Using an Internal 56 Kbps CSU/DSU

Using an Internal 56 Kbps CSU DSU

Problem

You want to configure an internal 56 Kbps CSU/DSU.

Solution

The configuration for an internal 56 Kbps 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)#end
Router2#

 

Discussion

There are several options available for configuring internal 56 Kbps CSU/DSU cards in a router, and they all use the service-module command, similar to the T1 module. The above example shows how to set the CSU/DSU to a line speed of 9,600 bps using the clock rate option. This option takes as an argument the line speed in Kilobits per second, with valid options being 2.4, 4.8, 9.6, 19.2, 38.4, 56, or 64. The default line speed is 56 Kbps. You can also configure the module to automatically adapt to whatever the line speed might be using this auto keyword:

Router2(config-if)#service-module 56k clock rate auto

This is particularly useful if the line speed frequently changes for some reason.

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 will assume 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 56 Kbps modules sets the network type to be either a dial-up switched-56 (also sometimes called Centrex) or a leased line. Please refer to Chapter 13 for a brief discussion of switched-56 circuits. To configure the CSU/DSU to support a leased line, you 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 only to support 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 only support the switched network type, and do not let you change this option.

Unfortunately, not all switched networks are created equal. Just about everybody has encountered the problem in which sometimes 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 56 Kbps network doesn't require echo canceling, 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. It is important to note that enabling the echo-canceling feature will increase call setup times noticeably.

An interesting problem can occur on 64 Kbps DDS circuits. The problem is that these types of circuits often use in-band signaling for error conditions. This in particular means that there is a sequence of bits that the carrier's equipment will interpret as the control code to take the circuit out of service. It is only a matter of time in this type of situation before a series of data bits in a packet duplicates this 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 only useful on 64 Kbps DDS circuits. Also, always bear in mind that this feature does jumble user data, so you must use the same option on both ends of the circuit.

See Also

Recipe 16.3