GPRS
—General Packet Radio Service
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Outline
Introduction
GPRS Applications
GPRS normal service procedures
GPRS Architecture
GPRS protocol layering
GPRS data link layer and coding schemes
GPRS packet transfer
Limitations of GPRS
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Introduction
The General Packet Radio System (GPRS) is a new service that
provides actual packet radio access for mobile Global System for
Mobile Communications (GSM) and time-division multiple access
(TDMA) users.
The main benefits of GPRS are that it reserves radio resources only
when there is data to send and it reduces reliance on traditional
circuit-switched network elements.
Theoretical up to 171.2 kbps transmission speed are achievable using
all eight timeslots at the same time.
No dial-up connection is necessary, GPRS facilitates instant
connections whereby information can be sent or received immediately
as the need arises.
GPRS facilitates several new applications that have not previously
been available over GSM networks due to the limitations in speed and
message length.
GPRS fully enables Mobile Internet functionality by allowing
interworking between the existing Internet and the new GPRS
network.
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GPRS applications
Communications
Use the mobile communications network purely as a pipe to access messages
or information.
— E-mail; fax; unified messaging; intranet/Internet access
Value-added services (VAS)
Refer strictly to content provided by network operators to increase the
value of their service to their subscribers.
— E-commerce; banking; financial trading; Retail; ticket purchasing;
Location-based applications
Provide the ability to link push or pull information services with a user's
location.
— Navigation; traffic conditions; airline/rail schedules; location finder
Vertical applications
Apply to systems utilizing mobile architectures to support the
carrying out of specific tasks within the value chain of a company.
— Freight delivery; fleet management; sales-force automation
Advertising
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GPRS normal service procedure (continue)
Provision
— GPRS services: Point To Point (PTP) and Point To Multipoint (PTM).
— Quality of Service (QoS):
1) priority; 2) reliability; 3) delay; 4) user data throughput;
5) scheduled repeated transmission.
Withdrawal
GPRS-Attach and GPRS-Detach
— Attach: MS indicates its presence to the PLMN (public mobile network)
for the purpose of using the GPRS services.
— Detach: MS indicates to the PLMN that the MS will no longer be using
the GPRS services
Registration
By registering the service parameters, the subscriber optimizes the the
its present, dynamic service profile within the limitations of the static
subscription profile.
Erasure
The subscriber erases previously registered service parameter values
from the service profile.
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GPRS normal service procedure
Interrogation
The subscriber interrogates the status/value of registered service
parameters as defined in the current service profile.
Activation
The subscriber activates each of the registered interworking profiles
independently within the limitations of the subscription profile.
De-activation
The subscriber de-activates the previously activated interworking
profiles independently within the limitations of the subscription profile.
Invocation and operation
The GPRS service is invoked upon transmission or reception of GPRS
data by subscribers.
PIN and Password Management
— Correct subscriber identification has been confirmed by entry of the
current GSM PIN when GPRS-Attach operates.
— GPRS services is offered to a subscriber with the subscription option of
using a password to control the services.
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GPRS reference architecture
GPRS can be thought of as an overlay network onto a GSM network.
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GPRS elements
New terminals (mobiles):
— Required to handle the enhanced air interface and packetize traffic.
— A GPRS terminal can be one of three classes: A, B, or C.
BSC/BTS:
— BSC is required to provide a physical and logical data interface out of
the base station subsystem (BSS) for packet data traffic.
GPRS Network:
— Gateway GPRS Service Node (GGSN) performs mobility management
functions such as mobile subscriber attach/detach and location
management.
— Serving GPRS Service Node (SGSN) are interfaces to external IP
networks such as the public Internet, other mobile service providers'
GPRS services, or enterprise intranets.
GPRS Mobility Management:
Mobility management builds on the mechanisms used in GSM networks.
— Home location register (HLR)
— Visitor location register (VLR)
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GPRS protocol layering
Transmission plane protocol layering
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GPRS protocol layering
Transmission plane protocol layering
The transmission plane covers the protocols for user information
transmission and associated control procedures.
Between SGSN and GGSN
— GPRS tunnel protocol (GTP)
Between the SGSN and MS
— Sub-network Dependent Convergence Protocol (SNDCP)
— BSS GPRS protocol (BSSGP)
Between MS and BSS
— Physical Link sublayer (PLL)
— Physical RF sublayer (RFL)
— Radio Link Control (RLC)
— Medium Access Control (RLC/MAC)
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GPRS protocol layering
Signaling plane protocol layering
GPRS-specific mobility management protocol (GMM) is required
within MS and SGSN to support the mobility functionality.
