Signaling Units in SS7 Networks

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Questions and Answers

What does the Backward Sequence Number (BSN) indicate in a Message Signal Unit (MSU)?

The maximum length of the signaling information field

The total number of MSUs transmitted

The number of last MSU successfully received (correct)

The sequence number of the current MSU being sent

In a Message Signal Unit (MSU), what action is taken to prevent the appearance of the flag bit during transmission?

Transmitter deletes all zero bits

Bit stuffing is applied by inserting a '0' after five consecutive ones (correct)

Transmitter inserts an extra '1' after five consecutive zeros

The entire message is sent in a compressed format

Which component of a Message Signal Unit (MSU) is used to identify the length of the signaling information field?

Backward Indicator Bit (BIB)

Length Indicator (LI) (correct)

Cyclic Redundancy Code (CRC)

Forward Sequence Number (FSN)

What does the Forward Indication Bit (FIB) do in the context of a Message Signal Unit?

Remains unchanged until a retransmission occurs Signup and view all the answers

How many types of signaling units (SUs) are there according to the given information?

Three types Signup and view all the answers

What is the primary purpose of SLS (Signaling Link Selection)?

To select a path among alternative routes Signup and view all the answers

What does LSSU (Link Status Signal Unit) primarily carry?

Status information regarding link alignment Signup and view all the answers

What is the role of the Discrimination function in message handling?

To analyze messages for routing to local nodes Signup and view all the answers

Which signaling unit is transmitted when no other signal units are available?

FISU (Fill-in Signal Unit) Signup and view all the answers

What does the Status Field (SF) in LSSU indicate?

The current operational status of the link Signup and view all the answers

Which of the following is NOT a function of Level 3 signaling network?

Error detection Signup and view all the answers

How does Routing function ensure proper message delivery?

By ensuring control signals for a call follow the same route Signup and view all the answers

What is the main objective of Level 3 signaling network functions?

To provide network management functions Signup and view all the answers

What is the primary function of a Signaling Transfer Point (STP) in the SS7 network?

Routing messages to outgoing signaling links Signup and view all the answers

Which type of link connects signaling end points (SCP or SSP) to an STP?

A link (access) Signup and view all the answers

What is the primary goal of managing signaling links in the network?

To ensure no more than 10 minutes of non-availability in a year Signup and view all the answers

Which of the following STP types ensures redundancy within the network?

All Signal Transfer Points are paired Signup and view all the answers

What does the process of changeover in signaling traffic management do?

Directs traffic to alternative links when a link is unavailable Signup and view all the answers

What function does a Service Control Point (SCP) primarily serve in the SS7 network?

Accessing application-specific databases Signup and view all the answers

Which procedure is performed when there is link congestion in the signaling network?

Transfer restricted procedure Signup and view all the answers

What role does the MTP Level 3 play in the SS7 architecture?

Controlling traffic management and routing Signup and view all the answers

What role do Service Switching Points (SSPs) play in the SS7 network?

They create signal units and route calls to other SSPs Signup and view all the answers

Which type of link can connect two signaling end points that are fully associated within the same local network?

F link (fully associated) Signup and view all the answers

What is the main function of a Signaling Transfer Point (STP) in the network?

To route messages between signaling points like a router Signup and view all the answers

Which component of the SS7 network accepts queries for information from subsystems at different nodes?

Service Control Point Signup and view all the answers

What happens when a link or signaling point fails in the SS7 network?

Traffic is redirected away from the failed link or point Signup and view all the answers

What occurs during the changeback process in signaling traffic management?

Traffic is reestablished on a signaling link that has become available Signup and view all the answers

In signaling link management, what does automatic allocation entail?

Assigning signaling terminals to links automatically Signup and view all the answers

Which of the following protocols do SSPs use for database lookups?

TCAP Signup and view all the answers

What is the main function of the Initial Address Message (IAM)?

To establish a connection by providing necessary information Signup and view all the answers

Which protocol is specifically associated with managing voice and data call establishment and termination?

