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Network switching subsystem (NSS) (orGSM core network) is the component of aGSM system that carries outcall out andmobility management functions formobile phonesroaming on thenetwork of base stations. It is owned and deployed bymobile phone operators and allows mobile devices to communicate with each other andtelephones in the widerpublic switched telephone network (PSTN). The architecture contains specific features and functions which are needed because the phones are not fixed in one location.
The NSS originally consisted of the circuit-switchedcore network, used for traditionalGSM services such as voice calls,SMS, andcircuit switched data calls. It was extended with an overlay architecture to provide packet-switched data services known as theGPRS core network. This allows mobile phones to have access to services such asWAP,MMS and theInternet.
Themobile switching center (MSC) is the primary service delivery node for GSM/CDMA, responsible forrouting voice calls and SMS as well as other services (such as conference calls, FAX, and circuit-switched data).
The MSC sets up and releases theend-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real-time prepaid account monitoring.
In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent digitally encoded directly to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound is encoded digitally as apulse-code modulation (PCM) signal in a 64-kbit/s timeslot, known as aDS0 in America).
There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.
Thegateway MSC (G-MSC) is the MSC that determines which "visited MSC" (V-MSC) the subscriber who is being called is currently located at. It also interfaces with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed through a G-MSC. The term is only valid in the context of one call, since any MSC may provide both the gateway function and the visited MSC function. However, some manufacturers design dedicated high capacity MSCs which do not have anybase station subsystems (BSS) connected to them. These MSCs will then be the gateway MSC for many of the calls they handle.
Thevisited MSC (V-MSC) is the MSC where a customer is currently located. Thevisitor location register (VLR) associated with this MSC will have the subscriber's data in it.
Theanchor MSC is the MSC from which ahandover has been initiated. Thetarget MSC is the MSC toward which a handover should take place. Amobile switching center server is a part of the redesigned MSC concept starting from3GPP Release 4.
Themobile switching center server is a soft-switch variant (therefore it may be referred to as mobile soft switch, MSS) of the mobile switching center, which provides circuit-switched calling mobility management, and GSM services to the mobile phonesroaming within the area that it serves. The functionality enables split control between (signaling ) and user plane (bearer in network element called as media gateway/MG), which guarantees better placement of network elements within the network.
MSS andmedia gateway (MGW) makes it possible to cross-connect circuit-switched calls switched by using IP, ATM AAL2 as well asTDM. More information is available in 3GPP TS 23.205.
The termCircuit switching (CS) used here originates from traditional telecommunications systems. However, modern MSS and MGW devices mostly use genericInternet technologies and formnext-generation telecommunication networks. MSS software may run on generic computers orvirtual machines incloud environment.
The MSC connects to the following elements:
Tasks of the MSC include:
Thehome location register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs perpublic land mobile network (PLMN), though oneinternational mobile subscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time.
The HLRs store details of everySIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is theprimary key to each HLR record.
Another important item of data associated with the SIM are the MSISDNs, which are thetelephone numbers used by mobile phones to make and receive calls. The primary MSISDN is the number used for making and receiving voice calls and SMS, but it is possible for a SIM to have other secondary MSISDNs associated with it forfax and data calls. Each MSISDN is also aunique key to the HLR record. The HLR data is stored for as long as a subscriber remains with the mobile phone operator.
Examples of other data stored in the HLR against an IMSI record is:
The HLR is a system which directly receives and processesMAP transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around.
The HLR connects to the following elements:
The main function of the HLR is to manage the fact that SIMs and phones move around a lot. The following procedures are implemented to deal with this:
Theauthentication center (AuC) is a function toauthenticate eachSIM card that attempts to connect to thegsm core network (typically when the phone is powered on). Once the authentication is successful, the HLR is allowed to manage the SIM and services described above. Anencryption key is also generated that is subsequently used to encrypt all wireless communications (voice, SMS, etc.) between the mobile phone and the GSM core network.
If the authentication fails, then no services are possible from that particular combination of SIM card and mobile phone operator attempted. There is an additional form of identification check performed on the serial number of the mobile phone described in the EIR section below, but this is not relevant to the AuC processing.
Proper implementation of security in and around the AuC is a key part of an operator's strategy to avoidSIM cloning.
The AuC does not engage directly in the authentication process, but instead generates data known astriplets for the MSC to use during the procedure. The security of the process depends upon ashared secret between the AuC and the SIM called theKi. TheKi is securely burned into the SIM during manufacture and is also securely replicated onto the AuC. ThisKi is never transmitted between the AuC and SIM, but is combined with the IMSI to produce achallenge/response for identification purposes and an encryption key calledKc for use in over the air communications.
The AuC connects to the following elements:
The AuC stores the following data for each IMSI:
When the MSC asks the AuC for a new set of triplets for a particular IMSI, the AuC first generates a random number known asRAND. ThisRAND is then combined with theKi to produce two numbers as follows:
The numbers (RAND, SRES,Kc) form the triplet sent back to the MSC. When a particular IMSI requests access to the GSM core network, the MSC sends theRAND part of the triplet to the SIM. The SIM then feeds this number and theKi (which is burned onto the SIM) into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet (which it should if it is a valid SIM), then the mobile is allowed to attach and proceed with GSM services.
