UNIT II MAC, TELE COMMUNICATION AND SATELLITE SYSTEMS

Transcription

1 UNIT II MAC, TELE COMMUNICATION AND SATELLITE SYSTEMS Medium access Control Techniques- SDMA-TDMA-FDMA- CDMA- Comparison. Tele communication systems- GSM-DECT and TETRA. Satellite Systems- Routing, Localization and hand over. MOTIVATION FOR A SPECIALIZED MAC Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot hear the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is hidden Hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B Near and far terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance MTech CSE (PT, ) SRM, Ramapuram 1 hcr:innovationcse@gg

2 the signal of terminal B therefore drowns out A s signal C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) o segment space into sectors, use directed antennas o cell structure FDMA (Frequency Division Multiple Access) o assign a certain frequency to a transmission channel between a sender and a receiver o permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) o assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time Space Division Multiple Access (SDMA) SDMA is used for allocating a separated space to users in wireless networks. A typical application involves assigning an optimal base station to a mobile phone user. The mobile phone may receive several base stations with different quality. A MAC algorithm could now decide which base station is best, taking into account which frequencies (FDM), time slots (TDM) or code (CDM) are still available (depending on the technology). SDMA is never used in isolation but always in combination with one or more other schemes. The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division multiplexing (SDM) Frequency Division Duplex - FDD/FDMA General scheme, example GSM All uplinks use the band between and 915 MHz, all downlinks use to 960 MHz. MTech CSE (PT, ) SRM, Ramapuram 2 hcr:innovationcse@gg

3 TIME DIVISION MULTIPLE ACCESS (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., controlling TDM. Tuning in to a certain frequency is not necessary, i.e., the receiver can stay at the same frequency the whole time. Using only one frequency, and thus very simple receivers and transmitters Fixed TDM The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern. This results in a fixed bandwidth and is the typical solution for wireless phone systems Assigning different slots for uplink and downlink using the same frequency - time division duplex (TDD). Time division multiplexing for multiple access and duplex Classical Aloha Aloha neither coordinates medium access nor does it resolve contention on the MAC layer. each station can access the medium at any time random, distributed (no central arbiter), time-multiplex If two or more stations access the medium at the same time, a collision occurs and the data is destroyed Slotted Aloha Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries MTech CSE (PT, ) SRM, Ramapuram 3 hcr:innovationcse@gg

4 Carrier Sense Multiple Access (CSMA) sensing the carrier before accessing the medium this decreases the probability of a collision. o hidden terminals cannot be detected, o if a hidden terminal transmits at the same time as another, a collision might occur at the receiver. Types of CSMA non-persistent CSMA o stations sense the carrier and start sending immediately if the medium is idle. o If the medium is busy, the station pauses a random amount of time before sensing the medium again and repeating this pattern. In p-persistent CSMA o systems nodes also sense the medium, but only transmit with a probability of p, with the station deferring to the next slot with the probability 1-p, i.e., access is slotted in addition. In 1-persistent CSMA systems, o all stations wishing to transmit access the medium at the same time, as soon as it becomes idle. This will cause many collisions if many stations wish to send and block each other. Demand Assigned Multiple Access (DAMA) Also called Reservation Aloha. Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) Reservation can increase efficiency to 80% a sender reserves a future time-slot o sending within this reserved time-slot is possible without collision o reservation also causes higher delays o typical scheme for satellite links Examples for reservation algorithms: o Explicit Reservation according to Roberts (Reservation-ALOHA) o Implicit Reservation (PRMA) o Reservation-TDMA DAMA - Explicit Reservation (Reservation Aloha) Two modes: o ALOHA mode for reservation: competition for small reservation slots, collisions possible o Reserved mode for data transmission within successful reserved slots (no collisions possible) it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time Packet Reservation Multiple Access Implicit reservation a certain number of slots form a frame, frames are repeated stations compete for empty slots according to the slotted aloha principle once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send MTech CSE (PT, ) SRM, Ramapuram 4 hcr:innovationcse@gg

