Setting Payload Scrambling on an ATM Circuit
Problem
You want to enable payload scrambling on your ATM circuit to prevent user data from being interpreted as an in-band control sequence.
Solution
The command to enable scrambling varies depending on the type of circuit. For a T3 ATM circuit, you must use the command atm ds3-scramble, as follows:
Router2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router2(config)#interface ATM0/0
Router2(config-if)#atm ds3-scramble
Router2(config-if)#exit
Router2(config)#end
Router2#
For E3 circuits, scrambling is actually enabled by default, but you can disable it with the no form of the command atm e3-scramble:
Router3#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router3(config)#interface ATM0/0
Router3(config-if)#no atm e3-scramble
Router3(config-if)#exit
Router3(config)#end
Router3#
Both of the preceding commands will scramble the ATM cells at the Physical Layer Interface Module (PLIM) on the ATM interface. You can also opt to scramble only the ATM cell's payload as follows:
Router4#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router4(config)#interface ATM0/0
Router4(config-if)#atm scrambling cell-payload
Router4(config-if)#exit
Router4(config)#end
Router4#
Discussion
In Recipe 16.5, we mentioned that some types of network devices use in-band signaling to indicate that there are errors on the circuit. This can cause network problems if a switch responds to a sequence of bits in a legitimate packet. In ATM networks, the problem is slightly different, however. Here the issue is not with in-band signaling, but rather with clocking.
Some ATM equipment relies on the cell contents to maintain its clocking. If it sees a long sequence of "1"s or "0"s, it runs the risk of losing track of where the bit boundaries are. You can avoid these types of problems by enabling scrambling on ATM cells.
However, there are two basically different ways to scramble an ATM cell. On physical circuits, such as DS3 and E3 circuits that support ATM services, you will often need to enable bit scrambling across the whole cell, including the ATM header. But with other equipment, such as OC3 circuits, it is sufficient to scramble only the payload. Most Cisco ATM interfaces support payload scrambling.
Note that if you intend to use payload scrambling, you must enable it on all of your ATM devices. And for physical layer scrambling on DS3 or E3 circuits, you should consult your ATM vendor to find out whether scrambling is required.