ATM TECHNICAL FUNDAMENTALS

An ATM broadband network enables dependable delivery of multimedia traffic in wireline and/or wireless environments. Prior to ATM implementation, individual networks carried data, voice, and video traffic separately on individual channels or circuits.


ATM CELL

ATM networks employ a standard, fixed-size 53-byte cell comprised of a 5-byte header and a 48-byte payload or information field as the basic unit of transmission. The 5-byte header includes an error detection field and a Virtual Channel Identifier (VCI) or Virtual Path Indicator (VPI) for transporting a cell payload to a destination address.

Through utilization of a common cell format, ATM enables real-time services, public and private network interconnectivity, and global interoperability. ISDN employs STDM (Statistical Time-Division Multiplexing) for enabling transmission of frames via designated timeslots at specified intervals. In contrast to ISDN installations, the ATM protocol supports dynamic allocation of timeslots to cells on-demand for optimizing traffic throughput in high-performance network configurations.

ATM technology employs a priority switching technique for enabling ATM cells carrying delay-sensitive signals to access the first available timeslot. Because the ATM cell size is fixed and the buffer memory size is constant for each cell, switch queuing delays are predictable and jitter or the variation in signal delay is minimized. By contrast, signal delays degrade performance of real-time applications such as videoconferencing and interactive video-on-demand (IVOD) in networks such as Frame Relay (FR) and Ethernet that transport variable length packets.

The ATM Forum defines procedures for monitoring the effectiveness of network transmission based on cellular throughput. Cell Loss Ratio (CLR) describes the percentage of cells that are not transported to their destination addresses as a consequence of buffer overloads and network congestion. Cell Transfer Delay (CTD) refers to propagation and queuing delays experienced by cells transiting the network.
Cell Delay Variation (CDV) measures variations in transmission delay between adjacent cells. Minimum Cell Rate (MCR) refers to the lowest cell rate supported by ABR (Available Bit Rate) service. In addition, metrics for Cell Delay Variation Tolerance (CDVT) and parameters for Maximum Cell Transfer Delay (MCTD) are also defined. The effectiveness of QoS delivery in ATM networks depends on such variables as Cell Transfer Delay (CDT) and Cell Delay Variation (CDV).


ATM APPLICATIONS

ATM is a connection-oriented virtual network transmission and switching technology that combines the low-delay of circuit-switched networks with the bandwidth flexibility and high-speed of packet-switched networks. ATM is an enabler of basic and advanced applications such as remote sensing, 3-D (three-dimensional) interactive simulations, tele-instruction, biological teleresearch, and medical teleconsultations. Edge devices at the boundary of an ATM network convert non-ATM traffic streams into standard ATM cells.
ATM technology is implemented in backbone, enterprise, and edge switches as well as hubs, routers, bridges, multiplexers, servers, server farms, and NICs (Network Interface Cards) in high-end Internet appliances. The ATM Data Exchange Interface (DXI) enables fast access to public network services. A flexible and extendible networking solution, ATM technology supports network configurations that include DANs (Desk Area Networks), LANs, MANs (Metropolitan Area Networks), WANs (Wide Area Networks), and GANs (Global Area Networks).


ATM TRANSMISSION RATES

ATM technology enables wireline transmissions via optical fiber, twisted copper pair, and hybrid optical fiber and coaxial cable media. To support bandwidth-intensive operations, the ATM platform multiplexes and relays a diverse mix of network traffic via optical fiber at rates that include 155.52 Mbps (OC-3 or Optical Carrier-Level 3) and 622.08 Mbps (OC-12). ATM also sustains speeds at 2.488 Gbps (OC-48), 10 Gbps (OC-192), and 13.21 Gbps (OC-255). Optical Carrier (OC) levels are sets of signal rates that describe digital transmission speeds over a fiber optic plant. In the United States, these levels are based on multiples of the base rate of 51.84 Mbps (OC-1).