5 competition for this slots starts again as soon as the slot was empty in the last frame Reservation TDMA every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) Multiple Access with Collision Avoidance MACA Uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain o sender address o receiver address o packet size Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC) MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A MTech CSE (PT, ) SRM, Ramapuram 5 hcr:innovationcse@gg

6 Protocol machines for MACA MACA variant: DFWMAC in IEEE Polling If one terminal can be heard by all others, this central terminal (a.k.a. base station) can poll all other terminals according to a certain scheme The master can poll the slaves according to many schemes: o round robin (only efficient if traffic patterns are similar over all stations), o randomly, o according to reservations (the classroom example with polite students) etc. Example: Randomly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminal this cycle starts again after polling all terminals of the list Inhibit Sense Multiple Access (ISMA) / Digital Sense Multiple Access (DSMA). Current state of the medium is signaled via a busy tone the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) mechanism used for Cellular Digital Packet Data (CDPD) (USA, integrated into AMPS mobile system) MTech CSE (PT, ) SRM, Ramapuram 6 hcr:innovationcse@gg

7 Code Division Multiple Access (CDMA) all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel each sender has a unique random number, the sender XORs the signal with this random number the receiver can tune into this signal if it knows the pseudo random number, tuning is done via a correlation function Advantages: o all terminals can use the same frequency, no planning needed o huge code space (e.g ) compared to frequency space o interferences (e.g. white noise) is not coded o forward error correction and encryption can be easily integrated Disadvantages: o higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) o all signals should have the same strength at a receiver CDMA in theory Sender A sends A d = 1, key A k = (assign: 0 = -1, 1 = +1) sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1) Sender B sends B d = 0, key B k = (assign: 0 = -1, 1 = +1) sending signal B s = B d * B k = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space interference neglected (noise etc.) A s + B s = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A apply key A k bitwise (inner product) A e = (-2, 0, 0, -2, +2, 0) A k = = 6 result greater than 0, therefore, original bit was 1 receiving B B e = (-2, 0, 0, -2, +2, 0) B k = = -6, i.e. 0 Coding and spreading of data from sender A MTech CSE (PT, ) SRM, Ramapuram 7 hcr:innovationcse@gg

8 Coding and spreading of data from sender B Reconstruction of A s data Reconstruction of B s data MTech CSE (PT, ) SRM, Ramapuram 8 hcr:innovationcse@gg

9 Spread Aloha Multiple Access (SAMA) Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha each sender uses the same spreading code combination of CDMA and TDMA Comparison of SDMA, TDMA, FDMA, and CDMA mechanisms Approach SDMA TDMA FDMA CDMA segment sending time segment the spread the segment space into into disjoint time-slots, Idea frequency band into spectrum using cells/sectors demand driven or fixed disjoint sub-bands orthogonal codes patterns Terminals Signal separation Advantages Disadvantages Comment only one terminal can be active in one cell/one sector cell structure, directed antennas very simple, increases capacity per km² inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful all terminals are active for short periods of time on the same frequency synchronization in the time domain established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks every terminal has its own frequency, uninterrupted filtering in the frequency domain simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA MTech CSE (PT, ) SRM, Ramapuram 9 hcr:innovationcse@gg

10 GSM Groupe Spéciale Mobile => Global System for Mobile Communication Performance characteristics of GSM Communication o mobile, wireless communication; support for voice and data services Total mobility o international access, chip-card enables use of access points of different providers Worldwide connectivity o one number, the network handles localization High capacity o better frequency efficiency, smaller cells, more customers per cell High transmission quality o high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (cars, trains) Security functions o access control, authentication via chip-card and PIN Disadvantages of GSM no end-to-end encryption of user data no full ISDN bandwidth of 64 kbit/s to the user, no transparent B-channel 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 Mobile Services GSM offers o several types of connections voice connections, data connections, short message service o multi-service options (combination of basic services) Three service domains o Bearer Services o Telematic Services o Supplementary Services Bearer and Tele Services Reference Model Bearer Services Telecommunication services to transfer data between access points Specification of services up to the terminal interface (OSI layers 1-3) Different data rates for voice and data (original standard) o data service (circuit switched) synchronous: 2.4, 4.8 or 9.6 kbit/s asynchronous: bit/s MTech CSE (PT, ) SRM, Ramapuram 10 hcr:innovationcse@gg

