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.

Route And Route Set

A Route is a predetermined path in a signalling network, consisting of succession of different SEPs(Signalling End Point) / STPs for sending a signalling message from a “Originating Point Code (OPC)” to a particular Destination Point Code (DPC). For most of the practical purposes, a route corresponds to a LinkSet available at a particular Signalling Point to reach to a given DPC.

A RouteSet is a group of all possible Routes between two Signalling Points. 

As an example consider following network diagram

 

There are 2 Signalling End Points (SEPs) A and B . And 2 STPs P and Q

As shown in the above figure, for SEP A , STP P as well as  STP Q are the “adjacent Point Codes. The group links L0 to L3 between A & P form a Linkset (LS1). Similarly, the group links L0 to L3 between A & Q form another LinkSet (LS2).

When ‘A’ needs to send a signalling message to ‘B’, the message may traverse any of the following paths: A – P – B, A – P – Q – B and A – Q – B. However, from SEP A’s point of view, there are only two routes to choose from viz. A – P or A – Q.  Similarly from STP P’s point of view, to reach the DPC B, there are only two routes: P – B or P – Q – B. Thus, from SEP A’s point of view the two LinkSets are the two possible Routes to reach the DPC B.

Signalling Links And Linkset

A Signalling Link is the physical time slot within an E1 or T1 – connected at both ends to “signalling terminals”. A signalling link is a bi-directional channel used for exchange of signalling information (see ITU-T Q.702 for more details).

LinkSet is signalling relation between two “adjacent” SPCs (Signalling Point Codes). A LinkSet is a group of Links between two adjacent SPCs. In ITU-T SS7 (ITU-T MTP layers), a LinkSet can have maximum of 16 links (due to 4 bit SLS field) whereas in ANSI SS7 (ANSI MTP layers) a LinkSet can have maximum of 32 links (due to 5 bit SLS field).

Common Number Plans For GSM

E.164

E.164 is an ITU-T recommendation which defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers. E.164 numbers can have a maximum of fifteen digits. All mobile numbers that we use for calling are in this format.


Format of E.164 Address is as following

CC(country code) + NDC (network destination code) + SN(Subscriber Number)

For eg if you are India subscriber than CC will be 91, If in India you are Vodafone Delhi subscriber then NDC will be 9811 and SN can be any unique number under this CC NDC say 123456.
So you number will be CC+NDC+SN = 91 9811 123456

E.212

The IMSI conforms to the ITU E.212 numbering standard. An International Mobile Subscriber Identity or IMSI is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field in the SIM inside the phone and is sent by the phone to the network for identification. IMSI help network identify susbcriber and hence provide all requried services. E212 number can have maximum 15 digits.

Format of E.212 Address is as following

MCC(mobile country code) + MNC(mobile network code) + MSIN( mobile station identification number)

For eg if you are India subscriber than MCC will be 404, If in India you are Vodafone Delhi subscriber then MNC will be 10 and SN can be any unique number under this CC NDC say 1234567890.
So you number will be MCC+MNC+MSIN = 404101234567890

E.214

E.214 is a numbering plan used for delivering mobility management related messages in GSM networks. The E.214 number is derived from the IMSI E.212 numbers are composed of two parts. The first, the E.164 part, is made up of a country code followed by the network code. The second part of the number is made from the MSIN part of the IMSI which identifies an individual subscriber.



For eg if IMSI is  404101234567890 , then corresponding E214 address will be formedby replacing MCC(404) by CC(91) and replacing MNC(10) with NDC(9811) and keeping MSIN as is (as long as it is less than equal to 15 digits).

404 10 1234567890  becomes 91 9811 123456789

So basically number format of E.214 is similar to E164 , just that E214 is generated from IMSI and is not actual GT but a virtual GT.

E.214 numbers are routed separately from E.164 numbers since they are marked with a different Numbering Plan Indicator, however, it is possible to reuse the Global Title analysis tables used E.164 numbers everywhere except for the final destination network of the message. This saves considerable administrative work.