ISDN User Part (ISUP) Signup and view all the answers

In which scenario would the Address Complete Message (ACM) be relevant?

When a call is ringing back Signup and view all the answers

What does the Release (REL) message signify?

That the phone has hung up Signup and view all the answers

What role does the Transaction Capabilities Applications Part (TCAP) serve in the SS7 network?

Managing queries and responses between applications Signup and view all the answers

Which statement about TUP is correct?

ISUP has largely replaced TUP for call management in many regions. Signup and view all the answers

What type of messages does the Op, Maint and Adm Part (OMAP) cover?

Messages designed for network management and operations Signup and view all the answers

Which component is required to transport ISUP messages between concerned nodes?

Message Transfer Part (MTP) Signup and view all the answers

Study Notes

Signaling Units (SUs)

Message Signal Units (MSUs): These are the primary units used to transmit information across the SS7 network. They contain the data and control information for a call.
Link Status Signal Units (LSSUs): These units provide information about the status of the signaling link. They are used for network management purposes.
Fill-in Signal Units (FISUs): These units are transmitted when no other data is available to fill in empty space on the network. They help ensure consistent error monitoring.

MSU Structure

Flag: A distinct bit pattern used to mark the beginning and end of an MSU frame.
BSN: Backward Sequence Number - It tracks the most recently received and successfully processed MSU. Piggybacked acknowledgment is achieved by inverting the Backward Indicator Bit (BIB) if there's a negative acknowledgment.
FSN: Forward Sequence Number - It indicates the sequence number of the currently transmitted MSU. For retransmissions, the Forward Indication Bit (FIB) is inverted. FIB and BIB remain unchanged until a retransmission or a negative acknowledgment.
LI: Length Indicator - Indicates the number of octets in the Signaling Information Field (SIF), with a maximum value of 63.
SIO: Signaling Information Octet - Contains the Service Indicator which is the user's application type (e.g., ISDN User Part or Maint Data) .
SIF: Signaling Information Field - Contains the user data being transmitted within the MSU.
CRC: Cyclic Redundancy Code - Used for error detection. The CRC is calculated by the transmitting node and compared at the receiving node, helping to ensure data integrity.

LSSU Structure

Flag: Similar to MSU, the LSSU also employs flag patterns at the start and end of the frame.
BSN: Tracks the most recent MSU received.
FSN: Indicates the sequence number of the LSSU.
LI: Indicates the length of the Status Field, which is used instead of SIO and SIF for LSSUs.
SF: Status Field - Contains information on the signaling link's status, including:
- 000: Out of Alignment
- 001: Normal Alignment
- 010: Emergency Alignment
- 011: Out of Service
- 100: Processor Outage
- 101: Busy
CK: Cyclic Redundancy Check - Used for error detection.

FISU Structure

Flag: The FISU also utilizes flag patterns to mark the start and end of the frame.
BSN: Tracks the sequence number of the most recent MSU received.
FSN: Indicates the sequence number of the FISU.
LI: Specifies the length of the data field.
CK: Cyclic Redundancy Check - Used for verifying data integrity.

Signaling Network (Level 3) Functions

Message Handling:
- Discrimination: Analyzes the destination code within the address field and decides if a message should be routed to another node (STP) or if it should be distributed to a user part within the local node (SSP).
- Routing: Determines the path that a message will follow based on the signaling information field (SIF) and a pre-defined route table.
- Distribution: Determines the specific user part within a node that should receive the message based on information contained within the Service Information Octet (SIO).
Network Management:
- Traffic Management: Handles the flow of signaling traffic, including changeover, changeback, forced rerouting, controlled rerouting, signaling point restart, management inhibiting, and signaling traffic flow control.
- Link Management: Deals with activities like link activation, restoration, deactivation, link set activation, and terminal allocation.
- Route Management: Manages routing paths between signaling points, ensuring messages reach their destinations reliably.

