GSM: Overview formerly: Groupe Spéciale Mobile (founded 1982) now: Global System for Mobile Communication Pan-European standard (ETSI, European Telecommunications Standardisation Institute) simultaneous introduction of essential services in three phases (1991,1994, 1996) by the European telecommunication administrations (Germany: D1 and D2) seamless roaming within Europe possible

GSM: Overview today many providers all over the world use GSM (more than 200 countries in Asia, Africa, Europe, Australia, America) more than 1.2 billion subscribers in more than 630 networks more than 75% of all digital mobile phones use GSM (74% total) over 200 million SMS per month in Germany, > 550 billion/year worldwide (> 10% of the revenues for many operators) [be aware: these are only rough numbers…]

GSM Working frequencies 890-915 MHz UPLINK 935-960MHz DOWNLINK GSM 1900: 1850-1910 MHz UPLINK 1930-1990 MHz DOWNLINK GSM 1800: 1710-1785 MHz UPLINK 1805-1880 MHz DOWNLINK

Performance characteristics of GSM (wrt. analog sys.) Communication : mobile, wireless communication; voice and data services Total mobility international access, chip-card enables use of access points of different providers Worldwide connectivity one number, the network handles localization

Performance characteristics of GSM (wrt. analog sys.) High capacity better frequency efficiency, smaller cells, more customers per cell High transmission quality high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains) Security functions access control, authentication via chip-card and PIN

Disadvantages of GSM There is no perfect system!! no end-to-end encryption of user data no full ISDN bandwidth of 64 kbit/s to the user, no transparent Bchannel reduced concentration while driving electromagnetic radiation abuse of private data possible roaming profiles accessible high complexity of the system several incompatibilities within the GSM standards

GSM Services GSM offers several types of connections voice connections, data connections, short message service multi-service options (combination of basic services) Three service domains Bearer Services Telematic Services Supplementary Services

GSM Archeitecture

GSM Services Tele services: GSM mainly focuses on voice-oriented tele services via mobile phones. All these basic services have to obey cellular functions, security measurements etc. These comprise encrypted voice transmission message services basic data communication with terminals as known from the PSTN or ISDN (e.g., fax).

GSM Services The primary goal of GSM was the provision of high-quality digital voice transmission specially enabling mobile telephony of traditional bandwidth of 3.1 kHz Special codecs (coder/decoder) are used for voice transmission, while other codecs are used for the transmission of analog data for communication with traditional computer modems used in, e.g., fax machines. Multinumbering several ISDN phone numbers per user possible

GSM Services Another service offered by GSM is the emergency number (eg 911, 999). This service is mandatory for all providers and free of charge. This connection also has the highest priority, possibly pre-empting other connections, automatically be set up with the closest emergency center.

GSM Services Additional services Non-Voice-Teleservices group 3 fax voice mailbox (implemented in the fixed network supporting the mobile terminals) electronic mail (MHS, Message Handling System, implemented in the fixed network)

GSM Services A useful service for very simple message transfer is the short message service (SMS) alphanumeric data transmission to/from the mobile terminal (160 characters) using the signaling channel Sending and receiving of SMS is possible during data or voice transmission It can be used for “serious” applications such as displaying road conditions, e-mail headers or stock quotes, but it can also transfer logos, ring tones, horoscopes and love letters.

GSM Services The successor of SMS, the enhanced message service (EMS), offers larger message size formatted text transmission of animated pictures small images ring tones in a standardized way.

GSM Services MMS offers transmission of larger pictures (GIF, JPG, WBMP), short video clips etc. MMS comes with mobile phones that integrate small cameras. Another non-voice tele service is group 3 fax FAX is available worldwide fax data is transmitted as digital data over the analog telephone network according to the ITU-T standards T.4 and T.30 using modems.

GSM Services Supplementary services: these services offer various enhancements for the standard telephony service, and may vary from provider to provider.

GSM Services Typical services are user identification Call redirection Forwarding of ongoing calls Barring of incoming/outgoing calls Advice of Charge (AoC) etc. Standard ISDN features such as closed user groups and multiparty communication may be available.

GSM Services Bearer services: GSM specifies different mechanisms for data transmission Telecommunication services to transfer data between access pts Specification of services up to terminal interface--OSI layers 1-3 Different data rates for voice and data (original standard) GSM allowing for data rates of up to 9600 bit/s for non-voice services

GSM Services Bearer services permit transparent and non-transparent, synchronous or asynchronous data transmission. data service (circuit switched) synchronous: 2.4, 4.8 or 9.6 kbit/s Asynchronous: 300 - 1200 bit/s

GSM Services Today: data rates of approx. 50 kbit/s possible data service (packet switched) synchronous: 2.4, 4.8 or 9.6 kbit/s asynchronous: 300 - 9600 bit/s Today: data rates of approx. 50 kbit/s possible Transparent bearer services only use the functions of the physical layer (layer 1) to transmit data

GSM Services Data transmission has a constant delay and throughput if no transmission errors occur. Transmission quality can be improved with the use of forward error correction (FEC), which codes redundancy into the data stream and helps to reconstruct the original data in case of transmission errors.

GSM Services Transparent bearer services do not try to recover lost data for example, shadowing or interruptions due to handover. Non-transparent bearer services use protocols of layers two and three to implement error correction and flow control. These services use the transparent bearer services, adding a radio link protocol (RLP).

GSM Services This protocol comprises mechanisms of high-level data link control (HDLC) special selective-reject mechanisms to trigger retransmission of erroneous data. Using transparent and non-transparent services, GSM specifies several bearer services for interworking with PSTN, ISDN, and packet switched public data networks (PSPDN) like X.25, which is available worldwide.

