ATM Dịch vụ Chương này giải thích và tóm tắt một loạt các khái niệm và các cơ chế, cho phép ATM để hỗ trợ nhiều loại lớn các dịch vụ. Nó trình bày các khái niệm về chất lượng dịch vụ ATM từ quan điểm dịch vụ. Nó mô tả loại QoS và các lớp học giao thông, các tham số QoS cũng như các thông số lưu lượng sử dụng trong các thiết bị ATM. Chương này cũng giải thích cơ chế quản lý giao thông cơ bản, đảm bảo yêu cầu QoS được phân phối cho khách hàng | Chapter 4 ATM Services The chapter explains and summaries a range of concepts and mechanisms which allow ATM for supporting of large variety of services. It presents the concept of Quality of Service in ATM from the services perspective. It describes QoS categories and traffic classes QoS parameters as well as traffic parameters that are used in ATM devices. The chapter also explains basic traffic management mechanisms which ensure that requested QoS is delivered to the customers. The reading of the chapter is advised prior to discussing ATM applications. Before presenting different ATM services and applications one has to realize the great capabilities of ATM technology in terms of provisioning guaranteed bandwidth and strict QoS conditioning. In fact whenever ATM services are discussed the QoS objectives and mechanisms are concerned. ATM was designed to deliver the QoS in a way incomparable to any former technology. The state-of-the-art transmission and switching constraints implicated a very detailed and coarse definition of the ATM QoS. The unique combination of capabilities and mechanisms allow for ensuring that all types of applications can be given a suitable set of parameters. Therefore different services can be supported simultaneously with adequate QoS treatment. 59 ATM Basics Quality of Service Categories Since ATM supports transport of several different connection and traffic types as well as service types a service classification was developed. Initially ITU-T took into account three factors that are used to differentiate between services. Service classes introduced by ITU-T are dependent on the timing relationship between source and destination bit rate and the connection mode. Next ATM Forum presented its own classification of so called QoS service categories. The relations between two classification models are given in the Table 4-1. ATM Forum QoS category CBR VBR VBR VBR UBR ABR ITU-T service ciass Class A Class B Class c Class D Class X Class
