GSM system ‒ introduction 1/3. • GSM – Global System for Mobile
Communication. • Operates in 900 and 1800 MHz. • Access to the radio link is
based on ...
Modeling and Dimensioning of Mobile Networks: from GSM to LTE
Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski
Modeling and Dimensioning of Mobile Networks: from GSM to LTE
GSM
Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski
GSM system – introduction 1/3 • GSM – Global System for Mobile Communication • Operates in 900 and 1800 MHz • Access to the radio link is based on frequency division multiple access (FDMA) and time division multiple access (TDMA) • Each band available for the system is divided into channels with bandwidth of 200 kHz • For the GSM 900 system there are 124 available channels (separate for the uplink and downlink direction), and for the GSM 1800 374 channels
GSM system – introduction 2/3 Feature/Bandwidth
GSM 900
GSM 1800
Uplink (MHz)
890-915
1710-1785
Downlink (MHz)
935-960
1805-1880
124
374
Number of available channels
GSM system – introduction 3/3
GSM system – architecture 1/6 • In the GSM system three basic subsystems can be distinguished: o base station subsystem (BSS) o core network (CN) o user equipment (UE)
• Between particular elements of the system the interfaces are defined
GSM system – architecture 2/6 • Base station subsystem – includes system of base stations and their controllers • Base station provide optimum radio coverage of a given area and communicates with user equipment over air interface • The operation of the base station subsystem is controlled by the base station controller (BSC) • This manages radio resources allocation, controlls the settingup of calls, gathers results of measurements carried out by base station and mobile station • The BSC is also responisble for power controll and handover controll • Interface A enables the BSS system to be connected to mobile switching center (MSC) • Interface Gb connects BSS with packet switching element
GSM system – architecture 3/6 • The main elements of the core network are: o o o o o o o
mobile switching center (MSC) visitor’s location register (VLR) home location register (HLR) authentication ceter (AUC) equipment identification register (EIR) serving GPRS support node (SGSN) gateway GPRS support node (GGSN)
GSM system – architecture 4/6
GSM system – architecture 5/6 • MSC basic task is to control and regulate services provided by the system, circuit switching, and gathering billing information • VLR registry keeps information concerning mobile stations available in the area of one, or several, MSC switching centers • SGSN is the equivalent of the MSC switching center of packet switching • GGSN is an interface between the mobile packet network and external packet networks
GSM system – architecture 6/6 • HLR is a central database that contains details of each mobile phone subscriber authorized to use the GSM core network and includes authorization data • AUC generates sets of keys used in encryption of transmission, identifies the mobile station and the network, and controls and regulates the integrity of transmitted data • EIR is a data base that keeps a list of numbers identifying a given mobile station – IMEI (International Mobile Equipment Identity)
GSM system – time structure 1/3 • In the GSM system each carrier frequency is divided into eight time slots • Packet transmission is commenced every 4.615 ms and single bit lasts 3.69 µs • Typical packet, except access packet, has 148 bits, thus its duration is about 546 µs • The duration of a single time slot is 577 µs, wich allows for maintaining a steady interval between successively transmitted packets
GSM system – time structure 2/3
GSM system – time structure 3/3
GSM system – logical channels 1/4 • Logical channels can be divided into two categories: o control channels – are used to set up a connection in the radio network for transmission of control data o traffic channels – are used to transmit user data
• In the GSM system, speech signals are transmitted with traffic channels (TCHs). Speech can be transmitted at full rate, 13 kbps, or at half rate, 6.5 kbps
GSM system – logical channels 2/4
GSM system – logical channels 3/4 • Frequency correction channel (FCCH) – used by the mobile station to tune to the carrier frequency, a frequency correction burst is transmitted on the channel by generating unmodulated sine waves • Synchronization channel (SCH) – transmits base station identity code (BSIC), which allows the mobile station to identify the base station and to convey synchronization information • Broadcast control channel (BCCH) – used for transmission of control information such as: radio channel frequency used by a given cell, neighbor cell list, information on the paging channel, configuration of logical channels in the base station • Paging channel (PCH) – with the PCH the base station initiates a connection with the mobile station
GSM system – logical channels 4/4 • Random access channel (RACH) – used by mobile station for initial access to a system (with the RACH the mobile station initiates a connection with base station) • Access grant channel (AGCH) – used by the base station to assign resources to a mobile station requesting access to the network • Stand alone dedicated control channel (SDCCH) – used to provide a reliable connection for signaling and SMS messages, for authentication, and to provide information on location update • Slow associated control channel (SACCH) – supports the SDCCH channel, used for sending network measurement reports and information related to power control procedures • Fast associated control channel (FACCH) – coupled with the speech channel, used for immediate transmission of information related to, for example, cell handover
