SUBJECT D16-1) Question:What is the meaning of CBR, VBR, ABR, UBR?

Answer: They are service classes defined by ATM forum traffic management group. Each class is defined as follows:

1. CBR (constant bit rate)
   The CBR service classs is intended for real-time applications, i.e. those requring tightly constrained delay and delay variation, as would be appropriate for voice and video applications. The consistent availability of a fixed quantity of bandwidth is considered appropriate for CBR service. Cells which are delayed beyond the value specified by CTD(cell transfer delay) are assumed to be significantly less value to the application.

   For CBR, the following ATM attributes are specified:

   PCR/CDVT(peak cell rate/cell delay variation tolerance)
   Cell Loss Rate
   CTD/CDV
   CLR may be unspecified for CLP=1.

   Real time VBR
   The real time VBR service class is intended for real-time applications,i.e., those requring tightly constrained delay and delay variation, as would be appropriate for voice and video applications. Sources are expected to transmit at a rate which varies with time. Equivalently the source can be described "bursty". Cells which are delayed beyond the value specified by CTD are assumed to be of significantly less value to the application. Real-time VBR service may support statistical multiplexing of real-time sources, or may provide a consistently guaranteed QoS.

   For real time VBR, the following ATM attributes are specified:

   PCR/CDVT

   CLR

   CTD/CDV

   SCR and BT(sustainable cell rate and burst tolerance)

   Non-real time VBR
   The non-real time VBR service class is intended for non-real time applications which have 'bursty' traffic characteristics and which can be characterized in terms of a GCRA. For those cells which are transfered, it expects a bound on the cell transfer delay. Non-real time VBR service supports statistical multiplexing of connections.

   For non-real time VBR, the following attributes are supported:

   PCR/CDVT

   CLR

   CTD

   SCR and BT

   UBR (unspecified bit rate)
   The UBR service class is intended for delay-tolerant or non-real-time applications, i.e., those which do not require tightly constrained delay and delay variation, such as traditional computer communications applications. Sources are expected to transmit non-continuous bursts of cells. UBR service supports a high degree of statistical multiplexing among sources. UBR service includes no notion of a per-VC allocated bandwidth resource. Transport of cells in UBR service is not necessarily guaranteed by mechanisms operating at the cell level. However it is expected that resources will be provisioned for UBR service in such a way as to make it usable for some set of applications. UBR service may be considered as interpretation of the common term "best effort service".

   For UBR, the following ATM attributes are specified:

   PCR/CDVT

   ABR (available bit rate)
   Many applications have the ability to reduce their information transfer rate if the network requires them to do so. Likewise, they may wish to increase their information transfer rate if there is extra bandwidth available within the network. There may not be deterministic parameters because the users are willing to live with unreserved bandwidth. To support traffic from such sources in an ATM network will require facilities different from those for Peak Cell Rate of Sustainable Cell Rate traffic. The ABR service is designed to fill this need. See section D16-2 for more ABR information.

Note that the ITU specs have a different names for similar services classes. Here is a mapping as I understand them:

• Class A is CBR with accurate timing (eg phone calls)
• Class B is VBR with timing (eg packetised phone calls)
• Class C is VBR without accurate timing
• Class D is connectionless VBR without accurate timing
• Class X is UBR
• Class Y is ABR

SUBJECT D16-2) Question: More information on ABR?

Answer: The ATM Forum Traffic Management (TM) subworking group has defined an ATM service type called ABR which stands for Available Bit Rate. Using ABR traffic is not characterized using peak cell rate, burst tolerance, et.al., and bandwidth reseverations are not made. Instead traffic is allowed into the network throttled by a flow control type mechanism. The idea is to provide fair sharing of network bandwidth resources.

Competing approaches were intensely studied for quite some time. The debate included many top folks from industry. Extensive simulation work was done by (among others) Bellcore, Sandia Labs, NIST and Hughes Network Systems. Some simulations were done explicitly with TCP/IP traffic sources, although most used a more generic stochastic model.

The result of all this was the adoption in principle of a "rate-based" approach known as Enhanced Proportional Rate Control Algorithm (EPRCA). The term "rate based" means that the paradigm used involves adjustment by the network of the 'sending rate' of each VC. This is as opposed to a "credit based" or "windowing" approach, where the network communicates to each source (VC) the amount of buffer-space available for its use, and the source refrains from sending unless it knows in advance that the network has room to buffer the data.

ABR has a Peak Cell Rate, a guaranteed Minimum Cell Rate (per VC), and will do a fair share of the remaining available bandwidth (the specific mechanism for determining fair share is left for vendor latitude and experimentation). So you don't have explicit leaky bucket parameters for ABR.

Check the ATM Forum "Traffic Management 4.0" specification as well as the "ABR Addendum" for the complete specification of the ABR service type. The ATM Forum also had a high level discussion on ABR in the October 1995 issue of their 53 Bytes publication. Surf their WEB site at: http://www.atmforum.com/ to access these publications.

There are also several rate-control and flow-control papers in the March-April 1995 issue of IEEE Network, and in the May 1995 issue of IEEE Journal on Selected Areas in Communication. Most of the issues were covered very well.

The essential {CBR, VBR, ABR, UBR} service model itself dates back to Sept 1993 (although those names were not yet attached to the categories, and the definitions were not explicit):

        Natalie Giroux,
        "Categorization of the ATM Layer QoS and Structure of
        the Traffic Management Work"
        ATM Forum contribution 93-0837, Sept 1993.

There was also a special issue of Computer Communications Review (April 1995) that covered a lot of the ATM forum work. It contained an excellent description of the various ABR services as well as an analysis of the ABR rates at steady state.

SUBJECT D16-3) Question: What are DBR and SBR?

Answer:

• DBR - Deterministic Bit Rate
• SBR - Statistical Bit Rate

...another viewpoint... I don't think there was any 'not invented here' involved. CBR and VBR refer to the source (cell stream) characteristics, and DBR and SBR relate to the concept of "ATM Transfer Capabilities" (ITU-speak) or "service categories" (ATM Forum terminology). As there is *not* a one-to-one relationship between cell stream characteristics and the transfer capability used to transport the cells, it would have spawned (even more) confusion if the same names would have been used for these different things. DBR and SBR are included in the new version of ITU I.371. The I.371 also includes a traffic class not supported by the ATM Forum, called ABT (Available Block Transfer).

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