General Packet Radio Service (GPRS)

1
Chapter 3

• General Packet Radio Service (GPRS)

2
GSM

• A Brief Introduction

3
Architecture
4
Components/ Elements

• Mobile Station (MS)
• Base Station Subsystem (BSS)
• Base Transceiver Station (BTS)
• Base Station Controller (BSC)
• Mobile Switching Center (MSC)
• Four databases
• Home Location Register (HLR)
• Visitor Location Register (VLR)
• Equipment Identity Register (EIR)
• Authentication Center (AUC)

5
Standard Interfaces

• UmMS-BTS
• Abis BTS-BSC
• A-interface BSC-MSC
• SIM
• Subscriber Identity Module

6
Physical Channel
7
Physical Channel (Contd)
8
Logical Channel
9
Logical Channel (Contd)

• TCH (Traffic Channel)
• TCH/F
• TCH/H
• Signaling channels
• BCH (Broadcast CH)-gt3 CHs
• CCCH (Common Control CH)-gt3 CHs
• DCCH (Dedicated Control CH)-gt3 CHs

10
GSM Data
11
Evolution

• CSDCircuit-Switched Data
• SMS Short Message Service
• HSCSD High-Speed CSD
• EDGE Enhanced Data rate for GSM Evolution

12
Pre-GPRS Systems
13
SMS in GSM

• Two-way non-blocking packet-mode messaging
  service
• Allowing non-real-time (NRT) dialog between
  mobile terminals
• Transmission on GSMs signaling channel (e.g.,
  SDCCH)
• Up to 160 bytes (PTP/broadcast)
• Offered message categories
• SMS-mobile-terminated PTP (stored intermediately
  in the SMS service center (SMS-SC))
• SMS-mobile-originated PTP
• Cell broadcast SMS

14
HSCSD

• Aggregation of multiple slots in a GSM frame
• Speed ranges from 14.4 kbps to 64 Kbps
• Using the existing GSM infrastructure
• Affects to GSM
• Channel allocation
• Connection setup
• Handoff procedures
• Access to the fixed network

15
GPRS
16
About GPRS

• Initiated in 1994 by ETSI
• Services offered PTP and PTM
• Systems adopted GSM and IS-136
• Speed
• Single slot-gt14.4 kbps
• Multiple slot-gt115.2 kbps
• Web page browsing and Internet surfing
• Channel allocation
• Fixed/Static or dynamic

17
GPRS Network Architecture
18
Elements-SGSN

• Serving GPRS Support Node (SGSN)
• Functions similar to MSC/VLR
• Monitors user location
• Handles charging data
• Provides security via A5 ciphering algorithm
• Manages sessions, mobility, and logical channels
• QoS management policing and scheduling
• Network access control

19
Elements-GGSN

• Gateway GSN (GGSN)
• Inter-works GPRS network with external
  packet-switched networks
• Data formatting
• Address translation
• Protocol conversion
• Routing of packets
• Firewall, billing, and packet filtering

20
GSN

• GSN includes SGSN and GGSN
• A GPRS network consists of multiple SGSNs but
  only one GGSN for internetworking
• GTP (GPRS Tunnel Protocol)
• Communication protocol between two GSNs
• User data is encapsulated and tunneled through
  the IP backbone network

21
GPRS Coding Schemes

• Four schemes CS-1CS-4
• Convolutional Code

22
GPRS Protocol Stack (Data Trans.)
23
Protocol Description

• Layer 1/ Physical layer
• PLL (Physical Link sub-Layer)
• Data unit framing
• Data coding
• Error detection and correction
• RFL (Radio Frequency sub-Layer)
• Conforms to GSM 05 series recommendations
• Performs modulation and demodulation
• Specifies carrier freq., radio channel
  structures, raw channel data rates, transceiver
  characteristics, performance requirements

24
Protocol Description (Contd)

• Layer 2/ Data link layer
• LLC (Logical Link Control)
• Supports PTM transmission
• Provides secure logical pipe between MS and SGSN
• RLC (Radio Link Control)/ MAC (Medium Access
  Control)
• Provides access signaling
• Resolves access contentions
• Provides link level ARQ

25
Protocol Description (Contd)

• Relay function
• In the BSS, relays LLC PDUs between the Um and Gb
  interfaces
• In the SGSN, it relays packet data protocol (PDP)
  PDUs between the Gb and Gn interfaces

