Transcript Signaling and Network Control

NETW 704
Signaling &
Network Control
ISDN User Part (ISUP)
Dr. Eng. Amr T. Abdel-Hamid
Winter 2006
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ISUP
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Responsible for setting up and releasing trunks used for interexchange 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.
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ISUP (cont.)
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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
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Bearers and Signaling
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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.
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ISUP Signaling Mode
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ISUP Signaling Mode
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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.
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ISUP Protocol
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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.
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ISUP Message Flow
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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.
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Messages
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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.
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Message Timers
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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.
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Circuit Identification Codes
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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.
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CIC
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CIC (cont.)
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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.
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CID/DPC
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Enbloc and Overlap Address Signaling
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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 fixedlength 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.
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Enbloc
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Overlap
Signaling
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Overlap Signaling
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Overlap signaling is preferable because it decreases postdial 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.
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Circuit Glare (Dual-Seizure)
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Circuit Glare (Dual-Seizure)
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Resolving Glare
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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
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Circuit Glare (Dual-Seizure)
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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.
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ISUP Message Format
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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:
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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
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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
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Local Number Portability (LNP)
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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.
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LNP Specifications
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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.
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LNP Types
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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.
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LNP Solutions
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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.
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LRN
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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.
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Location Routing Number (LRN).
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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 unrecoverable costs 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.
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The History of Signaling
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Before 1889: Star connections between phones
1878: 1st Manual Exchange
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Less Wires
Busy Operators
Privacy and Security Allegations
1889: 1st Automatic Exchange (Strowger Exchange)
1896: Pulse Dial
1950s-1996s: Direct Distance Dialing then IDDD
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Manual Operator Vs Automatic Equivalent
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Call Routing
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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 longdistance call.
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Wireless Call Routing
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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.
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Wireless LNP
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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.
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Wireless LNP
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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.
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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.
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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).
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LRN Re-visited
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Number Portability
Administration Centers
(NPACs)
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Local Service
Management System
(LSMS)
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LNP Database
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Local Service Order
Administration (LSOA)
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Number Portability Administration
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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.
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Local Service Order Administration
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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.
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Local Service Management System
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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.
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LNP Database
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The database used to maintain LNP data can be one of
two types: an SCP or an integrated signal transfer
point (STP).
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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.
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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.
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Notes:
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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.
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Wireline LNP (not Ported)
LNP
4. 3916 is not ported
3. 935 is ported and forward it to LNP
2. Call is routed to end office
STP
STP
(514) 935-3916
SSP
SSP
5. Connect call as usual
(514) 935-0000
1. (514) 935-3916
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(514) 936
5. Take control of Call
LNP
SSP
4. 3916 is ported
(514) 935-3916
2. Call is routed to end office
SSP
STP
STP
SSP
(514) 935
(514) 935-3916
3. 935 is ported and forward it to LNP