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NETW 704. Signaling & Network Control. ISDN User Part (ISUP). Dr. Eng. Amr T. Abdel-Hamid. Winter 2006. ISUP. Responsible for setting up and releasing trunks used for inter-exchange calls. Created to provide core network signaling that is compatible with ISDN access signaling.

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Signaling network control

NETW 704

Signaling & Network Control

ISDN User Part (ISUP)

Dr. Eng. Amr T. Abdel-Hamid

Winter 2006


  • Responsible for setting up and releasing trunks used for inter-exchange calls.

  • Created to provide core network signaling that is compatible with ISDN access signaling.

  • Today, the use of ISUP in the network has far exceeded the use of ISDN on the access side.

  • ISUP provides signaling for both non-ISDN and ISDN traffic; used by basic telephone service phones.

  • The primary benefits of ISUP are

    • speed, increased signaling bandwidth, and standardization of message exchange. P

    • Provides faster call setup times than Channel Associated Signaling (CAS), it ultimately uses trunk resources more effectively.

    • Enables more call-related information to be exchanged.

Isup cont
ISUP (cont.)

  • Messages and parameters do vary between different countries, a given variant provides a standard means of exchanging information between vendor equipment within the national network, and to a large degree, at the international level.

  • ISUP consists of call processing, supplementary services, and maintenance functions.

  • Main components of ISUP:

    • Bearers and Signaling

    • ISUP Message Flow

    • ISUP Message Format

    • Message Timers

    • Circuit Identification Codes

    • Enbloc and Overlap Address Signaling

    • Circuit Glare

    • Continuity Test

Bearers and signaling
Bearers and Signaling

  • ISUP allows the call control signaling to be separated from the circuit that carries the voice stream over interoffice trunks.

  • The circuit that carries the voice portion of the call is known within the telephone industry by many different terms. Voice channel, voice circuit, trunk member, and bearer.

  • If the signaling travels on a single linkset that originates and terminates at the same nodes as the bearer circuit, the signaling mode is associated.

  • If the signaling travels over two or more linksets and at least one intermediate node, the signaling mode is quasi-associated.

Isup signaling mode1
ISUP Signaling Mode

  • The signaling mode used for ISUP depends greatly on what SS7 network architecture is used.

  • For example, North America uses hierarchical STPs for aggregation of signaling traffic. Therefore, most ISUP trunks are signaled using quasi-associated signaling.

  • U.K. uses quasi-associated signaling for some SSPs, they also heavily use associated signaling with directly connected signaling links between many SSPs.

Isup protocol
ISUP Protocol

  • A connection exists between ISUP and both the SCCP and MTP3 levels.

  • ISUP uses the MTP3 transport services to exchange network messages, such as those used for call setup and clear down.

  • Interworking with ISDN uses MTP3 and SCCP for transport.

Isup message flow
ISUP Message Flow

  • A core set of five to six messages represent the majority of the ISUP traffic on most SS7 networks.

  • A basic call can be divided into three distinct phases:

    • Setup

    • Conversation (or data exchange for voice-band data calls)

    • Release

  • ISUP is primarily involved in the set-up and release phases.

  • Further ISUP signaling can take place if a supplementary service is invoked during the conversation phase.


  • A core set of five to six messages represent the majority of the ISUP traffic on most SS7 networks. Yet, there are more than 50 messages that are used in the ISUP

  • A basic call can be divided into three distinct phases:

    • Setup

    • Conversation (or data exchange for voice-band data calls)

    • Release

  • ISUP is primarily involved in the set-up and release phases.

  • Further ISUP signaling can take place if a supplementary service is invoked during the conversation phase.

Message timers
Message Timers

  • ITU Q.764 defines the ISUP timers and their value ranges:

    • T7 awaiting address complete timer: Also known as the network protection timer. T7 is started when an IAM is sent, and is canceled when an ACM is received.

    • T8 awaiting continuity timer: Started when an IAM is received with the Continuity Indicator bit set. The timer is stopped when the Continuity Message is received.

    • T9 awaiting answer timer: started when an ACM is received, and is canceled when an ANM is received. If T9 expires, the circuit is released.

    • T1 release complete timer: T1 is started when a REL is sent and canceled when a RLC is received. If T1 expires, REL is retransmitted.

    • T5 initial release complete timer: T5 is also started when a REL is sent, and is canceled when a RLC is received. T5 is a longer duration timer than T1 and is intended to provide a mechanism to recover a nonresponding circuit for which a release has been initiated. If T5 expires, a RSC is sent and REL is no longer sent for the nonresponding circuit.

