Download
1 / 86
990 likes | 1.37k Views
GSM. (GPRS, EDGE, UMTS, LTE and…). Global System for Mobile communications. GSM History. GSM world coverage map. Differences Between First and Second Generation Systems. Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital
E N D
GSM (GPRS, EDGE, UMTS, LTE and…) Global System for Mobile communications
Differences Between First and Second Generation Systems • Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital • Encryption – all second generation systems provide encryption to prevent eavesdropping • Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception • Channel access – second-generation systems allow channels to be dynamically shared by a number of users
GSM network The GSM network can be divided into four subsystems: • The Mobile Station (MS). • The Base Station Subsystem (BSS). • The Network and Switching Subsystem (NSS). • The Operation and Support Subsystem (OSS).
Mobile Station • Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit • Mobile equipment (ME) – physical terminal, such as a telephone or PCS • ME includes radio transceiver, digital signal processors and subscriber identity module (SIM) • GSM subscriber units are generic until SIM is inserted • SIMs roam, not necessarily the subscriber devices
Base Station Subsystem (BSS) • BSS consists of base station controller and one or more base transceiver stations (BTS) • Each BTS defines a single cell • Includes radio antenna, radio transceiver and a link to a base station controller (BSC) • BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging • The BSC (Base Station Controller)controls a group of BTS and manages their radio ressources. A BSC is principally in charge of handovers, frequency hopping, exchange functions and control of the radio frequency power levels of the BTSs.
Network Subsystem (NS) • NS provides link between cellular network and public switched telecommunications networks • Controls handoffs between cells in different BSSs • Authenticates users and validates accounts • Enables worldwide roaming of mobile users • Central element of NS is the mobile switching center (MSC)
Mobile Switching Center (MSC) 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 (AuC) – 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
The Operation and Support Subsystem (OSS) • The OSS is connected to the different components of the NSS and to the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS. • However, the increasing number of base stations, due to the development of cellular radio networks, has provoked that some of the maintenance tasks are transferred to the BTS. This transfer decreases considerably the costs of the maintenance of the system.
GSM Channel Types • Traffic channels (TCHs) carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link. • Control channels (CCHs) carry signaling and synchronizing commands between the base station and the mobile station. Certain types of control channels are defined for just the forward or reverse link.
How a Cellular Telephone Call is Made • All base stations continuously send out identification signals (ID) of equal, fixed strength. When a mobile unit is picked up and goes off-hook, it senses these identification signals and identifies the strongest. This tells the phone which cell it is in and should he associated with. The phone then signals to that cell's base station with its ID code, and the base station passes this to the MSC, which keeps track of this phone and its present cell in its database. The phone is told what channel to use for talking, is given a dial tone, and the call activity proceeds just like a regular call. All the nontalking activity is done on a setup channel with digital codes.
Mobile unit initialisation • Mobile-originated call • Paging • Call accepted • Ongoing call • Handoff
GSM Radio interface • Frequency allocation • Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system: • The band 890-915 Mhz has been allocated for the uplink direction (transmitting from the mobile station to the base station). • The band 935-960 Mhz has been allocated for the downlink direction (transmitting from the base station to the mobile station).
Multiple access scheme • In GSM, a 25 MHz frequency band is divided, using a FDMA, into 124 carrier frequencies spaced one from each other by a 200 kHz frequency band. • Each carrier frequency is then divided in time using a TDMA. This scheme splits the radio channel into 8 bursts. • A burst is the unit of time in a TDMA system, and it lasts approximately 0.577 ms. • A TDMA frame is formed with 8 bursts and lasts, consequently, 4.615 ms. • Each of the eight bursts, that form a TDMA frame, are then assigned to a single user.
