Interview Q&A for O&M profile - Switch

Here are some of the questions which can be asked while interviewing for the above profile :-

Q1. Call Flow - between same MSC's, or When Subscriber is on Roaming ?

Q2. Call Forward Concept ?

Q3 Interfaces, Protocols & Signaling Between Different Network Entities ?
Ans -

Q4 Call Flow Through Channels, Details ?

Q5 About KPI reports, CGR Reports ?

Q6 What is Erlang & ASR ?

Expecting More Questios & Answers from Viewers !!!

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What is GPRS

GPRS (General Packet Radio Service)

It is a non-voice service added to existing TDMA networks, It is an enhancement to GSM or TDMA (IS-95) network. It uses existing cellular network infrastructure with software upgrade at base stations and the addition of a GPRS Gateway that connects the GPRS network to the Internet.
TDMA is the underlying transport mechanism used by GSM networks.
GPRS provides the transmission of IP packets over existing cellular networks.

GPRS Architecture -

Components :-

1. SGSN (Serving GPRS Support Node) - It monitors the state of the mobile station and tracks its movements within a given geographical area. It is also responsible for establishing and managing the data connections between the mobile user and the destination network.
2. GGSN (Gateway GPRS Support node) - It provides the point of attachment between the GPRS domain and external data networks such as the internet and Corporate Intranets. Each external network is given a unique Access Point Name (APN) which is used by the mobile user to establish the connection to the required destination network.
3. WAP Servers - Its used for General Information Services like Train Timetables, etc.
4. RADIUS Server - Remote Access Dial-in User Server. 

GPRS Mobile devices

The key use for GPRS is to send and receive data to a computer application such as Email, web browsing or even telemetry (telemetry refers to devices not being controlled by humans such as cash point machines or traffic monitoring cameras etc.). To use GPRS the service is 'dialed' in a similar manner to a standard data call (though there is no phone no.) at which point the user is 'attached' and an IP address is allocated. From then on data can flow to and from the Internet until either the network unattaches you (maybe because of a time-out, fault or congestion) or you manually unattach.

Some of the key issues are:

• Using GPRS will not stop you making or receiving voice calls.Current phones will usually suspend the data session while a voice call takes place.
• Battery life will be reduced when using GPRS.
• Do not keep it near your ear for long periods while data transfers are taking place.

As a business GPRS user you will have a choice of methods to connect to the GPRS network - by far the most common method will be via the Internet. For larger users you may connect your company LAN to the GPRS networks using leased lines or Frame Relay virtual circuits.

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When Mobile is turned ON

Initial Process When Mobile is turned ON :-

Network Attachment

Network attachment is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services, and making its location known to the network. The process starts when the mobile station is switched on, and ends when the mobile station enters the idle mode. In idle mode the mobile station does not have a traffic channel allocated to make or receive a call, but the Public Land Mobile Network (PLMN) is aware of the existence of the mobile station within the chosen cell.

The network attachment process consists of the following tasks:

Cell identification - When a mobile station is switched on, it attempts to make contact with a GSM PLMN by performing the following actions:

• Measure the BCCH (Broadcast Control Channel) channels.

• Search for a suitable cell.

The mobile station measures the signal strength of the BCCH channels received. It stores in list information about 30 of these BCCH channels, such as the signal strength and the frequency corresponding to these BCCH channels.

PLMN selection - A suitable PLMN is chosen.

PLMN Area - This area is the geographical area in which a particular PLMN operator provides land mobile communication services to the public. From any position within a PLMN area, the mobile user can set up calls to another user of the same network, or to a user of another network. The other network may be a fixed network, another GSM PLMN, or another type of PLMN. Other network users, and users of other networks, can also call a mobile user who is active in the PLMN area.

When there are several PLMN operators, the geographical areas covered by their networks may overlap. National borders normally limit the extent of a PLMN area.

Cell selection - Cell selection is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services.

Location update - In order to initiate a call or to receive a call, the mobile station tunes to the control channel (BCCH plus CCCH – Common Control Channel) of the chosen cell. Then, it registers its presence in this cell (registration process) by means of a location updating procedure.

No suitable cell found - If the mobile station is unable to find a suitable cell to access, it attempts to access a cell irrespective of the PLMN identity, and enters a "limited service" state in which it can only attempt to make emergency calls.

