The GSM system uses simplified protocols similar to the OSI protocol model, including the physical layer (L1), data link layer (L2), and application layer (L3). L1 is the bottom layer of the protocol model, and provides all the functions required by the physical media to transmit the bit stream. L2 guarantees the correct delivery of messages and the identification of individual calls. In the GSM system, L1 and L2 on the wireless interface (Um) are the TDMA frame and the LAPDm protocol, respectively. On the network side, the L1 used by the Abis interface and the A interface are both E1 transmission methods, and L2 is the LAPD and MTP protocols, respectively. At the Um interface, the MS has an L1 and L2 layer establishment process each time it calls, and on this basis, it establishes L3 communication with the network side. On the network side (A and Abis interfaces), L1 and L2 (except SCCP) are always connected. The communication messages of the L3 layer are divided into three parts according to different stages and functions: radio resource management (RR), mobility management (MM), and call control (CC).
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1. Establish RR connection
The functions of RR include physical channel management and logical channel data link layer connection.
In any case, the first message sent by the MS to the system is CH-REQ (channel request), which requires the system to provide a communication channel. The type of channel provided is determined by the network. CH-REQ has two parameters: establishment reason and random reference value (RAND). The establishment reason refers to the reason why the MS initiated this request. The reason in this example is that the MS initiated the call. Other reasons include emergency call, call re-establishment and paging response.
RAND is a random value determined by the MS, so that the network can distinguish the requests initiated by different MSs. RAND has 5 digits, which can distinguish up to 32 MSs at the same time, but there is no guarantee that the RAND value of two MSs that initiate a call must be different. To further distinguish the MS that initiated the request at the same time, it is also based on the reply message on the Um interface.
CH-REQ messages are processed inside the BSS. After receiving this request, BSC allocates a channel for MS to use according to the judgment of the wireless resources in the existing system. Whether the channel can be used normally requires BTS response confirmation. A pair of response messages CHACT (channel activation) and CHACK (channel activation confirmation) on the Abis interface complete this function. CHACT indicates all the attributes required to activate the channel, including channel type, operating mode, physical characteristics, and timing advance.
After the network prepares the appropriate channel, it notifies the MS and the IMMASS (immediate assignment) message completes this function. In IM-MASS, in addition to the channel-related information in CHACT, it also includes a random reference value RA, a reduced frame number T, a time advance TA, and so on. The RA value is equal to the random value sent by an MS received by the BSS system. T is a frame number with a smaller value range calculated based on the TD-MA frame number when CH-REQ is received. The RA and T values ​​are directly related to the MS requesting the channel, and are used to reduce request conflicts between MSs. TA is the calculated timing advance when BTS receives the CH-REQ information on the RACH channel for equalization. The MS determines the timing advance for the next message transmission based on TA.
The purpose of IMMASS is to establish a wireless connection between the MS and the system on the Um interface, that is, the RR connection. After the MS receives the IM-MASS, if the RA value and the T value meet the requirements, it will send a SABM frame on the new channel assigned by the system, which contains a complete L3 message (MP-L3-INF), this Messages have different roles on different interfaces. On the Um interface, the SABM frame is a message on the LAPDm layer requesting to establish a multi-frame response operation mode connection. After receiving the SANM frame, the system sends back a UA frame as a response to the SABM frame, indicating that a LAPDm channel has been established between the MS and the system; in addition, the message field of this UA frame contains the same L3 message that the MS received After the message is compared with the corresponding content in the SABM frame sent by itself, it is considered to be accepted by the system only when they are exactly the same. The L3 message contains the MS's IMSI. The IMSI is unique to each MS, which ensures that only one MS can access the system on this channel. On the Abis interface, this message is ESTIND (Establishment Indication), which is used to notify that the LAPDm connection has been established as a response to the IMMASS message.
In the SANM frame, the L3 message transparently transmitted to the MSC is the first L3 message of the A interface. Although the MTP connection of the A interface has been established before the call, for each call, an SCCP connection is also established at L2. The L3 message is included in the SCCP request chain-building message (CR) on the A interface. If the request is allowed, the first downstream message of the A interface will be included in the connection confirmation (CC) frame of the SCCP layer. For the SCCP layer, the exchange of CR and CC is the exchange of source reference address and destination reference address. Under the same signaling point code, different calls have different source and destination addresses. After the first message on the A interface is delivered, the RR connection is established between the MS and the system, and the RR entity notifies the MM sublayer that it has entered the dedicated mode. In dedicated mode, the MM sublayer and CC sublayer are responsible for sending all messages on the L2 layer. Except for error indication and local link release, all are handled directly by the RR sublayer.