Maximum number of simultaneous calls = [(124) × 8] / N = 330 (if N=3)
TDMS format Trail bits: synchronisation between mobile and BS. Encrypted bits: data is encrypted in blocks, Two 57-bit fields Stealing bit: indicate data or stolen for urgent control signaling Training sequence: a known sequence that differs for different adjacent cells. It indicates the received signal is from the correct transmitter and not a strong interfering transmitter. It is also used for multipath equalisation. 26 bits. Guide bits: avoid overlapping, 8.25 bits
Data rate • channel data rate in GSM (1/120 ms) × 26 × 8 × 156.25 = 270.8 33Kbps • User data rate Each user channel receives one slot per frame With error control
Traffic Channels • full rate channels offer a data rate of 22.8 kBit/s: • speech data: used as 13 kBit/s voice data plus FEC data • packet data: used as 12, 6, or 3.6 kBit/s plus FEC data • half rate channels offer 11.4 kBit/s: • speech data: improved codecs have rates of 6.5 kBit/s, plus FEC • packet data: can be transmitted at 3 or 6 kBit/s • Two half rate channels can share one physical channel • Consequence: to achieve higher packet data rates, multiple logical channels have to be allocated =) this is what GPRS does
Speech coding There are 260 bits coming out of a voice coder every 20 ms. 260 bits/20ms = 13 kbps These 260 bits are divided into three classes: • Class Ia having 50 bits and are most sensitive to errors 3-bit CRC error detection code 53, then protected by a Convolutional (2,1,5) error correcting code. • Class Ib contains 132 bits which are reasonably sensitive to bit errors--protected by a Convolutional (2,1,5) error correcting code. • Class II contains 78 bits which are slightly affected by bit errors– unprotected • After channel coding: 260 bits 456bits
Evolution from 2G 2G IS-95 GSM- IS-136 & PDC GPRS IS-95B 2.5G HSCSD EDGE Cdma2000-1xRTT W-CDMA 3G Cdma2000-1xEV,DV,DO EDGE TD-SCDMA Cdma2000-3xRTT 3GPP 3GPP2
Newer versions of the standard were backward-compatible with the original GSM phones. • Release ‘97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). GPRS provides data transfer rates from 56 up to 114 kbit/s. • Release ‘99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), IMT Single Carrier (IMT-SC), four times as much traffic as standard GPRS. accepted by the ITU as part of the IMT-2000 family of 3G standards • Evolved EDGE standard providing reduced latency and more than doubled performance e.g. to complement High-Speed Packet Access (HSPA). Peak bit-rates of up to 1Mbit/s and typical bit-rates of 400kbit/s can be expected.
the Base Station Subsystem (the base stations and their controllers). • the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network. • the GPRS Core Network (the optional part which allows packet based Internet connections).all of the elements in the system combine to produce many GSM services such as voice calls and SMS.
ITU’s View of Third-Generation Capabilities • Voice quality comparable to the public switched telephone network • High data rate. 144 kbps data rate available to users in high-speed motor vehicles over large areas; 384 kbps available to pedestrians standing or moving slowly over small areas; Support for 2.048 Mbps for office use • Symmetrical / asymmetrical data transmission rates • Support for both packet switched and circuit switched data services • An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic • More efficient use of the available spectrum in general • Support for a wide variety of mobile equipment • Flexibility to allow the introduction of new services and technologies
Third Generation Systems (3G) The dream of 3G is to unify the world's mobile computing devices through a single, worldwide radio transmission standard. However, 3 main air interface standards: W-CDMA(UMTS) for Europe CDMA2000 for North America TD-SCDMA for China (the biggest market)
UMTS (Universal Mobile Telecommunications System ) Services UMTS offers teleservices (like speech or SMS) and bearer services, which provide the capability for information transfer between access points. It is possible to negotiate and renegotiate the characteristics of a bearer service at session or connection establishment and during ongoing session or connection. Both connection oriented and connectionless services are offered for Point-to-Point and Point-to-Multipoint communication. Bearer services have different QoS parameters for maximum transfer delay, delay variation and bit error rate. Offered data rate targets are:144 kbits/s satellite and rural outdoor384 kbits/s urban outdoor2048 kbits/s indoor and low range outdoor
Core Network The Core Network is divided in circuit switched and packet switched domains. Some of the circuit switched elements are Mobile services Switching Centre (MSC), Visitor location register (VLR) and Gateway MSC. Packet switched elements are Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Some network elements, like EIR, HLR, VLR and AUC are shared by both domains.The Asynchronous Transfer Mode (ATM) is defined for UMTS core transmission. ATM Adaptation Layer type 2 (AAL2) handles circuit switched connection and packet connection protocol AAL5 is designed for data delivery.
Summary of UMTS frequencies: Universal Mobile Telephone System (UMTS) • 1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink and downlink, channel spacing is 5 MHz and raster is 200 kHz. An Operator needs 3 - 4 channels (2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network.1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired, channel spacing is 5 MHz and raster is 200 kHz. Tx and Rx are not separated in frequency.1980-2010 and 2170-2200 MHz Satellite uplink and downlink.
