GSM technical information

(1) .2 GSM system, technical specifications and its major properties
GSM standard norms of Chapter 12 Series: Series 01: The family of 02: 03 family business: network 04 Series: MS-BS and the Statute of the interface (air interface layer 2, 3) 05 Series: Wireless on the path to the physical Layer (the first air interface layer 1) 06 Series: voice coding norms Series 07: Taiwan's mobile terminal adapter 08 Series: BS to the MSC interface (A and Abis interface) Series 09: 10 interconnection Series: Series 11 available : Equipment and standardize the approval of the 12 series models: the operation and maintenance
(2) .3 GSM system, key technologies
The work of the band allocation
(2) -1. Band
China's public land-based digital cellular mobile communication networks using GSM communication system 900MHz band:
890 ~ 915 (Taiwan-made mobile, base station close)
935 ~ 960 (base-fat, Taiwan Mobile closed)
Duplex interval is 45MHz, for the work of the bandwidth 25 MHz, for the carrier frequency between 200 kHz.
With the development of the business, may need to extend down, or 1.8GHz frequency band to the GSM1800 transition, that is, the 1800MHz band:
1710 ~ 1785 (Taiwan-made mobile, base station close)
1805 ~ 1880 (base-fat, Taiwan Mobile closed)
Duplex interval is 95MHz, for the work of the bandwidth 75 MHz, for the carrier frequency between 200 kHz.
(2) -2. Channel spacing
Between the two adjacent channels to 200kHz. Each channel using Time Division Multiple Access (TDMA) mode, is divided into 8 slot, that is, 8-channel (full speed). Each channel bandwidth occupied by 200 kHz / 8 = 25 kHz.
GSM in the future semi-rate voice coding, each channel can accommodate 16 and a half rate channel.
(2) -3 Multiple Access Program
GSM communication systems used in multiple access technologies: Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), coupled with frequency-hopping technology.
In the GSM wireless transmission paths on a basic concept: the transmission unit is about 100-bit modulation of the sequence, calling it a "sudden impulse." Pulse duration of the priorities in the wireless spectrum also accounts for a limited part. In the window of their time and frequency within the window to send, we call space. Accurately, the gap at the center of the system frequency band between 200 kHz arrangements (FDMA), which every 0.577ms (more precisely, is 15/26ms) once (TDMA). Corresponds to the same space of time as a slot, it will be the duration of time as a unit, known as BP (sudden pulse cycle).
Such a gap in time / frequency graph with a long 15/26ms, wide-200KHz said that the small rectangular (see Figure). Unity, the GSM will be provided for in a band called the 200KHz bandwidth gap.
(2) -4 at the time and frequency domain space
In the GSM system, each carrier frequency is defined as a TDMA frame, which is equivalent to a channel FDMA system. Each frame, including 8 time slot (TS0-7). Each TDMA frame, a TDMA frame.
TDMA frame, is 3 hours 28 minutes 53 seconds 760 ms (2048 * 51 * 26 * 8BP or 2048 * 51 * 26 TDMA frame) for the number of the cycle. * 51 * 2048 per 26 TDMA frame for a super-high frame, each frame can be divided into ultra-high 2048 super-frame, a super-frame is 51 * 26 TDMA frame of the sequence (6.12 seconds), each of the super-frame Conducted by the Multi-frame composition. Multi-frame is divided into two types.
26 frame of the complex: it includes 26 TDMA frame (26 * 8BP), continuing a long time 120ms. Such a complex of 51 frames to form a super-frame. This frame for the resumption of carry TCH (and SACCH increase FACCH).
51 frame of the complex: it includes 51 TDMA frame (51 * 8BP), continuing a long time 3060/13ms. Such a complex of 26 frames to form a super-frame. This frame for the resumption of BCH and carrying CCCH.
(2) -5 wireless interface management
In the GSM communications systems, wireless channel can be used far less than the number of potential users, two-way communication channel can only be allocated when needed. This is the standard telephone network is very different from the telephone network regardless of whether or not to call in each terminal are connected to a switch.
In the mobile network, users need to call and the release of dynamically allocated wireless channel. Whether the call issued by the mobile station, or sent to mobile call, the process requires the establishment of specialized methods used to make mobile access systems, in order to get a channel. In the GSM, is in the process of access to the exclusive use of a mobile station - base stations to achieve the channel. The channel for the transmission and paging information base stations - mobile channel, together known as GSM common channel, at the same time as it carried from / to many of the mobile station. On the contrary, in a certain period of time allocated to a separate channel of the mobile station known as a dedicated channel. As a result of this distinction, the definition of Taiwan's two mobile-hong state:
Idle mode: mobile radio listening in the channel, this time it did not take up any channel.
Specific models: a two-way channel of communication need to be allocated to the mobile station so that it can make use of peer-to-peer infrastructure, two-way communication.
Access so that the process of moving from Taiwan to the idle mode-specific model.
(3) 4GSM Channel
GSM channel in the channel is divided into physical and logical channel, a physical channel for a time slot (TS), which is based on the logical channel between MS and the BTS message and a different kind of a different definition of logical channel, the logical channel Mapped to the physical transmission channel. BTS from the MS known as the downlink direction, in the opposite direction as the uplink.
