The S7 consists of various functional elements where the Message Transfer Part (MTP) is the common platform. MTP serves different user parts, such as Telephony User Part (TUP), Integrated Services Digital Network User Part (ISUP), and other functional elements like Signaling Connection Control Part (SCCP).
SCCP provides additional functionality to fulfill the need for extended services in certain applications. An example of an extended service is communication with the databases, Home Location Register (HLR) and Visitor Location Register (VLR), without any speech connection, e.g. communication during Location Updating.
The combination of the MTP and the SCCP is called the Network Service Part (NSP).
SCCP supports two network services:
- Connection Oriented (CO)
- ConnectionLess (CL)
The CO transfers many or long signaling messages between two nodes. In this case, it makes sense to “establish a logical connection” between the sender and receiver.
The CL transfers short messages, including routing information, to their destination.
Circuit-related vs. Non-circuit-related Signaling
Connectionless vs. Connection-Oriented Signaling
SCCP in GSM
The protocol between the Mobile services Switching Center/Visitor Location Register (MSC/VLR) and the Base Station Controller (BSC) is called Base Station System Application Part (BSSAP). BSSAP requires both the Connection Oriented (CO) and the ConnectionLess (CL) service.
The MSC/VLR, HLR, and GMSC communicate via the Mobile Application Part (MAP), using only the ConnectionLess (CL) mode. The Transaction Capabilities Application Part (TCAP) supports MAP. However, there is no need to describe TCAP functionality to understand how GSM works. While SCCP performs tasks similar to MTP; e.g. routing, SCCP allows only routing of MAP/BSSAP messages MTP and SCCP should be considered as two different networks that are linked together.
- CO – Connection-Oriented
- CL – Connectionless
- BSSAP – Base Station System Application Part
- BTAP – BSC/TRC Application Part
- CAP – CAMEL Application Part
- INAP – Intelligent Network Application Part
- ISUP – ISDN User Part
- MAP – Mobile Application Part
- TCAP – Transaction Capabilities Application Part
SCCP protocol classes
1. Connectionless (CL) Services:
- Class 0: Basic connectionless class
- Class 1: In-sequence delivery connectionless class
2. Connection-Oriented (CO) Services:
- Class 2: Basic connection-oriented class
- Class 3: Flow control connection-oriented class
SCCP Service Primitives
Service primitive – general syntax and examples
SCCP functional structure
Messages for Connection-Oriented Services:
Messages for Connectionless Services:
Parameters of some SCCP messages
1. Message Type = UDT (Unitdata)
2. Message Type = CR (Connection Request)
3. Message Type = DT1 (Data Form 1)
Overview of the structure of SCCP message
Detailed structure of SCCP message
Unitdata (UDT) message
SCCP enables an S7 to route MAP messages. Routing is always based on addresses. SCCP uses the following addresses:
- Calling address — identifies call origination
- Called address — identifies call destination
SCCP addressing is very flexible and makes use of three separate elements:
- Destination Point Code (DPC)
- Global Title (GT)
- SubSystem Numbers (SSN)
NA = Nature of Address
AI = Address Information
SSN = SCCP Subsystem Number
TT = Translation Table
NP = Numbering Plan
One, two, or three elements may be present in the address information for the called and the calling party. The form of the address depends on the service, application, and underlying network. The Address Indicator shows which information elements are present.
GLOBAL TITLE (GT)
The GT is of variable length, and can contain specified combinations of:
- Address Information (AI)
- Nature of Address (NA)
- Numbering Plan (NP)
- Translation Type (TT)
It does not contain information that allows routing in the signaling network. The translation function is required. The following sections contain details and typical values for some of the elements previously listed.
SUBSYSTEM NUMBER (SSN)
The terminating node examines the SSN to identify the concerned user (node).
8 GMSC, MSC
222 BSC (BSSAP) in case of ANSI signaling (GSM 1900)
224 HLR-R (HLR Redundancy)
254 BSC (BSSAP) in case of CCITT signaling (GSM 900)
3 ISUP (if ISUP uses SCCP)
Address Information (AI)
This is an address according to the numbering plan indicated. (See the example in Numbering Plan).
Nature of Address (NA)
NA indicates if the address is
3 – National
4 – International
format according to the numbering plan used.
Numbering Plan (NP)
NP indicates the numbering scheme from which the address originates:
1 – ISDN/Telephony Numbering Plan (E.163/E.164) e.g. MSISDN, GT address
7 – ISDN/Mobile Numbering Plan (E.214) e.g. IMSI, MGT (Location Updating)
Translation Type (TT)
A GT requires a translation function. The TT directs the message to the appropriate Global Title (GT) translation.
It is possible for the Address Information (AI) to be translated into different values for different combinations of DPCs, SSNs and GTs.
0 CCITT signaling
9 ANSI signaling
1-8 Used for the Service Center (SC) interface
10-254 MTS exchange property SMSFMOSMTRTYPE
Example: NA=4, NP=1, AI=49 172, TT=0
NP indicates a normal ISDN/Telephony number. NA indicates international format for AI. Therefore, 49 is the Country Code (CC) for Germany and 172 is the NDC for D2 operator, and the Translation Type (TT) is CCITT signaling.
SCCP address elements
Global Title Translation
AI – Address Information
NA – Nature of address
NP – Numbering Plan
TT – Translation Type
NS – Number Series
GTRC – Global Title Routing Case
PSP – Primary Signaling Point
SSP – Secondary Signaling Point
OWNSP – “Own” Signaling Point (terminating message)
Connectionless signaling sequence
Connectionless data flow
Data form 1 (DT1) message
Connection establishment data flow
Data transfer phase
Data transfer data flow
Connection release phase
Connection release data flow
SCCP Subsystem status management
SCCP addressing during a call to MS
SCCP addressing during location updating
SCCP addressing between MSC and BSC
Numbering Plan (NP)
- NP indicates how the global title address information is built up of different parts (for example, country codes, network codes, or a national significant number) according to the syntax and semantic, defined for that particular numbering plan.
- In GSM, two numbering plans are used: NP=1 (E.164, ISDN) and NP=7 (E.214, MGT, Mobile Global Title). E.164 (ISDN) numbering plan is normally used, while MGT is used to address some messages from the MSC/VLR to the HLR.
- E.164 is the numbering plan used in the PSTN/ISDN.
- Addresses using E.164 resemble “regular” phone numbers.
- The E.214 number (MGT) is derived from the subscriber’s IMSI by replacing the Mobile Country Code and Mobile Network Code by the ISDN Country Code and National Destination Code.