ATM Overview Basic ATM Concepts

by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC ISBN: 0849331390 Pub Date: 110198 Previous Table of Contents Next

Chapter 2 Basic ATM Concepts

Asynchronous transfer mode ATM technology promises the integration of the diverse services of voice, video, image and data. ATM is a technology that allows transmission and switching of fixed- length packets known as cells through an ATM switching network asynchronously. The fixed-length ATM cell structure defines the foundation of the ATM technology. This chapter provides a high-level overview of ATM concepts.

I. ATM Overview

There has been a growing interest in the development of integrated multi-service enterprise and service provider networks, which consolidate voice, video, imaging, and computer data traffic into a single network. The driving forces for this interest are the rapid rate of growth in non-voice traffic, the emergence of new strategic multimedia applications, and the significant cost savings which can be realized from network consolidation. A common direction for both enterprises and service providers is the evolution towards ATM networking. High-speed networks refer to networks with a link capacity of 100 Mbitss and above. They are also high capacity networks. If a large number of users are sharing the capacity, their behavior is not fundamentally different from lower speed networks. However, in the case of a small number of high- bandwidth users the latency may start to dominate performance. Thus it is critical to reduce or hide end- to-end latency. Table 2-1 lists various alternatives for high speed networking. A principal attribute of ATM is that it is equally suitable for departmental and campus local area networks LAN, metropolitan area networks MAN and wide area networks WAN. In order to attain the end-to-end integration of broadband services, interoperability of LANs, MANs and WANs is a key requirement. For the first time this one technology is positioned to provide an end-to-end solution for the user. Table 2-1 WAN or Wide Area Networks LANMAN Local B-ISDN Broadband ISDN FDDI Fiber Distributed Data Interface ANSI Fiber Channel SMDS Switched Multi-Megabit Services IEEE 802.6 DQDB Distributed-Queue Dual- Bus HIPPI High Performance Parallel Interface SONET Synchronous Optical NETwork — — Each application has different requirements, for example, some are more sensitive than others. Data related applications can tolerate delay, but not loss. On the other hand, audio and video can tolerate loss, but not delay. Table 2-2 provides information on various applications, their traffic characteristics and service requirements. One of the most significant promises of asynchronous transfer mode ATM technology is the integration of diversified services such as voice, video, image, and data. Some of the salient features of ATM can be listed as follows: 1 It integrates Voice, Video and Data 2 ATM uses short, fixed length packets called cells 3 It is a best effort delivery system 4 It provides bandwidth on demand 5 It is a connection oriented technology, i.e., every cell with the same source and destination travels over the same route 6 It has the potential to remove performance bottlenecks in today’s LANs and WANs 7 It knits local and wide-area networks and services into a seamless whole 8 It is possible to bill the customer on a per-cell basis 9 ATM is scalable i.e., it works at different speeds and on a variety of media 10 ATM architecture has an open-ended growth path, since it is not locked to any physical medium or speed There are certain disadvantages of ATM: 1 Overhead of cell header 5 bytes per cell 2 Complex mechanisms for achieving Quality of Service 3 Congestion may cause cell losses Application Traffic Characteristics Service Requirements Voice PCM coded Constant bit rate size: 64 Kbps long connection times uniform traffic pattern Maximum delay 200ms guaranteedfixed bandwidth low cell loss Browsing databases including graphics Size per picture: 100 Kbytes to 10 Mbytes bursty traffic Maximum delay: typically 10 to 100 ms Real-time video MPEG Variable bit rate size: 128 Kbps 1.5 Mbps bursty traffic pattern Low average delay, low delay variation, low cell loss 10 -10 required Traffic between workstations PCs and centralized data bases Peak rates of 10 Mbps or more short connection times, typically bursty traffic Maximum delay: 1 to 100 ms depends on the application Previous Table of Contents Next Copyr ight © CRC Pr ess LLC by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC ISBN: 0849331390 Pub Date: 110198 Previous Table of Contents Next ATM technology can also be viewed as the next stage in the evolution of packet switching and circuit- switching technologies. Packet switching as the name suggests is a method in which long messages are subdivided into short packets and then transmitted through a communications network. In case of circuit switching a dedicated communications path is established through one or more intermediate switching nodes. Digital data are sent as a continuous stream of bits and the data rate is guaranteed. Delay in this case is limited to propagation time. ATM combines the two switching techniques and takes advantage of the desirable characteristics of both techniques. The statistical multiplexing feature from packet switching allows efficient use of the available bandwidth while the connection oriented feature from circuit-switching provides predictable cell transmission delay. Another strong reason for selection of fixed-length for the ATM cell and the connection-oriented path selection is to be able to implement the cell-switching at the hardware level in order to achieve the high-speed data transmission requirement. ATM allows multiple logical connections to be multiplexed over a single physical interface. The information flow on each logical connection is organized into cells. Many data as opposed to voice or video applications are bursty in nature and can more efficiently be handled with some sort of packet- switching approach. It is important to note that ATM does not provide for error-control or flow-control mechanisms. Asynchronous, in our context, refers to the ability of the ATM network to send only the data associated with a connection when there are actual live data that need to be sent. In contrast, Synchronous Transfer Mode STM networks consisting of channels require a continuous stream of data and when the channel is idle a special bit pattern, called idle cell, must be sent in every time slot representing the channel. There are several disadvantages of the synchronous approach. First, it does not provide a flexible interface for meeting a variety of needs. At the high data rates offered by ATM, there could be a wide variety of applications supported by ATM involving many different data rates. One or two fixed-rate channel types in this case would not provide a structure that can easily accommodate this requirement. Another aspect of the inflexibility of the synchronous approach is that it does not lend itself to rate adaptation which is essential for channels containing hundreds of megabits per second. Figure 2-1 shows the overall hierarchy of function in an ATM-based network. This hierarchy is seen Figure 2-1 illustrates the architecture of the ATM model. The ATM layer consists of virtual channel and virtual path levels as illustrated in Figure 2-2. These two components are discussed in detail in the next subsection. Below that is the physical layer as shown in Figure 2-1. Based on functional differences the physical layer can be divided into three levels see Figure 2-2: Transmission path level: This level consists of network elements that assemble and disassemble the payload of a transmission system. The payload in the case of end-to-end communication is the end-user information. The payload for user-to-network communication may be signaling information. Cell delineation and header error-control functions are required at the endpoints of each transmission path. Digital section: This section of the physical layer consists of network elements that assemble and disassemble a continuous bit or byte stream. This refers to the exchanges or signal transfer points in a network that are involved in switching data streams. Figure 2-1 B-ISDN Protocol Reference Model. Figure 2-2 Functional view of various layers. Regenerator section level: We can regard this level as a portion of the digital section. An example of this level is a repeater that is used to simply regenerate the digital signal along a transmission path that is too long to be used without such regeneration. In this case there is no switching involved.

II. Basic Definitions for ATM Concepts