IS-41 Network Signaling, SS7 protocol Layers

1. IS-41 Network Signaling

2. IS-41 Network Signaling
• Signaling System NO. 7
– PSTN signaling protocol
– interconnection between a PCN and the PSTN
– There are six types of SS7 signaling links.
– Two Types, A-Link (access link) and D-Link (diagonal link)
STP
SCP HLR
STP
MSCSSP
Trunk
PSTN
PCN (PCS Network)
(SCP)
A-Link A-Link
A-Link A-Link
D-Link

3. SS7 protocol Layers
OMAP MAP
TCAP
SCCP
ISDN-UP
MTP Level 3
MTP Level 2
MTP Level 1
OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
The SS7 Layers

4. Signaling System No. 7
– MTP (Message Transfer Part) – Transfer Signaling information
– MTP Level 2 provides reliable transfer of signaling message
– MTP Level 3 provides message routing and network management
– SCCP (Signaling Connection Control Part)
– Provides additional functions such as GTT to the MTP
– TCAP (Transaction Capabilities Application Part)
– Provides the capability to exchange information between using non-circuit-related signaling
– ISUP-UP (Integrated Services Digital Network User Part)
– Establishes circuit-switched network connections
– OMAP (Operations, Maintenance, and Administration part)
– Application of TCAP
– MAP (Mobile Application Part)
– Application of TCAP
– Both IS-41 & GSM MAP are implemented at this layer

5. Interconnection and Message Routing
Type 2A with
SS7 interface
STPSTP
MSCTandem
PSTN PCN
Type S (D-Links)
STP
MSCTandem
PSTN PCN
Type S (A-Links)
Type 2A with
SS7 interface
STPSTP
MSC
End
Office
PSTN PCN
Type S (D-Links)
Type 2B with
SS7 interface
STP
MSC
End
Office
PSTN PCN
Type S (A-Links)
Type 2B with
SS7 interface

6. Interconnection and Message Routing
– SS7 message routing is performed at the MTP and the SCCP of a node
(SSP,STP or SCP)
– Signaling messages are delivered with the actual destination address at the
MTP
– MTP level receives messages from TCAP, SCCP, ISUP
– DPC (destination point code) of the message uniquely identifies the
destination node
– Routing to the destination node is determined by the MTP using lookup
tables

7. Mobility Management Using TCAP
• TCAP operation
– More than 50 TCAP operation are defined
– For three purposes
• Inter-MSC handoff, automatic roaming, operations, administration,
and maintenance
• TCAP message consists of two portions
– Transaction portion – define package type
• Query With Permission, Response
• Conversation With Permission, Unidirectional
– Component portion – define number and types of
components
• INVOKE, RETURN RESULT
• RETURN ERROR, REJECT

8. Mobility Management Using TCAP
VLR2
MSC2
STP4
VLR1
MSC1
STP1 STP2 STP3
HLR
PCN1 PSTN PCN2
RegistrationNotification (INVOKE)
RegistrationNotification (INVOKE)
RegistrationNotification (RETURN RESULT)
RegistrationNotification (RETURN RESULT)RegistrationCancellation (INVOKE)
RegistrationCancellation (INVOKE)
RegistrationCancellation (RETURN RESULT)
RegistrationCancellation (RETURN RESULT) QualificationRequest (INVOKE)
QualificationRequest (RETURN RESULT)
ServiceProfileRequest (INVOKE)
ServiceProfileRequest (RETURN RESULT)
T6
T2
T3
T4
T5
T1

9. IS-41 TCAP message flow for MS
registration
– Transaction 1
• When MS is in MSC2’s service area, MSC2 sends a Registration Notification
(INVOKE) to its VLR(VLR2)
– Transaction 2
• If MSC1 is net served by VLR2, then VLR2 sends a Registration Notification
(INVOKE) to the MS’s HLR
– Transaction 3
• HLR sends a Registration Cancellation(INVOKE) to the MS’s previously visited
VLR (VLR1)
– Transaction 4
• The cancellation propagates to MSC1
– Transaction 5
• After T2 is completed, VLR2 creates a registration record for MS
• Sends a Qualification Request(INVOKE) to HLR to check MS’s qualification for
receiving services
– Transaction 6
• VLR2 sends a Service Profile Request(INVOKE) to HLR to obtain the service profile
for the roaming MS

10. PCN/PSTN Call Control Using ISUP
STP
MSC
PCN
STP
Tandem
End
Office
1 2 3
6 7
4
5
Conversation using trunks (6) and (7)
IAM
COT
ACM
ANM
REL
RLC
IAM
COT
ACM
ANM
REL
RLC
Step 1
Step 2
Step 5
Step 3
Step 4
Step 6
1 2 3 4 5
5 4 2 3 1
Signaling path from
PSTN to PCN
Signaling path from
PCN to PSTN

11. PCN/PSTN Call Control Using ISUP
• Typical message flow for type 2A with SS7 land-to-mobile call
setup and release involving a tandem switch
– When MIN is dialed, end office (EO) notices that the number is for
wireless service
– Suppose that EO has HLR query capability
– EO sends a query message to obtain MS’s TLDN (temporary local
directory number)
– Messages exchanged among switches, VLR and HLR are TCAP
messages

12. PCN/PSTN Call Control Using ISUP
– Step 1
• After the MS’s TLDN is obtained, EO sends an Initial Address Message
(IAM) to the PCN MSC to initiate signaling for trunk setup.
• When IAM is sent, IAM timer is set at the EO.
• EO expects to receive a response from MSC within timeout period.
– Step 2
• If IAM sent from the EO to tandem specifies a continuity check, selected
trunk from tandem to EO.
• After continuity check is successfully completed, Continuity Message
(COT) is sent from EO to tandem, and trunk is set up.
• Same procedure could be performed when MSC receives IAM from
tandem.

13. Cont.
– Step 3
• When IAM arrives at MSC, MSC pages the MS
• One of following three events occurs
– MS is connected with another call
» This situation is referred to as call collision
» Call is processed with call forwarding or call waiting, or MSC
returns REL message to EO with a cause indicating the busy line
situation
– MS is idle
» MSC send Address Complete Message (ACM) to EO
» Message informs EO of MS information, charge indications, and
end-to-end protocol requirements
» When the EO receives the ACM, the IAM timer is stopped
– Ms does not answer the page
» MSC returns REL message to EO

14. Cont.
– Step 4
• When MS answers call, an Answer Message (ANM) is sent from MSC to
EO
• Call is established through trunk path (6)(7)
– Step 5
• EO sends a Release Message (REL) to indicate that specified trunk is being
released from the connection
• Trunk is not set to idle is EO until a Release Complete Message (RLC)
message is received
– Step 6
• When MSC receives REL from EO, relies with a RLC
• After RLC is sent, EO and tandem waits for 0.5 to 1 sec. before it seizes
the released trunk for the next call