mrn – 7 – gprs

Politecnico di Milano Facoltà di Ingegneria dell’Informazione

MRN – 7 – GPRS

Mobile Radio Networks Prof. Antonio Capone

GPRS (General Packet Radio Service)

A. Capone: Mobile Radio Networks 2

o  The GPRS is a packet data service

q At the radio interface the TDMA slots are used in a dynamica way

q Statistica multiplexing of multiple flows on the same slots

q The fixed part of the network is an IP (Internet Protocol) network

q New nodes are added on the core network which are basically IP routers

Network architecture


R

TE MT

BTS

BSC

PCU

SGSN

LR

SGSN

LR

Um Gb

Gn

GGSN

LR

Gn

Packet Data Network (IP, X25)

Gi

HLR

EIR

Gr

Gf

Other PLMN GGSN

Gp

FW FW

Protocols


GSM RF

MAC

RLC

LLC

SNDCP

IP

TCP/UDP

Application

GSM RF

MAC

RLC BSSGP relay

network service

layer 1 layer 1

layer 2

SNDCP GTP relay

network service

layer 1

LLC TCP/UDP

BSSGP IP

layer 2

layer 1

TCP/UDP

IP

GTP

IP

Um Gb Gn

MS BSS SGSN GGSN

SGSN (Serving GPRS Support Node)

o  It is basically an IP router that plays in the packet core network of the GPRS the same role of the MSC in the circuit switched core

o  It has additional functionalities wrt a standard IP router for the management of the interfaces and protocols towards the BSS, for mobility support, and for the forwarding of packets to GGSN



o  Functions: Ø  Authentication and encryption functions

(same of GSM) Ø  Together with the RR (Radio Resource

Management) of GSM it manages the radio resource allocation for GPRS packet flows

Ø  Routing management (IP layer) of the PLMN Ø  Encapsulation and tunneling

Ø  GTP (GPRS Tunneling Protocol) tunnel between SGSN and GGSN in which user packets are transported

Ø  The tunnel is identified by a TID (Tunnel Identifier) unique for each user which is derived from the IMSI



o  ... functions: Ø  Mobility management

Ø  It has the LR (Location Register) which is equivalent to the VLR for the MSC

Ø  The LR includes localization information and connection statistics

Ø  LLC (Logical Link Control) connection management Ø  It setup LLC connections with MSs which are

used for the transport of user data and signaling


GGSN (Gateway GPRS Support Node)

o  It the interworking node between the GPRS network (cellular packet core) and an external PDN (Packet Data Network)

o  The PDN is usually a public IP network (Internet) or a private one (Intranet – usually reached with a protected IPSec tunnel)

o  It encapsulates packets to mobile users using the GTP tunnels towards the SGSNs

o  It stores in its local LR the addresses of the SGSN currently visited by each user, the user profiles, and the PDP context

o  The PDP context is activated on request: Ø  The PDP context describes the characteristics of the

data access with the external network o  For example: the IPv4 address assigned to the MS,

the connection with the external network, the QoS parameters

o  Moreover, for the external interface it acts as a normal IP router


PCU (Packet Control Unit) o  The BSC needs to be modified for including the

functions needed for the control of the radio resources of the GPRS and for the implementation of the packet service protocols

o  It is usually located in the BSC (even if it could be placed in the BTS)

o  Functions: Ø  segmentation/reassembly of LLC frames Ø  scheduling of transmissions on the packet

channels of the radio interface (PDCH – Packet Data Channel)

Ø  ARQ protocol Ø  Control of channel access requests (requests and

grants) Ø  Packet data channel management (power control,

congestion, broadcast of control info, etc.) A. Capone: Mobile Radio Networks 9

GPRS (General Packet Radio Service)


Protocols

o  The GPRS protocol stack is characterized n  At the radio interface:

o  By the protocols for the dynamic sharing of resources by packet flows – medium access control (MAC) and link control (RLC and LLC)

o  Regulated channel access (requests and assignment)

o  The link control includes retransmission and in case of channel error

n  In the fixed network: o  IP transport o  Encapsulation of upper layers into a tunneling

protocol (GTP) that is also used for managing mobility


SNDCP (Subnetwork Dependent Convergence Protocol)

o  SNDCP has basically the task of adapting the upper layer protocol (typically IP) to the transport offered by GPRS

o  The transport offered to upper layer allows the transfer of variable length PDUs between SGSN and MS

o  It is based on OSI model and identify upper layer with SAPI (NSAPI – Network Service Access Point Identifier)

o  Functions: n  Header compression/decompression n  Segmentation/reassembly n  Multiplexing of multiple NSAPI on the same

