What is GPRS?

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LECTURE 6
GPRS

GPRS System Architecture
3

BTS

MS

BSC

Gn
GGSN

SGSN

MS
Um

BTS
BSC
A

HLR D
MSC

Gi
Intra PLMN backbone network (IP based)
Gp

What is GPRS?
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!  General Packet Radio Service is an overlay on top of GSM physical layer and network entities
!  It extends data capabilities of GSM and provides connection to external packet data networks through the GSM infrastructure
!  Short access time to the network for independent short packets (500 – 1000 bytes)
!  No hardware changes to the BTS/BSC !  Easy to scale !  Support for both voice/data and data only terminals !  High throughput (up to 200 kbps) !  User-friendly billing "
New Network Entities
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!  GPRS Support Node (GSN)
!  Responsible for delivery and routing of data packets between the mobile station and external packet network
!  Two types:
#  Serving GPRS Support Node (SGSN) #  Gateway GPRS Support Node (GGSN)
!  GPRS Register (GR) co-located with the HLR
!  Stores routing information and maps IMSI to a PDN address (IP address for example)

Serving GPRS Support Node (SGSN)
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!  It controls access to MSs that may be attached to a group of BSCs
!  This is called a routing area (RA) or service area of the SGSN
!  It is responsible for delivery of packets to the MS in its service area and from the MS to the Internet
!  It also performs the logical link management, authentication, and charging functions

Gateway GPRS Support Node (GGSN)
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!  It acts as a logical interface to the Internet !  Maintains routing information related to a MS, so
that it can route packets to the SGSN servicing the MS !  It analyses the PDN address of the MS and converts it to the corresponding IMSI

GPRS Signaling Plane
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!  GPRS employs out of band signaling in support of actual data transmission
!  Signaling between SGSN, HLR, VLR, EIR is similar to GSM and extends only the GPRS related functionality
!  Based on Signaling System 7
!  Between the MS and SGSN, a GPRS mobility management and session management (GMM/SM) protocol is used for signaling purposes

GPRS Transport Plane
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Application

IP / X.25
SNDCP LLC RLC MAC
GSM PL
MS

IP / X.25

TLLI
LLC Relay

SNDCP GTP LLC TCP/IP

RLC MAC GSM PL

BSSGP
Frame Relay L1bis

BSSGP

Frame

L2

Relay

L1bis

L1

Um

Gb

BSS

SGSN

TID Gn

IP / X.25 GTP
TCP/IP
L2 L1
Gi GGSN

SNDCP: Subnetwork Dependent Convergence Protocol BSSGP: BSS Gateway Protocol GTP: GPRS Tunneling Protocol

RLC/MAC and Physical Layers
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!  The physical layer is the same as GSM (uses GMSK) !  Functionalities include
!  Forward error correction and indication of uncorrectable code words
!  Interleaving of radio “blocks” !  Synchronization !  Monitoring of radio link signal quality !  Other functions similar to GSM

GPRS Vs GSM
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!  GPRS allows a MS to transmit on multiple time slots of the same TDMA frame unlike GSM
!  A very flexible channel allocation is possible since 1-8 time slots can be allocated per TDMA frame to a single MS
!  Uplink and downlink slots can be allocated differently – asymmetric data traffic
!  Some channels may be allocated for GPRS. These are called Packet Data Channels (PDCH)

Allocation of Radio Resources
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!  A cell may or may not support GPRS!! !  Radio resources are dynamically allocated between
GSM and GPRS services !  GPRS information is broadcast on the CCHs !  PDCHs may be dynamically allocated or de-allocated
by the network (usually the BSC) !  If a MS is unaware that the PDCH has been de-
allocated, it may cause interference. !  Fast release of PDCHs is achieved by a broadcast of
de-allocation message on a PACCH

Medium Access
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!  Uplink and downlink transmissions are independent !  Medium access protocol is called “Master-Slave
Dynamic Rate Access” or MSDRA !  Organization of time-slot assignment is done centrally
by the BSS !  A “master” PDCH includes common control channels that
carry the signaling information required to initiate packet transfer !  The “slave” PDCH includes user data and dedicated signaling information

