You can see in Figure 11-1 how the call starts with your phone system (your phone system may be as simple as a single phone attached to a jack), and then progresses from your local carrier, to your long-distance carrier, to the local carrier that handles the service for the person you are calling, and finally to the phone you are actually calling. This is one of the most basic calls that you make, and probably the type of call that experiences the greatest quantity of issues.

 Remember   Before you can pinpoint a problem to any variable in the diagram, you need to compare the call to at least one of the remaining five call types.

 Technical Stuff   The public switched telephone network, or PSTN, is a generic term for all the long-distance carrier (Points of Presence) POPs, and hardware that routes and directs phone calls, as well as the local carrier central offices (COs), and all of their hardware that routes and directs phone calls. While a call is being sent through the PSTN, it is aggregated onto circuits with other calls for part of the path it takes from origin to termination. Imagine the PSTN is like a huge freeway for phone calls. All calls use the same roads , sometimes only until the nearest offramp, and sometimes for hundreds of miles. If a problem on a call happens while it’s being routed through the PSTN, the issue is said to be in the switched network, even if the call originally started, or ended, on a dedicated circuit.

Switched inbound long-distance call

A switched inbound long-distance call is a very helpful call type because it eliminates your long-distance carrier as a potential cause of any issue. The long-distance portion of the PSTN is filled with hardware that routes and maintains the quality of your call. The greater the distance the call must travel through the PSTN, the more hardware is required to maintain the call. More hardware, of course, means that there are more potential points of failure, and so you are more likely to encounter a problem in the 28 or 2,800 miles where your long-distance carrier is handling your call.

As you can see in Figure 11-2, when someone calls directly into your phone system (not using a toll-free number), the caller doesn’t use your long-distance carrier. If you have a problem on both outbound long-distance calls and inbound long-distance calls, you can eliminate your long-distance carrier as the source.

Figure 11-2: A switched inbound long-distance call.

Switched local call (inbound or outbound)

A switched local call uses the same carrier and interacts with the same hardware (carrier hardware and your phone system) for both outbound and inbound calls. It doesn’t matter if you call your friend across the street or if someone calls you; the only carrier that generally ever sees your call is the local carrier you both use. Figure 11-3 offers a visual representation of this call type.

Figure 11-3: A switched local call.

You can see in Figure 11-3 that the call barely enters the PSTN and might pass through only one central office. If you have a problem with a local call, the possible sources are your phone system and the local carrier.

Dialing someone else’s toll-free number

When you dial someone else’s toll-free number, the only piece of the call that you have any responsibility for is your phone system. Your local carrier is responsible for identifying the carrier that receives the traffic, and then your local carrier forwards the call to that network. Figure 11-4 shows the path the call takes when you dial someone else’s toll-free number.

Figure 11-4: Dialing someone else’s toll-free number.

If you have a problem calling someone else’s toll-free number, but all of your other calls work fine, the problem probably has something to do with either the carrier that handles the traffic of the toll-free number, or the local carrier that terminates the call. Identifying which carrier is the problem is covered in the step-by-step troubleshooting section for toll-free numbers in Chapter 12.

 Remember   Many companies use the same long-distance carriers. If you use Sprint for your long distance and you dial the toll-free number of a company that also uses Sprint for long distance, you can possibly see a similar problem on your outbound calls. In this case, you should focus on troubleshooting your out-bound long-distance calls on the Sprint network. When Sprint resolves your outbound issue, the toll-free problem for the company you are dialing to will probably also be fixed.

Someone is dialing your switched toll-free number

The last switched call type is an inbound call to your toll-free number. The diagram for this call, shown in Figure 11-5, looks quite a bit like Figure 11-1. It includes the same path.

Figure 11-5: Someone dialing your switched toll-free number.

The main aspect that separates a toll-free issue from a regular outbound issue is the fact that the call is being sent to your long-distance carrier based on a database check done by the originating local carrier. Aside from how the call is sent to your long-distance carrier, the path the call takes encountersall the same carriers and hardware it would if you had made an outbound call to the origination number.

Dedicated calling

You can compare calls over your dedicated circuits with switched calls in order to eliminate your local carrier as a possible source of the problem. Figure 11-6 shows how the local loop of your dedicated circuit passes through your local carrier, but doesn’t really interact with it.

