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ATM Networks and Security

by Dr. Eva Bozoki - May 30, 2000

ATM Networks and Security

1. Introduction


2. ATM Connections


3. ATM Protocol Basics


4. Attacks


5. Security Measures


6. Placement of the User Plane Security Services


7. Secure Call Setup Protocol


8. Conclusion






About the Author


2. ATM Connections

While T1 and T3 line customers purchase bandwidth sized for peak usage, ATM line customers pay only for the bandwidth they actually use. The reason for this difference lies in the multiplexing scheme used to allocate access. T-lines use the traditional time-division multiplexing, in which each customer is assigned a time slot that might or might not be used (thus wasting purchased bandwidth). ATM lines use asynchronous time-division multiplexing, whereby time slots are available to any user who needs them. These two schemes are illustrated on Figure 1.

Figure 1

Synchronous (a) and asynchronous (b) time-division multiplexing

In synchronous mode, each user is assigned a time slot that is synchronized to a user input time slot. The ownership of data is determined by position in the data stream. If there is no user data for a given time slot, that time slot remains empty (wasted). In asynchronous mode, users who have data to transmit fill time slots, and ownership is specified in the header within each time slot. In this case, all time slots are filled.

In ATM networks, it is necessary to distinguish between the physical circuit and the logical virtual path (VP) and virtual channels (VC), as illustrated on Figure 2. A physical circuit can carry more than one virtual path, and each of those supports multiple virtual channels.

Figure 2

Physical circuit and logical virtual path and virtual channels

ATM cells travelling between two end nodes are assigned a VP and a VC. Because virtual channels are dynamic resources, they are usually allocated at call setup time by the signaling protocols. Virtual paths represent static resources and are more likely to be allocated as service provision. At the ATM switches, both the VP and the VC assignment may be changed according to mapping tables stored at the switches. The effect of a combined VP-VC switch is shown in Figure 3. Due to their hierarchical relationship, switching is always done on VPs first and then on VCs.

Figure 3

Virtual path and virtual channel switching


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