Cellular Networks Outline Fundamentals of cellular network Brief
Cellular Networks
Outline • Fundamentals of cellular network • Brief History of cellular network • GSM
Cell 1 A Cellular Network Cell 2 Mobile Switching Center (MSC) HLR Mobile User VLR Public Switched Telephone Network (PSTN) And internet Base Transceiver Station (BTS) Cordless connection HLR = Home Location Register Wired connection VLR = Visitor Location Register Copyright: A. Umar
Introduction to cellular networks • Cell covers specific geographical region • BTS(Base transceiver station) – Create cell (similar to Access point) • Mobile station(MS) – Attached to network through BTS • Mobile switching center (MSC) – Connect cell to wide area net – Manages call setups – handles mobility (HLR, VLR)
History • 1 G: Basic mobile telephony service – analog cellular technology – American Mobile Phone (AMPS) and NMT in Europe – Uses FDMA • 2 G: service for mass users – Digital cellular technology – D-AMPS: combines FDMA/TDMA – Global System for mobile Communication (GSM ) • Combines FDMA/TDMA – Code Division multiple Access(CDMA) • Uses CDMA
History… • 2. 5 G: Mobile Internet/data services together with voice services – Packet switching technology adding into 2 G – Two types : evolved from GSM • General Packet Radio Service(GPRS ) • Enhanced data rates for global Evolution (EDGE) • EDGE provides a better data rates using enhanced modulation – CDMA -2000 • Evolved from the CDMA
History… • 3 G: Broad Band internet, multimedia and emerging new applications – Universal Mobile Telecommunication System(UMTS) • GSM next step, using CDMA – CDMA-2000 1 x EV-DO(Evolution-Data Optimized) • Uses CDMA/TDMA • up to 3 Mbps – 144 kbps - 384 kbps for high-mobility, high coverage – 2 Mbps for low-mobility and low coverage
History … • 4 G: LTE ( Long Term Evolution ) – More on seminar
Overview Data Rates 2 Mbps 3 G (144 Kbps to 2 Mbps) 1 Mbps 100 Kbps 2. 5 G (10 -150 Kbps) 10 Kbps 2 G (9. 6 Kbps) 1 Kbps 1 G (<1 Kbps) 1980 1990 2000 Years Copyright: A. Umar 2010
• Cellular concepts
0 G Wireless (old days) • Mobile radio telephones were used for military communications in early 20 th century • Car-based telephones first introduced in mid 1940 s – Single large transmitter on top of a tall building – Single channel used for sending and receiving – To talk, user pushed a button, enabled transmission and disabled reception. Became known as “push-to-talk”. – CB-radio, taxis, police cars use this technology • IMTS (Improved Mobile Telephone System) introduced in 1960 s – Used two channels (one for sending, one for receiving) – No need for push-to-talk – Used 23 channels from 150 MHz to 450 MHz Copyright: A. Umar
First-Generation Cellular • Advanced Mobile Phone Service (AMPS) invented at Bell Labs and first installed in 1982 • Key ideas: – Exclusively analog – Geographical area divided into cells (typically 10 -25 km) – Cells are small: Frequency reuse exploited in nearby (not adjacent) cells – As compared to IMTS, could use 5 to 10 times more users in same area by using frequency re-use (divide area into cells) – Smaller cells also required less powerful, cheaper, smaller devices Copyright: A. Umar
Cellular Network Organization • Cell design (around 5 km radius) – Served by base station consisting of transmitter, receiver, and control unit – Base station (BS) antenna is placed in high places ( high rise buildings) • Operators pay per month for BS – Different frequencies assigned to each cell – Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern) • In North America, two 25 -MHz bands allocated to AMPS – One for transmission from base to mobile unit – One for transmission from mobile unit to base Copyright: A. Umar
cellular network concepts segmentation of the area into cells possible radio coverage of the cell (omni-directional) cell idealized shape of the cell • use of several carrier frequencies • not the same frequency in adjoining cells • cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc. • hexagonal shape of cells is idealized (cells overlap, shapes depend on geography) • if a mobile user changes cells handover of the connection to the neighbor cell
Cell Design E E A D F B C G A G D F E D F A C G B • Cells grouped into a cluster of seven • Letters indicate frequency use • For each frequency, a buffer of two cells is used before reuse • To add more users, smaller cells (microcells) are used • Frequencies may not need to be different in CDMA (soft handoff) Copyright: A. Umar
• How to accommodate many users?
