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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
GSM - Phase 2+
•HSCSD•EDGE•GPRS
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
General Packet Radio Service(GPRS)
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
The Solution in GSM: GPRS
• To associate the traditional GSM (circuit-switched) network with a packet-switched, all-IP network
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
The Solution in GSM: GPRS
• Same bandwidth occupation
• Same radio interface
• Enhanced protocols and funtionalities toenable packet access over the radio link
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
GSM vs. GPRS6. One operational state 6. Three operational states
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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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)
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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)
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
GSM vs. GPRS6. One operational state
7. Location updatewhenever MT changesLA
8. For voice traffic, PHY layer only, between MSC and BSS. For data, alsoL2
6. Three operational states
7. Finer localization (LA divided into RoutingAreas composed of several cells)
8. For both data and signaling, in the fixedpart of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP)
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
GPRS Entities Functions and Protocol Stack
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
Radio Interface
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
Packet Data Logical Channels
• Packet Common Control Channels(PCCCH)
Packet Random Access Channel(PRACH) - UL
Packet Paging Channel (PPCH) - DL
Packet Access Grant Channel (PAGCH) - DL
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
Packet Data Logical Channels
• Packet Dedicated Control Channels
Packet Associated Control Channel (PACCH) – UL/DL
Packet Timing advance Control Channel(PTCCH) – UL/DL
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
Data Flow
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
• 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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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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RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
Mobile Originated Packet Transfer
Packet Channel Request
Packet Uplink Assignment
Packet Uplink Assignment
Packet Resource Request
MT Network
PRACH
PAGCH
PACCH
PACCH
Optional
Optional
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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)
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
GSM vs. GPRS6. One operational state
7. Location updatewhenever MT changesLA
8. For voice traffic, PHY layer only, between MSC and BSS. For data, alsoL2
6. Three operational states
7. Finer localization (LA divided into RoutingAreas composed of several cells)
8. For both data and signaling, in the fixedpart of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP)
12
RETI RADIOMOBILI Copyright Gruppo Reti – Politecnico di Torino
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.)