 Remember   When you report trouble on your dedicated circuit, keep in mind that the local loop is the dedicated section your calls pass through. If you have constant echo, static, dropped calls, or if the entire circuit is completely down, you should refer to your circuit when you open the trouble ticket. If you have any problems that you can duplicate by dialing out on a regular phone line over your carrier’s switched network, such as calls failing to a certain area code, fax completion problems, static, or echo, you should open a trouble ticket with your carrier based on the call from your regular phone line. If the problem exists on both your switched and dedicated calls, but you open the ticket as a dedicated issue, the problem will be harder to diagnose.

Carriers do assign greater priority to dedicated circuits, but they also push to intrusively test your circuit as the first step in troubleshooting. The intrusive testing process prevents you from using any channel on your circuit during the testing. If you can open the trouble ticket on a call from a regular phone line, your carrier won’t be distracted by your T-1 and waste time testing your local loop.

Figure 11-6: Dedicated calling process.

 Remember   There are only three elements that interact at the individual channel (DS-0) level on a dedicated circuit:

• Your channel bank or multiplexer: This is the piece of hardware at your office that breaks the dedicated T-1 circuit down into 24 useable DS-0 circuits that transmit and receive your phone calls.

• Your carrier’s switch: The one piece of hardware in your carrier’s switch that you should be concerned with is the section that functions like your multiplexer.

• Echo cancellers: These pieces of hardware eliminate the echo on your phone calls. They may be in your long-distance carrier’s network between the POP and the local carrier, or within the local carrier network.

The next important area to note in the diagram is the point where the call enters your carrier’s network to the point where it’s delivered to the recipient’s phone. Figure 11-7 shows an expanded view of a dedicated call.

Figure 11-7: Expanded view of a dedicated call.

In Figure 11-7, you can now see that the local loop begins at your building and ends at the CFA (Carrier Facilities Assignment) point where the local loop enters your long-distance carrier’s network within their POP. (Check out Chapter 8 if you have any question about the CFA point.) The majority of a dedicated call actually isn’t dedicated at all, but is handled by the PSTN, where it’s routed and handled with every other call in your carrier’s network.

Comparing call types

Call type diagrams are great tools when you are troubleshooting. You should record the types of calls you have made and note whether they experienced the problem. Then you can refer to the diagrams and begin eliminating variables. For example, if you have static on local calls and inbound calls, but not on dedicated calls, you can distill the variables in the following manner, as shown in Figure 11-8.

Figure 11-8: Comparing the call types in diagram form.

 Remember   The speed at which a problem is resolved is directly related to how well you accept the results of your testing. It’s not uncommon for people to place the calls side by side and have a good indication of where a problem is, but not troubleshoot that section first. This is very common when all the signs indicate that the issue is with your hardware. Open a technician assistance ticket (also called a tech assist ticket ) with your carrier to schedule a time with their technician to walk you through testing and work with your hardware vendor. The help of your carrier’s technician aids your hardware vendor in isolating issues within your phone system, but your first call should be to your hardware vendor if you suspect that is the site of your issue.

When troubleshooting, look for similar paths. For example, when you compare the diagrams of the inbound calls and the local calls, you see that there are only two variables in common: your phone system and the network of your local carrier. Local calls are commonly handled by your phone system, your local carrier, and the phone system of whomever you are calling. If you simply call another person in your local area and have the same problem, you can eliminate the hardware of the first person you called as the potential problem. The only two variables left for you to investigate are your phone system and your local carrier.

Every inbound long-distance call that comes from various companies in different sections of the world has its own local and long-distance carrier. Therefore, you can quickly eliminate the other long-distance carrier and the other phone system. The only variables that can possibly be causing a problem are either your phone system or your local carrier.

After you consider the call from your dedicated circuit that did not have the problem, you can eliminate your phone system as a possible source of the issue. As long as you are dialing out from the same phone system over both your dedicated circuit and your switched lines, it’s very unlikely that your phone system is the source of the problem. You have now reduced the possible sources of the problem to just one entity (your local carrier). The next step is to call the local carrier and open a trouble ticket. It’s helpful to let the local carrier know about the tests you have done and that the problem doesn’t affect calls on your dedicated circuit, where they don’t interact at DS-0 level.