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Definition:-

Roaming is defined as the ability for a cellular customer to automatically make & receive voice calls, send & receive data, or access other services when traveling outside the geographical coverage area of the home network, by means of using a visited network.

If the visited network is in the same country as the home network, this is known as National Roaming. If the visited network is outside the home country, this is known as International Roaming (the term Global Roaming has also been used).

If the visited network operates on a different technical standard than the home network, this is known as Inter-standard roaming.

GSM Roaming, which involves roaming between GSM networks, offers the convenience of a single number, a single bill and a single phone with worldwide access to over 191 countries. The convenience of GSM Roaming has been a key driver behind the global success of the GSM Platform.

Jargon

HPLMN - Home Public Land Mobile Network (Home Network ie. say the state of West Bengal)
VPLMN - Visited Public Land Mobile Network (Visited Network - say the state where roaming - outside West Bengal)
Notional TAX - The Trunk Auto Exchange of a particular State (say for West Bengal it is Asansol; for Orissa it is Cuttack and so on).  Each state has a Level 1 TAX through which the calls for & from other states pass. 
PSTN Phone - Public Switched Telephone Network ie. a Landline Phone.
PSTN Charges - The fixed line phone charges either Local Call Charges or STD Charges
Air Time - The duration of the call ie. during which time the call is there in the Air.
POI - Point of Interconnect - This is where the mobile network is connected to the PSTN
SA - Service Area - The area served by a particular operator. Usual a state boundary.
MSC - Mobile Switching Center ie. Mobile Exchange.
BSC - Base Station Controller - It is an interface between the MSC on the one side and the BTS on the other.
BTS - Base Transeiver Stations (Towers)

Concepts

When a subscriber goes out his service area (say out of West Bengal), he is going out of his home network that is to say he is roaming in another service area (say Bihar). 

While in roaming he will be considered as a subscriber of that service area temporarily.
When he receives calls while on roaming, all his calls will reach the MSC of his Home NetWork in the first instance.  He will be traced in his visited network and the calls are therefore forwarded there.  This is because the caller expects his called party to be in his home network.  He may not be knowing that the called party is roaming or not and where he is roaming.
When he makes calls to his Home Network (HPLMN), he obviously knows that he is temporarily out of his home network and thus his calls are routed through his temporary home network ie. visited network (VPLMN).
When he makes calls within his visiting network which is his temporary home network, his calls are routed within that without being routed through his home network (HPLMN).
Even When he is making calls to networks other than his home network or the temporary home network, his calls are routed through the temporary home network only.

A CellOne West Bengal subscriber roaming in West Bengal (Making & Receiving Calls)
Home Network (HPLMN): West Bengal
Temporary Home Network or Visiting Network (VPLMN): West Bengal
Receives Calls from any where including West Bengal (HPLMN) and West Bengal (VPLMN): The caller's call reaches the HPLMN and from there it is transferred to VPLMN (West Bengal).
Makes Calls from West Bengal (VPLMN) to West Bengal (HPLMN): Call is transferred from West Bengal (VPLMN) to West Bengal (HPLMN).
Makes Calls from West Bengal (VPLMN) to other than West Bengal (other than HPLMN) say Tamil Nadu: Call from VPLMN is transferred to Tamil Nadu.

National TAX

Each state has one designated Trunk Auto Exchange (TAX) which is called a notional TAX.  All outbound/inbound calls to/from different states are to be routed through the Notional TAX.  Since the call is to traverse from one notional TAX to the other, it goes on STD route.  Generally the distance between any two TAXs will be more than 500 kms.  The STD charges beyond 500 km is Rs. 4.80p per Minute.  

Charging

Charging is again a very important and mind boggling issue.  We understand that there is a need to clarify the various situations so that the subscribers can make a prudent use of their cell phones while on roaming.  

Any charging in cellular services can have two components viz. (i) the Airtime (for the duration during which a call is in the air) & (ii) PSTN Charges (if the call is to traverse through PSTN Network).

For a mobile to mobile call within the same service area, there will only be air time charges and there will not be any PSTN charges.  

For call to reach out to a landline phone (within the State or outside), it has to traverse through the PSTN network.  Here there is a component of PSTN charge to be added to the Air time.  PSTN charges are determined by the distance.  Please refer to table of distances and the charges.

