of Frame Relay or SNA. Needless, to say ATM was defined along with various methods for inter- working with other technologies. ATM can serve as the underlying technol- ogy for both existing LAN and WAN network services. Furthermore, ATM has the ability to mimic different protocols what makes possible to slowly migrate from existing technologies to ATM. The network operator can deploy ATM on selective basis in key parts of its network. The set of ATM specifications defines methods for interworking with LANs such as LAN Emulation. Other technologies present in the WAN environment can be also a subject to interworking. They include Frame Relay, SMDS and ISDN. Interworking between ATM and IP has been the point of the greatest inter- est for many operators. Hence, a number of specifications describe different scenarios for ensuring successful ATM and IP integration. They include basic solutions such as CLIP as well as fairly advanced models including MPOA. With the advent of MPLS it has become clear that as data trans- mission is concerned ATM technology can be soon replaced with MPLS con- cept. In fact most of the ATM equipment already installed in ATM networks can be easily updated to MPLS. Unsurprisingly there is a lot of ATM relat- ed information within MPLS specifications. It has been widely accepted that at the current stage of MPLS development it is the ATM infrastructure that provides the best performance. The integration from ATM based platforms into MPLS capable solutions can be achieved in a number of ways. Not only ATM switching devices can be converted into MPLS switching nodes and ATM and MPLS may coexist within the same device and even at the same physical interface. It is also foreseeable to transmit MPLS traffic across net- work regions comprised of traditional ATM devices. To conclude, even today it can be easily noticed that ATM is a future proof solution ATM Basics 6 1.3.4 Maturity Stage ATM is a proven and mature technology. It has been more than a decade since the first commercial ATM deployments took place. From the emerging technology ATM turned into the alternative that has been chosen by the largest players in the telecommunication market. The current approach is to provide services by means of common platforms. The convergence is not just a marketing term any longer but the common practice for hundreds of operators providing advanced services that one can hardly imagine ten years ago. Nowadays almost any application generates a stream of bits with variable bit rate. This may refer to the transmission of compressed voice and video but also to the transmission of data structured in packets. However, there exist a tremendous amount of embedded equipment and applications that can be used even though operators upgrade some parts of the core infra- structure. ATM applications such as Circuit Emulation Service allow ATM to emulate existing services (e.g. T1/T3 or E1/E3) with the intention to sup- port interconnections between end devices. Thanks to its unique capabilities and the continuous evolution ATM is still considered as the technology of choice for many customers, especially when reliability, scalability and QoS are at stake. Chapter 1 7 ATM Basics 8 . 1 Chapter 1 What Is ATM The intent of this chapter is to provide the reader with the crucial concepts of ATM. It describes the historic background that influenced the way ATM devices. transmis- sion) but other features are similar to those of circuit switched networks (low and predictable delay, guaranteed bandwidth). Chapter 1 3 Fig. 1- 1 , The idea of asynchronous transmission 1. 3. treatment. They can tolerate both the delay and delay variation, but they strongly rely on reli- able transmission. The great advantage of ATM over other technologies is ATM Basics 4 Fig. 1- 2 ,