PMD Sub-layer The Physical Layer

Figure 5-6 Idle cell insertion for cell rate decoupling The ATM layer has to deal with a variety of cells, i.e., signaling cells, information data-carrying cells, network management cells, etc. The ATM layer examines these cells mostly the header, but sometimes even the contents and then decides what action must be taken. In ATM networks a switch is a network relay device that takes a cell from an input device, interprets relevant address information and gets the cell onto an output port. Unlike TCPIP, which is a router-based connectionless protocol, ATM is a switch- based, connection-oriented protocol. Previous Table of Contents Next Copyr ight © CRC Pr ess LLC by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC ISBN: 0849331390 Pub Date: 110198 Previous Table of Contents Next

A. ATM Layer Functions

The ATM layer provides a variety of functions, such as: 1 Connection Admission Control CAC including connection assignment and removal 2 Cell construction, reception and header validation 3 Cell payload type discrimination 4 Cell relaying, copying and forwarding using the VPIVCI 5 Cell multiplexing and demultiplexing using the VPIVCI 6 Cell loss priority processing 7 Usage parameter control UPC 8 Support for multiple QOS classes 9 Generic flow control GFC 10 Feedback controls through Operation And Maintenance OAM 11 Explicit Forward Congestion Indication EFCI Figure 5-7 Idle cell format for cell rate decoupling at the TC sub-layer. ATM layer is where the cell headers are actually built. The ATM network nodes act based on that information. The VPI and VCIs are interpreted at the ATM layer so that the cell can be sent to the proper destination across the network. The mixing of cells for voice, video and data on the UNI also takes place at this layer which are then separated at the destination. As discussed in Chapter 6, construction of a virtual path VP and virtual channel VC is a key concept in ATM. In general, an ATM device may be either a connecting point or an endpoint for a VP or VC. As shown in Figure 5-11, a virtual path connection VPC or a virtual channel connection VCC can exist