Gsm: Network Architecture
Their maximum allowed output power is AWE. Handheld terminals; their popularity is owed to their weight and volume, which is continuously decreasing. According to some specification these terminals may emit up to 0. AWE. However, as technology has evolved their maximum allowed power output is limited to 0. AWE. Base Station Subsystem The BBS provides the interface between the ME and the INS. It is in charge of the transmission and reception. It may be divided into two parts: * Base Station Controller (BBS): It controls a group of BITS and manages their radio resources.
A BBS is principally in charge of handcuffs, frequency hopping, exchange functions ND power control over each managed BITS. * Base Transceiver Station (BITS) or Base Station: it maps to transceivers and antennas used in each cell of the network. It is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BITS has between 1-16 transceivers depending on the density of users in the cell. INS:- Its main role is to manage the communications between the mobile users and other users, such as mobile users, SIDE users, fixed telephony users, etc.
It also includes data bases needed in order to store information about the subscribers and to engage their mobility. The different components of the INS are described below. * MS: the central component of the INS. The MS performs the switching functions of the network. It also provides connection to other networks * GEMSвЂў A gateway that interconnects two networks: the cellular network and the EST.. It is in charge of routing calls from the fixed network towards a GSM user. The GEMS is often implemented in the same machines as the MS. HAL: The HAL stores information of the subscribers belonging to the coverage area of a MS; it also stores the current action of these subscribers and the services to which they have access. The location of the subscriber maps to the SSE address of the Visitor Location Register (EVIL) associated to the MN. * EVIL: contains information from a subscriber’s HAL necessary to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MS, the EVIL associated to this MS will request information about the new subscriber to its corresponding HAL.
The EVIL will then have enough data to assure the subscribed services without needing to ask the HAL each time a communication is established. The EVIL is always implemented together with a MS; thus, the area under control of the MS is also the area under control of the EVIL. * Authentication Centre (Auk): It serves security purposes; it provides the parameters needed for authentication and encryption functions. These parameters allow verification of the subscriber’s identity. * Equipment Identity Register (EIRE): EIRE stores security-sensitive information about the mobile equipment’s.
It maintains a list of all valid terminals as identified by their International Mobile Equipment Identity (MIME). The EIRE allows then to forbid calls from stolen or unauthorized terminals (e. G. , a terminal which does not respect the specifications concerning the output RFC power). * GSM Networking Unit (GIGS): The KIWIS provides an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated. Operation and Support Subsystem (JOSS) It is connected to components of the INS and the BBS, in order to control and monitor the GSM system.
It is also in charge of controlling the traffic load of the BBS. It must be noted that as the number of BBS increases with the scaling of the obscurer population some of the maintenance tasks are transferred to the BITS, allowing savings in the cost of ownership of the system. Geographical areas A cell, as identified by its Cell Global Identity (CGI) number, maps to the radio coverage of a BITS. Similarly an LA as identified by its Location Area Identity (LA’) number , is a cluster of cells served by a single MUSCLE.
A group of LA under the control of the same MUSCLE defines the MUSCLE area. A Public Land Mobile Network (PLAN) is the area served by one network operator. Network operations In this paragraph, the description of the GSM network is focused on the different unction’s to fulfill by the network and not on its physical components. In GSM, five main functions can be defined: * Transmission: of data and signaling. Not all the components of the GSM network are strongly related with both types of types of TX.
While the MS, BITS and BBS, among others, are involved with data and signaling, components such as HAL, EVIL or EIRE registers, are only concerned with signaling. Radio Resources Management (ARM). * M ability Management (M * Communication Management (CM). * Operation, Administration and Maintenance (AM). Radio Resources Management (ARM) The role of the OR function is to establish, maintain and release communication links between mobile stations and the MS. The elements that are mainly concerned with the OR function are the MN and the BITS.
However, since the OR component performs connection management also during cell handcuffs, it also affects the MS which is the handcuff management component. The OR is also responsible for the management of frequency resources as well as varying radio interface conditions. Main component operations are: * Channel assignment, change and release. * Handcuff * Frequency hopping. * Power-level control. Discontinuous transmission and reception. * Timing advance. Handcuff The user movements may result a change in the channel/cell, when the quality of the communication is degrading; this is known as handcuff.
Handcuffs occur between: * between channels within a cell * between cells controlled by the same BBS * between cells under the same MS but controlled by different BPCS * Between cells controlled by different MACS. Handcuffs are mainly controlled by the MS. However to avoid unnecessary signaling, the first two types of handcuffs are managed by the respective BBS (thus, the MS is only notified of the handbook. To perform the handcuff the mobile station controls continuously its own signal strength and the signal strength of the neighboring cells.
The list of cells that must be monitored by the mobile station is given by the base station. Power measurements allow to decide which is the best cell in order to maintain the quality of the communication link. Two basic algorithms are used for handcuffs: * The ‘minimum acceptable performance’ algorithm. When the quality of the transmission degrades, the power level of the mobile is increased, until the increase of the power level has no effect on the quality of the signal. Upon this link layer hint, a handcuff is initiated. The ‘power budget’ algorithm. Here the handcuff pre-empty the power increase, to obtain a good SIR.
Mobility Management (MM) The MM component handles: * Location Management: Location is managed through periodically or on-demand. At power-on time, the MM signals an AIMS attach. On-demand location updates are signaled when the MN moves to a different PLAN or new location area (LA). The signal is sent to the new MUSCLE, which forwards it to the subscriber’s HAL. Upon authorization in the new MUSCLE, the subscriber’s HAL removes the registration entry to the MN at the old M SCANS It tater the update time interval, t MN NAS not registered, it is then deteriorated.
On power-off, the MN performs an AIMS detach. * Security and authentication: Authentication involves the SIMI card and the Authentication Centre. A secret key, stored in the SIMI card and the Auk together with a ciphering algorithm called AY, are used to authenticate the user. The MN and the Auk compute a SERE through AY using the secret key and a nonce generated by the Auk. If the two computed SERE are the same, the subscriber is authenticated. The efferent services to which the subscriber has access are also checked. Next the a security check is performed in the equipment identity (MIME).
If the MIME number of the mobile is authorized in the EIRE, the mobile station is allowed to connect the network. To assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMI) after its first location update procedure. Enciphering is another option to guarantee a very strong security. Communication Management (CM) The CM component manages: * Call control (C): it controls call setup, management and tear-down in relation to management of type of service. Call routing is the primary task for this component.