What is SS7 Protocol Stack?

A protocol is called a set of rules, whereas a protocol stack means a group of protocols running concurrently for implementing the network protocol suite. It determines the interconnectivity rules just like OSI and TCP/IP models for a layered network model.

Every module in the Protocol stack communicates with two layers, to make work and designing easier. The lower layers add more features to the upper layer.

The full form of SS7 is Signaling System 7, this protocol stack is arranged like a multi-layered stack which corresponds to the layers of the standard OSI model.

SS7 Protocol Stack

SS7 protocol stack

1. SS7 Level 1 [Physical layer]: The SS7 protocol stack of physical layer supports 56 or 64kbps of data rate and which carry raw signaling data, and it defines the physical and electrical characteristics of the link. It is virtually the same as layer 1 of the OSI model called level connectivity.

2. SS7 Level 2 [Data Link, MTP level 2]: Message transfer part level 2 provides a link layer function, making sure that the messages are exchanged reliably. It provides flow control, sequence checking, and error checking. In this layer, we use sequential numbering to determine if any messages are lost during transmission.

3. SS7 Level 3 [Network Layer, MTP level 3]: This layer depends on the services of Level 2 for providing routing, message distribution, and message discrimination, it includes node addressing, routing and congestion control, and alternate routing. From SPC to DPC through the network, the MTP level 3 uses multiple parallel routes by considering link loading and availability. This MTP Level 3 network layer helps in the creation of telephony network nodes that are interconnected by SS7 links.

[Upper Layers]

4. TUP (Telephone User Part):    This layer is used to set up a telephone call between two SS7 nodes. It provides telephone calls to be set up and torn down. It will establish immediately before a call is established and immediately before it terminates. When a call is in progress no TUP messages are exchanged. It is the first protocol designed to support Analog phones.

5. ISUP (Integrated Services User Part): It is the same as TUP, but it is a more sophisticated function available with primary rate ISDN. 
Integrated Services consists of:

1. Calling and called number notifications,
2. It can control billing rates,
3. Performs telephony functions, and
4. Control over whether the voice channel is used for voice, data, or fax.

6.  SCCP (Signalling Connection Control Part): It runs above MTP layers and provides facilities similar to UDP and TCP layers of TCP/IP. MTP does not deal with software applications it only receives and delivers messages from a node, SCCP allows call processing, advanced intelligent network (AIN), call return service, repeat dialing all these to be addressed explicitly and It also performs incremental routing with the help of  Global Title Translation (GTT) capability.  

SCCP provides four message classes:

1. Class 0: It indicates the connection-less non sequenced message.
2. Class 1: It indicates connection less sequenced delivery of messages.
3. Class 2: It indicates connection-oriented without flow control.
4. Class 3: It indicates connection-oriented with flow control.

7. TCAP (Transaction Capabilities Application Part): TCAP is used to implement functions unrelated to the origination and termination of actual telephone calls in the SS7 network. It also provides the information that can be transferred from an application at a switch location to another application in another network entity.

8. MAP (Mobile Application Part): It is the most complex SS7 component and is mainly used in GSM mobile telephone systems to pass information between the network components.

9. INAP (Intelligent Network Application Part): It is used to implement services within a network, that allows access to an SCP and also involves the use of an Intelligent Peripheral (IP). INAP messages are sent between network entities with the help of TCAP transactions.

10. OMAP (Operations and Administration Application Part): The OMAP is used by network administrators to control an entire network from a central point. OMAP provides facilities that include administration of system databases, performance monitoring, and maintenance access.

Terminologies Used

1. SSP: Signal switching point performs organization, termination, and switching of calls
2. SCP: Signal Control Point is a database that takes care of advanced call processing features.
3. STP: Signal transfer point performs routing functionalities and allow the exchange of signaling information with each other.
4. Tandem: It is called a Trunk Automatic Exchange that allows connection between two exchanges that are not having a direct trunk.
5. Trunk: the link between two exchanges.

Advantages of SS7

1. SS7 activates better voice circuit utilization.
2. When compared to voice transmission, it offers high reliability.
3. Signal links are available even during existing calls.
4. Does not require replacing old equipment for service expansions.
5. High-speed connection setup.
6. Data can be transferred without loss or duplication.

Disadvantages of SS7

1. Uses complicated switches.
2. Due to additional sub-networks, the cost is increasing.
3. Continued test procedures are required.