INLInk – A Cellular Intelligent Network

(This was an article I wrote in 1993.  INLink as a product is actually still around.  Check this out


INLink℠: A Cellular Intelligent Network

Annual Review of Communications

National Engineering Consortium

1992-93, Vol 46




Ira P. Gorelick

GTE Telecommunications Services

100 S. Ashley Drive

Tampa, Fl. 33601




The following describes the issues and assumptions that drive INLink.  INLink is a package of integrated intelligent network products that support a Cellular Intelligent Network. INLink is based on IS-41 and includes, SS7 transport, SS7/X.25 protocol conversion, message interoperability, and gateway services.



I remember seeing a nature show showing a duck swimming on a lake. From the surface that little duck looked so graceful and calm. Viewed from underwater, however, you can see the little ducks feet going like crazy. What appears smooth on the surface many take a great deal of effort underneath. The same is true for the cellular intelligent network. The end game is to make roaming appear easy to the subscriber. Achieving that appearance, however, requires many things to happen in a coordinated fashion.


To begin, we need to define some terms.

  1. The most important is “seamless roaming environment.” CTIA defines it as “the concept of a person using their Cellular Subscriber Station (CSS) anywhere and still being able to utilize all of their ‘subscribed to1 features, as if they were at home.”l


Unfortunately this definition is really only half of the story. The other half of seamless roaming is that the subscriber does not have to perform any action as they enter a visiting system. This creates an environment where the boundaries between systems are transparent to the subscriber.


  1. Another important term is the “cellular intelligent network.” The cellular intelligent network is defined as the integration into the unique cellular environment of the following elements; microprocessor based computer systems, high speed and highly reliable digital transport systems, highly efficient multi-layer messaging protocols, and mobility management applications.


It is only through the cellular intelligent network that true “seamless roaming” can be achieved. However, achieving a truly seamless roaming environment is easier said than done. A great deal must occur to bring true benefits to the cellular carrier and subscriber.


In the past the ability to make and receive calls in areas outside ones home market was achieved in one of two ways. Either through the use of third party, roaming service provider systems like Positive Validation Service (PVS)SM, and Follow Me Roaming (FMR)®. Or through the linking of switches of the same manufacture using that manufacture’s proprietary protocol. Both alternatives have served the industry well. They allowed the subscriber to make and receive calls. But these solutions forced the cellular carrier into; 1) dependence on a single switch manufacture for both the carrier and their roaming partners, 2) into the use of the very slow and valuable recent change port, and/or 3) dependence upon a third party roaming service provider.


To overcome these dependencies the cellular carriers spearheaded the development of the EIA/TIA sponsored Interim Specification 41 (IS- 41). IS-41 was intended to facilitate seamless roaming. It does this by allowing cellular switches to exchange information about subscribers, through a high speed standard interface regardless of switch type. IS-41 solved two problems at once: 1) it reduced the carriers dependence on a single switch manufacture and 2) it provided high speed access to information about a roamer. IS-41 also set the direction for the cellular industry by stating, “The minimal acceptable protocol configuration includes transaction capabilities based on the TCAP standard, ANSI Tl. 114-1988, and the 1984 CCITT Red Book Recommendation X.25.”^



Carriers envision the implementation of the seamless network will improve operational efficiency, increase revenue per subscriber, and provide a platform to implement new services.


There are many reasons to believe that the seamless network will fulfill its promise and deliver those benefits. By allowing direct switch to switch communication, the seamless network can reduce fraud and lower operational costs. The cost savings will be realized by reducing the need for validation and call forwarding services, commonly known as clearinghouses, roaming service providers, or third party systems. Additionally, the seamless network facilitates automatic roaming by eliminating the need for the cellular subscriber to enter special codes, or dial special numbers to have calls delivered. This will make receiving calls easier, and will thus increase revenue. The seamless network also provides a platform to implement new services such as PCS.