Approaches to Increase Capacity • Adding/reassigning channels - some channels are not used for traffic • Frequency borrowing – frequencies are taken from adjacent cells by congested cells. – High traffic cells borrows channel from low traffic cells • Cell splitting – cells in areas of high usage can be split into smaller cells – Decrease the transmission power Copyright: A. Umar
Cellular hierarchy • Femtocell- smallest unit of the hierarchy – Cover only few meters where devices are in the physical range of the user. e. g. WPANs • Picocells: covers few tens of meters. – E. g. WLAN • Microcells : covers hundreds of meters • Macrocells: several kilometer coverage • Megacells: national wide coverage. – E. g Satellites
• Global System for Mobile Communication (GSM)
GSM Overview • Formerly: Groupe Spéciale Mobile (founded 1982) • Now: Global System for Mobile Communication • Pan-European standard (ETSI, European Telecommunications Standardization Institute) • Goal : was to provide a mobile phone system that allows users to roam throughout Europe and provides voice services compatible to ISDN and other PSTN systems. • Today many providers all over the world use GSM (219 countries in Asia, Africa, Europe, Australia, America) – more than 5 billion subscribers in more than 800 networks – more than 80% of all digital mobile phones use GSM • Today more people use mobile phone system than the fixed telephones.
GSM Technologies • A 2 G cellular network (a digital network for voice communication) • Circuit switching for voice (mainly)/data (limited) transmission rate – Connection-oriented service: establish a communication path (channel) for point-to-point communication • Multiplexing – Frequency division multiplexing (FDM) plus Time division multiplexing (TDM) (adding to Space division multiplexing, SDM) – Uses 124 pair of channels per cell, each channel can support 8 users through TDM (992 users max actually 500 users) – Some channels are used for control signals, etc
GSM - TDMA/FDMA qu en cy 935 -960 MHz 124 channels (200 k. Hz) downlink fre 890 -915 MHz 124 channels (200 k. Hz) uplink higher GSM frame structures time GSM TDMA frame 1 2 3 4 5 6 7 8 4. 615 ms GSM time-slot (normal burst) guard space tail 3 bits user data S Training S user data 57 bits 1 26 bits 1 57 bits Prof. Dr. -Ing. Jochen H. Schiller www. jochenschiller. de MC 2011 guard tail space 3 546. 5 µs 577 µs
GSM Technologies. . • Mobility management – Two-tier architecture: HLR and VLR – Location area for location update together with paging for searching • Services – Mainly for voice communication – Data communication is very limited (i. e. , 9. 6 kbit/s) and supporting SMS
Performance Characteristics of GSM Comparing with 1 G cellular network • Communication – Mobile communication for voice and data services • Total mobility – International access, chip-card enables use of access points of different providers (roaming services) • Worldwide connectivity – One number, the network handles localization and interoperability • High capacity – Better frequency efficiency (frequency reuses), 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 – encryption, authentication via chip-card and PIN
GSM: Mobile Services • GSM offers –several types of connections • voice connections, data connections, short message service – voice communication services • Basic telephony • Emergency number – common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible) • Voice mailbox
GSM: Mobile Services… – Non-Voice-services • group 3 fax • electronic mail ( Message Handling System, implemented in the fixed network) • Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal (160 characters) using the signaling channel, thus allowing simultaneous use of basic services and SMS(almost ignored in the beginning now the most successful add-on!)
