Early history of ss7 signaling and Local Number Portability origination

In the early days of telecommunication, analog equipments were in service and were suited only for voice traffic. There was problems in data and video traffic. Individual companies were developing own methodologies for data traffic. As international telecommunication companies and vendors began investigating alternate technologies, the need for all-digital network came apparent.
As a result of series of studies and experiments performed by ITU-T, the final product of ss7 came into existence.


In the path of development, ITU-T developed signaling system #6 in mid 1960s. Later ss6 evolved to ss7 that became the standard signaling scheme for the whole world. The main advantages from the ss6 system was Common Channel signaling scheme with a single 64kbps device handling lot of traffic trunks, single medium cost hardware, simple message structure and very minimal or no delay.

The first deployment of ss6 in US used 2.4 kbps data links. These were later improved to 4.8 kbps. Messages were sent in form of data packets and were used to request connections between two central offices. The main feature of ss6 is usage of fixed signal lengths. The advantages in ss7 over ss6 are improved data rates from normal signaling link of 64 kbps or 56 kbps ranging to high speed links of 2 mbps. The signaling message length is also made variable with a maximum length providing more flexibility and versatility.

From 1980s, the implementation of ss7 started and ss6 was slowly phased out from service. In fact, the first usage of ss7 was not call setup or teardown. It was first used for accessing remote databases. In 1980s, the US telephone companies offered a new service called Wide Area Telephone Service (WATs) which uses a common 800 area code regardless of the destination of the call. This posed a problem for the switching equipment which used a area code to determine how to route the call through the PSTN.

To overcome this problem, a second number was assigned to every 800 number. This second number is used by the switching equipment to actually route the call through the voice network. This number was stored in a central database where all the central offices could access and it exists even today. If a person makes a call to 800 services, the central office through a data link would enquire the central database about the actual number to be used. Then the call is routed to the actual retrieved number.

Ss7 enabled LNP facility

During those period, if a customer wants to move from one operator to another operator, he has to surrender all of his 800 numbers and has to request them from new operator. The numbers issued by new operator will not be same and this leads to some loss and slowdown in work for the customers. With the advent of ss7, this got a solution with having all the numbers in the central database and accessing them from any of the central offices. In 1996 the FCC (Federal Communication Commission) approved LNP and made it as mandatory.

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