29.7 ATM FOR MULTIMEDIA

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When ATM is used for applications such as audio/video transmission in real time, it is necessary that at the receiving end, the cells are received without variable delay; the cells should be received at a constant speed. For example, PCM-coded speech produces a continuous stream of bits at the rate of 64kbps. To transmit PCM over ATM, the PCM data stream has to be assembled into cells and transmitted over the network; at the receiver, the cells should be received with constant delay. To achieve this, special protocols are required. The ATM adaptation layer (AAL) has been defined to provide the necessary services to handle such applications. The AAL provides the following services:

Segmentation and reassembly
Flow control and timing control
Handling of transmission errors
Handling of lost cells

The functions of the ATM adaptation layer are segmentation and reassembly, flow control, timing control, handling transmission errors, and handling lost cells.

ITU-T defined four classes of services and five types of AAL protocols:

Class A services require synchronization the source and the destination, have a constant bit rate, and are connection-oriented services. An example of a class A service is voice transmission. Type 1 AAL protocol is used for this class of services.

Class B services require synchronization between the source and the destination, but the data rate is variable, and they are connection-oriented services. An example of class B services is variable rate video transmission for video conferencing. Type 2 AAL protocol is used for this class of services.

Class C services require no synchronization between the source and the destination, the data rate is variable, and they are connection-oriented services. Data applications fall under this category. Type 3, 4 and Type 5 AAL protocols are used for this class of service.

Class D services require no synchronization between the source and the destination, the data rate is variable, and they are connectionless services. Connectionless data applications fall under this category. Type 3and 4 AAL protocol as are used for this class of service.

Networks based on ATM are now becoming popular because ATM provides simple yet efficient protocols to handle multimedia services.

Note

ATM adaptation layer provides support for multimedia services by defining five types of AAL protocols that support fixed and variable data rate services.

Summary

This chapter presented the ATM technology. Asynchronous Transfer Mode (ATM) is based on small packets to facilitate the fast switching required for multimedia services. ATM provides connection-oriented service. ATM packets are called cells. Each cell will be of 53 bytes, with 5 bytes of header and 48 bytes of user data. The main attraction of ATM is that it guarantees quality of service. The quality of service parameters such as delay and packet loss can be set. To achieve the desired QoS, virtual channel and virtual path are used. A virtual channel is established between two end systems. A bundle of virtual channels having the same end points is called virtual path. All cells belonging to a virtual path are switched together.

To support multimedia services in the ATM, the ATM adaptation layer is defined. The ATM adaptation layer has five classes of services to support voice, video, and data applications. Support for quality of service parameters and fast switching are the main attractions of ATM. Hence, ATM gained popularity in both LANs and WANs. In the future, ATM is likely to be used extensively in backbone networks.

References

R. Horak. Communications Systems and Networks, Wiley Dreamtech India Pvt. Ltd., 2003. Chapter 19 covers the details of broadband services; ATM is also covered.
http://www.atmforum.com Web site of ATM Forum. An excellent resource for ATM technology.
http://www.iec.org/online/tutorials You can get online tutorials on ATM from this link.

Questions

What are the advantages of a small packet size?
Explain the two types of virtual connections in an ATM network.
Explain the ATM cell structure.
Explain the functionality of different layers in the ATM protocol architecture.
Explain the different classes of services in the ATM adaptation layer.

Exercises

1.	Study the details of the ATM adaptation layer for supporting real-time voice communication and video conferencing.
2.	Study the ATM products of commercial vendors such as Cisco and Nortel.
3.	To support voice services with 64kbps PCM over ATM, which AAL protocol is used?
4.	In a network, both variable rate coding and fixed rate coding need to be used for video transmission. Which AAL protocols need to be used to support both these services?
5.	Calculate the overhead in the ATM cells.

Answers

1.	Type 1 ATM adaptation layer is used for supporting real-time voice communication and video conferencing.
2.	You can obtain the details at http://www.cisco.com and http://www.nortelnetowrks.com.
3.	To support voice services with 64kbps PCM over ATM, Type 1 AAL protocol is used.
4.	Type 2 AAL is used for variable rate video transmission. For fixed rate transmission, Type 1 AAL is used.
5.	The ATM cell has 53 bytes, out of which 5 bytes contain the header information. Hence, the overhead is 5/53 or about 10%.

Projects

Simulate the ATM cell. Write a program in C language that creates an ATM cell with the header for user-network interface.
Prepare a technical report on using ATM as the backbone for IP-based networks.

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