SIM card (Subscriber Identity Module), that is, subscriber identification card, is a personal information card for GSM digital mobile phones. It is made with A-level encryption method and stores the user's data, authentication method and key, which can be used by the GSM system to authenticate the user's identity. At the same time, users complete the connection with the system and the exchange of information through it.

Mobile phones can only be used with a SIM card installed. "SIM cards" come in different sizes, with exactly the same functions, and are suitable for different types of GSM mobile phones. The SIM card can be inserted into any mobile phone that complies with GSM specifications, and the call charges are automatically charged to the card user's bill, regardless of the mobile phone.

The use of SIM cards effectively prevents theft, parallel calls and eavesdropping of calls, and reliably guarantees users' normal communications.

In order to ensure that your mobile phone is not stolen if it is lost, each SIM card can be set with a personal password (PIN code) to lock the SIM card, which is set by the user himself. Only after entering the password correctly will the phone enter normal use. If you enter an incorrect personal password three times in a row, your phone will lock the SIM card. If this happens, please turn off your phone immediately and bring your phone and SIM card to the Wireless Office Business Hall to unlock it. If you continue to operate at this time, the SIM card will be automatically blocked, causing unnecessary losses to you.

USIM card is the third generation mobile phone card

USIM: Universal Subscriber Identity Module (global subscriber identification card)

Global Subscriber Identity Module (USIM), also It's called Upgrade SIM and is a component of the UMTS 3G network.

Many people believe that in the 3G era, most applications can only be implemented by mobile phones, and the limited resources on the card only need to implement the authentication function. Indeed, 3G applications are very complex, and most applications cannot be completed through STK cards alone. However, the USIM card can not only perform a simple authentication function. Facts have proved that it is gradually transitioning to a mobile commerce platform and even a multi-application platform. It is no longer necessary to implement other applications such as electronic wallets, electronic credit cards, and electronic bills on mobile phones. Difficult matter. This feature makes the USIM card a medium for cross-field cooperation and mutual penetration between different industries. For example, banks can participate in telecommunications operations, and vice versa.

In addition to supporting multiple applications, the USIM card has also upgraded its security algorithm and added a card-to-network authentication function. This two-way authentication can effectively prevent hackers from attacking the card. . At the same time, the USIM card's phone book function is more powerful and can store up to 500 phone numbers. For each phone number, the user can also choose whether to enter other information, such as email, aliases, other numbers, etc.

Despite the faltering steps, 3G is still coming to us step by step. The high 3G license fee may be one of the reasons why many operators are hesitant to move forward. More importantly, they hold a wait-and-see attitude towards 3G applications. Moreover, there are still many problems that need to be solved to implement multiple applications based on USIM cards. For example, the relevant specifications are not perfect enough, there is a lack of mobile phones that support such multiple applications, and more importantly, multi-party coordination between operators and relevant enterprises or government agencies will increase. The difficulty of this kind of application is great. In any case, the third generation mobile communication cards have made technical preparations in this regard. It is believed that multiple applications based on USIM cards will eventually be widely used in the 3G era.

As for the use of TD-SCDMA USIM cards on non-TD-SCDMA mobile phones, we have tested that on other 3G mobile phones, such as WCDMA models, the USIM card can be used as an ordinary SIM card. Use it to make calls and message functions on the GSM network, but on non-2G mobile phones, it shows "SIM card" registration failed.

It can be seen that the USIM card itself is a dual-mode card of TD-SCDMA and GSM (it is also stated on the USIM card), but it can only be used on 3G mobile phones such as K850i, E51 or licensed mobile phones with "3G version" such as N73 , N95.

International Roaming Analysis between WCDMA and GSM

The abstract article introduces the terminals of different standards and different types of user identification modules (SIM, USIM, ISIM) in GSM and WCDMA systems. compatibility relationship between them. It mainly discusses two different operating modes for realizing international roaming between WCDMA and GSM systems, and analyzes the specific call signaling processes and different encryption methods in different modes.

Keywords 2G/3G interoperability WCDMA GSM international roaming authentication and encryption

1 Introduction

We know that both GSM and WCDMA are based on the GSM-MAP core network , the GSM network can smoothly evolve to the WCDMA system. At present, many countries in Europe, Asia, and Africa have established WCDMA systems. The so-called international roaming between WCDMA and GSM means that GSM (or WCDMA) users roam to foreign WCDMA (GSM) networks and use the visiting WCDMA (GSM) network to provide them with business services.

