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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino

GSM - Phase 2+

•HSCSD•EDGE•GPRS


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From GSM to GPRS

Phase 1• Standard for GSM at 900 MHz providing:

Teleservices: voice, emergency calls, fax, SMSData (up to 9.6 kb/s)Supplementary services (e.g., call forwarding)DCS 1800

Phase 2• Full integration between GSM900 and DCS1800• Improved teleservices and bearer services• Further supplementary services (e.g., call hold, call

waiting, caller ID, multi-party call)• Half-rate speech codec


From GSM to GPRS

Phase 2+1. New services, e.g., high speed data rate

HSCSD GPRS

2. New techniques:GSM/Mobile Satellite Service dual modeMultiband operational capabilitiesNew codecs (e.g., Adaptive MultiRate (AMR))EDGE


General Packet Radio Service(GPRS)


Motivations

• Growing demand of data services due to Internet and enterprise intranets

• Internet and intranets are packet-switchingarchitectures using the TCP/IP protocol suite

• PSTNs/ISDNs tend to become local islands connected with the IP backbone

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The Solution in GSM: GPRS

• To associate the traditional GSM (circuit-switched) network with a packet-switched, all-IP network


The Solution in GSM: GPRS

• Same bandwidth occupation

• Same radio interface

• Enhanced protocols and funtionalities toenable packet access over the radio link


GPRS: Basic Characteristics

• Uses from 1 to 8 time slots on the samecarrier (max bit rate: 171.2 kb/s=8*21.4kb/s)

• User charging based on the amount of data transmitted, thus allowing for always-onconnections

• Interacts with IP

• Supports various levels of QoS


• Financial and economical transactions

• “Always-on” connections allow remote work

• Support of WAP (Wireless Application Protocol) terminals

• Logistic management

• Alarm managment and remote surveillance(but not very urgent)

• E-mail services

Possible Applications


• Specifically studied for:

Discontinuous transmission of packets shorter than 500B, several times per minute

Rare transmission of a few kilobytes of data

• May be not appropriate for large data transfers

Possible Applications


• Uses the GSM network infrastructure butintroduces a new logical network structurewhich is added to the GSM system

GPRS Architecture

• GPRS coexists with GSM and uses sameradio cells

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Introduces:

• Two new network nodes

SGSN (Serving GPRS Support Node): routerwith same role as the MSC but for a packetnetwork

GGSN (Gateway GPRS Support Node): Router interconnecting the GPRS network with other packet networks (PDN-PublicData Networks) or circuit-switchednetworks (similar to GMSCs)

GPRS Architecture


• A new network unit

PCU (Packet Control Unit): part of the BSS; it allows for a packet traffic transfer over the radio interface

• A new data base

GLR (GPRS Location Register): implementedat every SGSN and GGSN; it managesinformation related to the GPRS users. Itmaintains the user profile for each userunder the control of the associated SGSN (GGSN)

GPRS Architecture


GPRS Architecture

• HLR must be enhanced

Must contain GPRS user information(localization and subscription)

Must be able to communicate with MSCs as well as GSNs

• MSC/VLR, EIR, SMS-Service Center must be able to communicate with MSCs as wellas GSNs


Architecture: Intra-PLMN GPRS Backbone

MT

BSS

SGSN

GGSN SGSN

MSC/VLR

SMS-SC

SMS-GMSCHLR

GGSN

otherPLMN

PDN(IP,X.25,...)

GLR

PCU

IP connection

PLMN

GLR

GLR EIR


SGSN1. Implements all functions of a standard

router (e.g., security, routing, QoS)

2. In charge of user authentication (authentication performed as in GSM)

3. Manages data encryption (encryptionperformed as in GSM)

4. Routes packets to/from the MTs under its control

5. Manages user mobilityRETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino

SGSN6. Data packets transfer through the GPRS

backbone (encapsulation and tunneling)

7. Manages radio resources in conjuction with the RRM functions at the BSS, to provide the required QoS

8. Collects information useful for billing

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GGSN1. Implements all functions of a standard

router (such as for outer nets)

2. Routes packets to/from other networks

• if necessary, maps the IP addresses used in the GPRS network to the ones used outside