So also, for a call to reach to a mobile phone outside the Service Area, it has to traverse through the PSTN network between the two originating Service Area and the terminating Service Area.  Here again there is a component of PSTN charge to be added to the Air time.

ALL THE EXAMPLES ARE WITH REFERENCE TO CELLONE OF West Bengal
[the PSTN charges indicated hereunder are not valid from 01.05.2003 after the inception of IUC and declaration of new tariff structure]

CellOne to CellOne (Air Time Charges & STD Charges as applicable to the Plan should be taken)

For example: Considering Home Network (West Bengal) & Visiting Network/Roaming Network (West Bengal)
Notional TAX of Home Network (West Bengal) is Asansol
Notional TAX of Visiting Network (West Bengal) is Asansol

(a) Roaming CellOne Customer making a call to another CellOne of the roaming area: Only Outgoing Airtime charges.
(b) Roaming CellOne Customer making a call to another CellOne in Home NetWork: Outgoing Airtime Charges + PSTN STD Charges from notional TAX of Visiting Network (Asansol) to notional TAX of Home Network (Asansol) (Rs. 4.80 for Plan 325/Excel)
(c) Roaming CellOne Customer receiving a call from another CellOne of the roaming area: Incomg Airtime (Nil for Plan 325/Excel) + PSTN STD charges from notional TAX of Home NetWork (Asansol ) to notional TAX of Visiting Network(Asansol) (Rs. 4.80 per Minute for Plan 325/Excel).
(d) Roaming CellOne Customer receiving a call from another CellOne in Home Network: Incoming Air Time Charges + PSTN STD charges from notional TAX of Home Network (Asansol) to notional TAX of Visiting NetWork (Asansol) (Rs. 4.80 per Minute for Plan 325/Excel)
(e) Roaming CellOne  receiving a call from another Roaming CellOne  (both the caller and the called party are out of their home networks - they may be roaming in the same state or in different states):
If both of them are roaming in the same state (say West Bengal):
Calling Party Charges: Outgoing Airtime Charges (as per Plan) + PSTN charges from his Notional TAX of Visiting Network of the calling party (Asansol) to Notional TAX of Home Network of the Called Party (Asansol) (Rs. 4.80p per minute for Plan 325/Excel)
Called Party Charges: Incoming Airtime Charges (Nil for Plan 325/Excel) + PSTN charges from Notional TAX of Home Network called party (Asansol) to Notional TAX of Visiting Network of the called party (Asansol) (Rs. 4.80 p per minute for Plan325/Excel)
If both of them are roaming in different states (West Bengal CellOne in West Bengal and Tamil Nadu CellOne in Himachal Pradesh) - West Bengal CellOne in West Bengal calling Tamil Nadu CellOne in Himachal Pradesh)
Calling Party Charges: Outgoing Airtime (as per plan) + PSTN STD Charges from Notional TAX of West Bengal (Asansol) to Notional TAX of Tamil Nadu (Coimbatore) (Rs. 4.80 per Minute for Plan325/Excel)
Called Party Charges: Incoming Airtime (Nil for Plan 325/Excel) + PSTN STD charges from Notional TAX of Tamil Nadu (Coimbatore) to Notional TAX of Himachal Pradesh (Shimla) (Rs. 4.80 per minute for Plan325/Excel)

CellOne to Landline (Air Time Charges & STD Charges as applicable to the Plan should be taken)
(a) Roaming CellOne (in West Bengal) calling a Landline (in West Bengal) - Outgoing Airtime Charges + PSTN Local Charges (1.20 per 3 minutes)
(b) Roaming CellOne (in West Bengal) calling a Landline (in West Bengal) - Outgoing Airtime Charges + STD Charge from Notional TAX of Visiting Network (Asansol) to Notional TAX of Home Network (Asansol) (Rs. 4.80p per minute for Plan 325/Excel)
(c) Roaming CellOne (in West Bengal) receiving call from Landline (of West Bengal or of West Bengal or from any other state) - Incoming Airtime Charges (as per plan applicable, Plan 325/Excel: Nil) + STD Charge from Notional TAX of HomeNetwork (Asansol) to Notional TAX of Visting Network (Asansol) (Rs. 4.80p per Minute for Plan 325/Excel)
caution: this should not be quoted as an authority