Due to certain differences in the development situation of various countries, for example, some countries only have GSM networks (such as China), while other countries have only built WCDMA networks (such as Japan) without GSM networks. . For this reason, the 3GPP TS 22.100 specification points out that WCDMA terminals should be able to support access to WCDMA networks through GSM SIM cards. Of course, at this time, the WCDMA network can only provide users with the services that the GSM system can provide (WCDMA's unique services, such as video and high-speed data communications, cannot be provided). Whether users can access the WCDMA network through a GSM SIM card is controlled by the WCDMA network operator. At the same time, the 3GPP TS 22.101 specification points out that the UMTS (Universal Mobile Telecommunications System) system should allow WCDMA users to access the GSM network through GSM terminals. Let's analyze this issue below to analyze what conditions and some key processes are needed to achieve international roaming between these two systems.

2 UICC card in mobile terminal

We know that an important link in 3GPP terminal design is the design of Universal Integrated Circuit Card (UICC). UICC card is a removable smart card, which is used to store user information, authentication keys, phone books, short messages and other information.

In the GSM and 3GPP specifications, users must rely on the UICC card in the terminal if they want to use various services normally. If there is no UICC card in the terminal, the user can only use emergency call services (such as 110, 119).

Users only need to take out the UICC card from one terminal and insert it into another terminal to easily transfer the user's subscription information (including phone book) from one terminal to another terminal. middle.

UICC is the general name of smart cards with defined physical characteristics. The interfaces between UICC and terminals are standard.

UICC can include a variety of logical applications, such as Subscriber Identity Module (SIM, Subscriber Identity Module), Universal Subscriber Identity Module (USIM, Universal Subscriber Identity Module), IP Multimedia Service Identity Module (ISIM, IP Multimedia Service Identity Module). Of course, UICC can also include other applications (electronic wallets, etc.).

2.1 SIM in GSM

The SIM card is a smart card used by mobile terminals in the GSM network. It is used to store various parameters and related user information, such as user subscription information and authentication. keys, user preferences, and short messages. It should be noted that although we often interchange the terms UICC and SIM, UICC actually refers to the physical card, and SIM refers to an application that stores GSM user subscription information on the UICC card. SIM is widely used in GSM systems.

SIM includes the following information:

◆International Mobile Subscriber Identity (IMSI, International Mobile Subscriber Identity): User identity, used for access authentication.

◆Mobile Subscriber ISDN Number (MSISDN, Mobile Subscriber ISDN Number): The mobile phone number of a mobile subscriber.

◆Key Ki, encryption algorithm A3, A8: used for authentication.

◆Mobile Country Code (MCC, Mobile Country Code), Mobile Network Code (MNC, Mobile Network Code) belonging to the PLMN: network identification.

SIM applications have been standardized in the early stages of GSM. These specifications continue to be inherited in 3GPP (see 3GPP TS 11.11 and 3GPP TS 51.011).

2.2 USIM in WCDMA

USIM (see 3GPP TS 31.102) is another application on the UICC card. USIM provides another set of parameters different from SIM, which includes user subscription information, authentication information, payment methods, user short messages, etc. USIM is used in the Universal Mobile Telecommunication System (UMTS) network, that is, the WCDMA network.

When a terminal (including circuit switching function and packet switching function) wants to use WCDMA services, USIM must be used. Obviously, SIM and USIM can be stored in the same UICC card.

In addition to other information, USIM includes the following information:

◆International Mobile Subscriber Identity (IMSI, International Mobile Subscriber Identity): IMSI is a unique identifier assigned to each user. The logo is invisible to the user but visible to the network. IMSI is used as user identification for authentication purposes. In IP Multimedia Subsystem (IMS, IP Multimedia Subsystem), its private user identity is equivalent to IMSI.

◆Mobile Subscriber ISDN Number (MSISDN, Mobile Subscriber ISDN Number): One or more phone numbers assigned to the user are stored in this domain. In IMS, its public user ID is equivalent to MSISDN.

◆Encryption key (CK, Cipher Key) and integrity key (IK, Integrity Key): These keys are used for encryption and integrity protection of data in the air interface. USIM stores separate keys used in the circuit domain and packet domain.

◆Short Message Service (SMS, Short Message Service): USIM can store short messages and related data, such as sender, receiver, status, etc.

◆Short message parameters: This field is used to store configuration data related to SMS services, such as SMS center address, supported protocols, etc.

◆Multimedia Message Service (MMS, Multimedia Message Service) user connectivity parameters: This field is used to store configuration data related to the MMS service, such as MMS server address and MMS gateway address.

◆MMS user preference information: This domain is used to store user preference information related to MMS services, such as sending report flags, priorities, expiration information, etc.

