Technical description of international mobile roaming May 2010 Prepared by the Ministry of Economic Development of New Zealand and the Department of Broadband, Communications and the Digital Economy of Australia
1. Introduction This document describes the technical processes of international mobile roaming (IMR) for voice, voicemail, and data, and the key charges associated with each. It was developed as supporting documentation for Trans-Tasman mobile roaming: discussion document and Analysis of retail trans-tasman mobile roaming prices: methodology, assumptions and price data. For simplicity s sake, this document describes the processes that apply to GSM networks, but the Ministry of Economic Development and the Department of Broadband, Communications and the Digital Economy (the Agencies ) understand that the general approach is similar with W-CDMA networks. Table of contents 1. INTRODUCTION... 2 2. KEY NETWORK COMPONENTS... 3 3. KEY WHOLESALE CHARGE COMPONENTS... 4 4. VOICE... 5 4.1 When a roamer makes a voice call... 5 4.2 When a roamer receives a voice call... 6 5. VOICEMAIL... 7 5.1 When a roamer retrieves a message from their voicemail... 7 5.2 When a message is deposited in a roamer s voicemail... 8 6.... 9 6.1 When a roamer sends an... 9 6.2 When a roamer receives an... 10 7. DATA... 11 7.1 When a roamer connects to Internet... 11 7.2 When a roamer receives a multimedia message service... 12 ISBN: 978-0-478-35839-1 (PDF) ISBN: 978-0-478-35842-1 (HTML) ISBN: 978-0-478-35845-2 (RTF) 2
2. Key network components The key components of the mobile networks used to provide IMR services are explained in the following table. Table 1: mobile roaming network components Term (in alphabetical order) Applicable Explanation services Controller (BSC) Subsystem (BSS) Transceiver (BTS) GPRS Support Node (G- GSN) (G-MSC) GPRS Roaming Exchange (GRX) (HLR) transit operator (MSC) Serving GPRS Support Node (S- GSN) MSC ( G- MSC) Visitor (VLR) All All All Data Voice Data All Voice & Voice & Data All Receives and transits radio signals between a BTS and an MSC. Comprises a BSC and all BTSs associated with that BSC. Receives and transmits radio signals between a mobile device and a BSC. It is commonly referred to as a cell site or cell tower. Acts as a gateway between an S-GSN and other data networks such as the Internet or private networks. Receives incoming calls and, after consulting the HLR, delivers them to the MSC in whose area of control the recipient is located. Acts as a hub between a visited network and a home network when a roamer sends or receives data. Like international transit operators, GRX operators compete amongst themselves to serve mobile network operators. A database that stores details of each SIM card authorised to use a mobile network. Such details include the services the SIM card allows access to, and the SIM card s location at any given time. A mobile network will usually have only one HLR. Company which provides transit services to networks which originate international traffic. Routes voice calls and that come from a BSS to their destination. It is broadly the equivalent of an exchange in a fixed network. Is responsible for the delivery of data packets to and from mobile devices. Receives from, and sends to, other mobile networks. A database attached to a BTS that temporarily stores details of each SIM card that enters the BTS s coverage area. Such details include the services the SIM card allows access to. This allows both national and international roaming. 3
3. Key wholesale charge components There are two key wholesale charges involved in international mobile roaming the inter-operator tariff (IOT) and the mobile station roaming number (MSRN) termination fee. A description of these follows. The inter-operator tariff is the tariff the visited network levies on the home network for providing outgoing voice and services, as well as data services, to the home network s customers. Strictly speaking, the IOT is the default tariff which can be found on the GSM Association s website. However, the term is widely used in the industry to refer to all tariffs charged to the home network by the visited network, including where negotiations between networks have led to a reduced fee. The Agencies have used this broader definition of IOT. The IOT covers the costs the visited network has incurred through the following: Origination of voice calls,, and data services on their own network. Arranging for the transit and/or termination of voice calls, and data. Roaming-specific costs costs involved in negotiating the original roaming agreement, in testing the roaming platform, and in billing the home network once the roaming service is up and running. The MSRN termination fee is charged by the visited network for terminating voice and communications that the home network s end user receives while roaming on the visited network. This fee is charged to the international transit operator through which the incoming communication has come, rather than to the home network. The international transit operator will incorporate the visited network s termination fee into its own invoice to the home network. The MSRN termination fee will typically be the same whatever the geographical origin of the incoming communication. Also, the MSRN termination fee will not typically incorporate the visited network s roaming-specific costs, as these are already incorporated into the IOT. 4
