Dedicated circuits come in many sizes, and the first order of business is to isolate the level of the issue within the circuit. Here’s a rundown of the basic levels of circuitry you might be dealing with:
OC-12 or OC-3: Problems at these levels are usually related to large optical circuits. A problem at this level affects everything on the circuit. Alternatively, you may have a problem that just affects part of the circuit (one or two DS-1s on a specific DS-3), thus eliminating the OC-3 or the larger OC-12 (in which the OC-3 rides) as the possible source of the issue.
DS-3: Problems at this level affect all the lower-level DS-1s and DS-0s. If red lights or alarms are flashing on your DS-3 multiplexer where the coax cable enters, and all of your DS-1s are down, you can trace the source to the DS-3 level.
DS-1 (also called T-1): Problems at this level affect the entire DS-1 circuit and all the individual calls running on the circuit. If you have any individual DS-0 channel that is unaffected, your problem is not at the DS-1 level.
Tip Don’t get tripped up if someone uses the term T-1. T-1s and DS-1s are two names for exactly the same thing, like Bobby and Robert.
DS-0: Problems at this level affect the individual call channel. These problems are either isolated to individual DS-0 level interfaces on your phone system, the DS-0 level hardware at your carrier, or somewhere within the public switched telephone network (PSTN). You know you’re dealing with a DS-0 problem if the problem only affects specific calls; it won’t affect the entire DS-1 circuit.
Tip The good news is that troubleshooting anything above the level of a T-1 (DS-1) circuit involves the same carriers and quantity of hardware. The greatest change in variables occurs when troubleshooting issues at the T-1 level and individual channel (DS-0) level.
If your local carrier provides your dedicated circuit, be sure to include the carrier in your troubleshooting process. Most dedicated circuits for voice calls terminate at your long-distance carrier, and because of that, the local carrier has only a supporting role in the circuit. As you isolate dedicated circuits from the DS-3 or DS-1 level down to the individual DS-0 channel level, take note of the variables at work and who is responsible for each one.
Remember Begin troubleshooting your dedicated circuit only after you confirm that your issue doesn’t exist anywhere else. Use Chapters 11 and 12 to compare call types and confirm the source of your telecom problem. Your dedicated circuit only involves a few miles of cabling and a few pieces of hardware between your company and your carrier. If calls that don’t use the dedicated circuit also experience the same problem, you need to report it to your carrier as a switched problem.
In fact, you may have a brand-new problem on your hands if you open a trouble ticket for your dedicated circuit and the problem could be effectively resolved as a switched issue. Put simply, troubleshooting the dedicated circuit places your circuit at unnecessary risk; you may end up taking down the circuit entirely, leaving your business without phone service for the duration of testing.
If your issue affects a variety of your long-distance calls, from switched outbound calls to dedicated inbound calls, I advise you (nay, I entreat you) to open your trouble ticket on the switched outbound issue, because it provides the greatest focus for your carrier.
Your multiplexer (MUX), the carrier’s multiplexer (at the level of your circuit — DS-3 MUX for a DS-3 circuit, OC-12 MUX for an OC-12 circuit, and so on), and the physical connection between the two pieces of hardware handled by the provider of your local loop interact at the DS-3 level and higher.
Remember If alarms ring on your DS-3 multiplexer, and the entire network span is down, a DS-1-level issue is not bringing down your circuit.
Similarly, if you have a problem with a T-1 circuit or on an individual channel on a single T-1, the problem can’t be caused by the provider that supplies your local loop or with the DS-3 MUX. This is because DS-3 and DS-1 circuit problems don’t interact at the individual DS-0 channel level. If your entire DS-3 has static on it, you must investigate the multiplexers on either end of your local loop. If you have a continuity issue and the span is down, the problem might lie with any of the cross-connects created by the local loop provider, or it could be a defect in the multiplexer at either end.
The primary elements of hardware interacting at the DS-1 level that can negatively affect the entire DS-1 circuit are
Your DS-1-level multiplexer.
The echo cancellers within the circuit.
The card that provides the DS-level multiplexing at your carrier.
If you have individual local loops for each of your T-1 circuits, your local loops are possible culprits.
Just as you do when troubleshooting at the DS-3 level, you must consider both the equipment that receives the circuit within your phone system and your carrier’s hardware as the most likely culprits causing both quality and continuity issues. The copper wire that makes up the local loop is also a possible source of DS-1-level problems. If someone cuts through the cable that provides juice to your DS-1 circuit, everything riding through it is down hard. At the DS-1 level of a circuit, you encounter your first increase in software interaction.
Remember If your local loop is a DS-3 circuit, or any circuit larger than the level of the issue you have identified, the source of the problem is most likely not the wiring or optical cable used to provide your local loop. If your DS-3 circuit has continuity, it is a safe bet that the DS-1s within your DS-3 are safely being received by your carrier and delivered to your phone system without incident. The local loop is essentially an empty pipe as far as your local loop provider is concerned; the local loop provider interacts only at the highest level of the circuit to maintain the clocking and keep the circuit stable. Your local loop provider ignores the T-1s or individual channels (DS-0s) within the local loop, except for the possible interaction of echo cancellers at the T-1 or individual DS-0 level if the circuit is an ISDN circuit.
Individual channel issues generally indicate a problem with either your multiplexing hardware or the cards your carrier uses in its switch to perform the multiplexing. You can identify a DS-0 issue pretty easily. If your DS-1 is working fine and many channels on the DS-1 can send and receive calls without any problem, there are really only two main variables and one minor variable to evaluate in determining the source of the problem.
The main variables at the DS-0 level are the multiplexers on either side of the circuit that break down the T-1 into individual channels. If your third DS-0 on your T-1 has static, is unavailable, or if you can’t seize a dial tone on it, either your hardware or your carrier’s hardware is failing for that channel. If you can isolate both pieces of hardware and the problem doesn’t clear up, the only other likely candidate is possibly an echo canceller somewhere in your circuit that is malfunctioning or dying a slow death. The purpose of an echo canceller (also referred to as an echo can) is to eliminate the echo heard on a call. If the device is failing or was provisioned incorrectly, it can prevent your call from completing with good quality, or at all.
You can also encounter problems at the individual call level caused by software compatibility in the outpulse signaling and start. If your system is configured for loopstart and your carrier is set for E&M Immediate, you will have problems making calls. This type of configuration issue causes calls to suddenly disconnect, fail to connect at all, and otherwise fouls things up. Configuration issues at the individual call level can also cause frame slip errors and errors that can take down your entire circuit. Chapter 8 covers line coding and framing in detail, as well as the symptoms you will encounter due to frame slips and errors on your circuit.