Signalling System No. 7

Giriş

SS7 mobil telefon altyapılarında sinyalleşme için kullanılır. Açıklaması şöyle. SS7'den önce örneğin SS5 vardı

Signalling System No 7 (SS7), which is called Common Channel Signalling System 7 (CCSS7) in the US or Common Channel Interoffice Signaling 7 (CCIS7) in the UK, is a system that connects one mobile phone network to another.

It was first developed in 1975 and has many variants. Most networks use protocols defined by the American National Standards Institute and the European Telecommunications Standards Institute.

Açıklaması şöyle

SS7 is a set of protocols allowing phone networks to exchange the information needed for passing calls and text messages between each other and to ensure correct billing. It also allows users on one network to roam on another, such as when travelling in a foreign country.

Şeklen şöyle

MTP 1 – Layer 1 Physical interface

MTP 2 – Layer 2 Data Link

MTP 3 – Layer 3 Network

Buraya kadar her şey ortak

1. PSTN Açısından

TUP – Telephone User Part. Açıklaması şöyle

- Message Transfer Part carries SS7 signals between nodes.

- Telephone User Part carries voice calls between users.

- Data User Part carries digital data between users.

2. Intelligent Network Açısından

1. SCCP – Signaling Connection Control Part

Açıklama yaz

2. TCAP – Transaction Capabilities Application Part

Açıklama yaz

3. INAP - Intelligent Network Application Protocol or Intelligent Network Application

Açıklaması şöyle. Normal trafik içindir. Bazı yerlerde INAP yerine SIN kelimesi de kullanılıyor

Intelligent Network Application Part (INAP) is a Signaling System 7 (SS7) telecommunication protocol that provides the intelligent service logic control among various Functional Entities (FEs) in Intelligent Network (IN). 

Yani INAP HTTP protokolü gibi düşünülebilir.. INAP türevleri var. Bunlar Ericsson INAP, Nokia INAP, Siemens INAP or Huawei INAP olabilir. SMS mesajları sanırım INAP ile gönderiliyor

CAP - CAMEL Application Part

CAMEL kelimesi Customized Applications for Mobile network Enhanced Logic anlamına gelir. Açıklaması şöyle. Yani farklı ülkelerde roaming gibi şeylerde kullanılır.

The CAMEL Application Part (CAP) is a signaling protocol used in the Intelligent Network (IN) architecture.

...

CAMEL is used when the subscriber is roaming between networks, allowing the home network to monitor and control calls made by the subscriber. CAMEL provides services such as prepaid roaming services, fraud control, special numbers (e.g., 123 for voicemail that works everywhere) and closed user groups (e.g., office extension numbers that work everywhere).

As with CAMEL, CAP has been defined in four phases, each of which has an accompanying specification that builds upon the previous phase. 

Açıklaması şöyle. Yani INAP yerine CAP geldi.

Customized Applications for Mobile network Enhanced Logic also known as CAMEL was developed as a standard for mobile intelligence across different vendor equipments for GSM network. What this means is that the end user should be able to roam between different networks (maybe in different countries) and be reachable at the same number and should receive only one bill from the original service provider (Home Operator).

Before CAMEL, the GSM networks used INAP (Intelligent Network Application Part) for injecting intelligence in GSM networks. The limitations of INAP was that it did not support mobility management. CAMEL solved the mobility management problem and provided with much more functionality. With the evolution of 3G standards, CAMEL has evolved to provide new functionalities that will be discussed later.

Camel'ın safhaları var. Bunlardan birisi

Camel Phase 3

4. MAP

Açıklaması Mobile Application Part. Yukarıdaki şekilde MAP görünmüyor. O yüzden yeni bir şekil ekliyorum. USSD mesajları sanırım MAP ile gönderiliyor.

Diğer Bazı Kavramlar

MSISDN

Bildiğimiz telefon numarasıdır. Açıklaması şöyle. Yani 905xx xxx xx xx  şeklindedir.

In GSM and its variant DCS 1800, MSISDN is built up as

• MSISDN = CC + NDC + SN
• CC = Country Code
• NDC = National Destination Code, identifies one or part of a PLMN
• SN = Subscriber Number

IMSI

Açıklaması şöyle. Bence MSISDN ve IMSI aynı şeyler.

An International Mobile Subscriber Identity (IMSI) is a unique 15-digit number that identifies every mobile network user globally. The IMSI is divided into two parts. The first part is a six-digit or five-digit number based on the North American or European standards, respectively. The initial set of numbers identifies the mobile network operator in a specific country with whom the user is subscribed to. The second part of the IMSI number is allocated by the network operator to uniquely identify the subscriber.

Mobile Switching Centre - MSC

Açıklaması şöyle. Yani kullanıcının önündeki bir çeşit router

A Mobile Switching Center (MSC) is a core part of the GSM/CDMA network system. It acts as a control center of a Network Switching Subsystem (NSS). The MSC connects calls between subscribers by switching the digital voice packets between network paths. It also provides information needed to support mobile service subscribers. Based on the size of the mobile operator, multiple MSC can be implemented.

Service Switching Point - SSP

Örneğin Nokia'nin bir SSP'si var. Açıklaması şöyle. Çağrının başlayıp bittiği yer

What are SSP, STP and SCP Signaling System 7 nodes?

There are three main types of signaling nodes in a SS7 network: Service Switching Points (SSPs), Signal Transfer Points (STPs) and Service Control Points (SCPs). SSPs originate or terminate a call and are the initial point on the SS7 network. The control signals are routed through various STPs, which operate as interconnected switches on the SS7 network. The SCPs determine how to route a call or set up and manage some special feature.

SCPs and STPs usually exist as a collection of discrete nodes so that service can continue if one network point fails. The SCPs may also communicate with a Service Data Point, which stores the user database and directory. 

Service Control Point - SCP

Açıklaması şöyle. Yani bağlantının sağlanması için gerekli bilgileri bilen bir çeşit veri tabanı. Ben GSM-R projesinde bir SCP'de çalıştım

When a subscriber starts to make a call, this request is received by the network's Mobile Switching Centre (MSC). The MSC then sends a message that 'queries' the SCP's database. Note that the essential element of any CAMEL solution is a Service Control Point (SCP). This unit effectively hosts a database which holds the instructions needed for an intelligent application.

The SCP processes that query, comes up with an appropriate response and then sends a message back to the MSC telling what action it should take with the subscriber’s request for a specific service. The call is then connected in the most appropriate manner, a process which is transparent to the customer. A very good example of this process in action is short code dialing over a VPN (Virtual Private Network) where the user calls a colleague’s internal extension telephone number but is, in fact, routed to that person’s mobile phone which is roaming abroad.

Örnek bir akış şöyle. Burada SSP'den SCP'ye IDP MSISDN gönderildiği görülebilir

(MSISDN) -> SSP -> (IDP MSISDN) SCP

SCP -> (Continue/Release) SSP -> (MSISDN) SMSC

Initial Detection Point Message

INAP ile gönderilir. Açıklaması şöyle

... if the Service Switching Point (SSP) does require further instructions it sends an IDP to a Service Control Point (SCP) via an IN protocol. The protocol may be INAP but could be CAMEL, or bespoke INAP flavors such as Ericsson INAP, Nokia INAP, Siemens INAP or Huawei INAP.

Şeklen şöyle. Yani bağlantının yapılıp yapılamayacağını sorgular. Bağlantı için erişim, kontör kontrolü vs gibi herhangi bir şart kontrol edilebilir.