NSS/Circuit Switch Core Training

Understanding MSC Number Analysis of GSM Network

By

Arjay K. Panganiban

Prerequisites

• GSM/UMTS Basic knowledge• SS7 Application Layer Protocols

What is Number Analysis?

• GSM/UMTS refers to it as the treatment of the Core Network elements on numbering plans being requested/negotiated to establish services a network carrier provides.

• One of the most important functions of the Core Network. It is being done entirely in all elements e.g. (IN, SMSC/VAS, SGSN, HLR/AuC, MSC, EIR)

• On Circuit Switch Core it is used for call switching functions and mobility management.

Number Analysis – Scenarios

• MS – MS call• PSTN/other PLMN – MS call• MS – PSTN/other PLMN call• IN trigger services

- Originating CAMEL Subscription Information (OCSI)- Terminating CAMEL Subscription Information (TCSI)

• Value Added Services- MO/MT SMS- Unstructured Supplementary Services Data (USSD)- Supplementary Service Code Trigger (RBT)- Announcement playing

MS – MS Calls

MSC A MSC B

SGs/STPs

HLR

09229221234

SRI Request

SETUP

Provide MSRN Request

Provide MSRN ResponseSRI Response

IAM

Paging/Alerting

ACM

ANM

Hello..

MS-MS Calls Signaling Messages

• Setup – contains the subscriber initiated dialed number• SRI Request – After MSC A checks the B number contained in the SETUP message, It checks

how it will route the call. For this scenario, MSC A doesn’t know yet where the B number is located/roaming. It asks the HLR for routing. SRI contains the B number in E164I standard or in international MSISDN format. It also contains the address of the calling MSC.

• PRN Request – also known as provide MSRN request. HLR sends this request to MSC B for the latter to allocate a free MSRN number from its pool. This message contains the address of the calling MSC and the IMSI of the called number.

• MSRN - a directory number temporarily assigned to a mobile for a mobile terminated call.***• PRN Response – MSC B has allocated a free MSRN and informs the HLR of that number. After

the HLR receives the MSRN, it relays the information to MSC A thru SRI Response message.• SRI Response – This message could contain other data such as FTN, SS code trigger for RBT,

call barring, call release due to absent subscriber etc. For the scenario discussed, It gives MSC A the MSRN allocated by MSC B. MSC A then analyzes the MSRN and routes the call to MSC B.

• Initial Address Message (IAM) - message to initiate a call connection between to switches. In this scenario, between two MSCs. IAM contains the point codes of both MSCs the calling and the called number by MSC A and other information as required.

• Address Complete Message (ACM) – message sent by MSC B to MSC A informing the latter that the digits sent in the IAM message are valid and that a circuit connection has been established. Basically, when you here the RBT on your handset it means ACM was already sent by the called MSC to the calling MSC.

• ANM – message sent by MSC B to MSC A informing the latter that the call has been established.

*** data from wikipedia

PSTN/other PLMN – MS call

GMSC VMSC

SGs/STPs

SRI Request PRN Request

PRN ResponseSRI Response

IAM

Paging/Alerting

ACM

ANM

HLRSwitch of other PLMN/PSTN

IAM

Hello..

ACM

ANM

MS – PSTN/other PLMN call

GMSC VMSC

SGs/STPs

IAMACM

ANM

HLRSwitch of other PLMN/PSTN

ACM

ANM

IAM

SETUP

Paging/Alerting

Hello

CAP 2 IN trigger services - OCSI

MSC A

IN

09229221234

SETUP

IDP

Invoke Req.

RRBE, ACH, Continue, Connect etc.

Invoke Resp.

After the response of IN. MSC

transactions are the same with normal

postpaid transactions.

Subscriber is prepaid

CAP 2 IN trigger services - TCSI

IN

09229221234

SETUP

Subscriber is normal postpaid

MSC A MSC B

Subscriber has special services triggered by MT Call

SRI Request

SRI Response

First SRI Response Contains the service

key and SCP address the Called subscriber

is assigned

IDP

Invoke Req.

RRBE, ACR, Continue, Connect etc.

Invoke Resp.

After the response of IN. MSC

transactions are the same with normal

postpaid transactions.

IN trigger signaling messages

• IDP – Initial Detection Point. Message sent by the MSC to the designated SCP addressed by the called GT and the service key found in the profile of sub in HLR.

• Service Key – a unique service identifier in an SCP which can be used by the subscriber.

• RRBE – Request Report BCSM event. Signaling Message by the SCP to the MSC for the latter to send a report before, during and after the call.

• ACH – Apply Charging Report. Signaling message by the SCP to the MSC for charging of calls.

• ACR – Message from MSC to SCP to give details on the transaction initiated by the subscriber

• Continue – Message sent by the SCP informing the MSC to continue the call transaction.

• Connect – Message sent by the SCP informing the MSC to continue the call except that the caller or called number has been modified.

6 MSUs (720 Bytes)

VIZMINNCR

1 MSU = 40 Bytes

VIZMINNCR

SMSC Transaction Call Flow

SMS Delivery NotificationSMS Delivery Notification

Original file courtesy of CNOD - VAS

RBT Service

MSC A MSC B

SGs/STPs

HLR

09229221234

SRI Request

SETUP

Provide MSRN Request

Provide MSRN ResponseSRI Response (SS Code) IAM

Paging/Alerting

ACM

IAM

B num= 501639229221234

ACM

ANM

The Caller listens to the customized

RBT

ANM

RBT Switch

Thank You!