The USIM card has the following characteristics compared with the SIM card:

◆Compared with the one-way authentication of the SIM card (network authentication user), the USIM card authentication mechanism adopts two-way authentication rights (in addition to network authentication users, users also authenticate the network), which has high security.

◆Compared with the SIM card phone book, each contact in the USIM card phone book can correspond to multiple numbers or nicknames.

◆ Compared with the SIM card machine card interface speed, the USIM card machine card interface speed is greatly improved (230kbps).

◆Compared with the SIM card's support for logical applications, USIM can support 4 concurrent logical applications at the same time.

2.3 ISIM in 3GPP IMS

ISIM applications can also be implemented in UICC (see 3GPP TS 31.103). ISIM is only used in 3GPP IMS systems. It includes relevant parameters for user identification, user authentication and terminal configuration in the IMS system. ISIM can be stored on a UICC card alone with SIM or USIM. Of course, it can also be stored on a UICC card with SIM and USIM at the same time.

The main parameters included in ISIM are:

◆Private User Identity: There can only be one private user identity in ISIM

◆Public User Identity: The SIP (Session Initiation Protocol) URI of one or more public user identities can be stored in ISIM.

◆Home network domain URI: ISIM stores the SIP URI including the home network domain name, which is used to find the address of its home network during the registration process. Only one home network domain name URI can be stored in ISIM.

◆Long Term Secret: used for authentication purposes and used to calculate the integrity key and encryption key used between the terminal and the network. The IMS terminal uses the integrity key to protect the integrity of SIP signaling between the IMS terminal and the proxy call session control function (P-CSCF, Proxy-Call Session Control Function). If the signaling needs to be kept confidential, the IMS terminal will use the encryption key to encrypt and decrypt the SIP signaling between the IMS terminal and the P-CSCF.

In addition to ISIM, USIM can also be used to access the 3GPP IMS network, but the terminal software needs to be appropriately modified. Since the security level of SIM applications is low, the 3GPP IMS system does not allow access through SIM.

2.4 Summary

Currently UICC cards generally include both USIM and SIM modules, which are called composite USIM cards (it can be compatible with GSM terminals and WCDMA terminals). If the UICC It only includes the USIM module, so it is called a pure USIM card.

WCDMA terminals have backward compatibility on the machine card interface and are compatible with USIM cards (composite USIM cards and pure USIM cards) and GSM SIM cards.

GSM terminals are compatible with GSM SIM cards and WCDMA composite USIM cards, but are not compatible with pure USIM cards.

WCDMA dual-mode terminal can access GSM wireless network or WCDMA wireless network regardless of inserting SIM card or USIM card (composite USIM card or pure USIM card).

GSM terminals can only access GSM wireless networks by inserting a SIM card or composite USIM card.

SIM cards can be used in GSM, WCDMA, and TD-SCDMA systems.

USIM cards can be used in GSM, WCDMA, and TD-SCDMA systems.

If the user wants to use IMS services, the UICC card must include both USIM and ISIM. If there is only USIM, access to IMS can be achieved by modifying the software in the terminal (Release 5). Future standards will not exclude the need for only ISIM in the UICC card to access IMS.

3 Air interfaces of WCDMA and GSM

WCDMA evolved from the GSM system. They use the same core network, but their air interface parts are very different. Figure 2 is WCDMA R4 network structure diagram. It can be seen from the diagram that GERAN and UTRAN share the same core network.

Table 1 lists some of the most important differences between WCDMA and GSM on the air interface:

Table 1 Comparison of key parameters of WCDMA and GSM air interfaces

WCDMA

GSM

Multiple access mode

CDMA

TDMA

Carrier bandwidth

5MHz

200kHz

Modulation method

QPSK (forward), BPSK (reverse)

GMSK

Diversity mode

Multipath diversity (RAKE receiver)

Slow frequency hopping

Frequency reuse factor

1

1~18

Voice Coding

AMR

RPE-LTP-LPC

Channel Coding

Convolutional code, Turbo code

Convolutional code

3.1 Multiple access method

As can be seen from the table, WCDMA adopts In code division multiple access, users and channels are distinguished by different codes, which means that different users can communicate at the same frequency and in the same time slot at the same time. The GSM system uses time division multiple access. Users and channels are distinguished by different time slots. That is to say, at a certain time, a time slot can only be allocated to one user.