3. En(de)-capsulates packets

4. Filters packets coming from outside


GGSN5. Collects billing information

6. Records in its GLR the SGSN serving the users present in the network, their userprofile and PDP context (if any)

7. Creates upon request a dynamic IP addressand the PDP context for a user


PCU• Allows MT & SGSN to exchange data packets

• Provides dynamic radio resources allocation for GSM CS and GPRS

• Could be located anywhere between the SGSN and the BTS; usually located at the BTS


PCUMain functionalities:

1. Segmentation/reassembly of LLC frames

2. ARQ

4. Control functions

• Medium access control (requests management and allocation grants)

• Broadcast of control information

• Power control

5. Physical channels scheduling


GSM vs. GPRS1. Circuit-switched voice and

data services

2. Network nodes are switching centers (MSCs). Each MSC is in charge of the MTs in its control area

3. Gateway MSC

4. BSS

5. User identified byMSISDN (phone no.)

1. Paket-switched data services

2. Network nodes are IP routers enhanced with mobility management functionalities

3. GGSN

4. BSS enhanced with aPCU

5. User identified by anIP address


GSM vs. GPRS6. One operational state 6. Three operational states

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• GPRS coexists with GSM

Use of the same radio cells

Voice traffic has priority

• Point-2-point, Multicast and “Group Call”connections

• Datagram (e.g., IP) services

GPRS Services


• MSs are classified as follows:

Class A: simultaneous access to GSM and GPRS services

Class B: GSM/GPRS access but not simultaneous

Class C: GPRS functionalities only

GPRS Services


• While on, more than one operational state (only one in GSM) is possible:

Idle: MT is unreacheable (no data/signaling tx/rx neither paging)

Standby: signaling tx/rx and paging are possible but unicast data tx/rx is not

Ready: MT can tx/rx data; no need forpaging

MT Operational States


standby

idleDetach: either explicit bythe MT or implicit by the SGSN (e.g., due tomissing RA update)

Attach (authentic., localiz., contextcreation)

Ready timeout

PDU Tx/Rx

Implicit detach(Standby timeout) or SGSN change

on the SGSN side only

Possible State Transitions

ready


PDP Context• Packet Data Protocol Context: created for

every MT ready or in standby, wishing toexchange traffic with exterior nets

• Contains: employed protocol (e.g., IPv4)MT’s IP addressrequired QoSthe GGSN address to be used as a gateway to the exterior net


PDP Context

• MT must require the PDP context activation to its SGSN, which asks the GGSN for the PDP context creation

• PDP context is stored by the GGSN, the SGSN and the MT

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GSM vs. GPRS6. One operational state

7. Location updatewhenever MT changesLA

6. Three operational states

7. Finer localization (LA divided into RoutingAreas composed of several cells)


Mobility Management: Localization

• A Location Area (LA) is divided intoRouting Areas (RAs), each of which iscomposed of several cells

• Each LA (RA) is identified by a LAI=LAIdentifier (RAI=RA Identifier) transmitted over the BCCH


Mobility Management : Localization

• If SGSN knows the MT’s cell, no need for paging but a Location Update must be performed when MT changes cell

Convenient only during data transfer tominimize delay

• If SGSN knows the MT’s RA, paging is performed over the RA

A Location Update is necessary when the MT’s RA changes (convenient for MTs in standby)


Idle: MT is unreachable

Standby: MT position is known within an RA

Ready: MT position is known within a cell and defined by the Cell Global Identity(CGI=CI+RAC+LAC)

Mobility Management: Localization


• GPRS Location Update:

Ready MT updates its location at every cell change

Standby MT updates its location when it changes RA

• For classes A and B, some GSM and GPRS procedures can be combined:

RA change (with SGSN) and LA change (with VLR)

Since GPRS localization is more precise, GSM paging can be performed by the SGSN serving the MT

GPRS Attach/Detach and GSM Attach/Detach

Remarks


GPRS Access: An ExampleA GPRS subscriber in idle state and wishing to

tx/rx data: 1. Performs an attach procedure

MT sends its ID (TLLI if available or IMSI), its classmark and information used for ciphering

If current reference SGSN is other than the previous one, authentication, ciphering initialization and location update (@ HLR, VLR, GLR, GGSN) are required

MT enters the Ready state: SMS exchange and multicast msgs reception

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GPRS Access: An Example

2. Activates its PDP context

MT requires PDP activation to its serving SGSN, which requires the reference GGSN for PDP context creation