Circuit identification codes
Circuit Identification Codes

  • The separation of signaling and voice create the need for a means of associating the two entities.

  • ISUP uses a Circuit Identification Code (CIC) to identify each voice circuit.

  • For example, each of the 24 channels of a T1 span (or 30 channels of an E1 span) has a CIC associated with it. When ISUP messages are sent between nodes, they always include the CIC to which they use. Otherwise, the receiving end would have no way to determine the circuit to which the incoming message should be applied.

  • Because the CIC identifies a bearer circuit between two nodes, the node at each end of the trunk must define the same CIC for the same physical voice channel.

Cic cont
CIC (cont.)

  • ITU defines a 12-bit CIC, allowing up to 4096 circuits to be defined. ANSI uses a larger CIC value of 14 bits, allowing for up to 16,384 circuits.

  • An association must be created between the circuit and the SS7 network destination.

  • This association is created through provisioning at the SSP, by linking a trunk group to a routeset or DPC.

  • The CIC must be unique to each DPC that the SSP defines.

  • A CIC can be used again within the same SSP, as long as it is not duplicated for the same DPC.

  • CIC 0 used many times throughout an SS7 network, and even multiple times at the same SSP.

  • Unidentified Circuit Codes

    • When a message is received with a CIC that is not defined at the receiving node, an Unequipped Circuit Code (UCIC) message is sent in response. The UCIC message's CIC field contains the unidentified code. The UCIC message is used only in national networks.

Enbloc and overlap address signaling
Enbloc and Overlap Address Signaling

  • When using ISUP to set up a call:

  • The Called Party Number (CdPN) can be sent using either enbloc or overlap signaling.

  • In North America, enbloc signaling is used. Europe, both methods are used.

  • Enbloc Signaling: The enbloc signaling method transmits the number as a complete entity in a single message. When using enbloc signaling, the complete number is sent in the IAM to set up a call. Enbloc signaling is better suited for use where fixed-length dialing plans are used, such as in North America.

  • Overlap Signaling: Overlap signaling sends portions of the number in separate messages as digits are collected from the originator. Using overlap signaling, call setup can begin before all the digits have been collected. When using the overlap method, the IAM contains the first set of digits. The Subsequent Address Message (SAM) is used to transport the remaining digits.

Overlap signaling1
Overlap Signaling

  • Overlap signaling is preferable because it decreases post-dial delay. As shown in the preceding example, each succeeding call leg is set up as soon as enough digits have been collected to identify the next exchange.

  • overlap signaling is less efficient in terms of signaling bandwidth.

Circuit glare dual seizure1
Circuit Glare (Dual-Seizure)

  • Resolving Glare

    • When glare is detected, one node must back down and give control to the other end. while the other call must be reattempted on another CIC.

    • There are different methods for resolving which end takes control. For normal 64-kb/s connections, two methods are commonly used:

      • the point code and CIC numbers are used to determine which end takes control of the circuit. The node with the higher-numbered point code takes control of even number CICs, and the node with the lower-numbered point code takes control of odd numbered CICs.

      • prior agreement between the two nodes about which end will back down.

      • when glare occurs. One node is provisioned to always back down, while the other node is provisioned to

Circuit glare dual seizure2
Circuit Glare (Dual-Seizure)

  • Avoiding Glare

    • glare conditions can be minimized by properly coordinating the trunk selection algorithms at each end of a trunk group.

    • A common method is to perform trunk selection in ascending order of the trunk member number at one end of the trunk group, and in descending order at the other end.

    • use the "Most Idle" trunk selection while the other end uses the "Least Idle" selection.

    • The idea is to have an SSP select a trunk that is least likely to be selected by the SSP at the other end of the trunk group.

Isup message format
ISUP Message Format

  • The User Data portion of the MTP3 Signaling Information Field contains the ISUP message, identified by a Service Indicator of 5 in the MTP3 SIO field.

  • Each ISUP message follows a standard format that includes the following information:

    • CIC: The Circuit Identification Code for the circuit to which the message is related.

    • Message Type: The ISUP Message Type for the message (for example, an IAM, ACM, and so on).

    • Mandatory Fixed Part: Required message parameters that are of fixed length.

    • Mandatory Variable Part: Required message parameters that are of variable length. Each variable parameter has the following form: Length of Parameter, Parameter Contents

  • Because the parameter is not a fixed length, a field is included to specify the actual length.