11 o data service (packet switched) synchronous: 2.4, 4.8 or 9.6 kbit/s asynchronous: bit/s Tele Services Telecommunication services that enable voice communication via mobile phones All these basic services have to obey cellular functions, security measurements etc. mobile telephony Emergency number o common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible) Multinumbering o several ISDN phone numbers per user possible Non-Voice-Teleservices o group 3 fax, voice mailbox, electronic mail, Short Message Service (SMS) Supplementary services Services in addition to the basic services, cannot be offered stand-alone Similar to ISDN services besides lower bandwidth due to the radio link May differ between different service providers, countries and protocol versions Important services o identification: forwarding of caller number o suppression of number forwarding o automatic call-back o conferencing with up to 7 participants o locking of the mobile terminal (incoming or outgoing calls) GSM SYSTEM ARCHITECTURE GSM is a PLMN (Public Land Mobile Network) several providers setup mobile networks following the GSM standard within each country components o MS (mobile station) o BS (base station) o MSC (mobile switching center) o LR (location register) subsystems o RSS (radio subsystem): covers all radio aspects o NSS (network and switching subsystem): call forwarding, handover, switching o OSS (operation subsystem): management of the network Radio Sub System (RSS) All radio specific entities Mobile Stations (MS) o all user equipment and software needed for communication with a GSM network o Subscriber Identity Module (SIM) all user-specific data that is relevant to GSM o International Mobile Equipment Identity (IMEI) o Personal Identity Number (PIN) o PIN Unblocking Key (PUK) o Authentication Key Ki o Cipher Key Kc o International Mobile Subscriber Identity (IMSI) o Temporary Mobile Subscriber Identity (TMSI) o Location Area Identification (LAI) Base Station Subsystem (BSS) o controlled by a base station controller (BSC) MTech CSE (PT, ) SRM, Ramapuram 11 hcr:innovationcse@gg

12 o functions necessary to maintain radio connections to an MS, o coding/decoding of voice o rate adaptation to/from the wireless network part o contains several BTSs Base Transceiver Station (BTS) o can form a radio cell or, several cells using sectorized antennas o connected to MS via the Um interface (ISDN U interface for mobile use) o connected to the BSC via the Abis interface. Base Station Controller (BSC) o manages the BTSs o reserves radio frequencies o handles the handover from one BTS to another within the BSS o performs paging of the MS o multiplexes the radio channels onto the fixed network connections at the A interface O interface o Signalling System No. 7 (SS7) based on X.25 (dashed lines) A interface o circuit-switched PCM-30 systems Network and Switching Subsystem (NSS) Mobile services switching center (MSC) o Gateway MSC (GMSC) additional connections to other fixed networks, such as PSTN and ISDN o Interworking Functions (IWF) Used to connect to public data networks (PDN) such as X.25. o Standard Signaling System No. 7 (SS7) o The MSC (mobile switching center) plays a central role in GSM switching functions additional functions for mobility support management of network resources interworking functions via Gateway MSC (GMSC) integration of several databases Functions of a MSC specific functions for paging and call forwarding termination of SS7 (signaling system no. 7) mobility specific signaling location registration and forwarding of location information provision of new services (fax, data calls) support of short message service (SMS) generation and forwarding of accounting and billing information Home Location Register (HLR) o Mobile Subscriber ISDN number (MSISDN) o International Mobile Subscriber Identity (IMSI) o Location Area (LA) o Mobile Subscriber Roaming Number (MSRN) Visitor Location Register (VLR) o local database for a subset of user data, data about all user currently in the domain of the VLR MTech CSE (PT, ) SRM, Ramapuram 12 hcr:innovationcse@gg

13 Operation SubSystem (OSS) enables centralized operation, management, and maintenance of all GSM subsystems Authentication Center (AUC) o generates user specific authentication parameters on request of a VLR o authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system Equipment Identity Register (EIR) o registers GSM mobile stations and user rights o stolen or malfunctioning mobile stations can be locked and sometimes even localized Operation and Maintenance Center (OMC) o different control capabilities for the radio subsystem and the network subsystem MTech CSE (PT, ) SRM, Ramapuram 13 hcr:innovationcse@gg

