mrn – 7 – gprs

Politecnico di Milano Facolt di Ingegneria dellInformazione

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

qAt the radio interface the TDMA slots are used in a dynamica way

qStatistica multiplexing of multiple flows on the same slots

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

qNew nodes are added on the core network which are basically IP routers

Network architecture

A. Capone: Mobile Radio Networks 3

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

A. Capone: Mobile Radio Networks 4

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

A. Capone: Mobile Radio Networks 5

SGSN (Serving GPRS Support Node)

o Functions: Authentication and encryption functions

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

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

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

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

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

A. Capone: Mobile Radio Networks 6

SGSN (Serving GPRS Support Node)

o ... functions: Mobility management

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

The LR includes localization information and connection statistics

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

used for the transport of user data and signaling

A. Capone: Mobile Radio Networks 7

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: 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

A. Capone: Mobile Radio Networks 8

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

channels of the radio interface (PDCH Packet Data Channel)

ARQ protocol Control of channel access requests (requests and

grants) Packet data channel management (power control,

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

GPRS (General Packet Radio Service)

A. Capone: Mobile Radio Networks 10

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

A. Capone: Mobile Radio Networks 11

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

oCan 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

A. Capone: Mobile Radio Networks 13

RLC (Radio Link Control)

oRLC implements a ARQ scheme on RLC blocks

o The ARQ is a Selective Reject oRLC blocks are numbered module 128 o Transmission window is 64 oCumulative ACK and NACK oAlso an unacknowledged mode is

possible

A. Capone: Mobile Radio Networks 14

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)

A. Capone: Mobile Radio Networks 15

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

A. Capone: Mobile Radio Networks 16

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)

A. Capone: Mobile Radio Networks 17

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)

A. Capone: Mobile Radio Networks 18

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

A. Capone: Mobile Radio Networks 19

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)

A. Capone: Mobile Radio Networks 21

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 co