LLC connection A. Capone: Mobile Radio Networks 12

LLC (Logical Link Control)

o  Layer 2 protocol similar to LAPD o  It is used to implement a virtual and

encrypted point-to-point link between SGSN and MS

o Can operate in two modes n  Acknowledged

o  Reliable transport service (the ARQ service of RLC is used)

n  unacknowledged o  No retransmission or error detection o  It is used for signaling and SMS


RLC (Radio Link Control)

o RLC implements a ARQ scheme on RLC blocks

o  The ARQ is a Selective Reject o RLC blocks are numbered module 128 o  Transmission window is 64 o Cumulative ACK and NACK o Also an unacknowledged mode is

possible


MAC (Medium Access Control)

o  It allows the dynamic sharing of GPRS channels at the radio interface

o  It manages packet multiple access in uplink and dynamic multiplexing in the downlink

o  In the uplink transmissions are scheduled by the PDU

o  Transmission requests are send by MSs using a contention based access (see next slides)


Physical layer

o  GPRS: 4 coding schemes up to 20 kb/s per channel

o  EDGE (Enhanced Data Rate for Global Evolution): advanced GPRS physical layer with higher layer modulation


Core network

o  BSSGP (BSS GPRS Protocol) n  It allows the exchange of signaling between

SGSN and BSC n  It is transported using the lower layer

transport service o  Network service:

n  It allows the packet transport between SGSN and BSS that are usually connected by dedicated links

n  It is usually a packet transport service like Frame relay (with a Permanent Virtual Circuit between SGSN and BSC)


GTP (GPRS Tunneling Protocol)

o  It allows the transparent transfer of packets (IP) between GGSN and SGSN crossing the IP network of the GPRS provider

o  It is similar to tunneling protocols of IP world (like IPSec) but it makes use of a transport layer: n  TCP for reliable tunnels n  UDP for best effort traffic (most common)


Physical and logical channels

o  The physical channel used by GPRS is the PDCH (Packet Data Channel)

o  The PDCH occupies one slot per frame o  Multiple PDCHs are usually managed by the

GPRS MAC/RLC on different time slots o  The minimum transmission unit is the Radio

Block o  The Radio Block includes 456 coded bits

provided by the MAC/RLC layers to physical layer which are transmitted usually in 4 normal bursts

o  The dynamic resource assignment is based on the Radio Block and not on the physical time slot


Logical channels

o  PPCH – Packet Paging Channel o  PRACH – Packet Random Access Channel o  PAGCH – Packet Access Grant Channel o  PNCH – Packet Notification Channel o  The set of the above channels is called PCCCH

(Packet Common Control Channel) o  PBCCH – Packet Broadcast Control Channel o  PDTCH – Packet Data Traffic Channel (Up or

Down) o  PACCH – Packet Associated Control Channel o  PTACCH – Packet Timing Advance Control

Channel A. Capone: Mobile Radio Networks 20

Mapping of logical channles

o  A multi-frame of 52 frames is used o  48 frames are used for transmitting 12

Radio Blocks, 2 for the PTACCH, 2 are idle

o  The mapping of logical channel into physical channels is dynamic (packet based)

o  The transported information (and then the logical channel) is indicated in the MAC header of the radio block (payload type)


MAC: access control to common physical channels

o  MAC layer assigns resources dynamically o  The resources in the uplink are requested by MS and

identified with a TBF (Temporary Flow Block) o  MSs can use multiple time-slots (PDCHs)

simultaneously o  The set of PDCHs that can be used is communicated

(assigned) to the MS by the MAC layer together with an USF (Uplink State Flag) of 3 bits

o  The possibility of using a PDCH in the uplink is regulated with the transmission of the USF in the header of the MAC packets transmitted in the downlink

o  USF=111 means FREE and corresponds to the PRACH which can be used to send TFB requests; replies to requests are transmitted in the downlink on the PAGCH


