# Preface This compendium aims to explain the most important concepts in the subject. The content is mostly based on the lecture slides and the book: "From GSM to LTE-Advanced PRO and 5G". The course focuses on: GSM, GPRS, UMTS, LTE, SIP & IMS, 5G and Over the Top Services. The compendium is structured around explaining each of these items in detail. Everyone is allowed to edit this compendium, if you see any errors feel free to correct them. # GSM ## Mobile Station The mobile station is made up of two key elements: - The __mobile equipment__ - The __Subscriber Identity Module (SIM) Card__ A mobile station can for example be a mobile phone. The mobile euqipment is the various hardware components of the mobile phone, for example:radio, screen, battery, processor, operating system, camera and so on. The SIM card is a component which mainly serves the purpose of identifying the user of the mobile equipment. ## GSM Subsystems GSM has three different subsystems: - __The Network Subsystem (NSS)__, also called the "core network" contains all nodes and functionalities that are necessary for switching of calls, for subscriber management and mobility management. - __The Base Station Subsystem (BSS)__, also called the "radio network" contains all nodes and functionalities that are necessay to wirelessly connect mobile subscribers over the radio interface to the network. - __The Intelligent Netowk Subsystem (IN)__ comprises SCP databases that add optional functionality to the network. For example prepaid service, SMS mesages and data services via GPRS and UMTS.  ## The Network Subsystem The most important responsibilities of the NSS are call establishment, call control and routing of calls between different fixed and mobile switching centers and other networks. It is also responsible for subscriber management. The NSS consissts of the following nodes: Mobile switching center (MSC), VLR (Visitor Location Register) and HLR (Home Location Register). ### The Mobile Switching Center (MSC) The MSC is the central element of a mobile telecommunication network and all connections between subscribers are managed by it. The management activities to establish and maintain a connection are part of the Call control (CC) protocol, which is generally responsible for the following tasks: - Registration of mobile subscribers - Call establishment and routing. - Forwarding of SMS messages. As subscribers can roam freely in the network, the MSC is also responsible for the Mobility Management (MM) of Subscribers. This activity comprises the following tasks: - Authentication of subscribers - Location update. - Handover. The MSC is also responsible for billing, not prepaid however. ### The Visitor Location Register (VLR) - Each MSC has an associated Visitor Location Register (VLR), which holds the record of each subscriber that is currently served by the MSC. - These records are only copies of the origianl records, which are stored in the HLR. - The VLR is mainly used to reduce signalling between the MSC and the HLR. If a subscriber roams into the area of an MSC, the data are copited to the VLR of the MSC and are thus locally available for every connection establishment. - Can be independent hardware, but is usually implemented as a software component in the MSC. - When a subscriber leaves the coverage area of an MSC, their record is copied from the HLR to the VLR of the new MSC, and is then removed from the VLR of the previous MSC. ### The Home Location Register (HLR) - The HLR is the subscriber database of a GSM network. - It contains a record for each subscriber, with information about the individually available servuces. - The International Mobile Subscriber Identity (IMSI) is an internationally unique number that identifies a subscriber and is used for most subscriber-related signalling in the network. - The IMSI is stored in the subscriber's subscriber identity module (SIM) card and in the HLR and is thus the key to all information about the subscriber. - Contains the AuC (Authentication Center) used for authentication of subscribers. ### The Short Messaging Service Center (SMSC) - Used to store and forward short messages. ## The Base Station Subsystem (BSS) and Voice Processing The BSS containts two central nodes: the Base Tranceiver Station (BTS) and the Base Station Controller (BSC). ### Base Tranceiver Station (BTS) Connects mobile stations to the GSM network via the air interface. Two access methods: Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). ### Base Station Controller (BSC) Responsible for: - Connection Establishment and release (signalling channels). - Call maintenance (handover). - Controlling transmission power. - Controlling of timing advance. ## Mobility Management and Call Control There are three processes that allow a subscriber to roam throughout the network ### Cell Reselection and Area Update Location area update is performed whenever an MS registers with the network and whenever an MS roams into a cell with a new location area code (LAC). Paging is the procedure of finding the exact cell of a mobile device. ## The Signalling System Number 7 Signaling System No.7 (SS-7) is a telecommunications signaling architecture traditionally used for the set up and tear down of telephone calls. It has a robust protocol stack that uses out-of-band signaling to communicate between elements of the public switched telephone network (PSTN). In recent years it has been superseded by the Diameter signaling protocol on all-IP networks. The SS-7 standard defines three basic types of network nodes: - Service Switching Points (SSPs): Are switching centers that are more generally reffered to as network elements and that are able to establish, transport or forward voice and data connections. - Service Control Points (SCPs): Are databases and application software that can influence call establishment of a connection. - Signalling Transfer Points (STPs): Are responsible for the forwarding of signalling messages between SSPs and SCPs as not all network nodes have a dedicated link to all other nodes in the network. # SIP SIP is a protocol operating at the application level. Can run on either TCP or UDP. ## SIP identification Public URI/Address of Record : Used to identify users FQDN : FQDN (Fully qualified domain names) are identifiers for individual devices.

## SIP Transactions Transaction is basically an exchange of messages and the type depends on the initiating request. There are three types of transactions: REGULAR transactions : May be initiated by any type of SIP method __except__ INVITE, ACK or CANCEL. A regular transaction can for example start with a BYE request. INVITE-ACK transactions : ACK is a request method sent in response to a final response of an INVITE. Consists of two sub-transactions when successful: INVITE transaction and ACK transaction. CANCEL transactios : Inititated by CANCEL request. Always connected to a previous transaction