PLMN selection mechanism - The particular PLMN to be contacted can be selected either in one of the following modes:

• Automatic mode - In automatic mode, the mobile station will choose which PLMNs to try all by it. The automatic mode is based on the existence of the preferred list, which is stored in a non-volatile memory in the SIM. This list includes a number of PLMN identities in order of preference and is under control of the user. The most preferred is usually the home PLMN. The list is filled in by the user through a mechanism to be specified by the mobile station manufacturer.

• Manual mode - In manual mode, the user is presented a list containing all found PLMNs. The user chooses one of the PLMNs from the list.

Cell selection criteria - The mobile station attempts to find a suitable cell by passing through the list in descending order of received signal strength; the first BCCH channel which satisfies a set of requirements is selected. The requirements that a cell must satisfy before a mobile station can provide service from it, are:

• It should be a cell of the selected PLMN - The mobile station checks whether the cell is part of the selected PLMN.

• It should not be "barred" - The PLMN operator may decide not to allow mobile stations to access certain cells. These cells may, for example, only be used for handover traffic. Barred cell information is broadcast on the BCCH to instruct mobile stations not to access these cells.

• The radio path loss - This loss between the mobile station and the selected BTS must be below a threshold set by the PLMN operator.

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(GSM) Mobile to Mobile Call Flow, Within Same MSC/BSC

Hi Friends,

Here I tried to explain very basic call flow from mobile to mobile.

To make it simple following assumptions have been taken. A) Call is in same MSC and BSC. B) No Authentication, C)No Ciphering , D) Messages on Ater and Air interface are not consider, etc.



Call Flow Mesage Details:-

1. CM Service Request – Message contains Global Cell Identity, Kind of Service required by subscriber e.g Voice, SMS, Fax, USSD. Mobile Identity e.g TMSI/IMSI.
2. CM Service Accept – Message is in reponse of CM Service Request.
3. Set Up - Message contains dailed number from Mobile A (i.e Mob B number) and speech Codec.
4. Send Routing Info-Request (SRI) - After analysis of dialed numbers received from A in set up message, MSC recognises its own MSISDN series and send SRI to HLR to get the Mobile station roaming number(MSRN). This messge contains B MSISDN, Interrogation type e.g Basic Call etc.
5. Assignment Request - Message is to seize a channel for voice call for Mob A between MSC and BSC. Message contains Circuit Identification Code(CIC).
6. Assignment Complete - Acceptance from BSC end that same CIC has been seized.
7. Provide Roaming Number Request - HLR checks in its database to which VLR, Mob B is registered & ask for roaming number from that VLR. Message contains IMSI, MSISDN, VLR ID of Mob B.
8. Provide Roaming Number Response - Message is response of roaming no. request and contains roaming number of Mob B. This message is for HLR. In this VLR checks its database & allocate one free MSRN from its MSRN pool for B number.
9. Send Routing Info-Response - In this HLR forward same roaming number which it receives in above message to MSC.
10. Paging Request - In this VLR checks the MSRN recieved from HLR & search which MSISDN is against that MSRN, now MSC checks the VLR database of that MSISDN to get the LAC & TMSI/IMSI of that MSISDN (Mob B) & initiates the paging request message to that particular LAC on the basis of TMSI/IMSI in that BSC. BSC now page to that LAC on the basis of IMSI/TMSI of Mob B. Message contains LAC, IMSI/TMSI, DPC of BSC.
11. Paging Response - Mob B response to that paging with GCI (Global Cell Identity) to BSC that inturn forwarded to MSC.
12. Assignment Request - Message is to seize a channel for voice call for Mob B between MSC and BSC. Message contains Circuit Identification Code(CIC).
13. Assignment Complete - Acceptance from BSC end that same CIC has been seized.
14. Set Up - It is used to display Mob A number on Mob B handset.
15. Alert - This message indicates Mob A that Mob B is connected & its ringing. The Ring Back Tone is send from MSC to Mob A.
16. Connect - Mob B answers the call & connect message from Mob B is send to MSC, inturn MSC sends connect message to Mob A indiacting that phone has been answered & connects those 2 voice channels which were seized during the assignment complete message.
17. Connect Acknowledge - To acknowledge the connection.

Coments are expected.
Hope it has been informative for you.


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GSM IDENTIFIERS

IMEI - International Mobile Equipment Identity
Its an internationally-unique serial number allocated to Mobile Station (MS) hardware at the time of manufacture. It is registered by the network operator & stored in Aunthetication Center (AuC) for Validation purpose. This number consists of type approval code, final assembly code and serial number of the mobile station. The network stores the IMEI numbers in the Equipment Identity Register (EIR).

