B-ISDN Services Broadband ISDN

164 Communication Networks Copyright © 2005 PragSoft

12.1.3. B-ISDN Protocol Architecture

Figure 12.138 shows the reference model for the B-ISDN protocol architecture. The architecture is partitioned by three planes. The User Plane facilitates the transfer of user information and associated control signals. The Control Plane is responsible for connection control functions such as call setup and release. The Management Plane is responsible for management functions relating to the system as a whole and coordination between the planes Plane Management, and for management functions related to resources and parameters residing in its protocol entities Layer Management. Figure 12.138 B-ISDN protocol architecture reference model. Higher layers Higher layers ATM Adaptation Layer ATM Layer Physical Layer Control Plane User Plane Management Plane Plane Management Layer Management The Physical Layer is responsible for the physical transmission of ATM cells. To achieve some degree of physical medium independency, this layer is divided into two sublayers. The Physical Medium Sublayer covers the medium-dependent aspects of the physical layer and provides bit transmission and bit timing functions. The Transmission Convergence Sublayer is independent of the physical medium and provides five functions: • Generation and recovery of transmission frames similar to frame structures described in Chapter 11 for narrowband ISDN. • Transmission frame adaptation which involves the packing of cells into transmission frames and their extraction upon arrival. • Cell delineation to enable the receiving-end identify the boundaries of cells whose information fields have been scrambled prior to transmission and are descrambled by the receiver. • HEC sequence generation by the transmitter, and verification by the receiver. • Cell rate decoupling through insertion of idle cells by the transmitter in order to adapt the cell rate to the playload capacity, and their removal by the receiver. www.pragsoft.com Chapter 13: Broadband ISDN and ATM 165 The ATM Layer is responsible for the transfer of cells across the network and is independent of the physical medium. It provides four functions: • Cell multiplexing by the transmitter in order to support multiple logical channels, and their demultiplexing by the receiver. • Virtual path and virtual channel identifier translation explained in the next section. • Cell header generation by the transmitter, and its extraction by the receiver. • Generic flow control for the control of traffic in a customer network by placing flow control information in cell headers. The ATM Adaptation Layer AAL packages information from higher layers into ATM cells to be transported by the ATM layer at the transmitter end, and extracts information from ATM cells for passing to higher layers at the receiver end. It consists of two sublayers. The Segmentation and Reassembly Sublayer handles the segmentation of information from higher layers into smaller units so that they can be transported by ATM cells, and their subsequent reassembly upon arrival. The Convergence Sublayer specifies the services that AAL provides to higher layers and is service-dependent. The physical and the ATM layer provide the same functions for the user and the control plane. The ATM adaptation and higher layers, however, may provide different functions for these two planes.

12.2. Asynchronous Transfer Mode

The recommended switching technology for B-ISDN is the Asynchronous Transfer Mode ATM. Given the high reliability of optical fiber for transmission of digital information, the significant error control overheads involved in earlier protocols such as X.25 become very questionable. Like frame relay, ATM takes advantage of this increased reliability to improve network performance by providing a highly streamlined protocol stack. ATM uses two methods to achieve this. First, it transmit all its information in fixed-size small packets, called cells, hence simplifying the processes of packaging and unpackaging user information. Second, unlike X.25 which requires error control and flow control functions to be performed in a link-by-link fashion, it only requires end-to-end support of these functions.

12.2.1. Channels and Paths

Figure 12.139 illustrates the layered structure of B-ISDN. The higher layer functions are application-dependent and beyond the scope of this chapter. The 166 Communication Networks Copyright © 2005 PragSoft ATM transport network is divided into two layers, both of which are hierarchically organized. Figure 12.139 Layered structure of B-ISDN. B-ISDN Higher Layer Functions ATM Transport Network ATM Layer Transport Functions Physical Layer Transport Functions Virtual Channel Level Virtual Path Level Transmission Path Level Digital Section Level Regenerator Section Level The ATM layer transport functions are divided into virtual channel level and virtual path level. A Virtual Channel VC denotes the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier VCI. This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit. A Virtual Path VP denotes the transport of ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier VPI, which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points. This two layer approach results in improved network performance. Once a virtual path is setup, the additionremoval of virtual channels is straightforward. The physical layer transport functions are divided into three levels of functionality. The transmission path connects network elements that assemble and disassemble the transmission system payload. This payload may contain user or signalling information to which transmission overheads are added. The digital section connects network elements such as switches that assemble and disassemble continuous bitbyte streams. The regenarator section is simply a portion of a digital section which connects two adjacent repeaters along a transmission path which is otherwise too long to sustain the signal.