Copyright. GSM - Phase 2+ From GSM to GPRS. From GSM to GPRS. Motivations. General Packet Radio Service (GPRS) HSCSD EDGE GPRS

Transcription

1 Copyright HSCSD EDGE GPRS GSM - Phase 2+ Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare: oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. From GSM to GPRS From GSM to GPRS Phase 1 Standard for GSM at 900 MHz providing: Teleservices: voice, emergency calls, fax, SMS Data (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 Phase New services, e.g., high speed data rate HSCSD GPRS 2. New techniques: GSM/Mobile Satellite Service dual mode Multiband operational capabilities New codecs (e.g., Adaptive MultiRate (AMR)) EDGE Motivations General Packet Radio Service (GPRS) Growing demand of data services due to Internet and enterprise intranets Internet and intranets are packet-switching architectures using the TCP/IP protocol suite PSTNs/ISDNs tend to become local islands connected with the IP backbone

2 The Solution in GSM: GPRS The Solution in GSM: GPRS To associate the traditional GSM (circuitswitched) network with a packet-switched, all-ip network Same bandwidth occupation Same radio interface Enhanced protocols and funtionalities to enable packet access over the radio link GPRS: Basic Characteristics Uses from 1 to 8 time slots on the same carrier (max bit rate: kb/s=8*21.4kb/s) User charging based on the amount of data transmitted, thus allowing for always-on connections Interacts with IP Supports various levels of QoS Possible Applications 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) services Possible Applications GPRS Architecture 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 GPRS coexists with GSM and uses same radio cells Uses the GSM network infrastructure but introduces a new logical network structure which is added to the GSM system

3 Introduces: Two new network nodes SGSN (Serving GPRS Support Node): router with same role as the MSC but for a packet network GGSN (Gateway GPRS Support Node): Router interconnecting the GPRS network with other packet networks (PDN-Public Data Networks) or circuit-switched networks (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): implemented at every SGSN and GGSN; it manages information related to the GPRS users. It maintains the user profile for each user under the control of the associated SGSN (GGSN) GPRS Architecture GPRS Architecture Architecture: Intra-PLMN GPRS Backbone HLR must be enhanced Must contain GPRS user information (localization and subscription) Must be able to communicate with MSCs as well as GSNs PLMN MT SMS-SC MSC/VLR SMS-GMSC GLR EIR HLR SGSN GGSN IP connection GLR MSC/VLR, EIR, SMS-Service Center must be able to communicate with MSCs as well as GSNs BSS PCU GGSN other PLMN SGSN GLR PDN (IP,X.25,...) SGSN 1. 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 (encryption performed as in GSM) 4. Routes packets to/from the MTs under its control 5. Manages user mobility SGSN 6. 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

4 GGSN 1. 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 GGSN 5. Collects billing information 6. Records in its GLR the SGSN serving the users present in the network, their user profile and PDP context (if any) 7. Creates upon request a dynamic IP address and the PDP context for a user 4. Filters packets coming from outside 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 Main functionalities: PCU 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 1. 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 by MSISDN (phone no.) GSM vs. GPRS 1. Paket-switched data services 2. Network nodes are IP routers enhanced with mobility management functionalities 3. GGSN 4. BSS enhanced with a PCU 5. User identified by an IP address GSM vs. GPRS 6. One operational state 6. Three operational states

5 GPRS Services GPRS Services 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 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 MT Operational States 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 for paging Possible State Transitions Implicit detach (Standby timeout) or SGSN change on the SGSN side only PDU Tx/Rx Ready timeout standby ready idle Attach (authentic., localiz., context creation) Detach: either explicit by the MT or implicit by the SGSN (e.g., due to missing RA update) PDP Context PDP Context Packet Data Protocol Context: created for every MT ready or in standby, wishing to exchange traffic with exterior nets Contains: employed protocol (e.g., IPv4) MT s IP address required QoS the GGSN address to be used as a gateway to the exterior net 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

6 GSM vs. GPRS Mobility Management: Localization 6. One operational state 7. Location update whenever MT changes LA 6. Three operational states 7. Finer localization (LA divided into Routing Areas composed of several cells) A Location Area (LA) is divided into Routing Areas (RAs), each of which is composed of several cells Each LA (RA) is identified by a LAI=LA Identifier (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 to minimize 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) Mobility Management: Localization 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) GPRS Location Update: Remarks 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 GPRS Access: An Example A 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

7 GPRS Access: An Example GSM vs. GPRS 2. Activates its PDP context MT requires PDP activation to its serving SGSN, which requires the reference GGSN for PDP context creation 6. One operational state 7. Location update whenever MT changes LA 6. Three operational states 7. Finer localization (LA divided into Routing Areas composed of several cells) 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, also L2 8. For both data and signaling, in the fixed part of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP) User Plane Application GPRS Entities Functions and Protocol Stack IP/X.25 SNDCP LLC Relay Relay SNDCP GTP UDP/TCP LLC IPv6 IP/X.25 GTP UDP/TCP IPv6 RLC MAC RLC MAC BSSGP NS BSSGP NS L2 L2 GSM RF GSM RF L1bis L1bis L1 L1 MT BSS SGSN GGSN Um Gb Gn 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 other GSNs 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 packets before sending it to another network Packets destined to another MT whithin same PLMN are routed by the SGSN (do not have necesserily to pass through a GGSN) All packets from/to the MT are en/decapsulated from/into a PDU of lower layer (SNDCP) protocol at the SGSN