IMSI - International Mobile Subscriber Number
When a subscriber registered with a network operator, a unique subscriber IMSI identifier is issued & stored in the SIM (Subscriber Identity Module) of the MS. A MS can only functional fully, if it is operated with a valid SIM inserted into a MS with a valid IMEI.
The total length of the IMSI is 15 digits and it consists of the following elements:

MCC = Mobile Country Code (three digits)
MNC = Mobile Network Code (two digits)
MSIN = Mobile Subscriber Identification Number (ten digits)
 
TMSI - Temporary Mobile Subscriber Identity
Its used to protect the true identity (IMSI) of a subscriber. It is issued by & stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) updates takes place. The issues TMSI only has validity within a specific LA. The TMSI is used for security purposes, so that the IMSI of a subscriber does not have to be transmitted over the air interface. Its a temporary identity, which regularly gets changed.

MSISDN - Mobile Subscriber International ISDN number
It represents the True or dailed number associated with the subscriber. It is assigned to the subscriber by the network operator at registration & is stored in SIM. Its possible for a MS to hold multiple MSISDNs, each associated with different services. It contains the following elements,

CC= Country code (33=France, 358=Finland, etc.)
NDC= National Destination Code
SN= Subscriber Number

MSRN - Mobile Subscriber Roaming Number
It is temporary, location-dependent ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in VLR & associated HLR but not in the MS. The MSRN is used by VLR associated MSC for call routing within MSC/VLR service area.

CC = Country Code (of the visited country)

NDC = National Destination Code (of the serving network)

SN = Subscriber Number

LAI - Location Area Identity
Each location area within PLMN (Public Landline Mobile Network) has an associated Intenationally unique identifier (LAI). The LAI is broadcasted regularly by the BTSs on the Broadcast Control Channel (BCCH), thus uniquely identify each cell within an associated location area (LA). It structure is as follows,

MCC= Mobile Country Code (of the visited country)

MNC= Mobile Network Code (of the serving PLMN)

LAC= Location Area Code

CI - Cell Identifier
Its an identifier assigned to each cell within a network. however, CI is only unique within a specific LA. When combined with the internationally unique LAI for its associated LA, the Global Cell Identity (GCI) is produced which is also internationally unique.

BSIC - Base Station Identity Code
Each BTS is issued with a unique identity, the BSIC & is used to distinguish neighbouring BTSs. It is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station.



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What is GTT

GTT - Global Title Translation

First of all what is GT (Global Title)
*Global Title* (GT) is an *address* used in the *SCCP* protocol for routing signaling messages on telecommunications networks. In theory, a global title is a unique address which refers to only one destination, though in practice destinations can change over time.

OR

The routing address within an ISDN network is termed the point code. Global titles identify end terminals that may be beyond the ISDN network. To route information the Global Title is translated into a point code, this is typically conducted at a STP (Signalling Transfer Point).

GTT can also be defined in terms of routing as SCCP routing,

I mean for SCCP routing we usually requires GTT not only to STP but also to Switch that supports SS7 protocol, e.g.

In any MSU (a Signal Unit or data in SS7 n/w) if we have an OPC as well as DPC then we don't need to find out SSN for the destine node (that mean the STP will not manipulate the data its receiving from the Originating node it will simply route the data in the format in which it was receiving (this is called MTP routing that we usually don't perform) while in case of SCCP routing we do have an idea about only OPC not about DPC, to find out DPC we extract the some other parameters such as SSN, NAI, NP, and TT from the MSU (this all parameters comes under SCCP part of SS7 architecture (This is called SCCP routing).

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BASIC ISUP CALL FLOW

• 
  
  Initial Address Message (IAM) — First message sent to inform the partner switch (here MSC2) that a call has to be established on the CIC contained in the message. Contains the called and calling number, type of service (speech or data) and many more optional parameters.
  
  

• 
  
  Subsequent Address Message (SAM) — In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.
  
  

• 
  
  Address Complete Message (ACM) — Message returned from the terminating switch (here MSC2) when the subscriber is reached and the phone starts ringing.
  
  

• 
  
  Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.
  
  

• 
  
  Release (REL) — Sent to clear the call when a subscriber goes on hook.
  
  

• 
  
  Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again. This is also sent (without a preceding Release message) if the terminating switch determines that the call cannot be completed. The terminating switch also sends a Cause Value to explain the reason for the failure, e.g., "User busy".
  
  

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