Global System Services for Mobile Communications (GSM) is a collection of standard apps and features available to mobile phone subscribers around the world. The GSM standard is defined by 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators. Common standards make it possible to use the same phone with different company services, or even browse to different countries. GSM is the most dominant mobile phone standard in the world.
The design of this service is complicated because it must be able to find mobile phones anywhere in the world, and accommodate relatively small battery capacity, limited input/output capabilities, and weak radio transmitters on mobile devices.
Video GSM services
Access the GSM network
To gain access to GSM services, users need three things:
- Billing relationship with mobile phone carrier. This is usually either where services are paid for before they are consumed (prepaid), or where bills are issued and resolved after the service has been used (postpaid).
- The phone is GSM-compatible and operates on the same frequency as the carrier. Most phone companies sell mobile phones from third-party manufacturers.
- The customer identity module (SIM card), which is activated by the carrier after the billing relationship is created. After activation, the card is then programmed with the Integrated Mobile Customer Network Customer Integrated Service Number (MSISDN) (phone number). Personal information such as contact numbers of friends and family can also be stored on the SIM by the customer.
After customers register, information about their identity (phone number) and what services are allowed for their access are stored in the "SIM record" in the Home Location List (HLR).
After the SIM card is loaded into the phone and the phone is turned on, it will search for the nearest mobile telephone pole (also called Base Transceiver Station/BTS) with the strongest signal in the carrier frequency band. If the pole can be successfully contacted, then said there is coverage in the area. The phone then identifies itself to the network via a control channel. Once this is completed successfully, the phone is said to be plugged into the network.
The main feature of the phone is the ability to receive and make calls in any area of ​​coverage available. This is generally called roaming from the customer's perspective, but is also called visiting when describing the underlying technical process. Each geographic area has a database called Visitor Location Register (VLR), which contains details of all the phones currently in the area. Every time a mobile phone sticks, or visits, a new area, Visitors The Location List should contact Home Location List to get details for that phone. The current mobile phone location (that is, where the base station is located) is inserted into the VLR record and will be used during a process called paging when the GSM network wants to locate the phone.
Each SIM card contains a secret key, called Ki, which is used to provide authentication and encryption services. This is useful for preventing theft of services, and also to prevent "over the air" peering user activity. The network does this by utilizing the Authentication Center and is completed without transmitting the key directly.
Each GSM phone contains a unique identifier (different from a phone number), called the International Mobile Device Identity (IMEI). This can be found by pressing * # 06 #. When the phone calls the network, its IMEI can be checked against the Equipment Identity Checklist to locate stolen phones and facilitate monitoring.
Maps GSM services
Voice all calls
All Outgoing
Once the phone has successfully connected to the GSM network as described above, calls can be made from the phone to other phones in the global Public Public Phone Network.
The user dials the phone number, presses the send or talk button, and the phone sends a call setup request message to the mobile phone network via the nearest base mobile phone transceiver base station (BTS).
The call setup request message is handled next by the Mobile Switching Center, which checks the customer records stored in the Visitors Location List to see if outgoing calls are allowed. If so, the MSC then routes the call in the same way that the telephone exchange is not in a fixed network.
If a customer uses a prepaid rate (sometimes known as Pay As You Go or Pay ), then additional checks are made to see if the customer has enough credit to continue. Otherwise, the call is rejected. If the call is allowed to continue, then it is continuously monitored and the appropriate amount is deducted from the customer's account. When credit reaches zero, calls are disconnected by the network. Systems that monitor and provide prepaid services are not part of GSM standard services, but as examples of smart network services that mobile operators can apply apart from the GSM standard.
Incoming call
MSC Gateway Contacts
When someone calls a cell phone, they call the phone number (also called MSISDN) associated with the phone user and the call is diverted to the Mobile Operator's Gateway Mobile Switching Center. The MSC Gateway, as the name implies, acts as an "entrance" from the outside of the Public Switched Telephone Network to the provider's network.
As mentioned above, phones are free to roam anywhere on the carrier network or on roaming partner networks, including in other countries. So the first job of the MSC Gateway is to determine the location of the current mobile phone to connect the call. This is done in consultation with the Home Location Register (HLR), which, as described above, knows which Visitor Visitor Register (VLR) is associated with the phone, if any.