Improved operational efficiency is achieved through direct switch to switch communication. In today’s environment the only method of direct switch to switch communication is SS7 and IS-41 Rev. A. Under Rev. A the Mobile Switch has the responsibility of translating the Mobile ID Number (MIN) into a Destination Point Code (DPC), which is then used to route the message through the SS7 network.


While direct switch to switch communication seems fairly straight forward, there are several issues that present itself. First, by sending IS-41 messages directly between switches, there is no way to validate a subscriber against the National Negative File, which is one of the cellular industries key validation authorities. Second, because mobile switches do not have sophisticated data processing capabilities, direct switch to switch communication would make some types of fraud detection (i.e., clone detection) more difficult. Third, under Rev A, the cellular carrier must maintain all the MIN to DPC translations in each mobile switch. Finally, direct switch to switch communication will result in a reduction of visibility into the message flow, compared to what carriers have today.


One of the issues, the need to maintain the MIN to DPC translation tables in each switch, will go away when Rev. B is released by the mobile -switch manufactures. Rev. B will eliminate this requirement by providing for the use of SS7 and Global Title Translation (GTT). GTT is the holy grail of the intelligent network. With GTT the carrier will not have to maintain a database that translates the MIN to a DPC. With GTT the carrier only needs to know the DPC of the network node it is connected to, most likely an STP. The SS7 network takes the responsibility of translating the MIN to a DPC.


From GTE’s perspective Cellular Carriers want SS7 for some of the same reasons that wireline carriers want SS7. More sophisticated network management functions, and the possibility of introducing addressing schemes that use global title translations. Additionally, SS7 is fast, has high availability, and has very good congestion abatement controls. Cellular carriers also look to the possibility of using the networks of SS7 service providers (lECs, LECs, or Independent SS7 service providers) to form national SS7 based cellular networks. And finally, SS7 will provide a uniform carriage service protocol for both cellular transactions and ISDN-UP related transactions.


One of the major hurdles, however, facing the evolution to the cellular intelligent network is that not all Cellular Carriers will implement IS-41 and SS7 at the same time, or in the same way. Thus there will continue to be a significant requirement to have “interoperability gateways” between the Non IS-41, IS-41/X.25, and IS-41/SS7 environments, for validation and call delivery services. Since there also may be various SS7 networks, there is also a requirement to have gateways to other SS7 networks.


INLink Services

The solution to these problems is to offer carriers a package of integrated “transitional” solutions that give them the freedom to move at their own pace based on individual business and market needs. This package of services not only addresses the issues of new services but also address the problems of uneven IS-41 implementation. GTE TSI has labeled this package of services INLinkSM.


Employing standard interfaces, INLink consists of four “building blocks” or basic capabilities: network transport, network protocol translation, IS-41 to Non-IS-41 interoperability, and intersystems gateway. These building blocks provide a high quality, ubiquitous network to subscribers, independent of carriers size, location, switch type, technology features, or network transport protocol. The goal is not only to provide the physical network and service connections necessary to see the cellular industry through this period of rapid evolution, but to provide a platform for achieving the opportunities inherent in the cellular intelligent network.


A basic element of INLink is SS7 transport. This transport provides connectivity between all relevant cellular network elements. It employs both A-Links and D-Links. It also puts in place a pair of Signal Transfer Points (STP) that are dedicated to the cellular intelligent network. SS7 A-links, circuits that connect an STP to a cellular switch, are the basic service used to facilitate direct switch-to-switch communications. SS7 BLinks are similar to A-links but are used to connect a cellular carriers STP to the cellular intelligent network STP. These links will carry the IS-41 messages to and from the cellular STP. The cellular STP will route the IS-41 messages to the appropriate location. This will be similar to an ALink and an SSP (e.g., Operator Switches) accessing the SS7 network for LIDB service. The SS7 network transport also provides global title translation.


