Signalling Point code (SPC)

In the CCS7 network each signalling node or entity is identified by an unique code called as Signalling Point Code (SPC in short).This is a unique value within a network.

In case of ITU-T SS7, the SPCs are 14 bit long, while in ANSI SS7, the Point Codes are 24 bit long. Thus, in case of ITU-T, the maximum number of unique SPCs within a “signalling network” are limited to 2¹⁴ (16384), whereas in ANSI SS7 networks there can be upto 2²⁴ (16777216) unique SPCs.

There are different formats / styles in which the Point Codes are represented.

The ITU Point Codes are usually represented in simple decimal number format.

However the ANSI Point Codes are usually broken up into three parts and represented in the form of combination of the three parts. The three parts are A) the Network ID, B) the Cluster ID and C) the Member ID. All three parts are 8 bits long. The ANSI Point codes are represented as the equivalent decimal values of three parts separated by hyphen (e.g. 8-241-23 :: Network ID-Cluster ID-Member ID). This format is sometimes also called as the 8-8-8 format.

On the similar lines, the ITU Point Codes are also represented in Network – Cluster – Member ID format. The 14 bits of the ITU PC are divided in 3 bits of Network ID, 8 bits of Cluster ID and 3 bits of Member ID. Thus, this format is sometimes also called as 3-8-3 format.

In case of ANSI, the Network ID, by definition, is not permitted to be zero. In case of ITU, there is no such restriction. Hence the logical division among Network ID, Cluster ID and the Member ID may lead to a value of zero in Network ID portion of the ITU point code.

There are no standard mechanisms to convert ITU Point Codes into ANSI and vice versa. This is inherently because of the difference in the lengths of the PCs (14 bits in ITU v/s 24 bits in ANSI).

Usually, at the boundaries of two networks where one of the networks is in ITU domain while the other is in ANSI, the “Gateway” STPs are used. The Gateway STPs keep an internal mapping of ITU point codes represented by a dummy ANSI point code and vice versa.