Logical GPRS Channels
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!  Analogous to GSM, GPRS has certain traffic and control channels
!  PDTCH $ Packet Data Traffic Channel !  PBCCH $Packet BCCH !  PNCH: Packet Notification Channel !  PRACH: Packet Random Access Control Channel !  PAGCH : Packet Access Grant Channel !  PACCH: Packet Associated Control Channel
!  Use to send ACKs for received packets
!  PTCCH: Packet Timing-advance Control Channel is used for adaptive frame synchronization

Uplink Data Transfer
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PRACH or RACH PAGCH or AGCH PACCH PACCH

Packet Channel Request Packet immediate assgt. Packet Resource Request Packet Resource assgt.

PDTCH PACCH PDTCH PACCH

Frame Transmission Negative ACK Retransmission Acknowledgment.

Random access Transmission
BTS BSC

Comments on Uplink Data Transfer
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!  If a MS does not get an ACK, it will back off for a random time and try again
!  The Master-Slave mechanism utilizes a 3 bit “uplink status flag” or USF on the downlink
!  A list of PDCHs and their USF are specified !  The Packet Resource or Immediate Assignment message
indicates what USF state is reserved for the mobile on a PDCH !  Assignment can also be done so that a MS can send packets uninterrupted for a predetermined amount of time

Downlink Data Transfer

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PPCH or PCH

Packet Paging Request

PRACH or RACH PAGCH or AGCH PACCH PACCH or PAGCH

Packet Channel Request Packet immediate assgt. Packet Paging Response Packet Resource assgt.

PDTCH PACCH PDTCH PACCH

Frame Transmission Negative ACK Retransmission Acknowledgment.

Paging Transmission
BTS BSC

Comments on Downlink Data Transfer
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!  Data transmission to a mobile can be interrupted if a high priority message needs to be sent
!  Instead of paging, a resource assignment message may be sent to the MS if it is already in a “ready” state (see later)

LLC
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!  The TLLI (Temporary Logical Link Identity) is used to identify a MS in the LLC header
!  A logical link is created between the MS and the SGSN
!  Performs sequence control, error recovery, flow control and encryption
!  It has an acknowledged mode (with retransmission for network layer payloads) and an unacknowledged mode (for signaling and SMS)
!  Supports various QoS classes

Attachment Procedure
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!  Before accessing GPRS services, the MS must register with the GPRS network and become “known” to the PDN
!  The MS performs an attachment procedure with an SGSN
!  Authentication !  Check with GR etc.
!  It is allocated a temporary logical link identity (TLLI) by the SGSN
!  A PDP (packet data protocol) Context is created for the MS

PDP Context
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!  For each session, a PDP context is created
!  PDP Type: (e.g. IPv4) !  The PDP address assigned to the MS !  The requested QoS !  The GGSN address that serves the point of access to the
PDN
!  PDP context is stored in the MS, the SGSN, and the GGSN
!  A user may have several PDP contexts enabled at a time
!  The PDP address may be statically or dynamically assigned (static address is the common situation)

SNDCP:Sub-Network Dependent

Convergence Protocol
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Signaling

SMS

PDP

SNDCP
LLC TLLI
RLC/MAC

GSM PHY

!  Supports a variety of network protocols (IP, X. 25, CLNP etc.)
!  All network layer packets share the same SNDCP
!  It multiplexes and demultiplexes the network layer payload
!  It forms the interface between the LLC and the network layer
!  Handles packets based on QoS

At the Mobile Station

BSS Gateway Protocol BSSGP
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!  Primary function is to relay radio related, QoS, and routing information between the BSS and SGSN
!  Paging requests from SGSN !  Support flushing of old messages from BSS
!  Also carries the LLC frame from the MS to the SGSN !  Many MS LLCs can be multiplexed over one BSSGP
!  Gb interface is different from A interface
!  Data transfer is unconfirmed between BSS and SGSN

Packet Transformation Data Flow
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PH Segment

User Data Segment

Segment

Network SNDCP

Frame

FH

FCS

Segment Segment Segment Segment

Block

BH

BCS Tail

Convolutional Encoding

Burst Burst Burst Burst

456 bits 114 bits

LLC LLC RLC interface
RLC/MAC
Physical

GPRS Tunneling Protocol (GTP)
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!  Why GTP? !  Allows multi-protocol packets to be tunneled through
the GPRS backbone !  A Tunnel ID (TID) is created using signaling plane
that tracks the PDP context
!  Multiplexing different payloads !  Use in mobility management

