2G (GSM) Evolution

By Saroj Dhakal

2G (GSM) Evolution• Limits of GSM

• limited capacity at the air interface:• Data transmission standardized with only 9.6kbit/s• advanced coding allows 14,4kbit/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 QoS functions

• Connectionless (CNLS)– No predefined route -packets routed individually– No guarantee of delivery– Difficult to implement QoS

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 3G 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 ElementsThe 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/AuC 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)

– 28kbps (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 CS1 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 69,2kbps (compare to 22.8kbps 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

Thank you………………………………..