The following service categories have been defined by Asynchronous transfer mode:

1) Real-time service:

• Constant bit rate (CBR)
• Real-time variable bit rate (rt-VBR)

2) Non-real-time service:

• Non-real-time variable bit rate (nrt-VBR)

• Available bit rate (ABR)

• Unspecified bit rate (UBR)

• Guaranteed frame rate (GFR)

1) Real-time service: Real-time applications typically involve a flow of information to a user that is intended to reproduce that flow at a source. For example, a user expects a flow of audio or video information to be presented in a continuous, smooth fashion. A lack of continuity or excessive loss results in significant loss of quality. Applications that involve interaction between people have tight constraints on delay. Typically, any delay above a few hundred milliseconds becomes noticeable and annoying. Accordingly, the demands in the ATM network for switching and delivery of real-time data are high.

• Constant bit rate (CBR): Constant bit rate (CBR) service is perhaps the simplest service to define. It is used by applications that require a fixed data rate that is continuously available during the connection lifetime and a relatively tight upper bound on transfer delay. CBR is commonly used for uncompressed audio and video information.

Examples of CBR applications include:

• Videoconferencing.

• Interactive audio (e.g., telephony).

• Audio/video distribution (e.g., television, distance learning, pay-per-view).

• Audio/video retrieval (e.g., video-on-demand, audio library).

• Real-time variable bit rate (rt-VBR): Real-time variable bit rate (rt-VBR) category is intended for time-sensitive applications; that is, those requiring tightly constrained delay and delay variation.

The principal difference between applications appropriate for rt-VBR and those appropriate for CBR is that rt-VBR applications transmit at a rate that varies with time. Equivalently, an rt-VBR source can be characterized as somewhat bursty. For example, the standard approach to video compression results in a sequence of image frames of varying sizes. Because real-time video requires a uniform frame transmis- sion rate, the actual data rate varies.

The rt-VBR service allows the network more flexibility than CBR. The net- work is able to statistically multiplex a number of connections over the same dedi- cated capacity and still provide the required service to each connection.

2) Non-Real-Time Services: Non-real-time services are intended for applications that have bursty traffic characteristics and do not have tight constraints on delay and delay variation. Accordingly, the network has greater flexibility in handling such traffic flows and can make greater use ofstatistical multiplexing to increase network efficiency.

• Non-real-time variable bit rate (nrt-VBR): For some non-real-time applications, it is possible to characterize the expected traffic flow so that the network can provide substantially improved quality of service (OoS) in the areas of loss and delay. Such applications can use the non-real-time vari- able bit rate (nrt-VBR) service.

With this service, the end system specifies a peak cell rate, a sustainable or average cell rate, and a measure of how bursty or clumped the cells may be.With this information, the network can allocate resources to provide rel- atively low delay and minimal cell loss.

The nrt-VBR service can be used for data transfers that have critical response- time requirements. Examples include airline reservations, banking transactions, and process monitoring.

• Available bit rate (ABR): To improve the service provided to bursty sources that would otherwise use UBR, the available bit rate (ABR) service has been defined. An application using ABR specifies a peak cell rate (PCR) that it will use and a minimum cell rate (MCR) that it requires. The network allocates resources so that all ABR applica- tions receive at least their MCR capacity. Any unused capacity is then shared in a fair and controlled fashion among all ABR sources.

The ABR mechanism uses explicit feedback to sources to assure that capacity is fairly allocated. Any capacity not used by ABR sources remains available for UBR traffic.

An example of an application using ABR is LAN interconnection. In this case, the end systems attached to the ATM network are routers.

• Unspecified bit rate (UBR): Unspecified Bit Rate (UBR) service is suitable for applications that can tolerate variable delays and some cell losses, which is typically true of TCP-based traffic. With UBR, cells are forwarded on a first-in-first-out (FIFO) basis using the capacity not consumed by other services; both delays and variable losses are possible. No initial commitment is made to a UBR source and no feedback concerning congestion is provided; this is referred to as a best-effort service.

Examples of UBR applications include:

• Text/data/image transfer, messaging, distribution, retrieval.

• Remote terminal (e.g., telecommuting).

• Guaranteed frame rate (GFR): The most recent addition to the set of ATM service categories is Guaranteed Frame Rate (GFR), which is designed specifically to support IP backbone subnetworks. GFR provides better service than UBR for frame-based traffic, including IP and Ethernet.

A major goal of GFR is to optimize the handling of frame-based traffic that passes from a LAN through a router onto an ATM backbone network. Such ATM networks are increasingly being used in large enterprise, carrier, and Internet service provider networks to consolidate and extend IP services over the wide area. While ABR is also an ATM service meant to provide a greater measure of guaranteed packet performance over ATM backbones, ABR is relatively difficult to implement between routers over an ATM network. With the increased emphasis on using ATM to support IP-based traffic, especially traffic that originates on Ethernet LANs, GFR may offer the most attractive alternative for providing ATM service.