Signaling System 7 (SS7)

History of SS7: 

CAS: Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, signalling always occurs in-band (i.e. over voice channels).

SS7: Starting from 1980 (mainly in Europe), CAS was being replaced by SS7. The use of stored program control (SPC) exchanges made this possible. Signalling messages are transmitted over separate signalling channels and SS7 technology is based on protocol stacks.

CAS has two serious draw-backs:

1) Setting up a circuit switched connection is very slow.
2) Signalling to/from databases is not possible (setting up a circuit switched connection to the database would be extremely inconvenient).

In CCS ( Common Channel Signalling) or SS7  packet-switched signalling network is separated from circuit switched connections. Consequently:
1) Signalling to/from databases is possible anytime.
2) End-to-end signalling is possible before call setup and also during the conversation phase of a call.
 

Signaling System 7 (SS7) is an architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN).

The network is built out of the following three essential components, interconnected by signaling link:

Signal Switching Points (SSPs): SSPs are telephone switches (end offices or tandems) equipped with SS7capable software and terminating signaling links. They generally originate, terminate, or switch calls.

Signal Transfer Points (STPs): STPs are the packet switches of the SS7 network. They receive and route incoming signaling messages towards the proper destination. They also perform specialized routingfunctions.

Signal Control Points (SCPs): SCPs are databases that provide information necessary for advanced call-processing capabilities.

 

Basic Signaling Architecture:





1. STPs W and X perform identical functions. They are redundant together, they are referred to as a mated pair of STPs. Similarly, STP's Y and Z form a mated pair.

2. Each SSP has inks one to each STP of a mated pair. All SS7 signaling to the rest of the world is sent out over these links. Because the STPs of a mated pair are redundant, messages sent over either link (to either STP) will be treated equivalently.

3. The STPs of a mated pair are joined by a link (or set of links).

4. Two mated pairs of STPs are interconnected by links (or sets of links).

SS7 Link Types:

A Links (Access Links):
A links interconnect an STP and either an SSP or an SCP, which are collectively referred to as signaling end points . A links are used for the sole purpose of delivering signaling to or from the signaling end points (they could just as well be referred to as signaling beginning points).

B Links (Bridge Links):
B links, D links, and B/D links interconnecting two mated pairs of STP's are referred to as either B links, D links, or B/D links. Regardless of their name, their function is to carry signaling messages beyond their initial point of entry to the signaling network towards their destination.

C Links (Cross Links):
C links are links that interconnect mated STP's. 

D Links (Diagonal Links):
Used to connect paired STPs at one in the hierarchy to paired STPs at another level in the hierarchy).

E Links (Extended Links):
Used to connect SSPs to alternate or remote STPs for increased resiliance and load sharing).

F Links (Fully Associated Links):
Used to connect SSP's when significant traffic flows between them).

SS7 Protocol Stack:





MTP1:Message Transfer Part 1
MTP2:Message Transfer Part 2
MTP3:Message Transfer Part 3
SCCP:Signaling Connection Control Part
TCAP:Transaction Capabilities Application Part
MAP:Mobile Application Part
INAP or CAP:Intelligent Network Application Part,CAMEL Application Protocol
ISUP:ISDN User Part


MTP1 (physical layer):

MTP1 is equivalent to the physical layer of the OSI model
MTP1 defines the physical and electrical characteristics of the signaling link.
The physical interfaces defined for SS7 include E-1 (2048kb/s; 3264kb/s channels), DS-1(1544kb/s; 24 64kb/s channels), V.35(64kb/s), DS-0(64kb/s) and DS-0A(56kb/s).


MTP2 (Data link layer):

MTP2 ensures reliable end-to-end transmission of a message across a signaling link that directly connects 2 nodes.
Implements flow control, message sequence validation, error checking and error recovery.


MTP level 2 frame formats:



MSU (Message Signal Unit): Contains signalling messages (User Part SIO) The received frame is MSU if LI > 2 (number of octets)

LSSU (Link Status Signal Unit): Contains signalling messages for link supervision The received frame is LSSU if LI = 1 or 2

FISU (Fill-In Signal Unit): Can be used to monitor quality of signalling link
The received frame is FISU if LI = 0


MTP3 (Network layer):

MTP3 provides message routing between signaling points in the SS7 network.
 
MPT3 reroutes traffic away from failed links and signaling points and controls traffic when congestion occurs.

