Wireless service providers may be the only vendors that encourage customers to roam. Without this capability, mobile communications are severely limited, if usable at all.
That’s why testing both the radio frequency and wire-based facilities is critical to the performance of wireless networks. Spectrum analysis and physical testing reward both the provider and the customer with network reliability. But an extra facet to testing wireless networks–analyzing signaling protocols–contributes even more to maximizing revenues and customer satisfaction.
Signaling protocols such as SS7 and Intermediate Specification-41 (IS-41, the mobile applications standard that runs on top of SS7) offer a series of important capabilities to wireless customers, including:
Authorizing mobile phone calls instantaneously.
Eliminating fraud in cellular networks.
Roaming between service providers.
Handing off wireless customers and their calling features, such as call forwarding and caller ID from one mobile telephone switching office (MTSO) or service provider to another.
If verified correctly, service providers can identify errors to avoid signaling problems that can lead to customer dissatisfaction. Invalid call process transactions, long response time to queries, and system errors (for example, the counts of unreceived electronic serial numbers transmitted seamlessly during each call to identify a wireless device) may occur if errors are not detected.
The cumulative effect of these errors may prevent customers from roaming; and if they cannot roam, they cannot access advanced features. As a result, wireless service providers cannot collect the revenue generated from those applications, and ultimately suffer customer turnover if unable to resolve problems quickly and permanently.
Analyzing SS7 and IS-41
To avoid customer dissatisfaction and lost revenue due to signaling errors, the SS7 and IS-41 signaling must be analyzed during network provisioning and troubleshooting. To start, wireless service providers should qualify the SS7 link for service. To do this, the T1 link must be checked for problems such as errored packets, cyclic redundancy check errors or bipolar violations (Figure 1).
Other important events to identify are bad link status signal units, discarded packets and negative acknowledgments from the switch (located within the MTSO). If both T1 and switch errors are identified, the integrity of the physical layer and the synchronization on those links should be checked with a loop-back test to locate the error.
Once this is performed and the problem is cleared, the SS7 link should be examined to see if errored packets still exist. Packets can also be generated and counted at the switch to ensure that all packets are being received across the link.
Once the SS7 link is qualified, the IS-41 protocol can be analyzed. IS-41 runs on top of SS7 between the MTSOs and other networks. IS-41 carries important network management and data-base information, which ensures accurate hand-off of call-specific customer data in wireless networks, such as home location register (HLR) and visitor location register (VLR) information. As a cellular customer crosses service-provider boundaries, the network can verify customer call information. For example, when a customer travels from Carrier 1 territory into Carrier 2 territory and wants to make a call, Carrier 2’s VLR queries Carrier 1’s HLR data bases through IS-41. This query determines if the customer has paid the bills, has a stolen cellular phone or has the correct set of calling services enabled, such as caller ID.
If the call is valid, Carrier 2 will get the thumbs-up signal from Carrier 1 and then the call can go through to the destination. This is the set-up process carried over IS-41 between networks.
The test applications begin by decoding IS-41 messages to verify proper user ID registration. The IS-41 messages convey registration and authentication information in conversations known as transactions.
Individual transactions can be monitored to ensure that registration and authentication proceed correctly. Monitoring individual transactions also allows the provider to analyze failed transactions more closely.
Query response times also must be measured. This is accomplished by marking the IS-41 query message containing customer information with a time stamp in the analyzer trace. Then the service provider can monitor the time stamp associated with the network response to analyze the time it takes for the network to handle a query and provide this response.
As an addition to this type of test, the analyzer should monitor the error codes in the IS-41 messages to examine the reasons behind call failure. IS-41 messages acknowledging a failed- call attempt carry codes known as return error codes. Return error codes indicate the reasons why call attempts fail or are blocked by the network.
The ability to decode return error codes provides new insight into these failure mechanisms. For example, a return-error code indicates there is a lack of switch capacity, congestion in the switch, unavailable trunk transmission facilities, unrecognized Mobile ID numbers, or a mismatch between the calling subscriber’s Mobile ID number and the subscriber’s entry in the HLR data base.
Such powerful troubleshooting capacity helps to rapidly identify the cause of network performance degradation or failure. This minimizes the impact of downtime on the customer’s level of satisfaction and the network’s capacity to earn revenues.
Conclusion
In today’s age of weekend vacations and business on the road, the ability to cross service-providers’ boundaries with a cellular phone or pager and continue to use advanced features is very important to mobile customers. By analyzing and verifying the performance of the SS7 and IS-41 protocols, wireless service providers can maximize their revenue potential from roaming and advanced services, enhance customer satisfaction and minimize customer turnover. Customers, likewise, are more satisfied, more productive when mobile and enjoy the full functionality of the service they have purchased.
About the Author
Peter Luff is a Marketing Manager with the TTC Internetwork Test Business Unit. He has been involved in the protocol analysis and network equipment arena for almost 15 years. Mr. Luff was educated in the United Kingdom, earning a B.S. degree in electrical engineering and a master’s degree in marine engineering. Telecommunications Techniques Corp., 20400 Observation Dr., Germantown, MD 20876, (301) 353-1550.
Copyright 1995 Nelson Publishing Inc.
November 1995
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