Logical channel is divided into two categories, business and the Channel Channel.
(3) -1. Business Channel (TCH):
After the code used to transmit voice or data customers in the uplink and downlink channel, point-to-point (BTS on a MS, or vice versa) to spread.
(3) -2. Channel:
Used to synchronize data transmission or signaling. According to the completion of the necessary functions of the Channel again defined as broadcasting, public and three specific Channel, which can be broken down into:
(3) -2-1. Security measures
GSM security system have been significantly improved, GSM and secrecy related to the functions of two goals: first, including to prevent unauthorized network access, (at the same time protect users from deceptive counterfeit); the second To protect the privacy of users.
To prevent unauthorized access through authentication (that is, insert the SIM card and mobile station provided by the user ID of the same security checks) to be achieved. From the operator side, which is the most important function, particularly in the case of international roaming, Internet access and control the user's record beyond its ability to pay.
To protect the privacy of users through different means, for transmission can be encrypted to prevent eavesdropping on communications in the wireless channel. Most of the signaling can also use the same method of protection against third parties who are called to understand. In addition, a temporary identification code to replace the user is unable to make the third-party tracking on GSM wireless users yet another mechanism.
(3)-2-2.PIN code
This is a simple method of authentication.
In the GSM system, customer information, such as signing were recorded in the SIM card. SIM card into a GSM terminal equipment, as will their telephone, call the billing records in the bill would name the SIM card. In order to prevent Daoda, billing errors have a bill in the SIM card PIN code set up operations (similar to the Password function on a computer). PIN code is from 4 to 8 digits, the median of its customers by its own decision. If a customer entered the wrong PIN code, it will give customers a prompt to re-enter if the 3rd consecutive input error, SIM cards have been closed, even if the SIM card will be pulled out or turn off the cell phone has no power required to operate To apply for by the operator to unlock the user.
(3) -2-3. Authentication
Authentication of the calculation as follows. RAND is one of the network side of the user's question, the only legitimate users will be able to give the correct answer SRES.
RAND is a network by the side of the AUC have a random number generator, a length of 128 bits, and its value at random from 0 to 2128-1 (tens of thousands of billion) within the framework of extraction.
SRES called to respond to symbols, the only user of the password parameter (Ki) of access to computing, 32-bit length.
Ki a very confidential way to store and the AUC in the SIM card, users do not understand their own Ki, Ki can be any length and format.
A3 algorithm for authentication algorithm, the decision by the operators, the algorithm is confidential. A3 algorithm is the only limit the length of the input parameters (RAND is a 128-bit) and the size of the output parameters (SRES must be 32-bit).
(3) -2-4. Encryption
In the GSM, transmission link in the encryption and decryption to deal with the location of all the special mode allows the data sent by a method of protection. Data is the customers can send information (voice, data, ... ...), and user-related signaling (such as the number being called to carry the news), even with the signaling systems (such as carrying ready to switch wireless measurements news) .
Encryption and decryption is a wireless emergency 114-bit pulse code with a special algorithm resulting 114-bit encryption or a different sequence for computing (A5 algorithm) completed. For each of the encrypted sequence of sudden, A5 enter the two calculations: the number of frames, and the other is a mobile station and the network agreed to between the key (called Kc), see figure. Uplink and downlink on the use of two different sequences: for every emergency, a series of mobile station for the encryption and decryption of the BTS as a sequence; sequence and the other for the BTS encryption, and as a Taiwan Mobile's decryption sequence.
(3) -2-4-1. Frame:
Frame coding, as a series of three values add up to a total of 22 bits.
For a variety of wireless channel, each frame of a sudden, all different, all in the same direction on a given emergency communications each using a different encryption sequence.
(3)-2-4-2.A5 algorithm
A5 algorithm must be within the framework of the international provisions of the algorithm can be described by the expertise of more than 22 parameters (frame number) and more than 64 special parameters (Kc) to generate more than 114 of the two long sequences of the black box.
(3) -2-4-3. Kc key
Prior to the beginning of encryption, key Kc must be mobile and the network agreed. GSM select the authentication key calculation during the Kc; key and then stored in the SIM card of the permanent memory. On the network side, the "potential" key to the store also called on the MSC / VLR, for the beginning of the use of encryption.
By the RAND (and for the same authentication) and Ki algorithm for calculating the Kc algorithm A8. A3 and the algorithm (by the RAND and Ki authentication algorithm for computing the SRES) similar to the decision by the operator of choice.
(3) -2-4-4. To protect the identity of users
The encryption of confidential information is very effective, but can not be used in the wireless path on the protection of each exchange of information. First of all, the public can not be used for encryption channel; Second, when the mobile station to a dedicated channel, the network did not know the identity of the user, can not be encrypted. Third parties may In both cases, the identity of the user to hear frame, so that the user's location at this time to roam. This is the user's privacy is harmful, GSM in order to ensure that the confidentiality of the introduction of a special function.
Where possible, through the use of temporary mobile user identity, TMSI alternative user IMSI, can be protected. TMSI by the MSC / VLR distribution, and we will continue to be replaced, the replacement cycle by the network operator set up.