GSM: Mobile Services…. – Supplementary service : May differ between different service providers, countries and protocol versions • • • Caller identification: Caller. ID forwarding of caller number: call. Diverting automatic call-back conferencing with up to 7 participants locking of the mobile terminal (incoming or outgoing calls)
Architecture of the GSM System • GSM is a PLMN (Public Land Mobile Network) – Main components • • MS (mobile station) BS (base station) MSC (mobile switching center) LR (location register) – Subsystems • RSS (radio subsystem): covers all radio aspects • NSS (network and switching subsystem): call forwarding, handover, switching • OSS (operation subsystem): management of the network
Components Mobile Station switching center Base Transceiver Station Management Database
OMC, EIR, AUC GSM: overview HLR NSS with OSS VLR MSC GMSC VLR fixed network MSC BSC RSS Prof. Dr. -Ing. Jochen H. Schiller www. jochenschiller. de MC 2009
GSM: system architecture radio subsystem MS network and switching subsystem fixed partner networks MS ISDN PSTN MSC Um BTS Abis BSC EIR SS 7 BTS VLR BTS BSS HLR BSC A MSC IWF ISDN PSTN PSPDN CSPDN
System architecture: radio subsystem MS network and switching subsystem – MS (Mobile Station) – BSS (Base Station Subsystem): consisting of MS Um BTS Abis BTS • Components BSC MSC • BTS (Base Transceiver Station): sender and receiver • BSC (Base Station Controller): controlling several transceivers • Interfaces A BTS BSS BSC MSC – Um : radio interface – Abis : standardized, open interface with 16 kbit/s user channels – A: standardized, open interface with 64 kbit/s user channels
Radio subsystem • The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers • Components – Base Station Subsystem (BSS): • Base Transceiver Station (BTS): – radio components including sender, receiver, antenna – if directed antennas are used one BTS can cover several cells • Base Station Controller (BSC): – switching between BTSs, controlling BTSs, – mapping of radio channels (Um) onto terrestrial channels (A interface) • BSS = BSC + sum(BTS) – Mobile Stations (MS) • MS=ME(mobile Equipment ) + SIM( subscriber identity module)
Base Transceiver Station and Base Station Controller • Tasks of a BSS are distributed over BSC and BTS • BTS comprises radio specific functions • BSC is the switching center for radio channels
Mobile station • Terminal for the use of GSM services • Mobile Equipment (ME) – represents physical terminals, such as a mobile or PDA. • Subscribers identity module (SIM) – Stores all user specific data(Static) – i. e Card-type, subscribed service, personal identity number(PIN) , PIN unblocking key (PUK), International mobile subscriber identity(IMSI) , phone book. • MS=ME + SIM
System architecture: network and switching subsystem network subsystem fixed partner networks ISDN PSTN MSC SS 7 EIR HLR VLR MSC IWF ISDN PSTN PSPDN CSPDN • Components • MSC (Mobile Services Switching Center): • IWF (Interworking Functions) • ISDN (Integrated Services Digital Network) • PSTN (Public Switched Telephone Network) • PSPDN (Packet Switched Public Data Net. ) • CSPDN (Circuit Switched Public Data Net. ) • Databases • HLR (Home Location Register) • VLR (Visitor Location Register) • EIR (Equipment Identity Register) • Protocol • SS 7 (signaling system no. 7)
Network and switching subsystem • NSS is the main component of the public mobile network GSM – Switching, Hand over , mobility management ( localization ), • Components – Mobile Services Switching Center (MSC) • controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC – Databases (important: scalability, high capacity, low delay) • Home Location Register (HLR) central master database containing user data, permanent and semipermanent data of all subscribers assigned to the HLR (one provider can have several HLRs) • Visitor Location Register (VLR) local database for a subset of user data, including data about all user currently in the domain of the VLR
Mobile Services Switching Center • The MSC (mobile services switching center) plays a central role in GSM switching functions ( b/n BSC) Hand over functions for mobility support management of network resources interworking functions via Gateway MSC (GMSC) integration of several databases. 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 – – –
GSM: elements and interfaces radio cell MS BSS MS Um radio cell MS BTS RSS BTS Abis BSC A MSC NSS MSC VLR signaling VLR GMSC HLR IWF O OSS EIR AUC OMC ISDN, PSTN PDN
Operation subsystem • enables centralized management and maintenance of all GSM subsystems • Components – Authentication Center (AUC) • Used to protect user identity and data transmission. • authentication parameters and Encryption keys are generated and stored • May be situated in special protected part of the HLR – Equipment Identity Register (EIR) • registers GSM mobile stations devices • Black list(stolen or locked ), Gray list( malfunctioning MS) • White list (Valid devices) – Operation and Maintenance Center (OMC) • control and monitor radio subsystem and the network subsystem entities via the O interface. • Traffic monitoring , status report of the network entities
Databases recap Mobile Switching Center (MSC) is at core; consists of several databases • Home location register (HLR) database – stores information about each subscriber that belongs to it • Visitor location register (VLR) database – maintains information about subscribers currently physically in the region • Authentication center database (Au. C) – used for authentication activities, holds encryption keys • Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station Copyright: A. Umar
GSM frequency bands Type Channels Uplink [MHz] Downlink [MHz] GSM 850 128 -251 824 -849 869 -894 GSM 900 0 -124, 9551023 876 -915 921 -960 890 -915 880 -915 935 -960 925 -960 classical extended 124 channels +49 channels GSM 1800 512 -885 1710 -1785 1805 -1880 GSM 1900 512 -810 1850 -1910 1930 -1990 GSM-R 955 -1024, 0124 876 -915 921 -960 876 -880 921 -925 exclusive 69 channels - Please note: frequency ranges may vary depending on the country! - Channels at the lower/upper edge of a frequency band are typically not used
Example coverage of GSM networks (www. gsmworld. com) T-Mobile (GSM-900/1800) Germany AT&T (GSM-850/1900) USA O 2 (GSM-1800) Germany Vodacom (GSM-900) South Africa
ETMTN (GSM-900) Ethiopia
Localization and calling • To always know where a user currently is , GSM performs periodic location updates even if the MS is not in use( as long as it is logged on to the GSM network ). • HLR always contains info about the current location • VLR of the MSC informs the HLR about the location change.
