Migrating to SS8

Telecommunications networks around the world are relying more and more on SS7 for communication between switches, databases and other network elements. Although SS7 sends more signaling data per call and at much higher speeds than older tone-based signaling methods, it is not without problems. In fact, SS7's addressing problems are so severe that a new signaling system may be required.

A Global Problem SS7 is used around the world, but there is not a global SS7 network. Rather, SS7 is a collection of national networks. Each national SS7 network may differ in significant ways. Most important, the basic form of SS7 addressing ends at a country's border, requiring the more complex global title addressing. Even then, global title translation is more complex and less available between countries than within countries.

The basic SS7 address is a point code, a number that identifies a specific switch, database or other device on an SS7 network. Point codes in the United States and Canada are 24 bits long, allowing more than 16 million addresses. However, in other countries, point codes are different lengths: 14 bits in many, but also 16 bits (in Japan) or 24 bits (in China). The critical limiting factor is that point codes, no matter what their lengths, are reused in each country and cannot be used for communication between national SS7 networks.

SS7 also provides global title addressing, which takes a phone or card identifier and uses it for routing. For example, a wireless phone may be identified by a MIN or international mobile station identity (IMSI). These numbers also identify the phone's HLR. Through global title translation, messages can be routed through the SS7 network to the HLR without the originator of the message knowing the actual point code of this system.

This routing is performed by signal transfer points (STPs) that examine part or all of the global title and translate it into the point code of the next STP or destination. Global titles can be used internationally, if (and it is a big if) the international gateway is programmed to understand the specific global title, and the global title is supported by both national SS7 networks.

Internet Approach Compare SS7's approach to the Internet. Both have 2-level addressing schemes. Most people are familiar with the higher-level Internet addressing scheme (an e-mail address such as crowed@cnp-wireless.com or a web site address such as http://www.cnp-wireless.com). These addresses contain a domain name (cnp-wireless.com) that can be converted into a TCP/IP numeric address through a domain name query. These numeric addresses usually are hidden from users, but configuring an Internet connection usually requires entering at least one numeric TCP/IP address of the form xxx.xxx.xxx.xx. The Internet method for routing messages is first to present the domain name to a domain name server and then to use numeric routing on the TCP/IP numeric address that it provides. This contrasts with the SS7 approach where global title translation may occur in a piecemeal fashion at each STP or international gateway in parallel with routing of the message. Another striking contrast is that TCP/IP provides a completely global address, allowing people around the world to access Internet resources without regard to the country in which they are physically located.

These differences would be of purely academic interest if the SS7 approach did not cause any problems. However, the national scope of an SS7 point code is potentially a fatal problem, seriously affecting the development of international roaming and other new services. Wireless networks are avoiding the problem in the short term by using U.S. point codes to extend the ANSI (U.S./Canadian) SS7 network into other countries to allow international roaming. However, this forces countries that want to roam with the United States to support two parallel SS7 networks, one running the national SS7 protocol for wireline interconnection and free phone queries and another, ANSI, network for international roaming.

Beyond the cost and political problems that extending the ANSI network internationally causes, the distributed nature of global title translation makes it difficult and costly to implement each new global title, even though requirements for new global titles are occurring all the time. Every STP and international gateway in the world may have to be reconfigured to support each new global title. LNP has created a further mess for global title translation by forcing several global title translations to be cloned. One global title takes a potentially ported number and routes to the LNP database, and the other takes the routing number (location routing number) and routes to the destination switch or database.

An "Internet" approach to addressing would make the SS7 network much more effective. First, point codes should be global so that any network element around the world can be addressed. Second, global title translations should be performed by the originating network element by a query to a local database based on the type of address (directory number, credit card number, IMSI) and the type of destination required (calling name database, HLR, short message center). The local database may query other databases transparently but eventually will obtain and return the global point code of the destination. Routing then can be performed using the global point code, which can be stored to avoid subsequent queries.

New Protocol? It may not be possible to make such drastic changes to SS7 because the addressing concepts are implemented in such a low level in the network. However, it probably will become obvious to more people that something needs to be done, even if a completely new protocol needs to be developed. Global title addressing used to be one of the coolest parts of SS7, but now it is having a chilling effect on the implementation of new applications and even on the maintenance of existing capabilities when new concepts such as number portability have to be embraced.

TCP/IP almost certainly is not the right protocol for the telephony network. It is optimized for relatively large packet data transmissions, not signaling. Using the same protocol in telephony as in the Internet also would increase the danger of leaks. Just imagine the impact of "denial of service" attacks by a hacker who discovered a way to send a flood of TCP/IP messages from his basement computer into the telephony network. Further, although some of the addressing concepts of TCP/IP are significantly superior to SS7, the Internet protocols have their own problems, including routing inefficiencies and potential exhaustion of their current address format.

Eventually, telecom carriers are going to have to look at a replacement for SS7. "SS8" will have to be designed with a global viewpoint, or it will just be SS7 d ej a vu. Although a new protocol will be a major expense for the telecom industry, it has the potential to stem the death from a thousand cuts that SS7 currently is causing the industry.