14 Radio interface GSM implements SDMA using cells with BTS and assigns an MS to a BTS. FDD is used to separate downlink and uplink GSM TDMA frame, GSM time slots Bursts, normal burst, guard space, GSM TDMA frame, slots, and bursts Frame Hierarchy MTech CSE (PT, ) SRM, Ramapuram 14 hcr:innovationcse@gg

15 Protocols channel coding and error detection/correction forward error correction (FEC) voice activity detection (VAD) radio resource management (RR) BTS management (BTSM) Mobility management (MM) temporary mobile subscriber identity (TMSI) international mobile subscriber identity (IMSI) call management (CM) o call control (CC) o short message service (SMS) o supplementary service (SS) dual tone multiple frequency (DTMF) pulse code modulation (PCM) Signaling system No. 7 (SS7) BSS application part (BSSAP) Localization automatic, worldwide localization of users. system always knows where a user currently is, and the same phone number is valid worldwide The HLR always contains information about the current location VLR currently responsible for the MS informs the HLR about location changes Roaming Changing VLRs with uninterrupted availability of all services is also called roaming Three types o within the network of one provider, o between two providers in one country (national roaming) o between different providers in different countries (international roaming). MTech CSE (PT, ) SRM, Ramapuram 15 hcr:innovationcse@gg

16 To locate an MS and to address the MS, several numbers are needed Mobile station international ISDN number (MSISDN) o follows the ITU-T standard E.164 for addresses as it is also used in fixed ISDN networks o Consists of country code (CC) national destination code (NDC) subscriber number (SN) o e.g., for Germany 179 for the address of the network provider International mobile subscriber identity (IMSI) o Consists of a mobile country code (MCC) the mobile network code (MNC) the mobile subscriber identification number (MSIN) Temporary mobile subscriber identity (TMSI) o To hide the IMSI o GSM uses the 4 byte TMSI for local subscriber identification o a VLR may change the TMSI periodically Mobile station roaming number (MSRN) o hides the identity and location of a subscriber o contains the current visitor country code (VCC), the visitor national destination code (VNDC) the identification of the current MSC together with the subscriber number o helps the HLR to find a subscriber for an incoming call Calling Mobile Terminated Call (MTC) MTech CSE (PT, ) SRM, Ramapuram 16 hcr:innovationcse@gg

17 Mobile Originated Call (MOC) Message flow for MTC and MOC Handover two basic reasons for a handover The mobile station moves out of the range of a BTS o signal level decreases o error rate may grow o diminish the quality of the radio link load balancing o (MSC, BSC) may decide that the traffic in one cell is too high and shift some MS to other cells MTech CSE (PT, ) SRM, Ramapuram 17 hcr:innovationcse@gg

18 Four Types of handover in GSM Intra-cell handover o Within a cell, narrow-band interference could make transmission at a certain frequency impossible. o The BSC could then decide to change the carrier frequency Inter-cell, intra-bsc handover o The mobile station moves from one cell to another, but stays within the control of the same BSC. o The BSC then performs a handover, assigns a new radio channel in the new cell releases the old one Inter-BSC, intra-msc handover o As a BSC only controls a limited number of cells; GSM also has to perform handovers between cells controlled by different BSCs. o This handover then has to be controlled by the MSC Inter MSC handover o A handover could be required between two cells belonging to different MSCs o Now both MSCs perform the handover together Handover decision depending on receive level MTech CSE (PT, ) SRM, Ramapuram 18 hcr:innovationcse@gg