In WCDMA, channelization codes and scrambling codes are used respectively. The channelization code is implemented by orthogonal variable spreading factor (OVSF, Orthogonal Variable Spreading Factor). OVSF has good cross-correlation. property, that is, different codes are completely orthogonal. The scrambling code is implemented through a pseudo-random sequence. The pseudo-random sequence has good autocorrelation, that is, there will be a large peak during synchronization.

3.2 Carrier bandwidth

In WCDMA, the spreading chip rate is 3.84Mbps, so the signal bandwidth after modulation is 5MHz. WCDMA is a Code Division Multiple Access (CDMA, Code Division Multiple Access) frequency division duplex (FDD, Frequency Division Duplex) system, so the total uplink and downlink occupy a total of 10MHz bandwidth. This is why WCDMA is called wideband CDMA.

For GSM, the final rate of information after channel coding is 270.8Kbps. After Gaussian Minimum Shift Keying (GMSK), the signal bandwidth is 200KHz. GSM is Time Division Multiple Access (TDMA). Access) frequency division duplex system, so the total uplink and downlink occupy a total bandwidth of 400KHz.

3.3 Modulation method

WCDMA system uses binary phase shift keying (BPSK, Binary Phase Shift Keying) and quaternary phase shift keying (QPSK, Quadrature Phase Shift Keying) , for BPSK, each bit (0 or 1) is mapped to phase 0 or π, while QPSK maps the two bits into phases 0, π/4, π/2, and 3π/4 respectively. At this time, the frequency of the modulated signal remains unchanged.

The GSM system uses the GMSK modulation method. GMSK is a continuous phase modulation. A Gaussian filter is added before MSK modulation. Its purpose is to make the main lobe of the modulated signal roll off faster. The frequency of the modulated signal is varied.

3.4 Diversity method

Diversity is to improve the reliability of the communication system. In the WCDMA system, CDMA's inherent anti-multipath fading ability is used to perform Maximal Ratio Combining (MRC) on the multipath signals reflected from different directions through the RAKE receiver, thereby combining the multipath signals that are originally harmful to communication reliability ( Multipath signals will cause multipath fading (ie, frequency selective fading). Multipath signals become signals that are beneficial to communication.

Slow frequency hopping technology is used in the GSM system. In layman's terms, information is transmitted on different frequencies, so that the impact on the signal caused by a certain frequency being in deep fading can be overcome.

3.5 Speech coding and channel coding

Speech coding and channel coding have always been the focus of research in information theory. Speech coding is to program the lowest possible bits on the basis of being understandable. rate. Channel coding ensures the reliability of information transmission by adding redundant bits.

The speech coder in the WCDMA system uses adaptive multi-rate (AMR, Adaptive Multi-Rate) coding technology. Channel coding in WCDMA systems includes convolutional codes and Turbo (proposed in 1993) codes. Turbo codes are usually used in data communication environments due to their large interleaving depth (resulting in increased transmission delay) and super error correction capabilities. .

The GSM speech coder uses Regular Pulse Excited-Long Term Prediction-Linear Predictive Coding (RPE-LTP-LPC, Regular Pulse Excited-Long Term Prediction-Linear Predictive Coding) technology. The channel coding in GSM uses convolutional codes.

3.6 Summary

Through the above description, we can draw a very simple conclusion, that is, when the terminal is within the coverage of a certain cellular network, if the terminal wants to work normally, it must The prerequisite is that the terminal and the base station must be of the same standard. That is to say, when the terminal is covered by a WCDMA base station, the terminal must be a WCDMA terminal (WCDMA/GSM dual-mode terminals are of course no problem); when it is covered by a GSM base station, the terminal must be a GSM terminal (WCDMA/GSM dual-mode terminals are obviously no problem).

4 Two ways to achieve international roaming between WCDMA and GSM

At present, there are two main ways to achieve international roaming between WCDMA and GSM: one is to rent an aircraft or card domestically Call forwarding roaming service; second, bring your own dual-mode terminal to achieve automatic roaming between GSM and WCDAM abroad.

Below we will take the GSM and WCDMA international roaming between China and Japan as examples for analysis.

4.1 Rent an aircraft, rent a card, and call forwarding roaming

When Chinese GSM users want to roam to Japan, since Japan is a WCDMA network, the user activates a domestic mobile phone, rent a card, and call forwarding. For the roaming service, the mobile phone rented in the business hall is a Japanese WCDMA mobile phone, and the user's GSM mobile phone number is transferred to the rented mobile phone. This kind of call forwarding is unconditional call forwarding.

Suppose user A wants to go to Japan and applies for the call forwarding and roaming service of renting an aircraft and card, and his number is forwarded to terminal B. When domestic user C calls user A.