After successful authentication, the MSC sends the encryption keyKc to thebase station controller (BSC) so that all communications can be encrypted and decrypted. Of course, the mobile phone can generate theKc itself by feeding the same RAND supplied during authentication and theKi into the A8 algorithm.
The AuC is usually collocated with the HLR, although this is not necessary. Whilst the procedure is secure for most everyday use, it is by no means hack proof. Therefore, a new set of security methods was designed for 3G phones.
In practice, A3 and A8 algorithms are generally implemented together (known as A3/A8, seeCOMP128). An A3/A8 algorithm is implemented in Subscriber Identity Module (SIM) cards and in GSM network Authentication Centers. It is used to authenticate the customer and generate a key for encrypting voice and data traffic, as defined in 3GPP TS 43.020 (03.20 before Rel-4). Development of A3 and A8 algorithms is considered a matter for individual GSM network operators, although example implementations are available. To encrypt Global System for Mobile Communications (GSM) cellular communications A5 algorithm is used.[1]
TheVisitor Location Register (VLR) is a database of the MSs (Mobile stations) that have roamed into the jurisdiction of the Mobile Switching Center (MSC) which it serves. Each mainbase transceiver station in the network is served by exactly one VLR (oneBTS may be served by many MSCs in case of MSC in pool), hence a subscriber cannot be present in more than one VLR at a time.
The data stored in the VLR has either been received from theHome Location Register (HLR), or collected from the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSC and, where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface. Whenever an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates the HLR of the mobile subscriber, apprising it of the new location of that MS. If VLR data is corrupted it can lead to serious issues with text messaging and call services.
Data stored include:
The primary functions of the VLR are:
EIR is a system that handles real-time requests to check theIMEI (checkIMEI) of mobile devices that come from the switching equipment (MSC,SGSN,MME). The answer contains the result of the check:
The switching equipment must use the EIR response to determine whether or not to allow the device to register or re-register on the network. Since the response of switching equipment to ‘greylisted’ and ‘unknown equipment’ responses is not clearly described in the standard, they are most often not used.
Most often, EIR uses the IMEI blacklist feature, which contains the IMEI of the devices that need to be banned from the network. As a rule, these are stolen or lost devices. Mobile operators rarely use EIR capabilities to block devices on their own. Usually blocking begins when there is a law in the country, which obliges all cellular operators of the country to do so. Therefore, in the delivery of the basic components of the network switching subsystem (core network) is often already present EIR with basic functionality, which includes a ‘whitelisted’ response to all CheckIMEI and the ability to fill IMEI blacklist, which will be given a ‘blacklisted’ response.
When the legislative framework for blocking registration of devices in cellular networks appears in the country, the telecommunications regulator usually has a Central EIR (CEIR) system, which is integrated with the EIR of all operators and transmits to them the actual lists of identifiers that must be used when processing CheckIMEI requests. In doing so, there may be many new requirements for EIR systems that are not present in the legacy EIR:
Other functions may be required in individual cases. For example, Kazakhstan has introduced mandatory registration of devices and their binding to subscribers. But when a subscriber appears in the network with a new device, the network operation is not blocked completely, and the subscriber is allowed to register the device. To do this, there are blocked all services, except the following: calls to a specific service number, sending SMS to a specific service number, and all Internet traffic is redirected to a specific landing page. This is achieved by the fact that EIR can send commands to several MNO systems (HLR,PCRF,SMSC, etc.).
The most common suppliers of individual EIR systems (not as part of a complex solution) are the companies BroadForward, Mahindra Comviva, Mavenir, Nokia, Eastwind.
Connected more or less directly to the GSM core network are many other functions.
Thebilling center is responsible for processing the toll tickets generated by the VLRs and HLRs and generating a bill for each subscriber. It is also responsible for generating billing data of roaming subscriber.
Themultimedia messaging service center supports the sending of multimedia messages (e.g., images,audio,video and their combinations) to (or from) MMS-bluetooth.
Thevoicemail system records and stores voicemail.
According to U.S. law, which has also been copied into many other countries, especially in Europe, all telecommunications equipment must provide facilities for monitoring the calls of selected users. There must be some level of support for this built into any of the different elements. The concept oflawful interception is also known, following the relevant U.S. law, asCALEA.Generally, lawful Interception implementation is similar to the implementation of conference call. While A and B are talking with each other, C can join the call and listen silently.
Cellular network standards | |||||||||
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| 0Gradio telephones (1946) | |||||||||
| 1G (1979) |
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| 2G (1991) |
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| 2G transitional (2.5G, 2.75G, 2.9G) |
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| 3G (1998) IMT-2000 (2001) |
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| 3G transitional (3.5G, 3.75G, 3.9G) |
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| 4G (2009) IMT Advanced (2013) |
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| 5G (2018) IMT-2020 (2021) |
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