In today's raging battle over next-generation wireless stand-ards, most of the attention is focused on the air interface. However, selecting the optimum RF modulation techniques is only the first link in a chain that reaches from the mobile subscriber to another subscriber, usually on a landline telephone.

A closer examination of the chain of events reveals a series of signaling protocols used to supervise and control all of the resources necessary to process the connection. As wireless services such as short message service (SMS), caller ID, selective call forwarding, voice mail and automatic ring-back proliferate, the signaling protocols to support and implement these features grow exponentially complex. One message failure can prevent a revenue-generating call. A serious signaling error can take a network off the air, costing millions of dollars in lost revenue.

With the new generation of telecom signaling protocol analyzers, carriers create a portal into the links that hold their networks together -- from the antenna to the landline telco.

The heart of the PCS network is the mobile switching center (MSC), which manages mobile handset registration and the set-up and routing of subscribers' phone calls, whether those calls are to another wireless subscriber or to a landline customer. The MSCs connect not only to base-station controllers (BSCs), but also to the LEC and IXC networks. It is at the MSC that the health of the wireless network can be monitored best.

Signaling Process Unfortunately for carriers, the signaling process is elaborate. A sophisticated signaling protocol is necessary to support the series of events and actions required to set up, manage and tear down phone calls successfully. Handset roaming and base transceiver hand-off also add to the requirements.

For example, a traveling subscriber gets off an airplane and decides to check his messages. To him, it is a simple set of actions. He turns on his wireless handset and expects to receive notification of any waiting voice or text messages immediately. He then retrieves those messages and decides to return a call. This seemingly simple action actually causes a complex set of signaling transactions that can generate as many as 100 messages, each of which may consist of many components.

When the handset is turned on, it identifies itself to the mobile network. This requires signaling messages that check to see whether the handset is registered in the local HLR or local visitor location register (VLR). When the system determines that the handset currently is not registered in the local databases, then signaling messages are sent through the mobile network to update the local VLR from the subscriber's home HLR. These messages may have to go across the country or, in the case of GSM, all the way around the world.

Next, signaling messages are generated to set up a call to the SMS to retrieve any messages for the subscriber, traversing the mobile network to the subscriber's home network.

Finally, signaling messages are generated to set up the voice call. Because more than 80% of mobile voice calls are made to landline phones, these signaling messages most likely will have to traverse the LEC's network and possibly an IXC's network as well. If a calling card is used or the call is a toll-free call, signaling messages will be generated to communicate with a service control point to validate the calling card or to translate the 800 number. These simple actions create many signaling messages that travel across many carriers' networks.

Testing the Network Although typical landline telco operators use only one signaling protocol throughout their networks, wireless carriers use four or five signaling protocols simultaneously. This increases the complexity of testing and trouble- shooting. It becomes critical that the test equipment be capable of decoding multiple protocols over multiple signaling links, simultaneously.

For example, in a GSM-based network, operators at a typical MSC might have to test 12 to 14 links running three different SS7-based protocols simultaneously:

* SS7 over the A-links to the LECs and IXCs

* SS7 with mobile application part (MAP) over the A+MAP-links supporting mobile internetworking

* SS7+MAP over the F-links to neighboring MSCs

* SS7 with base-station subsys-tem application part over the A-interface links to the BSCs.

In addition, many BSCs are collocated with the MSCs where the carrier also will be testing the proprietary A-bis interface between the BSCs and the base transceiver stations (BTSs).

It also is important to test the network. Any corruptions, excess delays, excess retransmissions, incorrect parameters and incompatibilities may prevent revenue-generating calls. A protocol analyzer is designed and optimized specifically for telecom signaling, with the appropriate features, functions and protocol support to analyze multiple links and decode multiple protocols simultaneously.

A protocol analyzer becomes an indispensable tool in solving many of the problems faced in operating and maintaining a wireless network:

* Quality control: analyzing CRC16, flag and length errors, and realignments to verify network quality and to determine whether errors are from switching or transmission equipment.

* Network optimization: analyzing link loads to verify load sharing and to identify peak load times; CIC use to determine distribution of calls or to identify blocked or noisy circuits; and call attempts vs. call completes to determine call success rates.

* Customer complaints: troubleshooting problems, such as dropped or blocked calls, or missing or non-working features (call forwarding, caller ID, SMS) by trapping key subscriber data, such as called number, calling number, mobile security number and international mobile security identity .

* Growth: determining when expansion is necessary and what network elements need to be added, such as voice trunks, BTSs, BSCs and LEC interconnects.

* Hardware expansions: verifying link alignment and link quality, protocol feature compatibility and proper optional configurations during and after installation of new switches and new network elements.

* New interconnects: verifying routing tables, translations and response times during and after installation of new interconnections with LECs, IXCs and other wireless providers.

* New software: verifying correct operation of features by comparing signaling data after the software update to the before picture baseline.

* Billing verification: verifying billing between carriers or for large end users by using a data source (signaling messages) independent from the automatic message accounting records.

* Fraud detection: analyzing signaling messages to correlate certain trends such as long calls (especially international) from a single phone or many short calls to a single phone (call resellers) and successive calls to every phone number in an exchange (computer hackers pinging for computers or competitors auditing your subscriber base).

Continuous testing of the signaling network with a protocol analyzer capable of decoding several protocols simultaneously will go a long way toward achieving high-quality service at a profit. Such a tool can help network operators install and maintain their networks effectively as well as provide valuable analysis needed to optimize the network.

While debates over third-generation (3G) standards continue, test equipment vendors are providing tools for both manufacturers and carriers to help ease the way to 3G. Here are a few such products introduced at PCS '98:

* An option to Hewlett-Packard's ESG-D series RF signal generators helps R&D engineers accelerate W-CDMA base-station and mobile receiver development by exercising the receivers to make sure they meet performance requirements. The option provides real-time data on uplink or downlink signals. The user interface lets engineers select symbol rate, long code and short code to ensure quick test setup.

* Anritsu has integrated its MG3672A digital modulation signal generator and MG0314A W-CDMA modulation unit that outputs test signals for evaluating W-CDMA base stations and devices during the R&D and production process. The units diffuse signals from short code and long code that comply with W-CDMA test specs. The instruments work together to generate multiplex signals for DS + QPSK modulation base-station transmission and mobile station transmission tests.

* Anritsu also introduced the MS8607A digital mobile radio transmitter tester that measures W-CDMA and IS-95 devices. The tester covers the 300kHz to 3GHz range, combines four instruments, and offers modulation analysis and code-domain power measurements.

* The latest addition to Racal Instruments' ProCLAIME protocol analyzers is the C-AIME, which allows for regression, conformance and CDG-22 stage 2 interoper-ability testing of cdmaOne mobiles as well as layer 2 and 3 logging and IS95B protocol testing, including handoff and power-control testing. The system acts as a script-driven cdmaOne network emulator with built-in logging of all signal procedures.