by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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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
Figure 5-8 Performance of the Header Error Checker for ATM networks.
Figure 5-9 Flow chart for the Header Error Correction algorithm.
Figure 5-10 State diagram for the Header Error Check mechanism.
A VPC or VCC is an ordered list of VP or VC links, respectively. Thus a VCC defines a unidirectional flow of ATM cells from one user to one or more other users. The ATM layer requires that the network
delivers the cells in the same order in which they are sent. In other words, the cell sequence integrity for a VCC must be preserved. However, the standards do not require a network to preserve the cell sequence
integrity for a VPC.
The physical layer of the ATM protocol architecture has a different implementation based on different media types. Similarly the ATM adaptation layer is, generally, active and present only at the end nodes of
the ATM network and does not have access to the cell header information. As a result, the ATM layer is responsible for performing the generic general functions, i.e.,
Network Resource Management NRM, Connection Admission Control CAC,
Priority Control PC, Usage Parameter Control UPC, and
Traffic Shaping TS.
Figure 5-11 Virtual paths and Virtual channels between End Points EP and Connecting Points CP.
No standard mechanisms have yet been defined to provide for these functions. These generic functions are supposed to provide the quality of service QOS to the users of the ATM network connections based on
their need and expectations. Some of these functions are discussed in greater detail in Chapter 8, which deals with ATM network management.
Table 5-2Traffic cell types Cell Type
VCI VPI
User Data N31
N0 Unassigned Cell
Meta-Signaling 1
Remote: Meta-Signaling 1
N0 Broadcast Signaling
2 Remote: Broadcast Signaling
2 N0