High Speed Circuit Switched Data 1/4 • High Speed Circuit Switched Data is an additional feature of the GSM network and was introduced in phase 2 • HSCSD technology enables a simultaneous application of several speech channels for a single data transmission link • A connection can be set up that makes use simultaneously of n channels (time slots) in the radio interface, where n takes on the values n = 1; 2; ...; 8 • A HSCSD connection can be set up only when the mobile station is capable of using several radio channels simultaneously • Additional modifications are needed in the BSS system that involve multiplexing of a component data stream in one 64 kbps channel of A interface
High Speed Circuit Switched Data 2/4 • A HSCSD connection can have a symmetrical configuration – the same number of speech channels is allocated for the uplink and the downlink direction – or a non-symmetrical configuration • A non-symmetrical configuration is chosen when the subscriber requirements cannot be accomplished in a symmetrical configuration • The maximum link transmission speed that can be achieved by the HSCSD technology depends on the number of channels used in the radio interface and on the applied coding • Due to a necessity of transmitting all channels included in a HSCSD connection in one link 64 kbps in interface A, the transmission speed is limited to 57.6 kbps
High Speed Circuit Switched Data 3/4 Data rate performance in radio interface [kbps]
TCH/F4.8
TCH/F9.6
TCH/F14.4
4.8 kbps
1
N/A
N/A
9.6 kbps
2
1
N/A
14.4 kbps
3
N/A
1
19.2 kbps
4
2
N/A
28.8 kbps
N/A
3
2
38.4 kbps
N/A
4
N/A
43.2 kbps
N/A
N/A
3
57.6 kbps
N/A
N/A
4
High Speed Circuit Switched Data 4/4
GPRS Packet Transmission 1/6 • The implementation of packet transmission in the GSM system requires changes in the structure of the system • New elements are included in the network – SGSN and GGSN nodes • According to the assumptions given in 3GPP specification, GPRS should allow: o pulse data transmission in which time interval between individual moments of transmission is considerably higher than the average transmission delay o frequent transmission (several times per minute) of small amount of data (bursty data transfer up to 500 octets) o occasional transmission of large volumes of data
GPRS Packet Transmission 2/6 • The GPRS technology makes it possible to transmit data in several channels. Within one packet connection, the mobile station as well as the base station can make simultaneous use of 8 time slots in a frame • All users of a packet service can share resources available for data transmission that are allocated, due to the asymmetry of traffic, separately for the uplink and for the downlink direction • The radio interface resources can be dynamically shared by speech service and packet data transmission depending on the configuration of the network
GPRS Packet Transmission 3/6 • Following logical channels, responsible for transmission of data and signalling information are introduced: o Packet Common Control Channel (PCCCH) • Packet Random Access Channel (PRACH) – used by the mobile station to initiate uplink transfer of user data or signalling information • Packet Paging Channel (PPCH) – used to page a mobile station preceding downlink direction packet transfer. The channel can also be used for establishing speech connections • Packet Access Grant Channel (PAGCH) – used in the packet transfer establishment phase to send resource assignment to a mobile station preceding packet transfer • Packet Notification Channel (PNCH) – used to send point to multipoint multicast notification information to a group of mobile stations preceding multicast packet transfer
GPRS Packet Transmission 4/6 • Following logical channels, responsible for transmission of data and signalling information are introduced: o Packet Broadcast Control Channel (PBCCH) – used to broadcast packet system information o Packet Data Traffic Channel (PDTCH) – allocated for user data transfer. Several PDTCH channels can be allocated to a given mobile station. They can be allocated temporarily to one or more mobile stations o Packet Associated Control Channel (PACCH) – used to transmit signalling information related to a given mobile station, such as those related to power control or packet reception acknowledgement messages o Packet Timing Advance Control Channel, Uplink (PTCCH/U) – used to ensure that the correct timing advance is maintained for each mobile station. In the uplink direction, the channel is used by a mobile station to send an access burst o Packet Timing Advance Control Channel, Downlink (PTCCH/D) – used to send packets in the downlink direction to assess the needed timing advance in order to achieve frame synchronization
GPRS Packet Transmission 5/6 • Four coding schemes (CS) have been defined for the GPRS transmission: CS-1, CS-2, CS-3 and CS-4 • Particular schemes are characterized by different user data transmission speed and by a various degree of error protection procedures • CS-1 coding, due to its highest level of protection (error correction), is used for channels with highest interference and for signalling channels • CS-4 coding enables the fastest data transmission speed – 21,4 kbps for one channel, but has no protection
GPRS Packet Transmission 6/6
Coding scheme
Coding efficiency
Data rate [kbps]
CS-1
1/2
9.05
CS-2
2/3
13.4
CS-3
3/4
15.6
CS-4
1
21.4
EDGE Packet Trasnmission 1/2 • Voice transmission in the GSM system and the GPRS packet transmission uses the Gaussian minimum shift keying (GMSK) modulation. This is a binary modulation with speed of 270.833 kbps • EDGE technology achives higher speed value with the same bandwidth of the radio channel thanks multivalue, eightlevel phase shift keying modulation (8PSK) • EDGE uses nine coding schemes: MCS-1 – MCS-9 • Each coding scheme is characterized by a different data transmission speed and data protection • Transmission speed for one PDTCH channel with MCS-1 coding is 8.8 kbps, for MCS-9 coding scheme is 59.2 kbps
EDGE Packet Trasnmission 2/2 Coding scheme
Coding efficiency
Modulation
Data rate [kbps]
MCS-9
1.0
8PSK
59.2
MCS-8
0.92
8PSK
54.4
MCS-7
0.76
8PSK
44.8
MCS-6
0.49
8PSK
29.6 27.2
MCS-5
0.37
8PSK
22.4
MCS-4
1.0
GMSK
17.6
MCS-3
0.85
GMSK
14.8 13.6
MCS-2
0.66
GMSK
11.2
MCS-1
0.53
GMSK
8.8