26
Protocol Description (Contd)

• SNDCP (Sub-Network-Dependent Convergence
  Protocol)
• Handles TCP/IP headers and data compression
• Deals with multiplexing, encryption, and
  segmentation
• BSSGP (BSS GPRS Protocol)
• QoS
• Routing between BSS and SGSN

27
GPRS QoS Issues

• Access delay 4 classes (UmGi)
• Priority 3 classes
• Reliability 5 classes
• Mean throughput 31 classes
• Peak throughput 9 classes (GiR)

28
Access Delay
29
The Others

• Priority classes high, medium, and low
• Reliability classes relates to prob. Of
  corruption, loss, mis-sequencing, duplication of
  packets
• Mean throughput classes class 1 (0.22bps)class
  30 (111kbps), class 31 (best effort)
• Peak throughput classes 8 kbps2048 kbps
• ltRemarkgt delay class and mean throughput are
  used by the SGSN for scheduling and admission
  control

30
MSDRA MAC Protocol

• MSDRA Master-Slave Dynamic Rate Access
• Channel concept
• PDCH (Packet Data CHannels)
• Channels are dedicated to packet mode transfer
• Uplinks and downlinks are basically used as
  independent channel resources
• Further divided into MPDCH (Master PDCH) and
  SPDCH (Slave PDCH)

31
Channel Concept (Contd)

• SPDCHs the channels on which user data and
  signaling is transferred
• Packet Traffic Channels (PTCHs)
• Dedicated to one MS or a group of MSs used for
  user data transfer
• Packet-Associated Control Channels (PACCHs)
• Transport signaling info related one MS
• Packet Data Broadcast Channels (PDBCHs)
• Downlink to broadcast user data

32
Channel Concept (Contd)

• MPDCHs accommodate Common Control Channels
  (CCHs) that carry the signaling information
  required to initiate packet transfer
• PRACH (Packet Random Access CH)
• Uplink to initiate data transfer of the MS
• PPCH (Packet Paging CH)
• Downlink to inform MSs about incoming packets
• PPRCH (Packet Paging Response CH)
• Uplink to respond to paging
• PAGCH (Packet Access Grant CH)
• Downlink to send CH reservation info to an MS
• PBCCH (Packet Broadcast Control CH)
• Downlink to broadcast all GPRS-specific info.

33
Phases of MAC

• MAC is based on a slotted ALOHA reservation
  protocol which has three phases on the uplink
• Contention phase
• Notification
• Transmission
• Two phases on the downlink
• Notification
• Transfer

34
PRACH Performance Analysis

• RACH and PRACH slotted ALOHA with retransmission
  ( of max. retrx.7)
• Assumption
• Equal new call arrival rates
• System is memoryless
• Capture effect is considered

35
Performance Analysis (Contd)

• For infinite number of users, PDF of interfering
  packets is Poisson
• where G is total access attempts
• Pc(Sun) the prob of success of a test packet
  given n interfering packets
• PC(Sun)Pr(PtgtzPn), z is the capture ratio

36
Performance Analysis (Contd)

• Pc(Sun) is given by
• Pc(Su) is given by
• The access throughput is

37
Performance Analysis (Contd)

• The prob that a packet will be successfully
  exactly k transmissions
• Average number of transmissions needed for a
  successful reception

38
Performance Analysis (Contd)
39
Channel Allocation for GPRS

• Packet Data Traffic CH (PDTCH)
• Packet Common Control CH (PCCCH)
• Packet Dedicated Control CH (PDCCH)

40
Allocation Algorithms

• Fixed resource allocation (FRA)
• Fixed resource allocation with queue capacity
  (FRAQ)
• Dynamic resource allocation (DRA)
• Dynamic resource allocation with queue capacity
  (DRAQ)

41
EGPRS (EDGE)

• Initiated in 1997
• Using the GPRS core network but a new air
  interface with 8 coding schemes
• 8-PSK modulation
• Max data rate 384 kbps

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References

• Book Wireless Personal Communications Systems by
  D. J. Goodman
• Paper GPRS-features and packet random access
  channel performance analysis by A. Gyasi-Agyei et
  al.
• Paper Concepts, services, and protocols of new
  GSM phase 2 General Packet Radio Service by G.
  Brasche et al.
• Paper Channel allocation for GPRS by Phone Lin
  et al.