1 Wireless Networks Lecture 18 GPRS: General Packet Radio Service (Part II) Dr. Ghalib A. Shah

2 Outlines  GPRS Protocol Architecture ►MS – BSS ►BSS – SGSN ►SGSN – GGSN ►GGSN – PDN  GPRS Air Interface  Data Routing and Mobility  Uplink Data Transfer  Downlink Data Transfer  QoS in GPRS

3 Last Lecture  Introduction to GPRS  GPRS Architecture  Registration and Session Management  Routing Scenario in GPRS  Channels Classification

4 Logical Channels  Mapped by the MAC to physical channels  Control channels for control, synchronization and signaling Common Broadcast Dedicated  Packet Traffic channels ►Encoded speech ►Encoded data

5 Control Channels  Packet Common Control Channel (PCCCH) ►When allocated in a cell, GPRS related mobiles camp on it ►Divded into Random Access (PRACH): MS initiate packet transfer or respond to paging messages Paging (PPCH): to page an MS prior to packet transfer Access Grant (PAGCH): send resource assignment to MS prior to packet transfer Packet Notification (PNCH): used to send a PTM-Multicast notification to group of MS

6 Control Channels  Packet Dedicated Control Channel (PDCCH) Slow Associated Control Channel (SACCH) –Radio measurements, power control and data –SMS transfer during calls Fast Associated Control Channel (FACCH) –For one Traffic Channel (TCH) –Carry Ack Stand-alone Dedicated Control Channel (SDCCH) –is used in the GSM system to provide a reliable connection for signalling and Short Message Service.

7 Control Channels  Packet Broadcast Control Channel (PBCCH) ►Frequency correction channels Allows the MS to synchronize their Local Oscillator (LO) to the Base Station LO, using frequency offset estimation and correction. ►Synchronization channel (MS freq. vs. BS) ►Broadcast control channel for general information on the base station

8 GPRS Architecture BTS MS BSC Gb SGSN Gf Gs Gr D EIR MSC/VLR HLR Gc Gn GGSN Gi PDN Gp GGSN Other GPRS PLMN

9 Protocol Architecture  Transmission Plane ►The protocols provide transmission of user data and its associated signaling  Signaling Plane ►Comprises protocols for the control and support of functions of the transmission plane

10 Transmission Plane  GPRS Backbone:SGSN GGSN ►GTP tunnels the user packets and related signaling information between the GPRS support nodes.  Sub-network dependent convergence protocol ►It is used to transfer packets between SGSN and MS  Data link layer ►LLC(MS-SGSN) ► RLC/MAC(MS-BSS)  Physical layer ►PLL:channel coding,detection of errors, forward error correction, interleaving, detection of physical link congestion ►RFL:modulation and demodulation

11 PLL RFL RLC MAC LLC SNDCP Network Layer Application PLL PHY RFL Layer MAC Network Service Relay RLC BSSGP MS BSS SNDCP : Sub-network dependent convergence protocol LLC : Logical link control RLC : Radio link control Um

12 Radio Link Control  Can provide reliability for MAC transmissions  Transparent mode ►No functionality  Acknowledged mode ►Selective Repeat ARQ ►Sender: Window ►Receiver: Uplink ACK/NACK or Downlink ACK/NACK  Unacknowledged mode ►Controlled by numbering within TBF ►No retransmissions ►Replaces missing packets with dummy information bits

13 Media Access Control (MAC)  Performs contention resolution between channel access attempts  Connection oriented  Connections are called Temporary Block Flows (TBF) ►Logical unidirectional connection between two MAC entities ►Allocated resources on PDCH(s) ►Temporary Flow Identity (TFI) is unique among concurrent TBFs in the same direction

14 MAC: Channel Access & Resource Allocation  Slotted Aloha ►Used in PRACH MSs send packets in uplink direction at the beginning of a slot Collision: Back off -> timer (arbitrary) -> re-transmit  Time Division Multiple Access (TDMA) ►Predefined slots allocated by BSS ►Contention-free channel access

