ATM OPERATIONS

SVCs (SWITCHED VIRTUAL CIRCUITS) AND PVCS (PERMANENT VIRTUAL CIRCUITS)

ATM networks sustain point-to-point links for direct connectivity, point-to-multipoint connections for broadcast and multicast services, and multipoint-to-multipoint connections for applications such as interactive videoconferencing and telecollaborative teleresearch. Path specifications for moving traffic across ATM networks are termed Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs).

SVCs are created virtually on a semi-permanent basis for enabling multimedia transmission. SVCs establish connections on a call-by-call basis for accommodating bursty transmissions and bandwidth on-demand. UBR (Unspecified Bit Rate) service for SVC (Switched Virtual Circuit) connections supports information delivery on a best-effort basis. SVC connections do not guarantee the availability of bandwidth for enabling QoS (Quality of Service) transmissions.

In comparison to SVCs, PVCs are static virtual connections between network endpoints that support always-available and assured bandwidth allocations for current and emergent network applications and services. As a consequence, PVCs enable stable, dependable, and reliable transmission of voice, video, and data traffic with QoS guarantees.


ATM SWITCHES

ATM networks consist of routers, servers, switches, and endpoint devices such as network nodes and stations. The ATM switch family includes workgroup, campus, enterprisewide, and next-generation switches that provide services in a variety of LAN, MAN, and WAN environments. For example, ATM switches enable LATMs (Local Area ATM Networks) to provision services to legacy workstations and support sophisticated network backbone operations for advanced academic and research networks.

ATM multiservice switches provide the underlying physical infrastructure for the network configuration and control network processing speed. These devices uniformly facilitate cell relay operations, sustain throughput and end-to-end network performance, interlink nodes on ATM networks, and route multiple cells concurrently to destination addresses. It is important to note that ATM switches also support diverse applications, services, and operations, and vary in structure, capacity, value-added capabilities, interoperability support, and traffic management functions in order to accommodate a wide range of E-government (electronic government), E-business (electronic businesses), telemedicine, teleresearch, and/or tele-education requirements.


User-to-Network Interfaces (UNIs), Network-to-Node and Network-to-Network Interfaces (NNIs), and Private Network-to-Node or Network-to-Network Interfaces (PNNIs)

ATM installations consist of a set of ATM switches or internetworking devices that are interconnected by point-to-point ATM interfaces. ATM interfaces or virtual connections include User-to-Network Interfaces (UNIs) and NNIs (Network-to- Node Interfaces or Network-to-Network Interfaces). UNIs are ATM protocols that define standard interfaces between customer premise equipment (CPE) and the network switch. For example, FUNI (Frame UNI) clarifies parameters for integrating legacy devices with ATM switching equipment in mixed-mode Frame Relay and ATM network configurations.

PNNIs (Private Network-to-Node or Private Network-to-Network Interfaces) are NNI protocols that define ATM interfaces within and between private networks. PNNIs determine approaches for routing ATM connection-oriented requests across an ATM network or between ATM networks.

Moreover, PNNIs employ signaling technologies to support SVCs and PVCs in multivendor environments, provision QoS guarantees, and foster distribution of reserved bandwidth. PNNIs also establish the format for the Broadband-Intercarrier Interface (B-ICI) between public networks for enabling seamless multicarrier multivendor multiservice ATM implementations.


ATM CLASS OF SERVICE (COS) AND QUALITY OF SERVICE (QOS)

ATM networks employ Classes of Service (CoS) for optimizing network performance and supporting applications with specified bandwidth or throughput requirements. ATM service classes resolve congestion problems and traffic management issues in order to ensure seamless transmission in multivendor environments. A Class of Service (CoS) refers to a category of ATM connections that features identical traffic patterns and resource requirements. Each class provisions a distinct level of service and associated QoS guarantees. Depending upon the format of the QoS service requested, the ATM network defines a series of CoS categories. The Variable Bit Rate (VBR) Class of Service consists of applications with specific requirements for delays and throughputs such as packetized voice and data applications. The real-time Variable Bit Rate (VBR-rt) Class of Service requires real-time support for provisioning applications such as video-on-demand (VOD) and voice-over-IP (VoIP). VBR-rt bandwidth requirements vary over time. However, delay and delay variance limits are clearly established.

The non-real-time variable bit rate (VBR-nrt) Class of Service eliminates the need for guaranteed delivery of applications such as multimedia e-mail, bulk file transmissions, and business and educational database transactions with minimal service requirements. Bandwidth for VBR-nrt applications varies within a specified range. However, delay and delay variance requirements are not fully defined. The Available Bit Rate (ABR) Class of Service requires the use of flow control mechanisms for ensuring allocation of bandwidth on-demand for non-real-time, mission-critical applications. With ABR applications, guaranteed minimum transmission rates are specified for the duration of the connection. In addition, ABR also establishes peak transmission rates for data bursts when bandwidth is available. As a consequence, the ABR service class tolerates delay variations. Applications grouped into this category allow priority traffic to consume bandwidth first. ABR applications include LAN emulation (LANE), file and data distribution, and LAN interconnections.

The Unspecified Bit Rate (UBR) Class of Service is equivalent to best-effort delivery in IP networks. Delay-tolerant UBR applications include Web browsing and IP transmissions. Because UBR applications require minimal network support, QoS guarantees and pre-established throughput levels are not defined. The Constant Bit Rate (CBR) Class of Service (CoS) requires utilization of a virtual channel with constant bandwidth for seamlessly transporting applications in accordance with pre-defined response time requirements. CBR applications include videoconferencing, telephony services, and television broadcasts.

In conjunction with establishing a CoS, ATM networks define cell rates and burst size to facilitate seamless network performance. For example, Peak Cell Rate (PCR) indicates the maximum rate at which cells transit the network for brief time periods. Sustainable Cell Rate (SCR) refers to the cell rate that is sustained for a specified period of time. Maximum Burst Size (MBS) defines the maximum number of back-to-back cells that transit the network.