NAMS Applications - SS#7 Fault Management System

Telecommunications networks have two components. The first component is the set of user lines and inter-office trunk lines utilized for end user communication. The second piece of the telecommunications infrastructure is a separate packet switched data network used for controlling phone calls. The current protocol used on this packet switched network is Signaling System Number 7 (SS#7). SS#7 is primarily used to initiate and terminate phone connections. It is also the vehicle for adding new features and services to the voice network such as call forwarding, last number re-dial, and call blocking. This page first overviews the SS#7 technology and then describes how Digilog’s Network Access and Monitoring System (NAMS) is used to provide centralized fault management capabilities in an SS#7 environment (see also Local Number Portability).

SS#7 networks are designed with a myriad of redundancies to increase their overall reliability. Major node types are deployed in redundant pairs so that the failure of one node does not impair service. Similarly, multiple diversely routed redundant links are installed between nodes so that if a circuit goes down, there is no impact on signaling.

Figure 1 is an overview of the SS#7 network architecture. These networks consists of three major node types. The Service Signaling Point (SSP) is the actual voice switch at the local Central Office (CO) to which end user phones are attached. The Signal Transfer Point (STP) is an intelligent packet switch used to route SS#7 control traffic throughout the network. The Service Control Point (SCP) is a fault tolerant computer system acting as a database node. An SCP holds critical information such as Calling Card data, and the proper translations between 800 numbers that users dial, and the actual phone numbers which get called.

Figure 1

The SS#7 call processing is distributed across the hierarchy of nodes shown in Figure 1. There are four main levels to this hierarchy.

The lowest level of the SS#7 hierarchy is the SSP. End user phones attach to the SSPs, and call processing starts with these voice switches. Figure 1 illustrates three SSP nodes which are connected to the Local STPs. Typically, there are many more SSPs attached to a Local STP pair than shown in Figure 1. In large metropolitan areas, there may be hundreds of these voice switches connected to a Local STP pair.

The next level up in the SS#7 architecture is the Local STP. Most call requests from an SSP go through the local STP. If possible, the user connection is completely set up by a local STP. There are multiple local STP pairs within one Regional Bell Operating Company (RBOC). Local STP pairs are deployed in each Local Access and Transport Area (LATA). The LATAs are geographic splits of an RBOCs coverage area defined as part of the original Bell breakup.

If a call requires connection outside the originating LATA, or requires some special handling, then the third level of the SS#7 hierarchy, the Regional STP, becomes involved. For calls between LATAs of one RBOC, the Regional STP routes the call to the proper destination STP. For other special handling, the Regional STP routes the request to the SCP.

The SCP is at the highest level of the SS#7 architecture. SCPs provide most of the real SS#7 network intelligence. Two of the most common functions performed by SCPs are credit card validation and 800 number translation.

A simple SS# 7 transaction is discussed in detail in the following paragraphs.

Interactions in the SS#7 network occur when someone at a phone attempts to initiate or terminate a connection. The SSP to which the phone is attached initiates the transaction. Figure 2 shows the SS#7 exchange for a simple case where the calling and called number are in the same LATA.

Figure 2

The Initial Address Message (IAM) is issued by the originating SSP toward its local STP. The IAM holds the target phone number and identifies the voice trunk to be used when the call has been established. The STP simply relays that message directly to the destination SSP since it is in the same LATA and therefore directly attached. The destination SSP acknowledges receipt of the IAM with an Address Complete Message (ACM). The SSP then rings the called phone. When the phone is picked up, the destination SSP sends an Answer Message (ANM) back to the originating SSP. At this point the actual voice trunk is connected and the call is complete.

More complicated SS#7 dialogues are common. For example, if the called number is outside the originating SSP’s LATA, the IAM is sent to the Regional STP for appropriate routing to the destination. Additionally, if the called number is an 800 number, then a request for translation to the true target phone number is sent to the SCP which is attached to the Regional STPs.

Besides the normal SS#7 interactions that occur within one RBOC to perform signaling, handoffs to interexchange carriers (long distance companies), and local independent phone companies also occur by interconnecting the various provider’s SS#7 networks. Such interconnections add to the overall complexity of the network, and are carefully run through batteries of certification tests before allowing live traffic to flow between the networks. These certification testers are complex multiport units that test the multiple redundant interconnections between the various carriers’ SS#7 networks.

As more intelligent features such as cellular roaming and call blocking are added to the voice network, the volume and complexity of the traffic carried by the SS#7 network is increasing. Unfortunately, the manpower available to manage these networks is being cut. Central Offices are being run by fewer people, with lights out operations becoming increasingly common.

The Digilog Network Access and Monitoring System (NAMS) is used to bring cost effective trouble shooting tools to complex SS#7 networks. The key to the NAMS solution is centralized remote control of the selection, access, and testing of SS#7 circuits. Figure 3 illustrates the NAMS solution.

Figure 3

As shown in Figure 3, the Digilog Network Supervisory System II (NSS II) hardware wraps around each STP in the network. Each STP site also has an HP37900 multiport test set. The HP37900 is capable of performance monitoring and protocol analysis on up to four SS#7 circuits. The NSS II can place the ports of the HP37900 on any of these SS#7 circuits.

Similarly, the NSS II can be used to provide the test access for a remotely controlled multiport certification tool such as BellCore’s LTU or Tekelec’s MGTS. This further reduces the manpower requirements at the remote site, and actually increases the accuracy of the circuit/test equipment interconnections.

Figure 4 shows the detail of how NSS II provides test access to an SS#7 circuit.

Figure 4

The combination of the NSS II line shelf and NSS II matrix shelf provide the basic test access capability. In Figure 4, each circuit emanating from the STP node is run through an access card in the NSS II line shelf. The access card provides the ability to bridge an STP or line side access bus onto the circuit. The access card can also break the circuit for interruptive activities such as SS#7 protocol certification testing.

Each line shelf access bus is connected to an NSS II matrix card. All test equipment such as the HP37900 shown in the diagram is connected to a matrix card. NSS II matrix cards provide the cross connect between the access buses from access shelves, and the attached test equipment. This NSS II configuration allows the test equipment to be shared across a large number of circuits.

The following diagram shows how NAMS remotely controls the NSS II nodes and remote test equipment.

Figure 5

The NAMS software at the central site provides the following:

• Configuration database holding circuit connectivity detail

• Path Graphics which schematically illustrate the circuit under test

• NAMS specific user Ids and passwords for logon security

• User capability classes which limit the functions each operator can perform

• Database regionalization limits the network elements an operator can access

• Remote control of the NSS II to perform a monitor or test access of one or more remote circuits

• Launch of HP37900 remote control windows to bring data to the central site

• Support of multiple simultaneous users, and multiple NSS II nodes

A typical SS#7 debug session involves using NAMS to select the circuit or circuits that need to be accessed, performing the access via the NSS II, and then launching the remote control window for the HP37900.

The Digilog NAMS software, the NSS II test access hardware, and the HP 37900 Signaling Test Set are effectively integrated, allowing a small group of central site technicians to test and perform remote protocol analysis throughout the SS#7 network. Problem detection and resolution take less time and fewer resources. Network maintenance costs are decreased. The more rapid problem resolution time increases customer satisfaction.