2. Establish MM connection Under normal circumstances, to establish an MM connection, you must first have an RR connection. The first step after the RR is established is authentication (AUTH), which is to identify the identity of the mobile user. There are two parameters in AU-THREQ (authentication request): CIP KEY No (encryption key number) and AUT RAND (authentication random value) Q CIPKEY No corresponding to the key Kc of each MS, calculated by the network Send to MS, the purpose is to get Kc directly from MS's IMSI and CIP KEYNo parameter in CM-SERV-REQ without calling AUTH process. ATU RAND is used by MS to calculate the authentication response value SRES. There are 4 data related to authentication and encryption stored in the SIM of MS: authentication algorithm A3, encryption sequence algorithm A8, encryption algorithm A5 and mobile user personal authentication key Ki. The relationship is as follows: Kc = A8 (RAND, Ki), SRES = A3 (RAND, Ki), encrypted data stream = A5 (user data, Kc). SRES is the response value of MS to AUTH REQ, which is transmitted in AUTH RES. The Ki value corresponding to each IMSI is stored in the network, and the network can authenticate the identity of the MS based on the calculated SRES value and the SRES value returned by the MS. Kc is used in the encryption process after authentication. The encryption algorithm A5 is specified by the network, but the MS must support this algorithm. In the encryption command CIP-M-COM, it pointed out the type of A5 algorithm supported by each MS, and also specified whether the IMEISV parameter was included in the MS's loopback message. After the MS identification and wireless channel transmission encryption process is completed, the MM connection required to establish the call has been established, and the call information transfer function can be provided to the higher layer (CC sublayer).
3. Establish a CC connection. The MS sends a SETU (establishment) message to the network, requesting the establishment of a call. The message includes: (1) The specific service type of the call request and the bearer capacity that the MS can provide, including information transmission requirements, sending methods, Coding standards and available wireless channel types; (2) The number of the called user, including the type and coding scheme of the called number.
The network receives the SETUP message, and if it accepts the request, it returns CALL PROC (call processing), indicating that the call is being processed, and the calling MS
Waiting. The network starts looking for the called user. If the called party is also a GSM system user, the way to access the network is similar to the calling party. The differences are as follows: (1) The called MS will only make a channel request after receiving the PAGINC (page) message sent by the network; (2) When the called MS establishes a CC connection with the network, the network will first send a downlink SETUP Message, the MS sends back a CALLCONF (call confirmation) message. After CALL, PROC or CALL, CONF, the CC layer connection between the network and MS is established.
Subsequent CC layer messages ALERT (ringing), CON-NECT (connection) and their response messages correspond to MS ringing and user actions. The network receives the called ALERT message, and then sends the same ALERT message to the calling MS, so that the calling party knows the current call connection status, that is, the ringing tone that is usually heard when making a call. After receiving the ringing tone, the caller waits for the called party to go off-hook. This action is reflected as a CONNECT message on the signaling connection. After the response to the CONNECT message is completed, both the calling and called parties enter the normal conversation state until one party shuts down and the conversation ends.
SDCCH or FACCH is used to transfer signaling. MS calls must be conducted on the TCH channel. To this end, the network assigns an MS
TCH channels are allocated in a similar way to IMMASS. The difference is that the origination of the assignment is started by the MSC's ASS-REQ (assignment request command). BSC activates the corresponding wireless channel according to the information of ASS-REQ, and determines the type of the wireless channel according to the corresponding information of the service specified in ASS-REQ. CHACT specifies radio resources, including channel frequency, time slot, and frequency hopping.
4. Connect the voice channel
The data transmission of the GSM system service adopts the circuit mode, and there is a physical path between the calling party and the called party. There are two requirements for establishing such a path: (1) Allocate certain channel resources for different routing segments to communicate; (2) Connect the various channels together.
Channel resources include the wireless channel of the Um interface and the PCM link channel of the A interface. The wireless channel is described by CHACT, and the terrestrial channel of the A interface is described by ASS-REQ.
The connection of each channel is a connection process. After receiving the ASS-REQ, the BSC connects the ground channel of the A interface and the wireless channel of the Um interface together. After receiving the CONNECT, the MSC connects the terrestrial channel of the A interface with the channel used in the network. In MS
There is a similar connection process inside. After receiving the ALERT message, the calling party connects the internal voice channel; when the called user (GSM user) sends CON-NECT, it connects the voice channel in the MS.
5. Call intermittent processing
5.1. Clear CC connection and MM connection When a party hangs up, it starts to clear the communication connection. It starts from the CC sublayer of L3, and finally reaches L1.