  • Optional Part: Optional fields that can be included in the message, but are not mandatory. Each optional parameter has the following form: Parameter Name, Length of Parameter, Parameter Contents

Local number portability lnp
Local Number Portability (LNP) included to specify the actual length.

  • LNP was defined in the Telecommunications Act of 1996 as the

    “ability of users of telecommunications services to retain, at the same location, existing telecommunications numbers without impairment of quality, reliability, or convenience when switching from one telecommunications carrier to another.”

  • The Telecommunications Act mandated that all telecommunications service providers provide, to the extent technically feasible, number portability in accordance with the requirements prescribed by the Commission.

Lnp specifications
LNP Specifications included to specify the actual length.

  • The following are some highlights from the FCC docket:

    • The solution must support existing services and features.

    • LNP must use the existing numbering resources efficiently.

    • LNP cannot require subscribers to change their telephone numbers.

    • There can be no unreasonable degradation in service (such as call setup delays) or network reliability degradation when subscribers switch carriers.

    • No carrier can have a proprietary interest.

    • The LNP solution must be able to accommodate location and service portability in the future.

    • There can be no significant adverse impact outside areas where number portability is deployed.

Lnp types
LNP Types included to specify the actual length.

  • There are three phases to LNP:

  • Service provider portability, enables a subscriber to select a new local service provider while keeping his or her existing telephone number. (Same Rate Center)

  • Service portability: This enables subscribers to change the type of service they have while keeping their telephone numbers. For example, if a subscriber changes from a Plain Old Telephone Service (POTS) line to an Integrated Services Digital Network (ISDN) service.

  • Location portability: enable a subscriber to move from city to city, or even state to state, while maintaining the same telephone number.

Lnp solutions
LNP Solutions included to specify the actual length.

  • There have been several proposals for providing LNP without implementing a database:

    • Call forwarding.

      • Rejected because of the delay imposed on the calling party while the carriers tried to route the call.

    • Query-on-Release (QoR). When a call is routed to a number that has been ported, the receiving switch identifies the number as being vacant and returns an SS7 REL with an appropriate cause code. The originating switch would then initiate a database query to determine if the number had been ported. This approach

      • reduces the traffic across the SS7 network

      • lessens the impact of the database queries

      • places unnecessary delays on setting up telephone calls to subscribers who have changed carriers.

LRN included to specify the actual length.

  • The solution that was chosen was the LRN method. The end-office switches in the rate center have a table identifying all NPA-NXXs, which have numbers in them that have been ported. The specific number is not provided in the database, so the switch must initiate a query if it is determined that the number dialed was to an NPA-NXX considered as ported.

Location routing number lrn
Location Routing Number included to specify the actual length.(LRN).

  • The LRN method places a higher demand on the SS7 network, but ensures there is no degradation of quality or service for the subscriber who changes carriers.

  • The LRN also imposes huge unrecoverablecosts on telephone companies.

  • Intelligent Network/Advanced Intelligent Network(IN/AIN) triggers should be used to initiate queries.

  • A trigger expands the call-processing capabilities of switches by triggering defined events to take place (like initiating an LNP query).

  • Example, if received dialed digits equal a specific value, a query is sent to obtain additional routing instructions.

  • This will require software upgrades in all switching equipment to support IN/AIN triggering.

The history of signaling
The History of Signaling included to specify the actual length.

  • Before 1889: Star connections between phones

  • 1878: 1st Manual Exchange

    • Less Wires

    • Busy Operators

    • Privacy and Security Allegations

  • 1889: 1st Automatic Exchange (Strowger Exchange)

  • 1896: Pulse Dial

  • 1950s-1996s: Direct Distance Dialing then IDDD

Manual operator vs automatic equivalent
Manual Operator Vs Automatic Equivalent included to specify the actual length.

Call routing
Call Routing included to specify the actual length.

  • Telephone switches are assigned blocks of numbers, with the first three digits (office code) identifying the particular switch or central office the subscriber number is served by.

  • When calls are routed, only the first six digits are used (area code/office code, or NPA/NXX). When the call is delivered to the correct destination central office,

  • The switches recognize the office code as their own and route the call by the last four digits (the subscriber number).

  • Telephone companies established rate centers by dividing the exchanges into geographical areas

  • When a call is placed, the area/office code is used to determine if the calling party is making a local or a long-distance call.

Wireless call routing
Wireless Call Routing included to specify the actual length.

  • Subscribers are “portable,” moving from cell site to cell site. Their billing is determined by a completely different plan and does not match the same system used by wireline providers.