Localization… • To locate the MS several numbers are needed. – Mobile subscriber international ISDN number (MSISDN) • Phone number E. g. +251917111213 – country code (CC)……… (+251) Ethiopia – National Destination code(NDC)…. ( 917 ) Jimma – Subscriber number (SN)………………. . (111213) individual – International mobile subscriber identity (IMSI) • Used by the network provider – Mobile country code (MCC) – Mobile network code (MNC)-code of network provider – Mobile subscriber identification number (MSIN) –
Localization… • Temporary mobile subscriber identity (TMSI) – To hide the IMSI by BSC – By the VLR • Mobile station roaming number (MSRN) – Temporary address to hide the identity and location of a subscriber by MSC • Visitor country code (VCC) • Visitor national destination code(VNDC) • International mobile Equipment identity (IMEI) – – Unique code to each mobile equipment device specific theft protection Stored in EIR( Equipment identity register ) Dial *#06#
Mobile Terminated Call(MTC) • • • 1: calling a GSM subscriber 2: forwarding call to GMSC 3: signal call setup to HLR 4, 5: request MSRN from VLR 6: forward responsible MSC to GMSC 7: forward call to current MSC 8, 9: get current status of MS 10, 11: paging of MS 12, 13: MS answers 14, 15: security checks 16, 17: set up connection HLR 4 5 3 6 calling station 1 PSTN 2 GMSC 10 7 VLR 8 9 14 15 MSC 10 13 16 10 BSS BSS 11 11 12 17 MS
Mobile Originated Call(MOC) • • 1, 2: connection request 3, 4: security check 5 -8: check resources (free circuit) 9 -10: set up call VLR 3 4 6 PSTN 5 GMSC 7 MSC 8 2 9 MS Prof. Dr. -Ing. Jochen H. Schiller www. jochenschiller. de MC - 2009 1 10 BSS
MS MTC/MOC BTS MS MOC BTS paging request channel request immediate assignment paging response service request authentication response ciphering command ciphering complete setup call confirmed assignment command assignment complete alerting connect acknowledge data/speech exchange
GSM Roaming • The ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services when travelling outside the geographical coverage area of the home network, by means of using a visited network. • Roaming Agreements between network operators required. – National Roaming(visited network in the same country as the home network) – International Roaming( visited network is outside the home country)
How Roaming Works • Subscribe for the roaming service • roaming agreement is needed b/n operators. • Phone support the radio frequency – e. g. 850, 900, 1800 and 1900 MHz • Switch on your phone in the foreign network. • Note that when roaming you have to pay both for calls that you make and receive.
GSM coverage and network info ETHIOPIA • Network Information • Operator: Ethiopian Telecommunications Corporation Also known as: ETMTN – Currently : Ethiotelecom • • Technology: GSM Frequency: 900 Launch Date: APR 1999 Services – Short Message Service Source : www. mobileworldlive. com
Roaming Partners: ETMTN • A Afghanistan, Albania, Algeria, Angola, Argentina, Armenia, Australia, Austria, Azerbaijan, B Bahrain, Bangladesh, Belarus, Belgium, Benin, Bosnia Herzegovina, Botswana, Brazil, Bulgaria, Burkina Faso, Burundi, C Cameroon, Canada, Chad, China, Comoros, Congo, Cote D Ivoire, Croatia, Cyprus, Czech Republic, D Denmark, Djibouit E Egypt, Equatorial Guinea, Estonia, F Finland, France, G Gabon, Gambia, Georgia, Germany, Ghana, Greece, Guinea Bissau, H Hong Kong, Hungary, I Iceland India, Indonesia, Iran, Iraq, Ireland, Israel, Italy, J Jamaica, Japan, Jordan, K Kazakhstan, Kenya, Korea, Kuwait, L Latvia, Lebanon, Lesotho, Liberia, Libya, Liechtenstein, Lithuania, Luxembourg, M Madagascar, Malawi, Malaysia, Mali , Malta Mauritius, Mexico, Mongolia, Morocco, Mozambique, N Namibia, Netherlands, Nigeria, Norway, O Oman, P Pakistan, Panama, Papua New Guinea, Philippines, Poland, Portugal, Puerto Rico, Q Qatar, R Romania, Russia, Rwanda, S Saudi Arabia, Senegal, Serbia, Seychelles, Sierra Leone, Singapore, Slovakia, Slovenia, South Africa, Spain, Sri Lanka, Sudan, Swaziland, Sweden, Switzerland, Syria, T Tanzania, Thailand, Togo, Trinidad and Tobago, Tunisia, Turkey, Turkmenistan, U Uganda, Ukraine, United Arab Emirates, United Kingdom, United States, Uruguay, Uzbekistan, Y Yemen, Z Zambia, Zimbabwe.