19 Inter BSC, Intra-MSC handover Security Security services offered by GSM Access control and authentication o authentication of a valid user for the SIM with a secret PIN o next step is the subscriber authentication based on a challenge-response scheme Confidentiality o voice and signaling encrypted on the wireless link (after successful authentication) o exists only between MS and BTS Anonymity o temporary identity TMSI (Temporary Mobile Subscriber Identity) o newly assigned at each new location update (LUP) o encrypted transmission Algorithms Algorithm A3 is used for authentication, A5 for encryption; implemented in the devices, identical for all providers A8 for the generation of a cipher key Authentication Based on the SIM, which stores o the individual authentication key Ki, o the user identification IMSI o the algorithm used for authentication A3. uses a challenge-response method: the access control AC generates a random number RAND as challenge the SIM within the MS answers with SRES (signed response) as response The AuC (Authentication centre) performs the basic generation of random values RAND, signed responses SRES, and cipher keys Kc for each IMSI forwards this information to the HLR The current VLR requests the appropriate values for RAND, SRES, and Kc from the HLR. the VLR sends the random value RAND to the SIM. MTech CSE (PT, ) SRM, Ramapuram 19 hcr:innovationcse@gg

20 Both sides, perform the same operation with RAND and the key Ki, called A3. The MS sends back the SRES generated by the SIM; the VLR can now compare both values. If they are the same, the VLR accepts the subscriber, otherwise the subscriber is rejected. Encryption MS and BSS can start using encryption by applying the cipher key Kc MTech CSE (PT, ) SRM, Ramapuram 20

21 Kc is generated using the individual key Ki and a random value by applying the algorithm A8. the SIM in the MS and the network both calculate the same Kc The key Kc itself is not transmitted over the air interface MS and BTS can now encrypt and decrypt data using the algorithm A5 and the cipher key Kc. New data services High Speed Circuit Switched Data (HSCSD) higher data rates are achieved by bundling several TCHs air interface user rate (AIUR) For n channels, HSCSD requires n times signaling during handover, connection setup and release. Each channel is treated separately General Packet Radio Service (GPRS) packet mode transfer for applications that exhibit traffic patterns such as frequent transmission of small volumes (e.g., typical web requests) or infrequent transmissions of small or medium volumes (e.g., typical web responses) point-to-point (PTP) packet transfer service PTP connection oriented network service (PTP-CONS) PTP connectionless network service (PTP-CLNS) QoS-profile o service precedence (high, normal, low), o reliability class and delay class of the transmission, o user data throughput. GPRS architecture o o o o o o o o o o GPRS support nodes (GSN) gateway GPRS support node (GGSN) packet data networks (PDN) serving GPRS support node (SGSN) GPRS register (GR) GPRS tunnelling protocol (GTP). subnetwork dependent convergence protocol (SNDCP) base station subsystem GPRS protocol (BSSGP) radio link protocol (RLC) packet data traffic channels (PDTCHs). MTech CSE (PT, ) SRM, Ramapuram 21 hcr:innovationcse@gg

22 DECT Digital Enhanced Cordless Telecommunications (DECT) digital European cordless telephone and digital European cordless telecommunications standard describes air interface between base-station and mobile phone GSM is designed for outdoor use with a cell diameter of up to 70 km, DECT can offer its service to some 10,000 people within one km2 Characteristics frequency: MHz channels: 120 full duplex duplex mechanism: TDD (Time Division Duplex) with 10 ms frame length multplexing scheme: FDMA with 10 carrier frequencies, TDMA with 2x 12 slots modulation: digital, Gaußian Minimum Shift Key (GMSK) power: 10 mw average (max. 250 mw) range: ca 50 m in buildings, 300 m open space System Architecture global network Local networks home data base (HDB) visitor data base (VDB) fixed radio termination (FT) portable radio termination (PT) Protocol Architecture Physical layer MTech CSE (PT, ) SRM, Ramapuram 22 hcr:innovationcse@gg

