Re: GSM SIM - What is a SIM - How it is used in 2G & 2.5G GSM (long)
Although a SIM is a microprocessor smart card (IC card) used in GSM mobile wireless telephony, all smart cards used in mobile wireless are not SIM's.
A SIM like any IC card is based on a micro-processor specifically designed to be tamper-resistant, composed of:
* a Central Processing Unit
* Random Access Memory
* Read Only Memory, where the Operating System is stored
* Electrically Erasable Programmable Read Only Memory, where customized applications and data are stored.
SIM cards like all IC cards are the result of the process of embedding micro-controllers into a plastic component. The position of the electrical contacts, the electrical characteristics, and the mechanical resistance of the cards are all defined by the International Standard Organization (ISO 7816-1 to 4). The mechanical constraints limit the size of the micro-controller itself to ¼" x ¼". However, the amount of memory made available increases over time.
Traditional SIM's (dimensions specified for GSM by ETSI SMG9 standards) come in 2 specified form factors today:
1. ID-1 format - is the same size as the credit cards in your wallet (specified by ISO-7610 specifications for financial transaction cards and and ISO 7816 standards for IC cards). Dimensions are 54 mm (2.12”) x 85 mm (3.35").
2. Plug-in format (is considerably smaller than the ID-1 format (but is actually punched or snapped out of the full size card body). Dimensions are 20.8 mm (.82") x 15 mm (.59”). It usually inserts inside a trap door in the handset. It is depicted here partially removed from the handset:
cp8.bull.net
Initially the SIM (Subscriber Identity Module) was specified as a part of the GSM standard to secure access to the mobile network and store basic network information. Today, SIM cards are used to customize mobile phones in several standards (GSM, DCS, PCS, Satellite).
In the near future SIM cards or derivatives of SIM cards (USIM or R-UIM) will be used in TDMA, SMR, and CDMA. Initially their use in these environments will be optional although there is a possibility that they will be mandated in all third generation standards for mobile wireless telephony.
In the future the SIM will play a role in providing plastic roaming from GSM to 3G systems and possibly plastic roaming from other 2G systems to 3G systems. It may provide for plastic roaming from 3G to 2G systems.
When the SIM card was invented, the objective was to use any SIM card in any handset, making the SIM the ultimate bridge to interoperability. As the years have passed, the role of the SIM Card has become increasingly important in the wireless service chain.
Today the SIM is the major component of the wireless market paving the way to value-added services. SIM cards now offer new menus, prerecorded numbers for speed dialing, sending of presorted short messages to query a database or to secure transactions and display of greeting messages or company logotypes.
All GSM (Global System for Mobile communications) carriers use smart cards to store authentication algorithms and keys, as well as service and user profiles. These smart cards are known as Subscriber Identity Modules (SIM).
As of January 1st, 2000, 250 million GSM wireless customers are using SIM cards worldwide to access to their wireless services. The SIM cards currently issued by wireless service providers support the following features:
* authentication algorithm and secret keys (network- specific)
* storage of the service profile and configuration data (preferred and forbidden roaming networks, short message service center address, available telephony services, voice mail number, service dialing number…)
* storage of user data, such as speed dial number and short messages (SMS)
* storage of SIM applications which can be either triggered at power-on or by specific events (reception of SMS, outbound call…)
The SIM operating system, file structure and content are compliant with the ETSI GSM specifications (GSM 11.11 and GSM 11.14).
SIM cards were first specified to support authentication and provide an exceptional barrier to subscription cloning. With an additional local authentication by means of PIN (private identification number), the use of SIM cards has tremendously reduced wireless fraud.
Since 1997, SIM cards are also used to store SIM applications compliant to the SIM Toolkit as specified in the GSM 11.14. SIM Toolkit technology allows for SIM cards to monitor the phone as a peripheral (screen, keyboard, etc.) and to insert menus within the phone menus transparently to the user. SIM Menu items trigger applications, which in turn can request information from the end-user (for instance a credit card account number, a dollar amount, a transaction date…). The information received can be digitally signed or encrypted by the SIM and sent to an application server via SMS.
The following detail includes excerpts from a classic overview of GSM called "Overview of the Global System for Mobile Communications" written by John Scourias of the University of Waterloo (CA). It focuses on the most basic roll of the SIM (Authentication & Security) in a GSM Network. The complete White Paper is available at:
ccnga.uwaterloo.ca
>> GSM Mobile Station:
The mobile station (MS) consists of the mobile equipment (ME) or terminal, and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.
GSM Network Subsystem:
The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signaling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signaling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.
The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.
GSM Authentication and Security:
Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be, is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the AuC. During authentication, the AuC generates a random number that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated.
The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key, together with the TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is XORed with the 114 bits of a burst (the two 57 bit blocks). Enciphering is an option for the fairly paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers.
Another level of security is performed on the mobile equipment itself, as opposed to the mobile subscriber. As mentioned earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR). The status returned in response to an IMEI query to the EIR is one of the following:
* White-listed - The terminal is allowed to connect to the network.
* Grey-listed -The terminal is under observation from the network for possible problems.
* Black-listed - The terminal has either been reported stolen, or is not type approved (the correct type of terminal for a GSM network). The terminal is not allowed to connect to the network.
Services provided by GSM:
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Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval <<
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- Eric - |
