ATM Service Categories

ATM Service Categories

AnATM network is designed to be able to transfer many different types of traffic simultaneously,
including real-time flows such as voice, video, and burstyTCP flows. Although
each such traffic flow is handled as a stream of 53-octet cells traveling through a virtual
channel, the way in which each data flow is handled within the network depends on the
characteristics of the traffic flow and the QoS requirements of the application. For
example, real-time video traffic must be delivered within minimum variation in delay.
In this subsection, we summarize ATM service categories, which are used by
an end system to identify the type of service required. The following service categories
have been defined by the ATM Forum:
• Real-time service
- Constant bit rate (CBR)
- Real-time variable bit rate (rt-VBR)
• Non-real-time service
-Non-real-time variable bit rate (nrt-VBR)
-Available bit rate (ABR)
-Unspecified bit rate (UBR)
-Guaranteed frame rate (GFR)
ReaI-Time Services The most important distinction among applications concerns
the amount of delay and the variability of delay, referred to as jitter, that the
application can tolerate. 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.
The 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)
The 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 transmission
rate, the actual data rate varies.
The rt-VBR service allows the network more flexibility than CBR. The network
is able to statistically multiplex a number of connections over the same dedicated
capacity and still provide the required service to each connection.


Non-Real-Tin1.e 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 of statistical multiplexing to increase network efficiency.
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 variable
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 relatively
low delay and minimal cell loss.
The nrt-VBR service can be used for data transfers that have critical responsetime
requirements. Examples include airline reservations, banking transactions, and
process monitoring.
At any given time, a certain amount of the capacity of anATM network is consumed
in carrying CBR and the two types of VBR traffic. Additional capacity is
available for one or both of the following reasons: (1) Not all of the total resources
have been committed to CBR and VBR traffic, and (2) the bursty nature of VBR

traffic means that at some times less than the committed capacity is being used. All
of this unused capacity could be made available for the unspecified bit rate (UBR)
service. This 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)
Bursty applications that use a reliable end-to-end protocol such as TCP can
detect congestion in a network by means of increased round-trip delays and packet
discarding. However, TCP has no mechanism for causing the resources within the
network to be shared fairly among many TCP connections. Further, TCP does not
minimize congestion as efficiently as is possible using explicit information from congested
nodes within the network.
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 applications
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.
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.