Network Sales and Services Handbook (Cisco Press Networking Technology)

ATM Service Classes represent a class of ATM connections that have uniform characteristics in terms of traffic pattern and QoS requirements. The following sections discuss the different ATM Service Classes implemented and their respective traffic parameters.

CBR

CBR is used by connections that need a fixed amount of bandwidth that is always available during the connection lifetime. The source can send cells at or below this fixed rate at any time and for any duration. ATM cell traffic is policed by the network ingress ATM switch according to the PCR, and cells presented in excess of the PCR are discarded.

CBR is ideal for use by real-time applications; that is, those applications requiring tightly constrained CTD and CDV, such as voice, video, or Circuit Emulation Services (CES).

VBRrt

VBRrt is for time-sensitive applications (that is, those requiring tightly constrained delay and delay variation), such as voice and video applications. Sources are expected to transmit at a rate that varies with time, which can best be described as "bursty" traffic.

Traffic parameters for VBRrt are as follows:

• PCR

• SCR

• MBS

Cells that are delayed beyond the CTD threshold are presumed to be of less value to the application and can be dropped. Real-time VBR service may support statistical multiplexing of real-time sources. ATM traffic is policed by the network ingress (incoming from the customer) ATM switch according to the PCR and the SCR.

Policing is accomplished in a two-stage process: In stage one, cells presented in excess of the PCR are discarded. In stage two, cells which exceed the average SCR over a period of time are tagged CLP=1 by the ingress switch. If the sending CPE switch has been idle for a period of time, it is permitted to burst to port speed at MBS without having cells tagged CLP=1. If this burst exceeds MBS, however, cells are marked CLP=1 and are therefore eligible for discard in case of network congestion.

Variable Bit Rate (Non-Real-Time) (VBRnrt)

VBRnrt is intended for use by applications having bursty traffic characteristics without tight constraints on delay (CTD) and delay variation (CDV).

Like VBRrt, VBRnrt traffic parameters are as follows:

• PCR

• SCR

• MBS

For those cells that are transferred within the traffic contract, the application expects a low CLR. For all cells, a bound on the CTD by the NSP is expected.

Available Bit Rate

Available Bit Rate (ABR) is for sources with the capability of dynamically reducing or increasing the sending data rate. This dynamic sending rate enables the source to exploit the changes in the ATM network, such as available bandwidth.

Traffic parameters for ABR are as follows:

• PCR

• MCR

No specific QoS parameter (PCR, MCR) is negotiated with ABR; however, it is expected that the end-system will adapt traffic shaping based on network conditions. This traffic shaping will enable the ABR traffic stream to experience a low cell loss ratio (CLR) and obtain a fair share of the available bandwidth. ABR service is not intended to support real-time applications.

Unspecified Bit Rate

Unspecified Bit Rate (UBR) is a "best effort" service used for non-critical applications. By their nature, these non-critical applications do not require controlled delay and delay variation, or a specified QoS. UBR sources are expected to transmit non-continuous bursts of cells and are often used to "fill in" gaps in the traffic stream created by other CoS having no traffic to send. ATM Cells that exceed the MCR are marked discard-eligible. Discard-eligible cells are allowed to pass through the network if bandwidth is available. If network congestion is encountered, these cells are discarded. ATM cells that exceed the PCR are immediately discarded by the network ingress ATM switch.