4. Voice 4.1 When a roamer makes a voice call When a roamer makes a voice call, the only networks involved in the carriage of the call are the visited network and the end network. If the end network is not the home network, but a third-party network, the home network is therefore not involved in the carriage of the call. 1. The visited network s MSC will transfer the voice call to the end network s G-MSC via the visited network s choice of international transit operator (assuming the end network is in another country). 2. The end network s G-MSC will query the end network s HLR, in order to determine the end network s MSC in which the recipient is located and using this information, determine the BSS in which the recipient s terminal can be found. 3. The recipient will then receive the voice call. Figure 1: Scenario roamer makes a voice call to a mobile phone in another country. Visited Network End Network Recipient s Although not involved in the technical process, the home network does of course become involved in the billing process. The visited network will charge the home network an IOT in order to cover the costs it has incurred for effecting the origination, and for arranging and paying for the international transit and the termination on the end network. In practice, it will also incorporate into the IOT roaming-specific costs. There is an exception to the description above. Some operators have implemented rerouting technologies that allow an outgoing voice call to be rerouted through the home network (much like an outgoing or uploaded data). 5
4.2 When a roamer receives a voice call When a roamer receives a voice call, all three networks (home network, visited network, and the originating network) will be involved in the carriage of the call. 1. The originating network, if not the home network, will send the call to the home network s G-MSC. 2. The home network G-MSC will then interrogate the home network s HLR and using the location information it finds (and in particular the relevant MSRN 1 ), send the voice call to an international transit operator for delivery to the relevant visited network G-MSC. 3. The visited network G-MSC concerned will query the VLR to determine the BSS in which the recipient s terminal can be found. 4. The recipient will then receive the voice call. Figure 2: Scenario roamer receives a voice call from a mobile phone. Originating Network Network Visited Network (end network) Caller s to ITO* - - Visitor * - if the Originating Network is in a different country from the Network. Regarding billing, the visited network will charge the international transit operator an MSRN termination fee in order to cover the costs it has incurred for effecting the termination. The international transit operator will incorporate this fee into its own invoice to the home network (this is known as cascade billing ). 1 A mobile station roaming number (or MSRN) is a temporary number assigned to a roamer s device by the visited network. It is different for every communication received by the roamer, even if s/he is in the same location. 6
5. Voicemail 5.1 When a roamer retrieves a message from their voicemail At a technical level, when a roamer checks voicemail, they are making a roaming call back to their home network. 1. The visited network s MSC will transfer the voice call to the home network s G-MSC via the visited network s choice of international transit operator. 2. The home network s G-MSC will query the home network s HLR in order to determine the details of the recipient, and upon recognising that the recipient is the home network s voicemail system, transfer the call to that system. 3. The roamer will then hear the voice message(s). Figure 3: Scenario roamer retrieves a message from their voicemail. Visited Network Network Voicemail Regarding billing, the visited network will charge the home network an IOT, in order to cover the costs it has incurred for effecting the origination, and for arranging and paying for the international transit. In practice, it will also incorporate into the IOT any roaming-specific costs. 7
5.2 When a message is deposited in a roamer s voicemail A roamer may be charged for messages deposited in their voicemail, independently of the fees they must pay to later retrieve (listen to) those messages. At a technical level what the roamer is paying for is, firstly, the carriage of the call over to the destination they are in and secondly, when the visited network discovers that the call is not being answered, the return carriage of the call back to the roamer s home network for deposit in the roamer s voicemail system back home. This two-way trip is referred to as tromboning. Figure 4: Scenario message deposited in a roamer s voicemail from a mobile phone. Originating Network Network Voicemail Visited Network (end network) Caller s to ITO* - - Visitor * - if the Originating Network is in a different country from the Network. Before leaving on a trip abroad the roamer can arrange an unconditional call diversion (that is, all incoming calls go straight to voicemail), in which case the roamer is not charged an international voicemail deposit fee. Regarding billing, the Agencies understand that the visited network will: charge the international transit operator (who will pass it on to the home network) an MSRN termination fee (even though the call is not actually terminated), and charge the home network directly an IOT, in order to cover the costs it has incurred for effecting a return origination, and arranging and paying for the return international transit (in practice, it will also incorporate into the IOT any roaming-specific costs). In addition, some operators have implemented rerouting technologies that can detect when a call is unanswered to allow a voicemail deposit to go straight to the home network s voicemail server without the visited network needing to arrange the return carriage of the call. 8