23 Physical layer modulation/demodulation generation of the physical channel structure with a guaranteed throughput controlling of radio transmission channel assignment on request of the MAC layer detection of incoming signals sender/receiver synchronization collecting status information for the management plane MAC layer maintaining basic services, activating/deactivating physical channels multiplexing of logical channels e.g., C: signaling, I: user data, P: paging, Q: broadcast segmentation/reassembly error control/error correction Data link control layer creation and keeping up reliable connections between the mobile terminal and basestation two DLC protocols for the control plane (C-Plane) o connectionless broadcast service: paging functionality o Lc+LAPC protocol: in-call signaling (similar to LAPD within ISDN), adapted to the underlying MAC service several services specified for the user plane (U-Plane) o null-service: offers unmodified MAC services o frame relay: simple packet transmission o frame switching: time-bounded packet transmission o error correcting transmission: uses FEC, for delay critical, time-bounded services o bandwidth adaptive transmission o Escape service: for further enhancements of the standard Network layer similar to ISDN (Q.931) and GSM (04.08) offers services to request, check, reserve, control, and release resources o necessary for a wireless connection o necessary for the connection of the DECT system to the fixed network main tasks o call control: setup, release, negotiation, control o call independent services: call forwarding, accounting, call redirecting o mobility management: identity management, authentication, management of the location register TETRA - TERRESTRIAL TRUNKED RADIO many different radio carriers assign single carrier for a short period to one user/group of users taxi service, fleet management, rescue teams interfaces to public networks, voice and data services very reliable, fast call setup, local operation TETRA - ETSI standard o formerly: Trans European Trunked Radio o offers Voice+Data and Packet Data Optimized service o point-to-point and point-to-multipoint o ad-hoc and infrastructure networks o several frequencies: MHz, MHz o FDD, DQPSK o group call, broadcast, sub-second group-call setup MTech CSE (PT, ) SRM, Ramapuram 23 hcr:innovationcse@gg

24 SATELLITE SYSTEMS Applications Traditional Weather forecasting o infra red or visible light Radio and TV broadcast satellites o it is cheaper to install and, in most cases, no extra fees have to be paid for this service Military satellites o much safer from attack by enemies Satellites for navigation o global positioning system (GPS) Mobile Communication Global telephone backbones o big cable in the sky o being replaced by fiber optical cables crossing the oceans o one-way, single-hop time delay of 0.25 s Connections for remote or developing areas o many places all over the world do not have direct wired connection to internet Global mobile communication Satellites using lower orbits extend the area of coverage mobile user link (MUL) gateway link (GWL) intersatellite links (ISL) Typical satellite system for global mobile telecommunications vertical handover o between a cellular network and a satellite system horizontal handover o within cellular Users should not notice the switching from, e.g., GSM, to a satellite network during conversation MTech CSE (PT, ) SRM, Ramapuram 24 hcr:innovationcse@gg

25 Basics Depending on the application, these orbits can be circular or elliptical Satellites in circular orbits always keep the same distance to the earth s surface following a simple law m is the mass of the satellite; R is the radius of earth with R = 6,370 km; r is the distance of the satellite to the centre of the earth f is the frequency of the rotation. To keep the satellite in a stable circular orbit, both forces must be equal Fg = Fc distance of a satellite to the earth s surface depends on its rotation frequency. Dependency of satellite period and distance to earth Important parameters in satellite communication inclination angle δ o angle between the equatorial plane and the plane described by the satellite orbit o inclination angle of 0 degrees means that the satellite is exactly above the equator elevation angle ε o angle between the center of the satellite beam and the plane tangential to the earth s surface footprint o area on earth where the signals of the satellite can be received propagation loss of the signals o Signal attenuation due to atmospheric absorption Refer below MTech CSE (PT, ) SRM, Ramapuram 25 hcr:innovationcse@gg

26 Four different types of orbits Geostationary (or geosynchronous) earth orbit (GEO) o distance of almost 36,000 km to the earth. o Examples: all TV and radio broadcast satellites, many weather satellites and satellites o operating as backbones for the telephone network Medium earth orbit (MEO) o distance of about 5,000 12,000 km o upcoming systems (e.g., ICO) use this class for various reasons Low earth orbit (LEO) o were mainly used for espionage o using altitudes of 500 1,500 km Highly elliptical orbit (HEO) o all satellites with noncircular orbits. o few commercial communication systems o perigee over large cities to improve communication quality. Van Allen radiation belts, o belts consisting of ionized particles o inner Van Allen belt heights of about 2,000 6,000 km o outer Van Allen belt about 15,000 30,000 km o make satellite communication very difficult in these orbits. MTech CSE (PT, ) SRM, Ramapuram 26 hcr:innovationcse@gg