Route calls
When HLR receives this request message, it determines whether the call should be diverted to another number (called redirection), or whether it will be transferred directly to the phone.
- If the previous mobile owner has requested that all incoming calls be diverted to another number, known as the Unconditional Call Forward Number (CFU), then this number is stored in the Home Location List. If that is the case, then the CFU number is returned to the MSC Gateway for direct routing to that destination.
- If the phone is not currently associated with the List of Locations Visited (since the phone has been shut down) then Home Location List returns a number known as the Call Forward Not Reachable (CFNRc) number to the MSC Gateway, and the call is forwarded to it. Many operators can assign this value automatically to the phone's voice mail number, so callers can leave a message. The phone can sometimes override the default settings.
- Finally, if the Home Location Register knows that the phone is roaming in a certain VLR area, it will ask for a temporary number (called MSRN) of the VLR (using IMSI as the reference number.This number is routed back to the MSC Gateway, and then used to route calls to the MSC on which the phone is called roaming.
Find and ring your phone
When the call arrives at MSC Visited, the MSRN is used to determine which phone in this area is called, ie the MSRN map back to the original phone number of the IMSI contacted. The MSC page of all the mobile phone poles in the area where the IMSI is registered to inform the phone that there is an incoming call for it. If the customer answers, the talk path is made through the Visiting MSC and the MSC Gateway back to the network of the person making the call, and the normal phone call follows.
Maybe also the phone call was not answered. If the customer is busy with another call (and call waiting is not used), the visited MSC will redirect the call to a predefined Call Force (CFB) number. Similarly, if the customer does not answer the call after a certain period of time (usually 30 seconds), then the MSC visited will redirect the call to a predefined Call Forward No Reply (CFNRy) number. Again, the operator may decide to set this value by default to mobile voice mail so that callers can leave messages.
If the customer does not respond to a paging request, either because it is out of range, or their battery has been exhausted/deleted, then the visited MSC will redirect the call to a predefined Called Off (CFNRc) number. Again, the operator may decide to set this value by default to mobile voice mail so that callers can leave messages.
Roaming users may want to avoid these forwarding services on the networks they visit because roaming charges will apply.
Voice charges
In the United States and Canada, callers pay a fee to connect to the MSC Gateway from the customer's phone company, regardless of the actual phone location. Since mobile numbers are given standard geographic numbers according to North American Numbering Plan, callers pay the same to reach fixed phones and cell phones in certain geographic areas. Mobile customers pay for connection times (usually using in-plan or prepaid minutes) for incoming and outgoing calls. For outgoing calls, any long distance charges are billed as if they are from GMSC, although they are MSC visits that complement the connection to PSTN. Plans that include national long distance and/or national roaming at no additional charge on popular "local" outgoing calls.
Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia and Argentina only charge their customers for outgoing calls. Incoming calls are free for mobile customers with the exception of receiving calls when a roaming subscriber as described below. However, callers usually pay a higher rate when calling a cell phone. Special prefixes are used to determine the phone number so callers are aware that they are calling a cell phone and therefore will be charged a higher rate.
From the caller's point of view, it does not matter where the mobile customer is, because the technical process for connecting calls is the same. If a roaming customer on a different corporate network, the customer, not the caller, can pay an additional fee for connection time. International roaming calls are often quite expensive, and as a result some companies require customers to give explicit permission to receive calls while roaming to certain countries.
When a customer roams internationally and calls are forwarded to his voice message, such as when his phone is dead, busy, or unanswered, he may actually be charged for two simultaneous calls simultaneously - first obtained from GMSC to VMSC and the second to get from VMSC to Call Forward Busy or Call Forward No Reply number (usually voice mailbox) in customer's country. However, some GMSCs' networks connect missed calls directly, keeping voice signals entirely within the home country and thereby avoiding multiple charges.
Greeting encoding
During a GSM call, the speech is converted from analog sound waves to digital data by the phone itself, and transmitted over the cell phone network in a digital way. (Although the older section of the Public Switched Telephone Line can use analog transmission.)