GSM Services Data transmission can be full-duplex, synchronous with data rates of 1.2, 2.4, 4.8, and 9.6 kbit/s or full-duplex, asynchronous from 300 to 9,600 bit/s.  

GSM Archeitecture A GSM system consists of three subsystems radio sub system (RSS) network and switching subsystem (NSS) operation subsystem (OSS)

GSM Archeitecture(RS) Radio Subsystem (RSS): comprises all radio specific entities, i.e., mobile stations (MS) base station subsystem (BSS) The figure shows the connection between the RSS and the NSS via the A interface (dash lines) and the connection to the OSS via the O interface (dashed lines).

GSM Archeitecture(RS) The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). SIM provides personal mobility, user can have access to subscribed services irrespective of a specific terminal

GSM Archeitecture(RS) When MS connects to GSM subsystem, SIM stores a temporary mobile(dynamic) cipher key for encryption in TMSI (Temp mobile subscribe ID) & LAI (Location Area ID) IMSI , card serial no, & type does not change while moving to other location PUK PIN Unblocking Key 128 bit authentication key by service provider IMSI is 15 digit no. 3 digit mobile country code, 2 digit mobile n/w code & 10 digit mobile subscriber identical code

GSM Archeitecture(RS) Inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services. The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI) SIM card contains the International Mobile Subscriber Identity (IMSI)

GSM Archeitecture(RS) IMSI used to identify the subscriber to the system, a secret key for authentication, and other information. IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number(PIN).

GSM Archeitecture(RS) Base station subsystem (BSS): GSM network comprises many BSSs, each controlled by a base station controller (BSC). BSS performs all functions necessary to maintain radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless network part. BSS contains several BSC & BTSs.

GSM Archeitecture(RS) Base station controllers (BSC): It provides all the control functions and physical links between the MSC and BTS. It is a high capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in BTS. A number of BSC’s are served by and MSC.

GSM Archeitecture(RS) Base transceiver station (BTS): handles the radio interface to the mobile station. BTS can form a radio cell or, using sectorized antennas, several MS are connected to BTS via Um interface, and to the BSC via the Abis interface. Um interface contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.) BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTS’s are controlled by an BSC.

GSM Archeitecture(NSS) Network Switching Subsystem: performs call processing & subscriber related functions & includes following functional units: Home location register (HLR) Visitor location register (VLR) Mobile switching center (MSC) Authentication center (AUC)– {OSS} Equipment identity register (EIR) – {OSS}

GSM Archeitecture(NSS) Home location register (HLR): database used for storage and management of subscriptions. stores permanent data about subscribers, including a subscribers service profile, location information and activity status. When an individual buys a subscription from provider, itis registered in the HLR of that operator.

GSM Archeitecture(NSS) Visitor location register (VLR) database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. VLR is always integrated with the MSC. When a MS roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR.

GSM Archeitecture(NSS) Later if the mobile station needs to make a call, VLR will be having all the information needed for call setup. Authentication center (AUC): provides authentication and encryption parameters to verify the users identity & ensure the confidentiality of each call.

GSM Archeitecture(NSS) Equipment identity register (EIR): database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized or defective mobile stations Mobile switching center (MSC): performs the telephony switching functions of System It controls calls to and from other telephone and data systems.

GSM Archeitecture(OSS) Operation and Support system: operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. Implementation of OMC is called operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system.

GSM Archeitecture(OSS) The purpose of OSS is to offer the customer cost-effective support for centralized, regional and local operational and maintenance activities that are required for a GSM network. OSS provides a network overview and allows engineers to monitor, diagnose and troubleshoot every aspect of the GSM network.

GSM If you've ever spoken into an electronic communication device be it desk top, mobile, or hands free, you've undoubtedly experienced acoustic echo — you hear your own voice coming back to you through the device, sometimes with significant delay. The problem can range from simply annoying to absolutely unbearable.

GSM If the echo's amplitude is low, it goes unnoticed, and is not problematic in the conversation; however, if the echo interval exceeds approximately 25 milliseconds (ms), it becomes audible to the speaker. Echo can be extremely disruptive to a conversation. It can so critically impair voice quality that it makes phones calls very unpleasant and distracting;

GSM ECHO TYPES Acoustic echo Hybride echo It originates in a local audio loop back that occurs when a microphone(s), pick up audio signals from a speaker(s), and sends it back to an originating participant. The originating participant will then hear the echo of the participant's own voice as the participant speaks Hybride echo The most prevalent cause of echo in PSTN is impedance1 mismatches within the hybrid where 4-wire phone circuits are converted to 2-wire circuits. This electrically generated echo occurs when the incoming energy from the far end speaker is reflected back toward the speaker as a slightly altered and delayed replication due to impedance mismatch in the hybrid

GSM Archeitecture ECHO CANCELLATION SOLUTIONS Acoustic Echo: Adaptive Digital’s voice quality enhancement algorithms are well suited to address all of the environmental issues required. In this instance, the acoustic noise reduction algorithm is particularly valuable in the presence of high level of background noise present. Acoustic Echo Canceller (AEC ) is used to cancel acoustic feedback between a speaker and a microphone in loud-speaking audio systems, teleconferencing devices, hands free mobile devices, and voice-controlled systems.

GSM Archeitecture Adaptive Digital’s High Definition Acoustic Echo Cancellation software, HD AEC™ product provides superior voice clarity and true full duplex performance under a wide set of difficult acoustic environments.