What is Signaling System-7 ?

Signaling System-7 (SS-7) is a set of protocols that is used to perform telecommunication all over the globe. It was developed in 1975. This protocol is not only limited to telephonic calls but also works like number translation, local number portability, prepaid billing, Short Message Service (SMS), etc are all possible because of this protocol only. 

In North America, it is known as Common Channel Signaling System-7 (CCSS-7). In the United Kingdom, it is known as C7 (CCITT number 7), and Common Channel Interoffice Signaling 7 (CCIS-7). In Germany, it is known as Zentraler ZeichengabeKanal Number 7 (ZZK-7). 

SS7 Probe :
SS7 probe is a physical device which makes it possible to record or to obtain information from a telecommunication network. A probe is capable of extracting the information from a network by monitoring the E1/T1 or SDH / SONET bearer channel. This work is done in a fully lawful manner. Lawful interception here means the delivery of calls and data to government-approved reception centers before completing the request. 

Example – The phone call made by us to any person is first sent to the established data centers before meeting the actual person. In the UK this interception is controlled by UK RIPA (REGULATION OF INVESTIGATORY POWER ACT 2000) and in Russia interception is done by Russia SORM. Both these agencies are set up legally. 

SS7 Vulnerability :
In 2014 a media report came out according to which there was a protocol vulnerability in SS7 by which anybody can track the movement of cell phone users from anywhere else in the world with a success rate of approximately 70%. In May 2017 O2, Telefonica which is a German-based company confirmed the use of SS7 vulnerabilities to achieve unauthorized withdrawals from the banks. 

SS7 Protocol Suite :
It has different protocols that are contained in it. The most important among them are : 

• Message Transfer Protocol (MTP) –
  It covers a portion of the OSI network layer and includes the following functions: network interface, information transfer, message handling, and routing to the higher levels. 
  
• SCCP –
  It completes the function of the OSI network layer, end to end addressing and routing, connectionless messages (UDTs), and management services for users. 
  
• Telephone User Part (TUP) –
  It is a link by link signaling system used to connect calls. 
  
• Transaction Capabilities Application Part (TCAP) –
  It is used to create database queries and call on advanced network functionality. It also links the Intelligent Network Application Part (INAP) for intelligent networks or Mobile Application Part (MAP) for mobile services. 
  

Message Transfer Part (M.P.T) :
It has 3 parts – 

1. Message Transfer Part-1 (MPT1) –
   It represents the physical layer, responsible for the connection of SS7 signaling points into the transmission network. 
   
2. Message Transfer Part-2 (MPT2) –
   It ensures the exact end to end transmission of a message. 
   
3. Message Transfer Part-3 (MPT3) –
   It provides routing functionality to transport messages to the destination. 
   

SCCP and TCAP : 
SCCP stands for Signaling Connection Control Part, Connection orientation, and Error correction facilities in telecommunications segmentation. 

TCAP stands for Transaction Capabilities Applications Part. It enables multiple concurrent dialogues between the same subsystem on the same machine. It uses transaction IDs to differentiate between them. 

MAP and INAP/CAP :
MAP stands for Mobile Application Part. It provides an application layer for the different GSM modes so that they can communicate with each other. 

INAP/CAP stands for Intelligent Application Part. It is an extended form of INAP and is a Customized Application For Mobile Network Enhanced Logic (CAMEL). It is also an application layer protocol to communicate with the service control point. 

ISUP/TUP :
ISUP is used to set up telephone calls in the Public Switched Telephone Network (PSTN). In SS7 ISUP is the only voice-related messages that are used to generate voice calls between two voice circuit.