15 PLL RFL Phy Layer MAC Network Service Relay RLC BSSGP Phy Layer Network Data Link Service BSSGP IP LLC TCP/UDP Relay SNDCP GTP Phy layer Data Link Layer IP TCP/UDP Network Layer (IP or X.25) GTP RLC :Radio link control BSSGP:BSS GPRS Application protocol PLL :Physical link layer GTP :GPRS tunneling protocol RFL :Physical RF layer TCP :Transmission control protocol MAC:Medium access control UDP :user datagram protocol IP :Internet Protocol Transmission Plane BSSSGSNGGSN GmGb Gi

16 GPRS Air Interface Uplink Downlink Voice User1 Voice User2 GPRS User1 GPRS User2 GPRS User3 F1 F2 F3 F4 F1 F2 F3 F4 Time Slot Number Carrier Frequency

17 GPRS Air Interface  Master slave concept One PDCH acts as Master Master holds all PCCCH channels The rest of channels act as Slaves  Capacity on demand PDCH(s) are increased or decreased according to demand Load supervision is done in MAC Layer

18 Uplink Data Transfer MSBSS PRACH or RACH PAGCH or AGCH Random Access Transmission Packet channel Request PACCH PDTCH PACCH PDTCH PACCH Packet Immediate assignment Packet resource Request Packet resource assignment Frame Transmission Negative Acknowledgement Retransmission of blocks in error Acknowledgement

19 Downlink Data Transfer PRACH or RACH PAGCH or AGCH Paging Transmission Packet channel Request PACCH PACCH or PAGCH PDTCH PACCH PDTCH PACCH Packet Immediate assignment Packet paging response Packet resource assignment Frame Transmission Negative Acknowledgement Retransmission of blocks in error Acknowledgement Packet paging request PPCH or PCH MSBSS

20 Mobility  A mobile station has three states in GPRS system: ►Idle ►Standby ►Active  The operation of GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions.

21  Data is transmitted between a mobile station and the GPRS network only when the mobile station is in the active state.  In the active state, the SGSN knows the cell location of the mobile station.  In the standby state, the location of the station is known only as to which routing area it is in.  In the idle state, the mobile station does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDZ) addresses allocated, The MS can receive only those multicast messages that can be received by any GPRS mobile station.

22 QoS Support  Assumes that IP multimedia applications are able to ►Define their requirements ►Negotiate their capabilities ►Identify and select available media components  GPRS specifies signaling that enable support for various traffic streams ►Constant/variable bit rate ►Connection oriented/connection less ►Etc.

23 QoS Profile for GPRS Bearers  4 parameters: ►Service precedence 3 classes ►Reliability parameter 3 classes ►Delay parameters 4 classes ►Throughput parameter Maximum and mean bit rates

24 QoS Profile for GPRS Bearers  QoS profile is included in Packet Data Protocol (PDP) context  Negotiation managed through PDP procedures (activation, modification and deactivation)

25 Conclusions  Same GMSK modulation as GSM  4 channel coding modes  Packet-mode supporting up to about 144 kbps  Flexible time slot allocation (1-8)  Radio resources shared dynamically between speech and data services  Independent uplink and downlink resource allocation

26 EDGE Airlink  Extends GPRS packet data with adaptive modulation/coding  2x spectral efficiency of GPRS for best effort data  8-PSK/GMSK at 271 ksps in 200 KHz RF channels supports 8.2 to 59.2 kbps per time slot  Supports peak rates over 384 kbps

27 Summary  GPRS Protocol Architecture ►MS – BSS ►BSS – SGSN ►SGSN – GGSN ►GGSN – PDN  GPRS Air Interface  Data Routing and Mobility  Uplink Data Transfer  Downlink Data Transfer  QoS in GPRS