2. Asynchronous Transfer Mode • ATM is the cell relay protocol designed by ATM Forum and adopted by ITU-T. • The combination of ATM & B-ISDN will allow high-speed interconnection of all the world’s networks. • In fact, ATM can be thought of as the “highway” of the information superhighway. • ATM operates at a speed of 155 & 622 Mbps & is a connection oriented network
3. Design Goals• Need for a transmission system to optimize the use of high- data-rate transmission media, in particular optical fiber.• A system that can interface with the existing systems, such as various packet networks, & to provide wide area interconnectivity between them without lowering their effectiveness or requiring their replacement.• design that can be implemented inexpensively• able to work with & support the existing telecommunications hierarchies• must be connection oriented to ensure accurate & predictable delivery.• to move as many of the functions to hardware as possible and eliminate as many software functions as possible
4. Packet Networks • Data communications today are based on packet switching & packet networks. A packet is a combination of data & overhead bits that can be passed through the network as a self contained unit. The overhead bits, in the form of a header & trailer, act as an envelope that provides identification & addressing information as well as the data required for routing, flow control, error control, …
5. ATM LANs Issues to be resolved • Connectionless Vs Connection-oriented • Physical Addresses Vs Virtual Connection Identifier • Multicasting & Broadcasting delivery • varying size & intricacy • To improve utilization
6. Cell Networks• Many of the packet internetworking problems are solved by adopting a concept called cell networking.• A cell is a small data unit of fixed size. In a cell network, all data are loaded into identical cells that can be transmitted with complete predictability & uniformity.• As packets of different sizes & formats reach the cell network from a tributory network, they are split into multiple small data units of equal length & loaded into cells.• Because of the same size, the problems associated with multiplexing different sized packets are avoided.
7. Advantages of Cells• High speed of the link coupled with the small size of the cells• A cell network can handle real time data• To a cell network, the smallest unit is a cell, not a bit.• Switching & multiplexing can be implemented in hardware rather than software
8. Asynchronous TDM • ATM uses asynchronous TDM (that’s why called as ATM) to multiplex cells coming from different channels. It uses fixed size slots. • ATM mux fill a slot with a cell from any input channel that has a cell, the slot is empty if none of the channels has a cell to send. • At the first tick of the clock, the multiplexer fills the slot with a cell from each channel. When all the cells from all the channels are multiplexed. The output slots are empty.
9. ATM Switching:Contentionless Time Division • Advantages • Non-blocking • Deterministic performance—probability of cell loss = 0 • Flexible port speeds – (DS-1, E-1, DS-3, E-3, 100M, 155M, 622M) • Hardware multicast without increasing fabric cell traffic • Low transit delays • Disadvantages • Limited scalability on single TDM fabric – Use time-space-time to expand
10. ATM Architecture ATM is a cell switched network. The user access devices, called the end points, are connected through a user-to-network interface (UNI) to the switches inside the network. The switches are connected through network-to-network interfaces.
13. ATM Layers in end-pointdevices & switchesThe end points use all three layers while theswitches use only the two bottom layers.
14. ATM Process Conventional Conventional ATM Telecom LAN Traffic Type Data, Voice, Video Voice Data Transmission Fixed Fixed Variable Unit Cell Frame Packet Switching Cell Circuit PacketConnection Type Connection-oriented Connection-oriented Connectionless Delivery Defined Classes Guaranteed Best Effort Access Dedicated Dedicated Shared Rate & Media Application Dependent Channel Dependent Protocol Dependent
15. ATM Process Conventional Conventional ATM Telecom LAN Traffic Type Data, Voice, Video Voice Data Transmission Fixed Fixed Variable Unit Cell Frame Packet Switching Cell Circuit PacketConnection Type Connection-oriented Connection-oriented Connectionless Delivery Defined Classes Guaranteed Best Effort Access Dedicated Dedicated Shared Rate & Media Application Dependent Channel Dependent Protocol Dependent
16. ATM is packet switching!• Switched or permanent connections• Traffic type independent (voice, data, interactive video)• Fixed length packet - 53 bytes (cell) header payload Fixed length packet = cell
17. Anatomy of an ATM Cell 8 7 6 5 4 3 2 1Byte 1 GFC (UNI) or VPI (NNI) VPIByte 2 VPI VCIByte 3 VCI HeaderByte 4 VCI PTI CLPByte 5 HEC 48 Bytes Payload VPI: Virtual Path Identifier CLP: Cell Loss Priority VCI: Virtual Channel Identifier HEC: Header Error Check PTI: Payload Type Indicator GFC: Generic Flow Control