MTP3 routes messages based on the Signaling Information Field(SIF) of an MSU. The routing label consists of the Destination Point Code(DPC), Originating Point Code (OPC) and Signaling Link Selection (SLC) field. When an MSU arrives at an SP and the DPC matches the point code of that SP, the message is sent to the appropriate user part (SCCP or ISUP).





SLS defines the signalling link which is used for transfer of signalling information.

CIC defines the circuit (used for a certain circuit switched connection) with which the ISUP message is associated.

DPC – Destination Point Code  (14 bit  16384 SPs) Termination point of application transaction Key information for routing within SS7 network DPC is inserted by the originating MTP ”user”.

OPC – Originating Point Code  (14 bit) Originating point of application transaction.

The ”network indicator” in the  SIO octet  determines whether the DPC or OPC is an international, national.


SCCP (Signalling Connection Control Part):


Essential for end-to-end signalling & database access.
Global Title Translation (GTT) for enhanced routing.
Sub System Number (SSN) analysis at destination.
Transport Service Classes.

SCCP provides connectionless and connection-oriented network services via MTP3 for the transfer of signaling messages between SSP's. While MTP3 provides point codes to allow messages to be addressed to specific signaling points, SCCP provides Subsystem Numbers (SSN) to let messages be addressed to specific applications at these signaling points. MTP transfers messages node-to-node while SCCP transfers messages end-to-end.

MTP will support only Connection Less Service but SCCP will support both Connection Less and Connection Oriented Service.





Global title translation (GTT): Global title translation (GTT) is the SS7 equivalent to IP routing. Translation examines the destination address (e.g. the number being called) and decides how to identify it over the telephone network. This process can include global title analysis, which is the act of looking up the number and finding a result address.

Some of SCCP Messages: UDT, UDTS, XUDT, XUDTS etc.


GTT Structure:

A global title can be in a variety of formats, most of which are each defined in separate standards. The format parameter indicates which of the available formats are in use. Each format can include any of the subsequent parameters.
Numbering Plan Indicator

The Numbering Plan Indicator (NPI) describes which numbering plan will be used for the global title. The numbering plan chosen will aid the routing system in determining the correct network system to direct the message.

Ex: NP1(E.164) , NP7 (E,214)

The Type of Number (TON) or Nature of Address Indicator (NAI) parameter, which is of relevance to E.164 (regular telephone) numbers for example, indicates the scope of the address value, such as whether it is an international number (i.e. including the country code), a "national" or domestic number (i.e. without country code).

The translation type (TT) parameter is used in a network to indicate the preferred method of global title analysis . Normally in European networks, this parameter is set to 0 (the default) value. In North American mobile networks, different translation types are used for analysis of the IMSI and for messages between telephone systems. This parameter is valuable in complex routing problems, where the same number has to be routed differently depending on the circumstances.

Ex: TT=0, TT=25 etc.

Routing Indicator: It is used to determine routing method wether it is route onGT or route on SSN.

Classes of service in SCCP:

Class 0: Basic connectionless class. Each information block (SCCP message) is transmitted from one SCCP user to another SCCP user independently.
Class 1: Sequenced (MTP) connectionless class. All messages use the same SLS code.
Class 2: Basic connection-oriented class.
Class 3: Flow-control connection-oriented class. 

Transaction Capabilities Application (User) Part:


TCAP is used to connect external databases.

TCAP provides a means for SCP-to-SCP communication via STP's.
TCAP messages are used by SSPs to query an SCP (via an STP) to determine the routing numbers associated with a global title. The SCP uses TCAP to return a response containing the routing numbers back to the SSP via STP.
TCAP messages are transfered end-to-end using the services of SCCP.
TCAP is not limited to database access, it is also used to invoke other features from remote switches. TCAP services include freephone, calling card, and wireless roaming.


ISUP (Integrated Services User Part):

Establishment / release of circuit switched connections (basic call control) using link-by-link signalling.

End-to-end signalling between two exchanges (for this purpose SCCP + ISUP is used).

General (non-user-related) circuit management.

Call Setup using ISUP:



Some of ISUP messages : IAM,ACM,ANM,REL,RLC

IAM: Initial Address Message
ACM:Address Completeion Message
ANM: Answering Message
REL: Relese Request Message
RLC: Relese Completion Message