In order for the cellular industry to take full advantage of the Cellular Intelligent Network, the SS7 transport network must perform Global Title Translation (GTT). Otherwise, carriers must maintain a complete routing database in their switch. Global title is an address that can be used to route a message, but it is not a physical network address. The global title allows a node to request that the network “find” a particular node based on data residing in the network. Under IS-41, a mobile switch can request the network “find” another mobile switch associated with a particular MIN. Unfortunately GTT is not available with IS-41 Rev. A, and carriers must wait for IS-41 Rev. B, planned for early 1993. In order to bridge the gap until Rev. B is implemented, the cellular intelligent network provides the GTT through its host, thus additionally facilitating the migration to SS7.


Since many switches will continue to use X.25 network transport, an important service provided is the provision of Bi-Directional X.25/SS7 protocol conversion. This protocol conversion offers the ability to convert an IS-41/X.25 message to and from IS-41/SS7.


Perhaps the most important service offered is the ability to provide interoperability between various revisions of IS-41 and Non IS-41 mobile switches. This interoperability function is performed by a non-stop, fault tolerant, System Control Point (SCP) based host processor. The interoperability host SCP provides message translation capability that will secure basic interconnection to switches at any level of technology deployment. This interoperability host SCP provides the critical link between those carriers who have deployed the latest revision of IS-41/SS7 and all other carriers. The interoperability host SCP ensures that the ubiquity and market reach required by all cellular carriers is preserved during the current period of technology evolution.


In addition to pure interoperability, the cellular intelligent network provides a vast array of services intended to assist the cellular carrier in reducing their fraud exposure. Two of the most important services are Call Teardown and Tumbling Detection. Call Teardown is important because not all mobile switches will employ precall validation. Therefore it is possible for fraudulent users to make a call before they are detected as a fraudulent user. The call teardown service will notify the mobile carrier to disconnect fraudulent calls in progress. Another service also detects tumbling fraud, or the action of arbitrarily scrambling Electronic Serial Numbers and/or Mobile Identification Numbers to make a connection, which has quickly turned into one of the industry’s most prevalent types of fraud.


The forth building block or capability offered is the ability for cellular carriers to access other Network Services through an Intersystem Gateway. The cellular intelligent network provides access to other third party enhanced services, other cellular networks and other local or interexchange carriers performing Personal Communication Services (PCS). This gateway uses B-links for STP to STP connections, as well as X.25 network connections.


The fundamental concept of the cellular intelligent network is to establish the building blocks for seamless network services. As I have demonstrated today no cellular intelligent network can be considered complete without the provision of a package of integrated “transitional” solutions. These “transitional” products are based on IS-41 and SS7 standards and will bring the benefits of IS- 41 to more carriers. The services are designed to provide the cellular carriers with the benefits they are want, improved operational efficiencies and increased revenues independent of the transition plans of other carriers.


Subscribers also will benefit from the cellular intelligent network because roaming will be easier. The cellular intelligent network will support interface and translation to non IS-41 systems using Follow Me Roaming® (FMR) or Phone Me Anywhere8 M (PMA) pre-IS-41 call delivery services. If not all markets within the cluster are IS-41 Rev. A compliant, activation’s can still be established via Autonomous Registration, eliminating the need for the subscriber to dial an activation code (*18). This means that as long as their cellular phones are on, roamers will receive their calls automatically without having to initiate call delivery with a “star code” (*). The cellular  intelligent network also will allow subscribers to take their custom-calling features with them wherever they roam, and can even be connected to their long distance provider of choice as they commute within INLink service areas.


While INLink represents a set of services and capabilities, more importantly it represents an overall approach to the flexible implementation of future technologies. GTE’s vision has extended beyond simple transport to the creation of a cellular intelligent network that will truly service the cellular industry.



[I] CTIA Seamless Network, “Request For Information,” May 28, 1992

[2] EIA/TIA S-41.5 (Revision A), “Cellular Radiotelecommunications Intersystem Operations Data Communications,” January 1991

[3] ANSI Tl.110.1-1987, “American National Standard for Telecommunications – General Information,” Section 2.2, June 15, 1987