In addition to optical fiber, ATM supports landline information transport via hybrid optical fiber and coaxial cable (HFC) connections, and ordinary twisted copper pair found in public networks such as the PSTN (Public Switched Telephone Network) at lower speeds. For example, ATM enables transmission at 1.544 Mbps or T-1 and DS- 1 (Digital Signal-1) in terms of the North American digital hierarchy. In the European Union, ATM supports information transport at 2.048 Mbps or E-1 (European-1) and DS-1 (Digital Signal-1) in accordance with European Union digital specifications. ATM also supports rates at 44.746 Mbps or T-3 and DS-3 in North America, and 34.368 Mbps or E-3 and DS-3 in the European Union. By enabling transport of concurrent voice, still images, video, and data traffic in local, municipal, and wider area configurations, ATM technology promotes development of an integrated and scalable multiservice network infrastructure that optimizes resource sharing and user productivity.


ATM PROTOCOL STACK

ATM services are based on protocol layer operations.The Physical Layer in the ATM protocol stack consists of the Physical Medium Sublayer and the Transmission Convergence Sublayer. These sublayers enable the use of diverse physical media, interfaces, and transmission speeds. In addition, these sublayers transform signals into electronic or optical formats, map and encapsulate IP packets into cells, and provision multiplexing services for transmitting cells over the same physical link. In addition, the Physical Layer defines the process for routing and switching cells in accordance with Virtual Path Identifiers (VPIs) or Virtual Channel Identifiers (VCIs). The ATM Physical Layer corresponds to Layer 1 or the Physical Layer of the Open Systems Interconnection (OSI) Reference Model.

Situated directly above the Physical Layer, the ATM Layer employs the 53-byte cell as the basic transmission unit. The ATM Layer operates independently of the ATM Physical Layer. At the ATM Layer, ATM switches route cellular streams received from the ATM Adaptation Layer (AAL) to destination addresses in accordance with the Virtual Channel Identifier (VCI) or Virtual Path Indicator (VPI) contained in each cell header.

Situated directly above the ATM Layer, the ATM Adaptation Layer (AAL) facilitates cell segmentation and reassembly by dividing media streams received from upper protocol layers into 53-byte ATM cells for transmission over the physical link. For reassembly, the AAL reconstitutes original media streams from cells that were received via the Physical Layer. The reconstructed media streams are then ready for transport by upper protocol layers to destination addresses. The ATM Adaptation Layer (AAL) consists of five sublayers, ranging from AAL1 to AAL5. Each sublayer has specific and overlapping responsibilities for enabling dependable and robust transmissions. AAL1 supports cell segmentation and cell reassembly and the transmission of data, video, and high-quality audio signals for enabling real-time applications. AAL2 enables transport of connectionoriented VBR (Variable Bit Rate) packetized video-over-ATM and voice-over-ATM. Defined in the ITU-T I.363 Recommendation, AAL Sublayers 3/4 (3 and 4) handle connectionless and connection-oriented VBR transmissions. As with AAL1, AAL5 supports cell segmentation and cell reassembly operations.

The ATM Forum describes the features and functions of the ATM protocol stack and clarifies procedures for interworking ATM with technologies that include DSL (Digital Subscriber Line), SONET/SDH (Synchronous Optical Network and Synchronous Digital Hierarchy), WDM (Wavelength Division Multiplexing), and DWDM (Dense WDM). In addition, approaches for enabling ATM to interwork with SMDS, Frame Relay, cable modem, MMDS (Multichannel Multipoint Distribution System), and LMDS (Local Multipoint Distribution System) implementations are also clarified. The ATM Forum works with other standards organizations such as the Digital Video Broadcasting/Digital AudioVisual Council (DVB and DAVIC) and the Full Service Access Network (FSAN) Consortium in developing specifications for ATM television broadcasts and APONs (ATM Passive Optical Networks). APONs interwork with FTTC (Fiber-to-the-Curb) and FTTH (Fiber-to-the-Home) broadband residential access networks.