N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 4 - ATM Technology                                                                                                                      Slide 23 of 46 _________________                                                                                  __________

                                                 ATM Classes of Service

                                        Constant Bit Rate (CBR)

                                        Real-Time Variable Bit Rate (rt-VBR)

                                        Non-Real-Time Variable Bit Rate (nrt-VBR)

                                        Available Bit Rate (ABR)

                                        Unspecified Bit Rate (UBR)

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The ATM Forum has defined four types of ATM classes of services: CBR, VBR, ABR and UBR. These specify the quality of service, including required bandwidth, allowable error rates and other characteristics, requirements and traffic characteristics of voice, data and video transmission. These services apply to virtual circuits (VCs) and virtual paths (VPs).

Constant Bit Rate (CBR) - This service is intended for real-time applications that are very sensitive to delay and jitter. Typical applications using this service are uncompressed real-time digitized voice or video, T-1 service and constant high-volume data transfer requiring a guaranteed response. The user equipment at the connection time specifies the necessary parameters, which the network then reserves for the duration of the connection. This service uses ATM Adaptation Layer 1 (AAL-1).

Real-Time Variable Bit Rate (rt-VBR) - This service is intended for bursty traffic. Typical applications are compressed voice and video (MPEG-2 traffic). The user equipment at the connection time specifies the necessary parameters, which the network then reserves for the duration of the connection. This service uses ATM Adaptation Layer 5 (AAL-5).

Non-Real-Time Variable Bit Rate (nrt-VBR) - This service is intended for bursty traffic. Typical applications are short bursty data messages such as bank and airline transactions, process control, LAN interconnection and Frame Relay traffic. The user equipment at the connection time specifies the necessary parameters, which the network then reserves for the duration of the connection. This service uses ATM Adaptation Layer 5 (AAL-5).

Available Bit Rate (ABR) - This service is intended for non-real-time applications that are capable of reducing and increasing their information rate according to network conditions. Typical applications are LAN internetworking and LAN emulation. The user equipment at the connection time specifies the necessary parameters, which the network then reserves for the duration of the connection. This service uses ATM Adaptation Layer 5 (AAL-5). Due to the additional traffic guarantees and flow control mechanism, this service is preferred over Unspecified Bit Rate (UBR) service.

Unspecified Bit Rate (UBR) - This service is intended for non-critical applications that may use leftover bandwidth and are typically not sensitive to delay. Typical applications are LAN interconnection, E-mail, FTP file transfers, telecommuting, and text/data/image transfer, messaging, distribution and retrieval of information. At connection time equipment does not specify maximum jitter or required bit rate. This is a best-effort service with no QOS guarantees. This service uses ATM Adaptation Layer 5 (AAL-5).