At this point, the MT is ready to transmit and/or receive data




8. For voice traffic, PHY layer only, between MSC and BSS. For data, alsoL2



8. For both data and signaling, in the fixedpart of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP)


GPRS Entities Functions and Protocol Stack


User Plane

Application

IP/X.25

SNDCP

LLC

RLCMAC

GSM RF

RLCMAC

BSSGPNS

GSM RF L1bis

IP/X.25

GTP

L2

L1

UDP/TCP

IPv6

L2BSSGP

NSL1bis L1

LLCUDP/TCP

IPv6

GTPSNDCP

MT BSS SGSN GGSNUm Gb Gn

Relay

Relay


GPRS Tunneling Protocol (GTP)

• GTP allows the transfer of user data packets through the GPRS IP backbone

• It takes care of

Encapsulation: all packets from/to otherGSNs are “encapsulated” into PDUs of the GTP

Tunneling: transfer of encapsulated packets through GPRS intra- and inter-PLMN backbones


Encapsulation• A GGSN encapsulates packets coming from

other networks and the SSGN serving the destination MT decapsulates them

• Vice-versa the GGSN decapsulates packetsbefore sending it to another network

• Packets destined to another MT whithin same PLMN are routed by the SGSN (do not havenecesserily to pass through a GGSN)

• All packets from/to the MT are en/decapsulated from/into a PDU of lowerlayer (SNDCP) protocol at the SGSN

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Tunneling

• PDUs of GTP containing packets belonging tothe same data connection are marked by the same identifier, calledTunneling Identifier(TID)

• The TID is derived from the IMSI and isunique for each user


GPRS Access: An Example

A GPRS subscriber wishing to tx/rx

1. Performs an attach procedure

2. Activates its PDP context

At this point, MT is ready to transmit/receive data


GPRS Access: An ExampleWhen MT is transmitting1. At MT, the IP datagram is compressed and

encapsulated into an SNDC PDU, that is sent through LLC, RLC/MAC and RF to the serving SGSN

2. When SGSN receives the data error-free, it tunnels the packet to the reference GGSN through the GPRS backbone

3. GGSN removes the tunneling and forwards the IP datagram to the Internet that delivers the data to the final destination


GPRS Access: An ExampleWhen MT is receiving1. The corresponding host sends the IP datagram to a

GPRS MT using the MT’s IP address2. Internet routing protocols are used to route data to

MT’s subnetwork3. GGSN extracts MT’s IP address and maps it to MT’s

current location 4. GGSN tunnels the packet through the GPRS

backbone to the SGSN serving the MT5. SGSN removes the tunneling, encapsulates the IP

datagram into an SNDC PDU and forwards it to BSS6. Packet is sent to MT through LLC, RLC/MAC and RF


Radio Interface


Packet Data Logical Channels

• Packet Broadcast Control Channel(PBCCH) – DL

• Packet Common Control Channels(PCCCHs)

• Packet Dedicated Control Channels(PDCCHs) – UL/DL

• Packet Data Traffic Channels(PDTCHs) – UL/DL

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• Packet Common Control Channels(PCCCH)

Packet Random Access Channel(PRACH) - UL

Packet Paging Channel (PPCH) - DL

Packet Access Grant Channel (PAGCH) - DL



• Packet Dedicated Control Channels

Packet Associated Control Channel (PACCH) – UL/DL

Packet Timing advance Control Channel(PTCCH) – UL/DL


Notice that...

• Packet Data Traffic Channels (PDTCHs) areunidirectional (either uplink or downlink) and uplink and downlik are not related to eachother

• In cells with light GPRS traffic, the common control channels (PP/PRA/PAG-CH) and the PBCCH can be shared with GSM


Data Flow


Radio Block

• Radio Block = RLC/MAC Header + Data RLC + BCS

• PHY layer segments 1 Radio Block into 4 normal bursts which are transmitted over the same time slot on 4 consecutive frames

In GPRS the physical channel isrepresented by a radio block


• Divided into multiframes of 52 (26x2) GSM frames each

• 48 frames are used to transmit 12 Radio Blocks

• 2 frames are devoted to signaling (e.g., timing advance parameter transmission)

• 2 frames are left idle

• The radio block is the minimum access data unit

Radio Interface

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4 framescorrespond to 1 radio block