The digital algorithm used to encode speech signals is called a codec. The speech codec used in GSM is called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR), Adaptive Multirate (AMR) and Wideband AMR also known as HD sound. All codecs except AMR operate with fixed data rates and error correction rates.
Data transmission
The GSM standard also provides a separate facility for transmitting digital data. This enables the phone to act like any other computer on the Internet, sending and receiving data via the Internet Protocol.
The phone can also be connected to a desktop computer, laptop, or PDA, to be used as a network interface (such as a modem or Ethernet card, but using one of the GSM data protocols described below instead of a PSTN-compatible audio channel or an Ethernet link to send data). Some GSM phones can also be controlled by standard AT commands defined via a serial cable or wireless connection (using IRDA or Bluetooth). The AT command can control anything from ringtones to a data compression algorithm.
In addition to public Internet access, other special services may be provided by mobile phone operators, such as SMS.
Circuit-switched data protocol
The circuit-switched data connection reserves a certain amount of bandwidth between two points for connection life, as traditional phone calls allocate quality audio channels between two phones during the call duration.
Two circuit-switched data protocols are defined in the GSM standard: Circuit Switched Data (CSD) and High-Speed ​​Circuit-Switched Data (HSCSD). This type of connection is usually charged per second, regardless of the amount of data sent through the link. This is because a certain amount of bandwidth is dedicated to the connection regardless of whether it is needed or not.
Circuit-switched connections have the advantage of providing a constant and guaranteed quality of service, which is useful for real-time applications such as video conferencing.
General Package Radio Service (GPRS)
General Packet Radio Service (GPRS) is a packet-switched data transmission protocol, which was incorporated into the GSM standard in 1997. It is compatible with systems that use pre-1997 standard versions. GPRS does this by sending packets to a local mobile phone mast (BTS) on a channel that is not used by a switched-circuit voice call or data connection. Some GPRS users can share one unused channel because each uses it only for occasional short bursts.
The advantage of packet-switched connections is that the bandwidth is only used when there is actually data being transmitted. Such connection types are generally billed by kilobytes rather than by the latter, and are usually a cheaper alternative for applications that only need to send and receive data sporadically, such as instant messaging.
GPRS is usually described as 2.5G technology; see the main article for more information.
Short Messaging Service (SMS)
The Short Message Service (better known as text messaging) has become the most widely used data application on mobile phones, with 74% of all mobile users worldwide already as active SMS users, or 2.4 billion people by the end of 2007.
SMS text messages can be sent by mobile users to other mobile users or external services receiving SMS. Messages are typically sent from mobile devices via the Short Message Service Center using the MAP protocol.
SMSC is the central routing center for Short Messaging. Many mobile service operators use their SMSC as a gateway to external systems, including the Internet, incoming SMS news feeds, and other mobile operators (often using de facto SMPP standards for SMS exchange).
SMS standards are also used outside the GSM system; see the main article for details.
Additional Services
See also GSM code for additional services.
- Call forwarding .
- Outbound Call Restrictions .
- Inbound Call Restriction .
- Feeding Feedback (AoC) . This GSM service estimates the cost of a call to be displayed on a user's mobile phone. This helps the user by preventing billing shocks and reducing the burden on the customer service department of the mobile network operator. However, in practice, this GSM service is rarely used because the calculations are more complex than the standard that allows. Instead, some operators provide customers with a balance notice and/or call cost notification sent at a predetermined time, such as at the beginning or end of each call. This notification can be read aloud using a speech-synthesis system, or it can be sent in textual form.
- Hold Call .
- Call Waiting .
- Multiparty service .
- Calling Line Identification Presentation (CLIP)/Restrictions (CLIR) .
- Closed User Group (CUG) .
- Explicit Call Transfer (ECT) . This service allows users who have two calls to connect these two calls together and release their connections to the other two parties.
See also
- GSM USSD Code - Unstructured Additional Service Data: standard GSM code list for network and SIM related functions
References
External links
- GSM Call Flow and Order Chart Detailed call flow chart depicting GSM call settings, location updates and handover.
- GSM Call Flow and Detailed Flowchart diagram tutorial
Source of the article : Wikipedia
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