6. 6.1 When a roamer sends an When a roamer sends an, all three networks (home network, visited network, and the end network) will be involved in the carriage of the. 1. The visited network s MSC will query its VLR to see what the roamer s home network is, and transfer the to the home network s G-MSC via the visited network s choice of international transit operator. 2 2. The home network s G-MSC will interrogate its HLR to learn what network the recipient is on. If the is being sent to a third-party network, the home network s G-MSC will then forward the message to the end network s G-MSC. 3. The end network s G-MSC will then query the end network s HLR, in order to determine the end network MSC in which the recipient is located, and using this information, determine the BSS in which the recipient s terminal can be found. 4. The recipient will then receive the. Figure 5: Scenario roamer sends an to a mobile phone. Visited Network Network End Network MSC ITO* MSC to the Recipient s Visitor * - if the End Network is in a different country from the Network. Regarding billing, the visited network will charge an -IOT to cover the costs it has incurred for effecting the origination and for arranging the initial international transit. In practice, it will also incorporate into the IOT any roaming-specific costs. 3 The visited network will typically bill the home network the same -IOT whatever the destination of the sent by the roamer (New Zealand, Australia or a third country). This is because, whatever the destination of the outgoing, the role of the visited network remains the same: simply to deliver the to the home network. 2 For international transit, the international transit operator is usually an SS7 signalling gateway operator. 3 This may include the cost of terminating to roamers on its network, since such termination is zerocharged. 9
6.2 When a roamer receives an When a roamer receives an, all three networks (home network, visited network and the originating network) are involved in the carriage of the. 1. The originating network, if not the home network, will send the to the home network s G-MSC. 2. The home network s G-MSC then interrogates the home network s HLR, and using the location information it finds (and in particular the relevant MSRN), sends the to the international transit operator for delivery to the recipient. 3. The visited network MSC concerned will then query the VLR to determine the BSS in which the recipient s terminal can be found. 4. The recipient will then receive the. Figure 6: Scenario roamer receives an from a mobile phone. Originating Network Network Visited Network (end network) Sender s to ITO* MSC MSC Visitor * - if the Originating Network is in a different country from the Network. Regarding billing, by convention the visited network will not charge an termination fee. 10
7. Data The Agencies understand that, for data roaming, the visited network provides the bearer ( bit-pipe ) and the home network provides the service (internet access, WAP access, etc). The visited network cannot technically tell the difference between any data service provided by the home network. For example, the visited network cannot tell the difference between an internet session and a multimedia message service (MMS). As a result, the visited network will bill the home network for all the bits that pass via its network at the same rate (whether through origination/upload or through termination/download). There can, nevertheless, be some technical differentiation between certain data roaming services. Two examples are provided below. 7.1 When a roamer connects to internet When a roamer initiates a connection to the internet through their mobile device, a connection will be set up from the mobile device through to the internet, passing via the visited network and home network. 1. The device first connects to the S-GSN of the visited network. The S-GSN will establish a connection with the G-GSN of the home network, via a GRX. 4 2. The G-GSN of the home network will then open the connection to the internet. The roaming customer may now surf the web and send-e-mails. Traffic may pass via an international transit operator. Figure 7: Scenario roamer connects to internet. Visited Network Network Serving GPRS Support Node GPRS Roaming Exchange GPRS Support Node Internet Regarding billing, the visited network will charge the home network a data IOT to cover the costs it has incurred for effecting the origination and/or termination of the data from/to the mobile device and for arranging the initial transit to the GRX. In practice, it will also incorporate into the data IOT any roaming-specific costs. 4 A GRX is a private IP network interconnecting GPRS mobile operators, in a manner much more secure than the internet could. There are many GRX providers (e.g. Belgacom, C&W, Comfone) and together all the GRXs ensure that all GPRS mobile operators are connected. 11
The charging interval for the data IOT can be per kb, 10 kb, 100 kb, and so on. There can also be a staggered reduction in the price, whereby, for example, the first 100 kb is charged at a higher rate than later units of kb, during a given connection. 7.2 When a roamer receives a MMS When a roamer receives a MMS the following process occurs. 1. The originating network s MMS-C passes the MMS to the home network s MMS-C. 2. The home network s MMS-C interrogates the home network s HLR in order to determine the current location of the customer (the precise S-GSN to which they are connected). Upon discovering that the customer is on a given S-GSN of the visited network, the MMS-C will activate an end-to-end connection with the terminal of the roaming customer, via the G-GSN of the home network, the GRX, and the S-GSN of visited network. 3. The MMS is downloaded from the MMS-C of the home network, to the handset of the customer. Figure 8: Scenario roamer receives a MMS from a mobile phone. Originating Network Network Visited Network (end network) Sender s to ITO* MMS GPRS Support Node GPRS Roaming Exchange Serving GPRS Support Node * - if the Originating Network is in a different country from the Network. Regarding billing, because a visited network cannot tell the difference between an internet connection and an MMS, it will apply the exact same billing principles as in the internet connection example above. Thus the visited network will charge the home network a data IOT, in order to cover the costs it has incurred for effecting the termination of the MMS data to the visiting roamer. For example, if the MMS is 40 kb, the visited network will bill the home network as it would for an internet connection of 40 kb. Similarly, the visited network will apply any discounts it has negotiated with the home network on its data IOT. 12