9.7 Integrated Solutions

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With the advent of broadband Internet access, CSU-DSU functionality has been embedded into routers via optional add-in interface cards for T1 and FT1 service supporting nonchannelized data rates of N x 64 Kbps or N x 56 Kbps. An integrated solution has fewer components and fewer points of failure—one less power supply and fewer cables—which makes for greater reliability, space savings, and simplified management. When WAN bandwidth requirements or service provider pricing change, network administrators can easily change WAN services either by changing the software configuration or replacing the WAN interface card. Because the same card can be used on a vendor’s other router platforms, the number of spare units can be reduced, and the WAN interface card can be redeployed from one platform to another.

At installation, the router’s setup utility prompts a series of basic configuration questions. A Web browser interface tool provides point-and-click initial configuration for the CSU-DSU card. CSU-DSU statistics, such as 24-hour history, are provided by using the command line interface (CLI). Loopbacks, bit error rate tester (BERT) test patterns, alarm counters, and performance reports are also accessible from CLI, while front-panel LEDs provide indications for carrier detect, loopback, and alarm functions for quick troubleshooting. A manual loopback button allows users at remote sites to easily initiate a T1 network line loopback by pushing a recessed button on an interface card front panel without having to access the router via a console port or Telnet. The types of diagnostic information provided by the integrated router/CSU-DSU are summarized in Table 9.2.

Table 9.2: Diagnostic Information Provided by the Integrated Router/CSU-DSU

	Network line loopback (user initiated) Recessed push button for network line loopback (toggle on/off) Network line loopback (carrier initiated) Network payload loopback Local DTE loopback
Loopbacks	Remote line and payload loopback
Self	Self test activated by user 0 in 1: Transmits an all-zeros test pattern to verify B8ZS line encoding. The remote end may report a loss of signal when using AMI line coding. 1 in 1: Transmits an all-ones test pattern for signal power measurements. 1 in 2: Transmits an alternating ones-and-zeros pattern for testing bridge taps. 1 in 5: Transmits the industry-standard test pattern loopback request. 1 in 8: Transmits a test pattern for stressing timing recovery of regenerators. 3 in 24: Transmits a pattern for testing the ones-density tolerance on AMI lines. Quasi random word (QRW) test pattern to simulate user data.
Test patterns (BERT)	User-programmable 24-bit test patterns. Loss of network signal Loss of network frame
Network (T1) alarms	Receive from network CRC Bipolar violations (BPV) Out of frame (OOF) Errored seconds Burst errored seconds Severely errored seconds
Performance reports/error counters	Ft and Fs framing errors for SF framing FPS framing errors for ESF framing 24-hour history stored in 15-minute increments Data carrier detect (CD) Loopback (LP)
LEDs	Alarm (AL)

Through the router’s operating system, there is the capability for remote configuration via Telnet from a CLI of both the router and CSU-DSU. This eliminates the need for out-of-band management as well as the need to learn and use a second set of command syntax for an external CSU-DSU. With the integrated approach, CSU-DSU management becomes simply another menu selection in the overall router management. If there is a failure at any level in the data transport stack, the router will analyze the failure by layer and indicate the lowest layer that is responsible. If the problem is at the CSU-DSU layer, the router will indicate it so the system administrator can quickly focus on the problem.

The integrated device is manageable as a single SNMP entity through an element-level management system. The CSU-DSU interface card comes with an SNMP agent that supports the standard MIB II, CSU-DSU MIB, and T1 MIB. All T1 performance statistics can be monitored. In addition, the router generates the appropriate SNMP traps in response to CSU-DSU alarms.

When the CSU-DSU and router are not integrated, locating the cause of a data transport failure can be quite complex. Consider the case where a technician has improperly configured the line coding on a standalone CSU-DSU. The CSU-DSU will experience bit errors, and the router will fail to bring up a link. Thus, the CSU-DSU and the router will each report a problem. The network administrator will be confronted with two separate failure reports and may not know which problem to address first.

When a standalone CSU-DSU loses its T1 network connection, usually the end of network connectivity for any attached router is lost as well. But with an integrated CSU-DSU, the ISDN basic rate interface (BRI) can be configured as a backup link. In addition, the console port can provide a second backup, since it can be configured to dial out through a connected modem to a remote network.

Despite the many advantages of integrated CSU-DSUs, a strong case can still be made for dedicated devices. As noted, CSU-DSUs provide a natural handoff point between the carrier and the customer premises. Carriers can perform diagnostic and loopback functions up to the CSU-DSU, even if there is no WAN edge device, such as a router. However, if the router malfunctions, all onboard components will become unavailable along with it, preventing the carrier from isolating the cause of the failure. If the CSU-DSU fails—because of a lightning hit on the carrier circuit, for example—CSU-DSU spares can be put into place quickly without having to reconfigure the router and possibly disrupting Internet connectivity while loading a new address table.

Dedicated CSU-DSUs can also reduce the ongoing cost of WAN management. By standardizing on a single WAN-interface management strategy, such as physical and data link network monitoring systems, it does not matter what equipment is connected behind it. Local technicians can easily verify the health of the link by checking the front-panel LEDs of the CSU-DSU before they start troubleshooting back-end routers and systems. Many test functions are available at the front panel of the CSU-DSU, allowing for diagnostics without compromising router management security.

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