MAC: access control to common physical channels


Mac header payload

MS3 MS2 vuoto MS1

offset

USF =Free

USF =MS1

USF =MS2

USF =MS2

down

up

PRACH: random access for request transmissions

Mobility management

o  Mobility management is similar to GSM and depends on the MS state: n  idle

o  MS cannot transmit or receive and it is considered unreachable by the network

n  stand-by o  MS cannot transmit or receive packets, but is can

receive paging messages and exchange signaling with the network; the MS is localized on the basis of the Routing Area (RA) associated to a SGSN

n  ready o  MS can transmit and receive and its position is

tracked on cell basis (CGI – Cell Global Identity)


Mobility management

o  The switch from idle to ready and back is based on the GPRS attach/detach procedure

o  The switch form ready to stand-by is based on an inactivity timer that is reset at each LLS transmission

o  The switch from stand-by to ready is triggered by LLC transmissions

o  The handover (ready state) and location update procedures of GPRS are similar to that of GSM and the reference node is the SGSN


Mobility management

  IDLE –  not attached to GPRS –  MS is not reachable IDLE

READY

STANDBY

GPRS Attach GPRS Detach

PDU Transmission READY timer expiry Force to STANBY

STANBY timer expiry

  READY –  MS known down to Cell by SGSN –  May receive/transmit packets –  No Packet paging required

–  MS remains in READY state until “READY Timer” expires or GPRS Detach

  STANDBY –  MS known down to Routing Area by

SGSN

–  MS attached to GPRS –  May receive Packet paging –  No data reception or transmission


Session management

o GPRS manages connection within a Packet Data Protocol (PDP) context

o  PDP context is activated with a setup procedure

o During setup a session with an external IP network is established and an IP address is assigned (static/dynamic, private/public)


GPRS: application scenario

MTTE

R

SGSN

GGSN

BSCBTS

BG

GGSN

SGSN

Gn

Gb

Router(AP)

Firewall

Gi

PDNInternet

Host 1.2.3.4

IP packethost=1.2.3.4

Um

Other GPRSoperator

HPLMN

Gp

EdgeRouter

LAN

Intra PLMN IP-backbone


GPRS: application scenario


MTTE

R

SGSN

GGSN

BSCBTS

BG

GGSN

SGSN

Gn

Gb

RouterAP

Firewall

Gi

PDNInternet

Host .6.5.4.2IP packet

Host 6.5.4.3

Um

Other GPRSoperator

HPLMN

Gp

RADIUSServer

LAN

EdgeRouter

Host 6.5.4.3

Optional Tunnele.g. IPsec

Intra PLMN IP-backbone

GPRSIP Backbone

SGSN

BorderGateway

ChargingGateway

GGSN

DNS

Inter PLMNBackbone

ISPExternal Network

GPRS handset

MSC/VLR HLRBSC

SMSC

Procedures: GPRS Attach

1. GPRS Attach Request 2. Authentication 3. Update Location (GPRS) 4. Insert Subscriber Data 5. Location Update (GSM) 6. GPRS Attach Accept 7. Attach complete

1 2 6

3 4

5 2

7


Procedures: Routing Area Update

BSC

GPRSIP Backbone

SGSN1

SGSN2 GGSN

DNS

BSC

GPRS handset

GPRS handset

1. RA Update Request (old RAI) 2. DNS Query: IP @ for old RAI 3. SGSN Context Request 4. SGSN Context Response 5. Forward Packets 6. Update PDP Context Request: IP @ of new RAI 7. Update PDP Context Response 8. RA Update Accept

1

2 3 4 5

6 7 8


GPRSIP Backbone

SGSN

BorderGateway

ChargingGateway

GGSN

DNS

Inter PLMNBackbone

ISPExternal Network

GPRS handset

MSC/VLR HLRBSC

SMSC

Procedures: PDP Context Activation

1. Activate PDP Context Req 2. DNS Query 3. Create PDP Context Req 4. Create PDP Context Rsp 5. Activate PDP Context Ack

1 2 5

3

4


GPRSIP Backbone

SGSN

BorderGateway

ChargingGateway

GGSN

DNS

Inter PLMNBackbone

ISPExternal Network

GPRS handset

MSC/VLR HLRBSC

SMSC

Procedures: Data Transfer

IP@ src IP@ dst Mobile Server SGSN GGSN

SNDCP IP

IP

GTP IP IP

G-CDR S-CDR


mrn – 7 – gprs

Documents