Mobility and Tunnel Management
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BTS

BSC LLC 1 BTS Abis BSC

HLR Gr
Gb

VLR
Gs
Gn SGSN

External data
network
Gi GGSN

LLC 2

GTP 1

GTP 2

BTS

BSC

SGSN

LLC 3

The two-level tunneling mechanism corresponds to a two level Mobility Management: LLC “tunnels” (or virtual circuits) correspond to small area mobility, while GTP tunnels correspond to wide area mobility.

Location Management

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!  MS can be in three states
!  In the IDLE state the MS is not reachable !  All PDP contexts are deleted
!  In the STANDBY state, movement across routing areas are updated to the SGSN but not across cells
!  In the READY state, every movement of the MS is indicated to the SGSN

READY

IDLE

Timer Expiry STANDBY

Why three states?
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!  If the MS updates its location too often, it consumes battery power and wastes the air-interface resources
!  If it updates too infrequently, a system-wide paging is needed; this is also a waste of resources
!  A standby state focuses the area
!  Chance of packets reaching are medium
!  A ready state pinpoints the area
!  Chances of packets reaching are high

Routing Area Updates
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!  Intra-SGSN RA Update
!  The SGSN already has the user profile !  A new temporary mobile subscriber identity is issued as part
of routing area update “accept” !  The Home GGSN and GR(HLR) need not be updated
!  Inter-SGSN RA Update
!  The new RA is serviced by a new SGSN !  The new SGSN requests the old SGSN to send the PDP
contexts of the MS !  The new SGSN informs the home GGSN, the GR, and other
GGSNs about the user’s new routing context

Mobility Management in GPRS
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!  Handoff Initiation
!  The MS listens to the BCCH and decides which cell it has to select
!  Proprietary algorithms are employed that use RSS, Cell ranking, Path loss, Power Budget, etc.
!  An option exists where the network can ask the MS to report its measurements and ask it to make a handoff (as in GSM)
!  Handoff Procedure
!  Very similar to Mobile IP
Steps in Mobility Management
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!  1. RA Update to new SGSN !  2. Communication between new and old SGSN !  3. Communication between new SGSN and Home-GGSN/HLR !  The Home GGSN “tunnels” packets to the new SGSN !  The HLR deletes old SGSN information and includes the new
SGSN information in the database !  The new SGSN decapsulates packets and forwards them to the
MS

BSC Mobility Management BSC BTS
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SGSN
Direction of movement

Intra PLMN backbone network (IP based)

BG

Inter PLMN

backbone

network
3

BSC BTS
SGSN 1
2

Home GGSN HLR PLMN BG
Data Network (Internet)
Router

Intra PLMN backbone network (IP based)
GGSN Server

SGSN
Visited PLMN

Corporate LAN

Limitations of GPRS
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!  Limited cell capacity for all users !  Speeds much lower in reality !  Sub-optimal modulation !  Transit delays !  No store and forward
#  Popular SMS does have it

GPRS-136: GPRS and IS-136
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!  Adaptation of GPRS to the IS-136 TDMA cellular standard
!  Very similar to GPRS
!  30 kHz physical channels instead of 200 kHz physical channels
!  Can use coherent 8-PSK in addition to π/4 – DQPSK to increase throughput over a limited area => EDGE
!  Hooks in the standard allow the possibility of 16-QAM, 16-PSK, or 16-DQPSK in the future

High Speed Circuit Switched Data (HSCSD)
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!  Use multiple circuit switched connections to transfer data!
!  Slow and expensive !  Inefficient use of radio resources !  Migration path to full fledged third generation (3G)
packet data services
!  Customer base development

EDGE
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!  Enhanced Data rates for GSM Evolution
!  Data throughput speeds of up to 384 kb/s using existing GSM infrastructure.
!  Idea: Use higher level modulation schemes like 8PSK
!  Disadvantage: Coverage is reduced
!  Adaptive bit rates