You want to configure an internal 56Kbps CSU/DSU.
The configuration for an internal 56Kbps CSU/DSU is similar to that of an internal T1 CSU/DSU:
Router2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router2(config)#interface Serial0/1 Router2(config-if)#ip address 192.168.99.25 255.255.255.252 Router2(config-if)#no shutdown Router2(config-if)#service-module 56k clock rate 9.6 Router2(config-if)#exit Router2(config)#exit Router2#
There are several options available for configuring internal 56Kbps CSU/DSU cards in a router, and they all use the service-module command, which is similar to the T1 module. This example shows how to set the CSU/DSU to a line speed of 9600bps using the clock rate option. This option takes as an argument the line speed in kilobits per second, with valid options of 2.4, 4.8, 9.6, 19.2, 38.4, 56, or 64. The default line speed is 56Kbps. You can also configure the module to automatically adapt to whatever the line speed might be by using the auto keyword:
Router2(config-if)#service-module 56k clock rate auto
This is particularly useful if the line speed changes frequently, or if you aren't sure what value the carrier uses.
Note, however, that this clock rate option does not imply that the internal CSU/DSU is the clock source for the circuit. By default, the CSU/DSU assumes that the clock signal comes from the network. If you want your router to supply the clock signal instead, you must configure it to do so as follows:
Router2(config-if)#service-module 56k clock source internal
Another important option for 56Kbps modules sets the network type to be either a dialup switched-56 (also called Centrex) or a leased line. Refer to Chapter 13 for a brief discussion of switched-56 circuits. To configure the CSU/DSU to support a leased line, use the keyword dds, which stands for Digital Data Service:
Router2(config-if)#service-module 56k network-type dds
And you can configure the module for switched-56 using the switched keyword:
Router2(config-if)#service-module 56k network-type switched
However, it is important to remember that some modules are designed to support only one network type. In particular, switched-56 uses two-wire signaling, while DDS uses four wires. All of Cisco's four-wire CSU/DSU modules (such as the WIC-1DSU-56K4) can support both switched or dds options, although the default is dds. However, the less common two-wire modules can support only the switched network type, and they do not allow you to change this option.
Unfortunately, not all switched networks are created equal. Just about everybody has encountered the problem in which you hear an echo of your own voice when talking on the telephone, particularly on long-distance calls. This is annoying for voice communications, but it can be devastating for digital data transmission. In the United States, AT&T's switched 56Kbps network doesn't require echo cancelling, while Sprint's does. You can specify that your carrier behaves like the AT&T network as follows:
Router2(config-if)#service-module 56k network-type switched Router2(config-if)#service-module 56k switched-carrier att
Or, for networks like Sprint's, use the argument sprint instead:
Router2(config-if)#service-module 56k network-type switched Router2(config-if)#service-module 56k switched-carrier sprint
You can also specify other as the argument, which behaves exactly the same as the att option, but presumably makes other network vendors feel better. Note that enabling the echo cancelling feature will increase call setup times noticeably.
An interesting problem can occur on 64Kbps DDS circuits. The problem is that these types of circuits often use in-band signaling for error conditions. This means that there is a sequence of bits that the carrier's equipment interprets as the control code telling it to take the circuit out of service. In this type of situation, it is only a matter of time before a series of data bits in a packet duplicates the pattern and causes problems.
You can use a simple algorithm called data scrambling to overcome this problem. This algorithm just jumbles up the bit patterns to ensure the control codes never appear on the line accidentally. You can enable this feature with the data-coding scrambled command:
Router2(config-if)#service-module 56k network-type dds Router2(config-if)#service-module 56k clock rate 64 Router2(config-if)#service-module 56k data-coding scrambled
We included both the network type and clock rate commands with the command that does the data scrambling to remind you that this option is useful only on 64Kbps DDS circuits. Always bear in mind that this feature does jumble user data, so you must use the same option on both ends of the circuit.
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