2 G GSM Evolution By Saroj Dhakal 2
2 G (GSM) Evolution By Saroj Dhakal
2 G (GSM) Evolution • Limits of GSM • limited capacity at the air interface: • Data transmission standardized with only 9. 6 kbit/s • advanced coding allows 14, 4 kbit/s • not enough for Internet and multimedia applications • Extensions • HSCSD (High-Speed Circuit Switched Data) • GPRS (General Packet Radio Service) • EDGE (Enhanced Data Rate for GSM Evolution) • EGPRS (EDGE and GPRS) • GERAN (GSM Interface to UMTS)
Circuit Switching Vs Packet Switching
Circuit Switching Vs Packet Switching • Advantages of Circuit Switching – More suited to time-sensitive applications – Less data overheads for routing requirements – Compatibility with existing circuit-switched networks (eg PSTN/ISDN) • Advantages of Packet Switching – More efficient use of existing bearers – More suited to bursty-type traffic such as Internet access.
Packet Routing Strategies • Connection-Orientated (CONS): – Packet route established prior to data transfer – Supports flow control – Supports Qo. S functions • Connectionless (CNLS) – No predefined route -packets routed individually – No guarantee of delivery – Difficult to implement Qo. S
Virtual Circuit Connections • Permanent Virtual Circuit (PVC): – Dedicated resource for specific user – Connectionless • Switched Virtual Circuit (SVC): – Setup on request – Temporary allocation of resources – Cleared on completion of session
The Requirement for Mobile Data • Requirement to develop additional revenue-generating streams • Exponential growth in Internet Access • Evolution towards 3 G services
Emerging Mobile Data Technologies
HSCSD (High-Speed Circuit Switched Data) • continuous use of multiple time slots • asynchronous allocation of time slots • gain: net data rate up to 115. 2 kbps (allocation of all 8 traffic channels) • Mainly software update • additional HW needed if more than 3 slots are used – advantage: constant quality, simple – disadvantage: channels blocked for voice transmission
GPRS (General Packet Radio Service) • What is GPRS? – Packet-switching technology – Transparent end-to-end data transport of data packets over a modified GSM network – Shares Radio Resources with GSM – New GPRS core network distinct from GSM – using free slots only if data packets ready to send (e. g. , 115 kbit/s using 8 slots temporarily) – standardization 1998, introduction 2001 – advantage: one step towards UMTS, more flexible data services – disadvantage: more investment needed
GPRS -Conceptual View • Key GPRS Features: Introduces new air interface coding schemes, Supports SMS over GPRS and Allows for shared usage of GSM control functions
GSM/GPRS Mobile Terminal (MT)
System Modifications • GPRS Mobile Station (MS) Modifications – New Mobile Stations will be required – Backward compatibility with GSM – SIM Card updates to include GPRS functionality • Base Station Subsytem (BSS) Modifications – BTS Requires software upgrade – Inclusion of a CCU in the BTS – BSC requires software upgrade and addition of Packet Control Unit Support Node (PCU) – Packet Control Unit (PCU) Functions • Provides physical and logical data interface out of the BSS for packet data traffic • LLC layer PDU segmentation/reassembly • Packet data transfer scheduling • Radio channel management function – Channel Codec Unit (CCU) Functions • • Data coding / decoding Forward error correction (convolution coding) Air interface interleaving Radio management
GPRS Core Network Elements The following GSM network changes are required: • New GPRS entities: – Serving GPRS Support Node (SGSN) – Gateway GPRS Support Node (GGSN) • Modified GSM entities: – HLR – MSC/VLR – SMS-GMSC
Databases –VLR/HLR • GPRS can share GSM database resources • Database software upgrades required to accommodate new GPRS functionality and parameters when interacting with GSM • SGSN acts as VLR for GPRS-specific VLR functions • EIR/Au. C will also require software upgrades for GPRS-specific authentication/authorization