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GPRS data link layer
Logical link control (LLC)
— Provides a logical link between the MS and SGSN.
— LLC layer protocol functionality is based on link access
procedure-D (LAPD).
Radio link control/medium access control (RLC/MAC)
— Provides services for information transfer over the physical
layer of the GPRS radio interface.
— RLC layer is responsible for the transmission of data block
across the air interface and the backward error correction
(BEC) procedures.
— MAC layer is derived from a slotted ALOHA protocol, which
performs contention resolution between channel access
attempts.
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GPRS packet transformation data flow
Packet (N-PDU)
PH
Segment
…
Info
FSC
LLC frame FH
Segment
RLC/MAC block
Network layer
SNDCP layer
User data
…
Segment
BH
Info
Segment
SNDCP layer
LLC layer
LLC layer
RLC/MAC layer
BCS Tail
Convolutional encoding
Normal
burst
Burst
Burst
Burst
Burst
PH: Packet header
FCS: Frame check sequence
FH: Frame header
BCS: Block check sequence
RLC/MAC layer
Physical layer
BH: Block header
Payload
Add
BCS
Add
precoded
USF
Add
tail bit
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Coding
Puncture
456
bits
GPRS RLC/MAC block structure
Payload
User
Data
USF
T PC
MAC header
RLC
header
RLC data
RLC data
block
BCS
Block
check
sequence
Payload
Control
USF
T PC
MAC header
RLC/MAC signaling information
BCS
RLC/MAC control
block
Block
check
sequence
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GPRS coding schemes
Scheme
Code
rate
SC-1
1/2
181
40
3
4
456
0
9.05
SC-2
 2/3
268
16
6
4
588
132
13.4
SC-3
 3/4
312
16
6
4
676
220
15.6
SC-4
1
428
16
12
0
456
0
21.4
Payload BCS
Pre-coded Tail
USF
bits
Coded
bits
Punct. Data rate
bits
(kb/s)
Maximal data rate = 8*21.4 = 171.2Kbps per user
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GPRS logical channels-PDCH
Packet Broadcast Control Channel (PBCCH)
Packet Broadcast Control Channel (PBCCH) — Broadcast
Packet Common Control Channel (PCCCH)
Packet Random Access Channel (PRACH) — Random access
Packet Paging Channel (PPCH) — Paging
Packet Access Grant Channel (PAGCH) — Access grant
Packet Notification Channel (PNCH) — Multicast
Packet Traffic Channel (PTCH)
Packet Data Transfer Channel (PDTCH) — Data
Packet Associated Control Channel (PACCH) — Associated control
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Mobile-Originated packet transfer
MS
PRACH/RACH
PAGCH/AGCH
PACCH
PACCH
PATCH
PDTCH
PACCH
PDTCH
PACCH
Packet channel request
Packet immediate assignment
Packet resource request
Network
Random
access
Packet resource assignment
RLC/MAC block USF
RLC/MAC data
Acknowledgement
Retransmission of data in error
Acknowledgement
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Transmission
Mobile-Terminated packet transfer
MS
PPCH/PCH
PRACH/RACH
PAGCH/AGCH
PACCH
PAGCH/AGCH
PDTCH
PDTCH
PACCH
PDTCH
PACCH
Packet paging request
Network
Packet channel request
Packet immediate assignment
Packet paging response
Random
access
Packet resource assignment
RLC/MAC block USF
Frame transmission
Acknowledgement
Retransmission of blocks in error
Acknowledgement
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Transmission
Throughput VS. Input load in GPRS
Throughput
The throughput of the GPRS system performs as a slotted ALOHA
system when the system is stable. When the system is overloaded, the
throughput saturates at a maximum value.
GPRS curve
Slo
tte
Input load
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d A
LOH
A c
urv
e
Limitation of GPRS
Limited cell capacity for all users
There are only limited radio resources that can be deployed for both
voice and GPRS calls.
Speeds much lower in reality
It is unlikely that a network operator will allow all timeslots to be
used by a single GPRS user.
Support of GPRS mobile terminate by terminals is no ensured
There has been no confirmation from any handset vendors that mobile
terminated GPRS calls.
Transit delays
GPRS packets are sent in all different directions to reach the same
destination. the result is that potential transit delays can occur.
No store and forward
There is no storage mechanism incorporated into the GPRS standard,
apart from the incorporation of interconnection links between SMS and
GPRS.
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