Asynchronous Transfer Mode
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ATM is a cell relay protocol designed to optimize fiber optic networks. It breaks data into fixed-size cells for uniform transmission. ATM aims to maximize bandwidth, interface existing systems, be inexpensive, support telecom hierarchies, ensure reliable delivery, and minimize software functions. Connections between endpoints are established through virtual paths and circuits identified by header fields. Cells contain a 5-byte header and 48-byte payload. Connections can be permanent or switched. ATM defines layers for applications, cell processing, and physical transmission. It supports various quality of service levels through parameters like cell error and loss rates.
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Asynchronous Transfer Mode
- 1. Unit-I ATM (ASYNCHRONOUS TRANSFER MODE)
- 2. ATM (Asynchronous Transfer Mode) Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T. A cell is a small data unit of fixed size. In a cell network, which uses the cell as the basic unit of data exchange, all data are loaded into identical cells that can be transmitted with complete predictability and uniformity.
- 3. FRAME RELAY NETWORK CELL RELAY NETWORK Why Cell Relay Network ?
- 4. Design Goals Among the challenges faced by the designers of ATM, six stand out. • Foremost is the need for a transmission system to optimize the use of high-data-rate transmission media, in particular optical fiber. In addition to offering large bandwidths, newer transmission media and equipment are dramatically less susceptible to noise degradation. A technology is needed to take advantage of both factors and thereby maximize data rates. •The system must interface with existing systems and provide wide-area interconnectivity between them without lowering their effectiveness or requiring their replacement. • The design must be implemented inexpensively so that cost would not be a barrier to adoption. If ATM is to become the backbone of international communications, as intended, it must be available at low cost to every user who wants it. • The new system must be able to work with and support the existing telecommunications hierarchies (local loops, local providers, long-distance carriers, and so on). • The new system must be connection-oriented to ensure accurate and predictable delivery. • Last but not least, one objective is to move as many of the functions to hardware as possible (for speed) and eliminate as many software functions as possible (again for speed).
- 5. ARCHITECTURE OF ATM NETWORK The user access devices, called the endpoints, are connected through a user-to-network interface (UNI) to the switches inside the network. The switches are connected through network-to-network interfaces (NNIs).
- 6. How it connects? Virtual Connection Connection between two endpoints is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs). A transmission path (TP) is the physical connection (wire, cable, satellite, and so on) between an endpoint and a switch or between two switches. A transmission path is the set of all highways that directly connect the two cities. Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual circuit and remain in their original order until they reach their destination. Think of a virtual circuit as the lanes of a highway
- 7. The lengths of the VPIs for UNIs and NNIs are different. In a UNI, the VPI is 8 bits, whereas in an NNI, the VPI is 12 bits. The length of the VCI is the same in both interfaces (16 bits). We therefore can say that a virtual connection is identified by 24 bits in a UNI and by 28 bits in an NNI
- 9. Cell The basic data unit in an ATM network is called a cell. A cell is only 53 bytes long with 5 bytes allocated to the header and 48 bytes carrying the payload (user data may be less
- 10. Connection Establishment and Release ATM has two types of connections: PVC and SVC PVC: PVC A permanent virtual-circuit connection is established between two endpoints by the network provider. The VPls and VCIs are defined for the permanent connections, and the values are entered for the tables of each switch. SVC: SVC In a switched virtual-circuit connection, each time an endpoint wants to make a connection with another endpoint, a new virtual circuit must be established. ATM cannot do the job by itself, but needs the network layer addresses and the services of another protocol (such as IP).
- 11. ATM Layers The ATM standard defines three layers. They are, from top to bottom, the application adaptation layer, the ATM layer, and the physical layer.
- 12. ATM Layers in Endpoint Devices
- 13. ATM Layer ATM Layer provides • Routing • Traffic Management • Switching • Multiplexing Services It processes outgoing traffic by accepting 48-byte segments from the AAL sublayers and transforming them into 53-byte cells by the addition of a 5-byte header.
- 14. ATM Header Format ATM uses two header format.
- 15. o Generic flow control (GFC). The 4-bit GFC field provides flow control at the UNI level. The ITU-T has determined that this level of flow control is not necessary at the NNI level. In the NNI header, therefore, these bits are added to the VPI. The longer VPI allows more virtual paths to be defined at the NNI level. The format for this additional VPI has not yet been determined. o Virtual path identifier (VPI). The VPI is an 8-bit field in a UNI cell and a 12-bit field in an NNI cell. o Virtual circuit identifier (VCI). The VCI is a 16-bit field in both frames. o Payload type (PT). In the 3-bit PT field, the first bit defines the payload as user data or managerial information. The interpretation of the last 2 bits depends on the first bit. o Cell loss priority (CLP). The I-bit CLP field is provided for congestion control. A cell with its CLP bit set to I must be retained as long as there are cells with a CLP of O. We discuss congestion control and quality of service in an ATM network in Chapter 24. o Header error correction (HEC). The HEC is a code computed for the first 4 bytes of the header. It is a CRC with the divisor x8 + x2 + x + 1 that is used to correct single-bit errors and a large class of multiple-bit errors.
- 16. Application Adaption Layer AAL1: AAL1 supports applications that transfer information at constant bit rates, such as video and voice. It allows ATM to connect existing digital telephone networks such as voice channels and T lines. AAL2: It is used for low-bit-rate traffic and short-frame traffic such as audio (compressed or uncompressed), video, or fax. A good example ofAAL2 use is in mobile telephony. AAL Type 3/4: supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25. AAL Type 5: is similar to AAL 3/4 with a simplified information header scheme. This AAL assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services that use AAL 5 are classic IP over ATM, Ethernet Over ATM, SMDS, and LAN Emulation (LANE). AAL 5 is a widely used ATM adaptation layer protocol. This protocol was intended to provide a streamlined transport facility for higher-layer protocols that are connection oriented.
- 17. QOS ATM A number of parameters are used to define the QOS. These include the 1. Cell Error Ratio, 2. Cell Loss Ratio, 3. Cell Misinsertion Rate, 4. Cell Transfer Delay, 5. Cell Delay Variation
- 18. …. A number of classes of QOS are supported by the network and fall into either a Specified QOS or an Unspecified QOS class. Among all the classes of QOS the network must only support the Specified QOS class 1 which is the circuit emulation service and constant bit rate video. A specified QOS may have two cell loss objectives, for the high and low priority traffic. The unspecified QOS has no specified objectives given to the user but may have internal parameters for the network but these may change during a call. Even though there are no QOS the call may have specified traffic parameters, in fact that would be desirable for the network. This type of traffic could be the so called best-effort traffic. This allows the network to respond to time variable resources. The unspecified QOS is optional for the network to support. Degradation of QOS may arise for many different reasons and one of these is the ATM switch. The buffer capacity could be a complex multiple queue system with an algorithmicaly defined service rule that could be based on priorities. The switch may thus introduce loss under heavy load. Of particular interest to JPL is that the buffering strategies for wide area low speed networks may be more complex than for high speed local area networks due to the detection of the PDU by a higher level. For compliant connections the QOS will be supported for at least the number of conforming cells as specified in the conformance definition. For non-compliant connections the network does not need to support and QOS. The QOS of a VPC will be the combination of the strictest set of QOS's of the underlying VCC's.
- 19. THANKS