Look Who's Calling

Like many wireless subscribers, Shekar Deshpande can't imagine life without caller ID.

"To me, there's nothing more important after call delivery than caller ID because I screen my calls very heavily," said Deshpande, Triton PCS vice president of engineering and technology development. "Everybody knows that they can reach you on the mobile phone. That's the first number they call."

He isn't alone. Although most wireless carriers give away caller ID as part of basic service, that doesn't mean the service isn't valuable to subscribers. One barometer is the number of wireline subscribers who are willing to pay for the service. In Texas, for example, 27% of Southwestern Bell's wireline customers had caller ID in mid-1996. A year later, that figure had grown to 47%. In Washington, D.C., one woman liked caller ID so much that she pulled the batteries out of her smoke detector to keep the caller-ID box running.

As wireless rates have dropped, caller ID has evolved from a means of controlling bills by screening calls to a time-management tool. But it isn't helpful when it works only intermittently or provides incomplete information.

Caller ID is implemented in much the same way in wireless networks as it is in wireline. For example, in the ISDN-user-part (ISUP) message, two fields control caller ID: the calling party's number (CPN) and the address-presentation indicator, which tells the network whether to present the CPN.

"Ninety-nine percent of the time, if CPN is available, it's there," said Mel Bailey, Triton PCS manager of network-operations support. "But if you opt not to see it or present it, it's always in the network. We turn it on or off based on this presentation-restricted parameter."

TROUBLE TRAIL If subscribers report problems with caller ID, a protocol analyzer can show if those fields are turned on or off. Other approaches include verifying that the feature is included in the subscriber's profile, attempting to duplicate the problem and looking at SS7 messages to determine how the call traversed the network.

"The only caveat on the wireless side would be how far you can take the call trace," said Kim Parker, Tekelec product manager, IN diagnostics. "In most SS7-surveillance systems, you're not doing (surveillance) all the way to the handset. You're taking it only once it's on the signaling network."

Roaming adds another potential challenge because it limits how far the surveillance system can track the call.

"When they're out of your network, they're not going to be in your (surveillance) system any more," Parker said. "You could do a partial call trace, so you could possibly see where the call didn't complete because it didn't complete on your network. Unless you're monitoring all those areas that that subscriber could roam to, you could get only part of that information."

When the roamer registers with the visited system, its VLR downloads the roamer's profile from the home system's HLR. As a result, the first troubleshooting question is whether the visited system's VLR and MSCs support caller ID.

"The serving system has to be able to support the transfer of that information because you have an extra 10 or more digits coming in," said Randy Snyder, telecom consultant and co-author of Mobile Telecommunications Networking With IS-41. "They have to support the transport of that information, and they have to support that feature while roaming."

In the case of two wireless networks, the roaming agreement should define the parameters that control how caller-ID information is passed between the networks.

"If they roamed into another system, and their caller ID wasn't working, there could be a problem in passing the profile data," Parker said. "When it was passed, either that information was left out or once it got to the other system, it wasn't copied to the correct place. It could be a switching error or a data-fill error."

PARAMETERS GALORE Regardless of whether the interconnection is between two wireless networks or wireless and wireline, one bottom-line question is whether all the networks that the call traverses have the ability to deliver caller-ID information.

"There are networks that have ISUP but do only the minimum mandatory parameters," Snyder said. "In the ISUP message, the basic initial address message has about 40 parameters in it. They get really complex and deliver all kinds of fancy stuff, but only three or four of them are mandatory."

Interconnectivity testing helps ensure that a new roaming partner or a new MSC can support caller ID. Because each network can differ slightly in how it interprets and implements standards, interconnection documents spell out exactly how each network is engineered.

"Each network has slight idiosyncrasies," Snyder said. "There's the standard, and you might be 90% compliant, but then there's all the extensions. In all these protocols, there are reserved fields and operational things, and different networks use them different ways. They require that certain information be passed back and forth for operations and maintenance or for the delivery of special custom features."

One of the more annoying caller ID glitches is getting an unfamiliar number. Worse still is calling back the displayed number only to find that it's the company's main number and not the caller's extension. Both are common situations when the caller is calling from a PBX, but there is a solution.

A PBX essentially is a private, small-scale telephone network. As a result, like a public wireless or wireline network, many can be configured to pass information such as the caller's extension, called the subaddress, instead of just the PBX's main number. The first step is to exchange information about how the networks are configured and which parameters contain which information.

"That's the same as the interconnection documents," said Randy Snyder, a telecom consultant and co-author of Mobile Telecommunications Networking With IS-41. "If you're a PBX, you're still interconnecting to a public network. There has to be an arrangement to pass this information. It has to be a piece of information that the equipment can provide, and you have to agree to provide it. There might individual fees associated with that."

So why don't more businesses configure their PBXs to pass subaddresses? Most business owners aren't telecom experts, so they wouldn't know to ask their PBX vendor about configuring their systems. Even for experts, it can be tricky.

"Both ISUP and IS-41 support the calling number and the subaddress so that you can have two numbers delivered to you," Snyder said. "You get the number coming in from the network: How did this number get to me? What line did it come in on? What's the real number of the calling party so that it's a real callback number? Sometimes there's no distinction, and it can be very confusing from a network perspective."