SS7 Components

Service Switching Point (SSP): These switches are responsible for originating, terminating, and switching voice calls. They communicate with the signaling network using SS7 protocols and can query remote databases to find routing information.
Signaling Transfer Point (STP): These act as routers within the SS7 network, responsible for forwarding messages between signaling points. They do not handle call termination. There are three types of STPs:
- National Signal Transfer Point (NSTP): Responsible for routing messages within a national network.
- International Signal Transfer Point (ISTP): Handles international communication, connecting to other countries' STPs.
- Gateway Signal Transfer Point (GSTP): Connects between different SS7 networks, facilitating global communication.
Service Control Point (SCP): These serve as a front-end to databases containing information relevant to SS7 operations. SCPs accept queries from other SS7 nodes and provide data, such as credit card validation, routing information, and other call processing information.

SS7 Network

The SS7 network is composed of various STPs interconnected by signaling links. STPs act as routers, directing messages toward their destination.

SS7 Signaling Links

Link Types:
- A Link (Access Link): Connects a signaling end point (SSP or SCP) to an STP.
- B Link (Bridge Link): Connects two STPs in different networks, typically used for inter-network communication.
- C Link (Cross Link): Connects two STPs in the same network for redundancy, creating a mated pair.
- D Link (Diagonal Link): Connects a secondary STP to a primary STP in a quad-link configuration.
- E Link (Extended Link): Connects an SSP to an alternate STP.
- F Link (Fully Associated Link): Connects two signaling end points within the same local network.

Architecture: MTP Level 3 (Network)

Level 3 of the MTP specifies how messages are routed between signaling points.
It includes network management functions that ensure the reliability and stability of the SS7 network, including dealing with link and node failures, traffic control, and congestion.

Signaling System 7 (SS7) - Level 4

This level of the SS7 protocol stack focuses on signaling connection control and defines the procedures for establishing and managing connections between signaling entities.

Protocol Architecture: Level 4

Signaling Connection Control Part (SCCP):
- It allows applications within a node to be addressed individually.
- It enables the establishment of connections for more complex signaling applications beyond basic call setup, such as routing information for specific types of services. It also provides the mechanisms needed for network management.
ISDN User Part (ISUP) / Telephone User Part (TUP):
- These protocols define the messages and procedures for setting up, maintaining, and tearing down voice and data calls within an integrated services digital network (ISDN).
- TUP was initially used for telephone services, while ISUP is primarily used for ISDN signaling.
Operations, Maintenance, and Administration Part (OMAP):
- OMAP is used for managing the overall SS7 network and includes tools for diagnostics, performance monitoring, and configuration.
Transaction Capabilities Applications Part (TCAP):
- This protocol is designed for handling non-circuit-related data exchanges between different SS7 applications.
- It facilitates queries and responses between SSPs and SCPs, enabling functions like credit card validation, routing lookups, and other database operations.

ISUP Messages

Initial Address Message (IAM): Contains the caller's and callee's number and other vital information needed to establish a connection.
Address Complete Message (ACM): Acknowledges the IAM, indicating that the circuit is reserved and the ringing tone is being sent to the called party.
Answer Message (ANM): Indicates that the called party has answered the call.
Release (REL): Signifies that the call is being terminated.
Release Complete (RLC): Acknowledges the REL message, ensuring that all nodes involved in the call have received the call termination indication.

Signaling System 7 (SS7) Basic Call Setup

The process of establishing a call can be broken down into steps:
1. IAM (Initial Address Message): The SSP sending the call request sends an IAM message to the STP.
2. ACM (Address Complete Message): The STP receives the IAM and sends an ACM message to the SSP indicating the call is being established.
3. Ring Back Tone: The calling party receives a "ring back tone" indicating the call is going through.
4. ANM (Answer Message): When the called party answers, the called party's SSP sends an ANM to the STP.
5. Connection Established: The call is now established, and communication can begin.

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Description

This quiz covers the different types of signaling units used in SS7 networks, including Message Signal Units (MSUs), Link Status Signal Units (LSSUs), and Fill-in Signal Units (FISUs). Additionally, it delves into the MSU structure, focusing on key components like the Flag, BSN, and FSN. Test your knowledge on these essential communication protocols!