Hand over • Hand off is used to provide continuity of services while a mobile unit moves from one cell to another. • hand off is necessary due to the limited power of the MS and BTS • Reason for hand over – > Low signal strength ( out of cell range ) – > Load balancing (too high in one cell)
Hand over… • Hand off parameters (measurements) – word error indicator( WEI) • Demodulation error – Received signal strength indicator (RSSI) – Quality indicator (QI) • Quality with respect to interference • The handoff algorithm will be based on the value of this parameters. (threshold value )
Types of handover • In general there are 3 types of handovers for cellular network. – Mobile controlled hand over (MCHO) • Mobile unit measures the parameters and make handover decision by itself. – Network controlled hand over (NCHO) • Base station measures the parameters and makes the handover decision. – Mobile Assisted hand over (MCHO) • Mobile unit measures the parameters and sends the values to the network. • Then the base station(BS) makes the hand over decision. • Used by GSM.
4 types of GSM handover 1 MS BTS 2 3 4 MS MS MS BTS BTS BSC BSC MSC 1. Intracell HO 2. Intercell /Intra-BSC HO 3. Inter-BSC/Intra-MSC HO 4. Inter-MSC HO
Handover decision receive level BTSold receive level BTSnew HO_MARGIN MS MS BTSold BTSnew
Hand off decisions algorithms • Using Relative signal strength – Switch to BTS with better signal • Using Relative signal strength and threshold – Switch to BTS with better signal plus threshold • Common hand of problems – False handoff(multipath propagation ) – Ping pond effect
Handover procedure MSC MS BTSold BSCold measurement report result BSCnew BTSnew HO decision HO required HO request resource allocation ch. activation HO command HO request ack ch. activation ack HO complete HO access Link establishment clear command clear complete
GSM Evolution review 63
Evolution of cellular communication Services • From voice communication to voice and data communication Technologies • From circuit switching to packet switching
High-speed circuit-switched data (HSCSD) • is an enhancement to the original data transmission mechanism of the GSM system, • four times faster than GSM, up to 38. 4 kbit/s. • circuit-switched mode. • Higher speeds are achieved as a result of superior coding methods, and the ability to use multiple time slots to increase data throughput.
High Speed Circuit Switched Data (HSCSD) 66
General Packet Radio Service (GPRS) • • • 2. 5 G is a packet oriented mobile data service. usage charging is based on volume of data. Use packet switching method. provides data rates of 56 -114 kbps GPRS extends the GSM Packet circuit switched data capabilities and makes the following services possible: – – "Always on" internet access Multimedia messaging service (MMS) Push to talk over cellular (Po. C/PTT) Instant messaging (IM)
Global Packet Radio Service (GPRS) 68
Evolution of GSM… • EDGE (Enhanced Data rate for GSM Evolution) – 2. 5 G , also known as Enhanced GPRS (EGPRS) – new modulation scheme, GMSK(Gaussian minimum-shift keying) & 8 PSK ( 8 phase shift keying) – 384 kbps is the maximum data rate – designed for service providers that may or may not migrate to UMTS • UMTS (Universal Mobile Telecommunications Systems) – – – 3 G 144 kbps for vehicular access 384 kbps for wide-area coverage (pedestrian) 2 Mbps for local coverage (stationary) WCDMA (wideband CDMA) Adopted by Europe and Japan 69
Evolution of CDMA… • CDMA 2000 1 X – 2. 5 G – use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. – up to 153 kbps • CDMA 2000 1 x. EV-DO (Evolution-Data Optimized) – 3 G – broadband Internet access – Uses CDMA/TDMA – up to 3 Mbps
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