27 GEO Orbit o km distance to earth surface, equatorial plane (inclination 0 ) complete rotation exactly one day, satellite is synchronous to earth rotation Advantages o can use fixed antenna positions, no adjusting is needed. o ideal for TV and radio broadcasting. o do not need a handover due to the large footprint o do not exhibit any Doppler shift because the relative movement is zero. Disadvantages: o Northern or southern regions of the earth have more problems receiving these satellites due to the low elevation above a latitude of 60, i.e., larger antennas are needed in this case. o Shading of the signals in cities due to high buildings and the low elevation further away from the equator limit transmission quality. o The transmit power needed is relatively high (some 10 W), problems for battery powered devices. o Cannot be used for small mobile phones. o high latency of over 0.25 s one-way biggest problem for voice and also data communication o many retransmission schemes which are known from fixed networks fail. o Due to the large footprint, either frequencies cannot be reused or the GEO satellite needs special antennas focusing on a smaller footprint. o Transferring a GEO into orbit is very expensive LEO Orbit ca km above earth surface typical duration of LEO periods are 95 to 120 minutes provide a high quality communication link by ensuring a high elevation for every spot on earth will only be visible from the earth for around ten minutes Further classification into o Little LEOs with low bandwidth services (some 100 bit/s) o big LEOs (some 1,000 bit/s) o broadband LEOs with plans reaching into the Mbit/s range Advantages: o using low transmit power in the range of 1W o The delay for packets delivered via a LEO is relatively low (approx 10 ms) o Smaller footprints of LEOs allow for better frequency reuse o LEOs can provide a much higher elevation in polar regions and so better global coverage. Disadvantages o handover necessary from one satellite to another o many satellites necessary for global coverage o more complex systems due to moving satellites MEO Orbit ca km above earth surface Advantages: o the system only requires a dozen satellites for global coverage o move more slowly relative to the earth s rotation allowing a simpler system design o satellite periods are about six hours o Depending on the inclination, a MEO can cover larger populations, so requiring fewer handovers. Disadvantages: o delay increases to about ms. o need higher transmit power o need special antennas for smaller footprints. MTech CSE (PT, ) SRM, Ramapuram 27 hcr:innovationcse@gg

28 Routing Satellites offering Inter Satellite Link (ISL) o traffic can be routed between the satellites. o one user sends data up to a satellite o the satellite forwards it to the one responsible for the receiver via other satellites. o The last satellite now sends the data down to the earth o only one uplink and one downlink per direction is needed o The ability of routing within the satellite network reduces the number of gateways needed on earth. Without ISL o all traffic is relayed to earth, routed there relayed back to a satellite. Localization similar to that of terrestrial cellular networks Home Location Register (HLR) o stores all static information about a user with current location Visitor Location Register (VLR) o The last known location of a mobile user Satellite User Mapping Register (SUMR) o stores the current position of satellites o a mapping of each user to the current satellite Registration of a mobile station o initially sends a signal which one or several satellites can receive. o Satellites receiving such a signal report this event to a gateway. o The gateway can now determine the location of the user via the location of the satellites. o User data is requested from the user s HLR, VLR and SUMR are updated Calling a mobile station o localization using HLR/VLR similar to GSM o connection setup using the appropriate satellite Handover in satellite systems caused by the movement of the satellites Intra satellite handover o handover from one spot beam to another o mobile station still in the footprint of the satellite, but in another cell Inter satellite handover o handover from one satellite to another satellite o mobile station leaves the footprint of one satellite Gateway handover o Handover from one gateway to another o mobile station still in the footprint of a satellite, but gateway leaves the footprint Inter system handover o Handover from the satellite network to a terrestrial cellular network o mobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc Comments & Feedback Thanks to my family members who supported me while I spent hours and hours to prepare this. Your feedback is welcome at GHCRajan@gmail.com MTech CSE (PT, ) SRM, Ramapuram 28 hcr:innovationcse@gg