Take the example that the calling MS hangs up first. The MS sends a DISCON-NECT (disconnected) message, indicating the originator of the call clearing and the reason for clearing. After receiving the DISCONNECT, the network stops all CC connection timers, clears the connection of the traffic channel in the network, and sends RELEASE (call release) to the MS to inform it that the network is releasing the connection at the CC layer. After receiving the message, the MS stops all CC connection timers, releases the MM connection, sends RELCMP to the network, and enters the "NULL" (idle) state itself. At this time, on the MS side, the L3 connection has been completely released, but the MS cannot remove the L2 layer connection by itself, and must wait for the network release command. After the network receives RELCMP (call release complete), it releases the MM connection and returns to the "NULL" state.
After the connection between the CC layer and the MM layer is released, the network initiates the release of the SCCP connection. The release and response messages are CLRCOM (clear) and CLRCMP (clear).
5.2. Release RR connection
The purpose of RR connection release is to deactivate the dedicated channel in use. After the dedicated channel is released, the MS returns to the IDLE state. The command for RR connection release is CHREL (channel release), including the reason for release (normal release, timeout, handover failure, etc.). After receiving the CHREL, the MS starts the timer and sends back a DISC message at the LAPDm layer, ready to disconnect. When the DISC message is confirmed by the system's UA message or the timer expires, the MS deactivates all channels and returns to idle mode.
After the RR connection is released, stop the system from sending DESACH (deactivated SACCH channel) on the accompanying channel SACCH of the TCH channel, and send RFCHREL (radio channel release) and its response on the TCH channel. Corresponding to RFCHREL, the L1 connection is also cleared to reduce or turn off the system's transmit power on that channel.
6. Others
6.1. Choosing TCH channel allocation time During a call, MS has used two different types of channels, SDCCH and TCH, for signaling and voice transmission.
The network can allocate TCH channels to MSs at different points in time according to the allocation principles used for SDCCH and TCH. There are three ways:
Early allocation, special early allocation and late allocation.
TCH can be assigned immediately after the CC connection is established, or it can be assigned after receiving the ALERT message. The former is called early allocation and the latter is late allocation. Sooner or later the allocation will affect the time the system occupies the SDCCH or TCH channel. The late allocated SD-CCH channel takes a long time, which may cause the TCH channel to be idle, and the call fails due to the lack of SDCCH channel resources, but it can increase the successful utilization rate of the TCH channel. After ALERT, both the calling and the called are in the connected state. Once the called user takes the opportunity, the TCH channel can be successfully used. In the early allocation, if the called user fails to connect, the TCH channel allocated to the calling user cannot be actually used, which reduces the utilization rate but increases the capacity of the SDCCH. In the early allocation, a TCH channel is directly allocated during IMMASS, but it is only used as a signal channel. After the CC connection is established, the channel mode modification command is used to change to the TCH channel. Early allocation does not specifically allocate a separate physical channel for the Xinjin channel, which maximizes the number of users who can talk at the same time, reducing the buffering process of call establishment. When the N channels available for communication in the system are all occupied, new users cannot be accessed. In fact, before the call, the MS and the network still need time for initial signaling communication. During this time, the original call user may have ended the call and can establish a new call. At present, the early allocation method is used less and the early allocation method is used more.
6.2. Identify MS identity
TMSI is a temporary identification code assigned to each mobile user by the network, and is only valid in one location area. In order to improve the confidentiality of MS users, signaling communication can first use TMSI instead of IMSI. If the network recognizes the TMSI number, the connection process can continue; if it cannot recognize the TMSI (MS enters another location area from one location area), it will request the MS to report the IMSI number again. If the number is valid, the communication continues, and the network will assign a new TMSI number to the mobile user. This connection process immediately follows the first L3 message of the A interface.
6.3 Redistribute TMSI
Regardless of whether the TSMSI currently used by the MS can be recognized by the system, for the sake of keeping the user's identity confidential, the network department can reassign a TMSI to the MS during each communication. The reassignment process of TMSI is generally after the encryption is completed and before SETUP is established. Corresponding to the TMSI reassignment command, the MS has a response to the TMSI assignment complete message.
6.4. Send power control information in advance According to the system configuration, the MS can decide whether to report the MS's processing capacity after AUTHREQ. The message name is
CLASSMRAKCHANGE, the content is the same as in the build instruction, but it explains the encryption algorithm supported by MS in more detail. In the establishment instructions, only specify whether to support A5 / 1, A5 / 2 and A5 / 3; and in CLAMARK-CH, further explain whether to support A5 / 4 ~ A5 / 7 algorithm. After receiving this message, the network first sends back the MSPWRCTRL message, indicating the power range that the MS can use and the transmit power required by the TRX corresponding to this MS. During the encryption process, using the information of the encryption algorithm, whether the MS needs to send this message in advance is explained by system message 3 on the network side.
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GSM system signaling connection process
< p> GSM system signaling connection process