  • Each subscriber is assigned to a home mobile switching center (MSC), which falls within a specific wireline rate center.

  • If a mobile subscriber calls a wireline number, the billing is determined by the distance from the MSC to the wireline number.

Wireless lnp
Wireless LNP included to specify the actual length.

  • The wireless subscriber’s mobile identification number (MIN) has been used for determining the home MSC and how they would be billed for calls when roaming.

  • The MIN is also used for identifying the carrier providing the wireless services. The first six digits of the MIN identify the service provider for that subscriber.

  • This means the MIN can no longer be used for call processing, because the MIN cannot be ported.

  • Wireless networks rely on the MIN for call processing, billing, and virtually every transaction related to a mobile subscriber.

  • Use of the MIN to address portability would require too many database queries and impacts global title databases.

Wireless lnp1
Wireless LNP included to specify the actual length.

  • The wireless industry has elected to change the identification of mobile subscribers by assigning two numbers:

    • Mobile directory number (MDN)

    • The mobile station identifier (MSID) can use the MIN format, but the MSID is not portable.

  • In international wireless networks, the MIN is not recognized. Instead, networks use what is called the international mobile station identifier (IMSI), which is recognized in any network overseas. These networks are usually based on (GSM) technology.

  • LNP offers an opportunity to use the IMSI format when assigning MSIDs to mobile subscribers.

  • Wireline networks have agreed on the IN/AIN triggers for querying databases.

  • wireless networks do not necessarily support IN/AIN.

  • The industry is looking at IS-41 and GSM protocols for querying the LNP database.

  • Both the IS-41 and GSM protocols are being modified to support additional parameters for LNP.

  • LNP has required new parameters to the ISDN User Part (ISUP).

Lrn re visited
LRN Re-visited querying databases.

  • Number Portability Administration Centers (NPACs)

  • Local Service Management System (LSMS)

  • LNP Database

  • Local Service Order Administration (LSOA)

Number portability administration
Number Portability Administration querying databases.

  • Managed by a third-party company with no interest in the telephone business.

  • NPAC:

    • responsible for receiving requests from recipient carriers for the porting of telephone numbers.

    • coordinate the porting of the number:

      • by sending the data to the donor network

      • confirming the request has been accepted.

      • downloading the ported number data (which is the new LRN for the telephone number and other routing information) to all of the other networks connected to that NPAC.

Local service order administration
Local Service Order Administration querying databases.

  • Order information regarding a number being ported is sent to the SOA system.

  • The SOA:

    • process a subscriber’s order and track the order through completion.

    • provides all departments with a single record location regarding a service order

    • used to coordinate and track service order activities.

  • SOA communicates this information to the SOA in the NPAC.

  • SOA tracks activities of an order and provides the specifics about when a number is to be ported and who the donor network is.

Local service management system
Local Service Management System querying databases.

  • Serves as the interface between the carrier networks and the NPAC. The LSMS is responsible for collecting

  • porting data and downloading it to the LNP databases.

  • The LSMS is usually a computer system with database storage and must be able to verify the data within the database with the data stored at the NPAC.

  • This is accomplished through periodic audits between the LSMS and the NPAC.

  • LSMS audits the LNP databases within its own network.

Lnp database
LNP Database querying databases.

  • The database used to maintain LNP data can be one of two types: an SCP or an integrated signal transfer point(STP).

  • The top-of-the-line SCP is only capable of 850 transactions per second, while some newer STPs are capable of 20,000 transactions per second and higher.

  • To calculate the number of queries (transactions) per second, remember that every call made to an NPA-NXX with a directory number that has been ported will require a database query.

Notes: querying databases.

  • The only connection to the SS7 network provided is the database itself.

  • LSMS does not connect to the SS7 network, and neither does NPAC.

  • Communications between these entities are through a private communications link using Ethernet and TCP/IP protocols.

  • The SS7 network uses the information provided by the LNP database to route calls through the network.

  • This function is much like the database function used in 800 services.

Wireline lnp not ported

LNP querying databases.



Wireline LNP (not Ported)

4. 3916 is not ported

3. 935 is ported and forward it to LNP

2. Call is routed to end office

(514) 935-3916



5. Connect call as usual

(514) 935-0000

1. (514) 935-3916

LNP querying databases.



(514) 936

5. Take control of Call


4. 3916 is ported

(514) 935-3916

2. Call is routed to end office




(514) 935

(514) 935

3. 935 is ported and forward it to LNP

(514) 935-3916