(1) After receiving the called user number A, the MSC sends the send_routing_info message to A's HLR through the No. 7 signaling network.

(2) In the HLR, you can see that user A has transferred the call to terminal B. At this time, the HLR returns the B number to the MSC through the send_routing_info message.

(3) MSC analyzes and learns that the number is an international number and sends an IAM message to TSMC, and then delivers it to Japan TMSC through ISC and international traffic relay company.

(4) After receiving the IAM message, Japan TMSC sends the send_routing_info message to the HLR of terminal B through No. 7 signaling.

(5) The HLR of terminal B knows the MSC currently providing services to terminal B, and then sends a provide_roaming_num message to the MSC to obtain the MSRN of terminal B.

(6) The MSC returns the MSRN of terminal B to the HLR through the provide_roaming_num_ack message.

(7) Then the HLR of terminal B sends B’s MSRN to TMSC through the send_routing_num_ack message.

(8) After learning the MSRN of terminal B, TMSC will continue the subsequent traffic connection through the IAM message.

Similarly, when Japanese WCDMA users roam to China's GSM network, they can also handle the service in their country.

4.2 Bring your own WCDMA terminal to realize international roaming from GSM to WCDMA

The simple authentication and encryption process for GSM users to access Japanese WCDMA network through WCDMA terminal:

After the Chinese user arrives in Japan and turns on the phone, he first initiates the location update process. After receiving the location update request from the Chinese user, the Japanese WCDMA MSC initiates a location update request to the user's HLR through the international Signaling No. 7 network and the Chinese Signaling No. 7 network. Then HLR sends Triplets (Kc, RAND, SRES) to Japan WCDMA MSC through the authentication request message. The authentication process at this time is the same as that of the GSM system. That is, after the MSC sends Kc and RAND to the terminal, the terminal uses RAND and Ki to obtain the SERS through the A3 algorithm, and returns the SERS to the MSC. The MSC compares the HLR and sends Check whether the SERS sent by the terminal is consistent with that sent by the terminal. If they are consistent, the authentication is passed. HLR will insert the relevant data of Chinese users into the Japanese WCDMA MSC/VLR and delete this information from the old MSC/VLR. If inconsistent, the user is rejected.

In fact, after the authentication is completed, the air interface encryption process should be carried out, but we do not use it in our country. In the GSM system, the encryption of the air interface is completed through the Kc and A5 algorithms. However, when the user roams to Japan, as shown in the figure above, the WCDMA terminal and WCDMA MSC will convert the received Kc into CK, IK, thereby achieving encryption and integrity protection, it can be seen that the security of its transmission is improved.

4.3 Bring your own GSM terminal to realize international roaming from WCDMA to GSM

Japanese WCDMA users only need to change a GSM terminal after roaming to China, and there is no need to change a USIM composite card . If the user is using a WCDMA/GSM dual-mode terminal, automatic roaming can be achieved. Let's briefly look at the authentication and encryption process in this scenario.

When a Japanese user roams to China and turns on the phone, the location update process is first performed. After receiving the location update request from the Japanese user, the Chinese GSM MSC initiates a location update request to the user's HLR through the No. 7 signaling network. . Note that Japan's HLR at this time is WCDMA HLR, which stores authentication quintets (RAND, CK, IK, XRES, AUTN), which must be converted into triplets (Triplets), that is, through CK , IK calculates Kc, and XRES calculates SERS. Then the HLR sends Triplets (Kc, RAND, SRES) to the Chinese MSC through the authentication request message. After receiving the triplets, the MSC sends RAND to the GSM terminal through the GSM BSS. The terminal can use the RAND to calculate CK, IK and RES. Then the terminal uses different conversion functions to convert CK, IK into KC and RES into SRES respectively. Then the terminal returns the SERS to the MSC. The MSC compares the SRES received from the HLR with the SRES received from the terminal. If they are consistent, the authentication is passed. The HLR inserts the user-related information into the GSM MSC/VLR and obtains it from the GSM MSC/VLR. Delete user-related information from the old VLR. Complete the location update process. If the comparison results are inconsistent, the user will be rejected. Although the air interface of my country's GSM system is not encrypted, in fact, in the specification, after the authentication is completed, the encryption process is performed, that is, the encryption operation is performed through Kc between the terminal and the GSM BSS.

5 Conclusion

Through the above analysis, we can see that since WCDMA and GSM have the same core network, as long as operators open services between WCDMA and GSM, users only need Automatic roaming can be achieved by replacing the original terminal. The difference is that during the air interface encryption process, the parameters in the authentication group need to be converted accordingly to suit the needs of the air interface.