Serving GPRS Support Node (SGSN) Functions: • Packet routing • MS Session management • Authentication and Ciphering • Mobility management • Billing information collection
Gateway GPRS Support Node (GGSN) • Interface between GPRS backbone and external PDNs. • PDP Conversion • IP address assignment management • Packet routing to/from SGSNs • Billing information collection
GPRS architecture and interfaces
GPRS Air Interface • GPRS Um Air Interface – – – New Packet logical channels defined New multi frame structure defined Ability for multiple users to share single or multiple timeslots Dynamic Resource Allocation Four new channel coding schemes • GPRS Logical Channels – GPRS shares GSM for many logical common control channel functions (e. g. FCCH, SCH) – New GPRS logical dedicated control channels are used (e. g. PDTCH : Packet Data Traffic Channel, PACCH: Packet Associated Control Channel and PTCCH: Packet Timing Advance Control Channel – Packet control channels are optional – GSM control channels may be substituted
GPRS Coding Schemes • 4 coding schemes have been defined for GPRS as shown in table. – CS-1 is mandatory for the BSS – CS-1 to 4 are mandatory for the MS
Protocols • A protocol is a set of rules, agreed by both sides, to allow meaningful communication to take place. Protocols are needed whenever systems need to pass information from one to another. • Data Tunneling: A tunnel is a point-to-point (PTP) virtual connection between two end points across a network through which data packets can be transported, transparent to the bearer network • GPRS Protocol Stacks • Transmission Protocol Stack • Network Access Signalling Protocol Stack • Core Network Signalling Protocol Stack
GPRS Mobility Management (GMM) States • GPRS defines 3 GMM states: – Idle -camped on but not attached – Standby -attached but not in data transfer mode – Ready -able to transmit/receive packet data
MM State Change Conditions • Idle to Ready: – GPRS attach • Ready to Standby: – Ready timer expired – Forced to Standby • Standby to Ready: – Packet transmission /reception required • Ready to Idle: – GPRS detach – Location cancelled (SGSN context) • Standby to Idle: – detach (SGSN context) – Location cancelled (SGSN context)
GPRS services • • Quality-of-Service Parameters: Reliability, Throughput, service priority End-to-End packet switched traffic(peak channel rates) – 28 kbps (full use of 3 time slots, FEC) – 171. 2 kbps (full use of 8 time slots, no FEC) • Average aggregate throughput of a cell (Source: H. Menkes, Wireless Web, Aug. 2002) – 95 kbps (for both up and downlink) – Assumptions: 1/3 reuse, realistic RF conditions, random traffic • Adaptive Coding Schemes(adaptive Forward Error Control –FEC) – – • CS 1: 9. 05 Kbps/slot CS 2: 13. 4 Kbps/slot CS 3: 15. 6 Kbps/slot CS 4: 21. 4 Kbps/slot (no Forward Error Correction) (current systems implement CS 1 and 2 only) Problems and limits – – – IP-based network=> high latency, no guarantees Limited data rate: 28 kbps(3 slot/CS-1) -64. 2 kbps(3 slot/CS-4) Latency/flow control problems with TCP Cell capacity. Sub-optimal modulation technique.
EDGE (Enhanced Data Rates for GSM Evolution) • Spectral efficiency depends on: – Size of frequency band – Duration of usage – Level of interference to others (power) • EDGE: – Adaptation of modulation depending on quality of radio path • GMSK (GSM standard– 1 bit per symbol) • 8 -PSK (3 bits per symbol) – Adaptation of coding scheme depending on quality of radio path (9 coding schemes) – Applied to a GSM channel, EDGE allows a maximum data rate of 48 kb/s per timeslot, giving the quoted figure of 384 kb/s per carrier (8 timeslots)
EDGE – Gain: data rate (gross) up to 69, 2 kbps (compare to 22. 8 kbps for GSM) – complex extension of GSM! • EDGE can be applied to HSCSD (ECSD) and GPRS (EGPRS) • EDGE will be expensive for operators to implement: – Each base station will require a new EDGE transceiver – Abis interface between BTS and BSC must be upgraded – New EDGE-enabled handsets required
Preview: UMTS system architecture (Release 3)
UMTS/GSM Network Architecture
- Slides: 29