Asynchronous Transfer Mode (ATM)

1. Asynchronous Transfer Mode (ATM) • 1

2. Introduction • A high-speed, connection oriented switching and multiplexing technology. • A transfer mode in which the information (voice, video and data) is organized into cells. • It is asynchronous in the sense that the information streams can be sent independently without a common clock. • Asynchronous transfer mode (= Statistical multiplexing) – Sources packetize data. Packets are sent only if there is data 2

3. Features • Enables high-bandwidth Distributed Applications (155Mbps, 622 Mbps) • Provides high transmission speeds for both LAN and WAN. • Supports both circuit switching and packet switching (cell switching). • Virtual Networking (share available bandwidth among multiple applications). • Support for multicasting and broadcasting. • Enables supply of bandwidth on demand and billing as per cell basis. • Scalable. 3

4. ATM’s Key Concepts • Hybrid form of switching technology. • ATM requires connections to be established before any communication takes place. • A connection is called a virtual circuit which could be permanent or switched. • Streamlined: minimal error and flow control.

5. Architecture of an ATM Network

6. Logical Connections • VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25 • VPC (Virtual Path Connection): a bundle of VCCs with same endpoints 6

7. Figure 5.2 7

8. TP, VPs, and VCs

9. Example of VPs and VCs

10. Connection Identifiers

11. Virtual Connection Identifiers in UNIs and NNIs

12. Advantages of Virtual Paths • Simplified network architecture • Increased network performance and reliability • Reduced processing and short connection setup time • Enhanced network services 12

13. ATM Layers

14. ATM Layers in End-Point Devices and Switches

15. Functions of the Layers

16. ATM Layers and Planes • ATM is a three dimensional model consisting of layers and planes. • Physical layer deals with physical media issues, further divided into -PMD and TC sub layers • ATM layer defines cell format and connection setup. • ATM Adaptation Layer segments and reassembles upper layer into and from cells, divided into -SAR and CS sub layers. • User planes deals with data transport, flow and error control. • Control plane deals with connection management. • The Layer and Plane management deals with resource management and interlayer coordination.

17. ATM Layer • Defines the cell format and connection setup.

18. Cell Switching • ATM packets are small and have a fixed size – Packets in ATM are called cells – Small packets are good for voice and video transmissions

19. ATM Cells • Why 53 Bytes? A 48 byte payload was the result of a compromise between a 32 byte payload and a 64 byte payload • Advantages – Low packetization delay for continuous bit rate applications (video, audio) – Processing at switches is easier • Disadvantages – High overhead (5 Bytes per 48)

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21. ATM Adaptation Layer (AAL) • Support non-ATM protocols • e.g., PCM voice, LAPF • AAL Services • Handle transmission errors • Segmentation/reassembly (SAR) • Handle lost and misinserted cell conditions • Flow control and timing control 21

22. AAL Protocols • AAL layer has 2 sublayers: • Convergence Sublayer (CS) • Supports specific applications using AAL • service dependant • Segmentation and Reassembly Layer (SAR) • Packages data from CS into cells and unpacks at other end 22

23. AAL Types

24. ATM Network Architecture • ATM Adaptation Layer (AAL) - Convergence sublayer • Different service classes are accessed via AAL-SAP • Associated with Convergence sublayer is SAR protocol • AAL types • 4 different AAL protocols or types • Originally correspond with 4 classes

26. ATM Network Architecture • ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer • AAL - Type 1 • Carries Class A Services • e.g. CBR packetised voice (traditional circuit switched) • 48 octet payload • 47 speech samples (samples acquired every 125 micro secs) • 1 octet holds sequence # and seq # check

27. ATM Network Architecture ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer • AAL - Type 2 • Carries Class B Services • Not fully developed yet, but some characteristics are:- • Transfer of Service Data Units with variable source bit rate • Transfer of timing information

28. ATM Network Architecture ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer • AAL - Type 3/4 • Carries Class C&D Services • Can operate in message or stream mode • Message mode entire message carried in one CS-PDU • Stream mode involves many CS-PDUs.

29. ATM Network Architecture • ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer • AAL - Type 5 (Simple and Efficient Adaptation Layer). • Carries Class C & D Services • Designed for same class of traffic as AAL3/4 • Simpler, less overhead • SAR-PDU for instance has no header / trailer • Essentially CS-PDU is segmented and segments mapped to ATM payload

30. Problems and Challenges • Interoperability • LAN Emulation over ATM • IP over ATM • Bandwidth management • Peak rate allocation method • Minimum throughput allocation method • Bursty traffic specification method • Latency/Bandwidth Tradeoff