ảo Mạch Dịch vụ thực tế được cung cấp bởi các phương tiện công nghệ ATM là hướng kết nối. Mặc dù các kết nối trong ATM được thực hiện trong nội bộ với chuyển mạch gói, người sử dụng cuối cùng là cung cấp với các mạch ảo. Từ chương này, người đọc có thể tìm hiểu các loại khác nhau của các mạch ảo được sử dụng trong mạng lưới ATM hiện đại. Chương này trình bày các khái niệm chung của các kết nối thường trực và chuyển sang và thảo luận về phương pháp để thiết lập. | Chapter 3 Virtual Circuits The actual service offered by means of ATM technology is connection-oriented. Although connections in ATM are implemented internally with packet switching the end user is provided with virtual circuits. From this chapter the reader may learn different types of virtual circuits that are in use in contemporary ATM networks. The chapter presents the general concept of permanent and switched connections as well discusses methods for setting them up maintaining and releasing them. As it was presented earlier the basic B-ISDN services require a compromise between pure packet switching and pure circuit switching. The actual service offered within an ATM network is connection oriented but internally it is implemented with packet switching not circuit switching. The term connection is used to name the logical circuit that is set-up between end-users having physical links to ATM network. A virtual connection is a concatenation of logical links between neighboring ATM switches. It is not defined by means of a time slot or a physical pair of wires but it is defined by table entries inside ATM switches. The term virtual is an equivalent word for logical as far as ATM circuits are discussed. Today a number of connection types are offered permanent virtual connections switched virtual connections and soft-permanent virtual connections. 49 ATM Basics . Permanent Virtual Connections Historically at the early stage of ATM deployment only permanent virtual connections PVCs were available. The simplest method for the establishing of a virtual connection in ATM is the exact configuration of parameters such as VPI VCI QoS category AAL type and others. In large networks this function is implemented within a centralized management system . Network Management System . PVCs are requested by the customers manually . by contacting the carrier directly with a fax and in most cases remain in place for several days or months. Permanent connections are .
