SS7 ISUP CALL CONTROL

The advent of the SS7 network allowed signaling required to set up and tear down a call for transmission across dedicated signaling links, rather than actual communication circuits (e.g., a phone conversation). Analog tones once used for call control have now been replaced by digital MSUs, which are generated and processed by the ISUP layer.

Two phases are required for call completion:

1. Call Setup and Connection
2. Call Release

Different types of messages are used during each phase. When a subscriber initiates a call, an MSU is generated for ISUP at the local SSP. ISUP then locates the destination SSP that will be used to make the circuit connection. The two SSPs communicate to open a circuit and connect subscribers.

ISUP message types used up to this point would be:

• Initial Address Message (IAM)
• Address Complete Message (ACM)
• Answer Message (ANM)

When either subscriber terminates a call, the respective SSP generates an ISUP message. This message notifies the other SSP that the circuit is no longer needed. The other SSP responds to confirm that the circuit has been released. This ends the call.

ISUP message types used during this second phase of ISUP call control are:

• Release (REL)
• Release Complete (RLC)

This is an example of a basic call setup and teardown performed by ISUP.

• Person A has placed a telephone call for Person B, whose local phone company owns SSP B.
• SSP A sends an IAM ISUP message to SSP B, which responds with an ACM once it identifies the desired subscriber's phone.
• Person B's phone is now ringing (and Person A can "hear it ringing").
• Person B picks up the phone, and SSP B generates an ANM message to let SSP A know that the call is now in progress (and the billing cycle can start).
• The conversation between Person A and B continues over the voice circuit.
• Person A then hangs up the phone and SSP A transmits a REL message to notify SSP B that the circuit is no longer needed.
• SSP B confirms this with an RLC, completing the release of the circuit and ending the call (and the billing cycle).

SS7 Overview

You have reached the end of our SS7 Tutorial. You should now be able to:

• Define the components of an SS7 network.
• Create a diagram of a generic SS7 network setup.
• Define each layer within an SS7 protocol stack.
• Differentiate various types of signaling units.
• Use the fields of a signaling unit to identify its type.
• Recall categories of signaling network management functions in response to phrases that describe the functions' purpose.
• Recall criteria for establishing a signaling link and keeping it in service.
• Determine whether a signaling unit was transmitted and received without error.
• Predict how MTP Level 3 selects a link for an outbound message from SCCP.
• Predict how SCCP's class of service will affect delivery of MSUs to remote points.
• Describe components of TCAP messages.
• Recall the purpose of messages used by originating and destination points during ISUP call control.

For information on Ulticom's SS7 offerings, please contact sales@ulticom.com.

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