by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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Chapter 5 ATM Protocols
Protocols for computer communications are defined as sets of rules and message exchanges between computer systems. In order to talk to another computer, it is essential to understand its communication
protocol. Typically, various communication functions involved in networking are divided into specific tasks and spread among the layers. The most well-known protocol architecture is the seven-layer Open
Systems Interconnection OSI model.
A protocol stack is a set of layers that incorporates standards of one system or another at each layer. There are many different kinds of protocol stacks. The connector on the back of a computer or the
communications board is referred to as the lowest layer while the applications program running in the local memory of the computer or the network device is referred to as the highest layer. The protocol
stack bridges the gap between the hardware i.e., the connector and the software i.e., the applications program.
As shown in Figure 5-1 a protocol stack includes a variety of components necessary for providing a complete solution for the user’s networking problems. Signaling protocols are necessary for controlling
the network. Other components include a method for monitoring the network performance and a means for managing both, the resources and the traffic on the network. It also includes a protocol for
internetworking with older networking technologies.
The protocol stack can often be proprietary, i.e., wholly developed and owned by a private company, and the company is under no obligation to reveal the internal functional details to anyone. IBM’s System
Network Architecture SNA is an example of a proprietary protocol. There are other protocols that are available to everyone and referred to as an open standard. ISO and the popular Transmission Control
ProtocolInternetwork Protocol TCPIP belong to this category where documentation is available from a variety of sources.
I. ATM Protocol Stack
ATM is an open standard based on the documentation available from the ITU-T and the ATM forum. It
Figure 5-1 Example of ATM protocol stack application.
ATM protocol stack is for communications over a high-speed network where information i.e., voice, video, audio and data is sent in an unchannelized fashion through cells. Figure 5-2 shows the three
primary layers of the ATM protocol reference model. The physical layer interfaces with the actual physical medium and performs the task of transmission convergence. The cell structure occurs at the
ATM layer. The virtual paths and virtual channels are also defined at this layer. Support for higher layer services such as circuit emulation, frame relay, Internetwork Protocol IP, SMDS, etc. is provided by
Figure 5-2 ATM layers implementing the various cell functions.
Figure 5-3 depicts the number of instances of defined standardized protocols or interfaces at each layer by boxes. It is clear that ATM is the pivotal protocol, such that for a large number of physical media,
several AALs and an ever-expanding set of higher layer functions there is only one instance of the ATM layer. Thus ATM allows machines with different physical interfaces to transport data independently of
the higher layer protocols using a common well-defined protocol.
Figure 5-3 Hardware to software progression for the ATM protocol model. Number of instances in
each layer is indicated by the boxes. Figure 5-3 also illustrates how ATM implementation moves from being hardware intensive at the lower
layers the physical and ATM layers to software intensive at the higher layers AALs and higher layers. Table 5-1 shows the various ATM layers and their functions.
II. The Physical Layer
1 The Physical Medium Dependent PMD sub-layer which provides the actual transmission of the bits, in the ATM cells.
2 The Transmission Convergence TC sub-layer which transforms the flow of cells into a steady stream of bits and bytes for the transmission over the physical medium.
A. PMD Sub-layer
The original transport medium for ATM was defined by ITU-T for B-ISDN architecture in the late 1980s. It was a fiber-based, scalable, high-speed network scheme referred to as Synchronous Digital
Hierarchy SDH. The SDH is standardized for use in the U.S. by the American National Science Institute ANSI and is known as the Synchronous Optical NETwork SONET
By 1992 the ATM forum declared that ATM did not have to be limited to high-speed fiber medium and other media such as coaxial cable, microwave links, etc. should be supported. Goralski 1995 and
Ginsberg 1996 discuss various mediums in detail with the emphasis on the fiber-based network. The overall philosophy of ATM network is that they should be fast and highly reliable. i.e., the transmission
errors should be fewer in magnitude. But it is also important that the transport media should be readily available. ATM is meant for providing unchannelized network support to a variety of users with
connections having different speeds. Thus it is more informative to group the allowed transport by physical medium rather than by technology. Figure 5-4 illustrates this framework where some transport
media are meant to be used in private ATM networks while others are anticipated to be used only in public ATM networks.
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