8 Tunneling PDUs of GTP containing packets belonging to the same data connection are marked by the same identifier, calledtunneling Identifier (TID) The TID is derived from the IMSI and is unique 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 Example When MT is transmitting 1. 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 Example When MT is receiving 1. The corresponding host sends the IP datagram to a GPRS MT using the MT s IP address 2. Internet routing protocols are used to route data to MT s subnetwork 3. 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 MT 5. SGSN removes the tunneling, encapsulates the IP datagram into an SNDC PDU and forwards it to BSS 6. Packet is sent to MT through LLC, RLC/MAC and RF Packet Data Logical Channels Packet Broadcast Control Channel (PBCCH) DL Radio Interface Packet Common Control Channels (PCCCHs) Packet Dedicated Control Channels (PDCCHs) UL/DL Packet Data Traffic Channels (PDTCHs) UL/DL

9 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 Packet Data Logical Channels Packet Dedicated Control Channels Packet Associated Control Channel () UL/DL Packet Timing advance Control Channel (PTCCH) UL/DL Notice that... Data Flow Packet Data Traffic Channels (PDTCHs) are unidirectional (either uplink or downlink) and uplink and downlik are not related to each other In cells with light GPRS traffic, the common control channels (PP/PRA/PAG-CH) and the PBCCH can be shared with GSM Radio Block Radio Interface 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 is represented 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

10 52 frames Radio Interface X idle or signaling frame 4 frames Radio Interface B0 B1 B2 X B3 B4 B5 X B6 B7 B8 X B9 B10 B11 X 4 frames correspond to 1 radio block Radio Block = 1 slot per frame!!! 456 (114x4) bit Adaptive coding depending on the radio channel conditions frame S0 S1 S2 S3 S4 S5 S6 S7 S0 S1 S2 Radio Block is segmented into 4 normal bursts 1 normal burst transmitted on 1 slot in 4 consecutive frame (same slot in all 4 frames) S3 Radio Block Header Includes: 1. RLC header + MAC header 2. Uplink Status Flag (USF): 3 bits used in DL to assign the corresponding uplink channel To 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 PRACH PRACH PRACH PRACH PRACH B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 UL PBCCH B0 B1 DL PRACH Fixed Allocation B2 B3 B4 PAGCH B5 B6 B7 B8 PRACH Dynamic Allocation B9 B10 B frame USF=111 MAC: Channel Access 1. MT transmits a burst on the PRACH (it may be 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 to 8 radio blocks) 3. For each granted PDTCH (Radio Block in a multiframe), the network assigns a USF to the 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)) or 2-phase access procedure depending on the amount of data to be transmitted A 2-phase procedure allows for resource negotiation

11 Mobile Originated Packet Transfer Network Originated Packet Transfer MT Packet Channel Request Network PRACH MT Packet Paging Request Network PPCH Packet Uplink Assignment PAGCH Packet Channel Request Packet Downlink Assignment PRACH PAGCH Optional Packet Resource Request Optional Optional Packet Resource Request Packet Downlink Assignment Optional Packet Uplink Assignment Packet Paging Response Packet Data Transfer MT Network Data Block PDTCH Data Block PDTCH Data Block (last sent in window) PDTCH Packet Uplink Ack/Nack Data Block PDTCH Data Block PDTCH Data Block (last) PDTCH Packet Uplink Ack/Nack (final) MAC: Radio Resource Allocation Two RR allocation techniques are supported: Dynamic Allocation (e.g., PDTCHs individually allocated through USF) Extended Dynamic Allocation (e.g., one USF allocates the associated PDTCH and the higher ones)...or just leave the PDTCH idle 1. 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 by MSISDN (phone no.) GSM vs. GPRS 1. Paket-switched data services 2. Network nodes are IP routers enhanced with mobility management functionalities 3. GGSN 4. BSS enhanced with a PCU 5. User identified by an IP address 6. One operational state 7. Location update whenever MT changes LA 8. For voice traffic, PHY layer only, between MSC and BSS. For data, also L2 GSM vs. GPRS 6. Three operational states 7. Finer localization (LA divided into Routing Areas composed of several cells) 8. For both data and signaling, in the fixed part of the network: L1 (e.g. SDH); L2 (ATM or Frame Relay); L3 (IP)

12 GSM vs. GPRS 9. Physical Channels: FDMA/TDMA 10. An MT can occupy 1 TCH only and that TCH is occupied by the MT for the whole call duration 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 is assigned to an MT just for the time of a packet transmission. Up to 8 MTs can be multiplied over the same time slot 11. Logical Channels: PDTCHs (Packet Data TCHs) and new Control Channels (PBCCH, etc.)