Radio Interface

B0 B1 B2 X B3 B4 B5 X B6 B7 B8 X B9 B10 B11 X

52 framesX idle or signaling

frame

• Radio Block = 1 slot per frame !!!• 456 (114x4) bit • Adaptive coding depending on the radio

channel conditionsRETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino

Radio Interface

S0 S1 S2 S3 S4 S5 S6 S7 S0 S1 S2

frame

• Radio Block is segmented into 4 normalbursts

• 1 normal burst transmitted on 1 slot in 4 consecutive frame (same slot in all 4 frames)

S3

4 frames


Radio Block HeaderIncludes:1. RLC header + MAC header2. Uplink Status Flag (USF): 3 bits used in DL

to assign the corresponding uplink channelTo dynamically allocate the PRACH: (USF=111)USF may identify up to 7(8) users multiplexed on the same time slot

3. Block Type Indicator (T): indicates which Logical CH is maped onto the PHY CH (PDCH)

4. Power Reduction (R) (in DL): for power control


Physical Channel Structure

B1B0 B3B2 B5B4 B7B6 B9B8 B10 B11PRACH PRACH PRACH PRACH

0 4 8 13 2117 26 30 34 39 43 47

B1B0 B3B2 B5B4 B7B6 B9B8 B10 B11PBCCH PAGCH

0 4 8 13 2117 26 30 34 39 43 47

PRACH Fixed Allocation

USF=111

PRACH Dynamic Allocation

PRACH

DL

UL

frame


1. MT transmits a burst on the PRACH (it maybe a response to a network paging)

2. Network assigns to the MT PDTCH(s) through the Packet Assignment Message

BSS assigns a min. no. of resources (up to8 radio blocks)

3. For each granted PDTCH (Radio Block in a multiframe), the network assigns a USF tothe MT

MAC: Channel Access


4. To actually allocate a PDTCH (i.e., a Radio Block (B(n) (n=0,..,11)) in an UL multiframe) to an MT, the associated USF is transmitted on the same PDTCH in the previous DL Radio Block (B(n-1))

5. 1- or 2-phase access procedure dependingon the amount of data to be transmitted

A 2-phase procedure allows forresource negotiation

MAC: Channel Access

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Mobile Originated Packet Transfer

Packet Channel Request

Packet Uplink Assignment

Packet Uplink Assignment

Packet Resource Request

MT Network

PRACH

PAGCH

PACCH

PACCH

Optional

Optional


MT Network

Packet Channel Request PRACH

Packet Downlink Assignment PAGCH

Packet Downlink Assignment PACCHOptional

Packet Resource RequestPACCHOptional

Packet Paging RequestPPCH

Packet Paging ResponsePACCH

Network Originated Packet Transfer


Packet Data Transfer

Data Block

Packet Uplink Ack/Nack

MT Network

PDTCH

PACCH

PACCH

Data Block

Data Block (last sent in window)

Data Block

Packet Uplink Ack/Nack (final)

Data Block

Data Block (last)

PDTCH

PDTCH

PDTCH

PDTCH

PDTCH

...or just leave the PDTCH idleRETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino

MAC: Radio Resource Allocation

• Two RR allocation techniques are supported:

Dynamic Allocation (e.g., 0-2-3 PDTCHs individually allocated through USF)

Extended Dynamic Allocation (e.g., oneUSF allocates the associated PDTCH and the higher ones)


GSM vs. GPRS1. Circuit-switched voice and

data services

2. Network nodes are switching centers (MSCs). Each MSC is in charge of the MTs in its control area

3. Gateway MSC

4. BSS

5. User identified byMSISDN (phone no.)

1. Paket-switched data services

2. Network nodes are IP routers enhanced with mobility management functionalities

3. GGSN

4. BSS enhanced with aPCU

5. User identified by anIP address




8. For voice traffic, PHY layer only, between MSC and BSS. For data, alsoL2



8. For both data and signaling, in the fixedpart of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP)

12


GSM vs. GPRS9. Physical Channels:

FDMA/TDMA10. An MT can occupy 1

TCH only and that TCH is occupied by the MT for the whole callduration

11. Logical Channels: TCHs(1 time slot) and Control Channels (BCCH, etc.)

9. Physical Channels: FDMA/TDMA

10. An MT can occupy up to 8 slots. A traffic channel isassigned to an MT just for the time of a packettransmission. Up to 8 MTscan be multiplied over the same time slot

11. Logical Channels: PDTCHs(Packet Data TCHs) and new Control Channels(PBCCH, etc.)

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