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ISUP Introduction

Overview

The Integrated Services Digital Network User Part (ISUP) is used for establishment of wired connections between exchanges. It includes messages associated with the connection and disconnection of calls. ISUP is the protocol used to support the signaling necessary to provide voice and non-voice services in telephone communications. It is an extension of SS7, used as the interface protocol for voice and data within, and for ingression or egression to/from the Public Switched Telephone Network (PSTN.)

Advantages of ISUP

There are numerous advantages to using ISUP for call processing and trunk group maintenance/management functions. These advantages include the following:

• Faster call setup

• Conservation of network resources

• Improved customer feature support as well as enhanced features

• Improved/Automated trunk management procedures

ISUP requires that the exchange be digital and support SS7. This does not mean that the exchange cannot interface with non-SS7 capable devices or exchanges. All that is required is that the exchange be able to interwork with other signaling types such as TUP, R2, R1, DTMF and/or SS7.

Definition

The ISDN User Part defines the protocol which is used to support the signaling functions required for both a) Non-ISDN voice/data communications and b) ISDN voice/data communications in North America, and for ISDN functionalities throughout the rest of the world. ISUP can also be used to support dedicated/private telecommunications systems consisting of either digital or analog or mixed networks. The data from ISDN messages is transferred to ISUP messages for call processing or data transfer. They are two separate protocols that function together. ISDN can be thought of as a user-side protocol while ISUP is strictly network side. ISDN may also be used between exchanges, but that is not the primary intent of the protocol. There is enough flexibility and spare capacity within the protocol to support present application requirements as well as, any foreseeable future requirements.

Currently the ISDN User Part is designed to use the services of the SS7 Message Transfer Part (MTP). The use of the SS7 Signaling Communication Control Part Network has been provided for in the specification but, at present, no procedures exist that require the use of the SCCP for ISUP. ISUP connects to the SS7 MTP as User Part 5. Because ISUP exists as a User Part, all Layer 4, 5, 6, and 7 functions are handled within the software that comprises ISUP.

End User Signaling

The exchange is required to provide connectivity and service to the end users of the service by the exchange. This end user’s ISDN connectivity is usually provided by the use of Primary Rate Interface. While ISUP is not the protocol used to the end user, it does support the services and functionalities provided to the end user by ISDN and other end user protocols.

Primary Rate Interface

The Primary Rate Interface (PRI) is implemented differently depending upon whether the user is following ITU-T or ANSI standards. Under ITU-T standards, the PRI consists of:

• 30 (64Kb) channels for voice/data,

• One, 64Kb channel for signaling/data (channel 16)

• One 64Kb channel for framing/synchronization (channel 0).

Under ANSI standards, the PRI consists of:

• 23 channels (64 Kb) for voice/data

• one (64 Kb) channel for signaling/data (channel 9).

PRIs are typically used between an exchange and a PBX or other device requiring high data transfer rates. The PRI in most of North America is 23B+D (1.544 Mb) and is equal to DS1, and is normally carried on a T1 carrier. In the rest of the world, the PRI is 30B+D (2.048 Mb) and is carried on an E1 carrier. Other configurations are possible. In North America, a single D channel may support its 23 B channels plus 24 B channels from each of an additional three PRIs. The PRI offers subscribers flexible bandwidth, high rates of data transfer, and a very stable platform.

Inter-Exchange Signaling

ISUP is used for the exchange of data between exchanges. Data is passed in messages between exchanges for call processing, maintenance, and circuit management. These messages are in the standard SS7 format of the MSU. The actual ISUP data is carried within the SIF.

An understanding of these messages is important as it allows you to know what type of data is being transferred between exchanges at any given moment in time. There are many messages defined in the ITU and ANSI standards. However, very few exchanges, if any, support all of them. It is a good idea to be familiar with the various messages even if they are not supported by your exchange. It is possible that one of these unsupported messages may be received at your exchange, and it is important to know how to deal with it. See the ISUP messages and parameters.

ISUP Call Control and Circuit Management API

The host manages ISUP call control and circuit management through the following mechanisms:

• Common Call Control API messages, using SS7 Parameter (ISUP) Jibs to pass data.

• Channel State

DS0 Status Change messages notify the host of service state changes and channel purges. The Service State Configure message allows the host to bring channels in and out-of-service.

• The PPL Event Indication and PPL Event Request messages allow the host to send and receive messages to and from PPL components.

The CSP sends PPL Event Indication messages to the host in response to various SS7-related call processing events, such as reception of an ACM, ANM, or CON (for ITU only) for ISUP.

Data is included in the SS7 Parameter ICB (ISUP). If you do not require this information, acknowledgement (ACK) the message and then ignore it.

The host can generate SS7-related call processing or management events using the PPL Event Request message.

You can modify the default call model to have the Called and Calling Party Numbers passed to the host as BCD encoded digits. See SCCP/TCAP for more information.

ISDN. Network architecture based on digital transmission, switching, and common channel signaling. Basic configurations include 2B+D called a Basic Rate Interface, and 23+D called a Primary Rate Interface. Outside North America, PRI is 30B+D.
See Integrated Services Digital Network User Part.
See Signaling System Number Seven.
Public Switched Telephone Network
Telephone User Part
See Integrated Services Digital Network.
MTP. Message Transfer Part of the SS7 signaling protocol.
See Message Transfer Part.
Call processing layer of the CSP.
PRI. One of two basic ISDN interface configurations providing 23 (30 outside of North America) Bearer channels (64 Kbps data) and 1 Data signaling channel (16 Kbps control and signaling data). Commonly refered to as ISDN PRI or 23B+D interface (30B+D outside of North America).
International Telecommunications Union (UN)
American National Standards Institute. ANSI administers and coordinates the U.S. voluntary standardization and conformity assessment system for the
telecommunications industry. More details on these standards may be
viewed at www.ansi.org.
International Telephone Union – Telecommunications sector (UN)
See Private Branch Exchange.
Digital communications link - voice, data, and video at a 1.544 Mbps rate (24 64 Kbps channels)
A digital communications link - voice, data, and video at a 2.048 Mbps rate; 30B/2D or 31B/1D (ISDN); non-North American version of T1.
Setting up, monitoring, and tearing down calls.
The condition of a component at a given instant. The current "context" of a PPL object, such as Idle, Seizing, Answered. In PPL files, a State is represented by a circle.
See Programmable Protocol Language.
A condition that, when met, drives a state machine from one state to another. A condition that drives a Component out of its current state by invoking an associated primitive.
See Information Control Block.
Positive acknowledgment to a message