Oct. 2002 GPRS Overview 1

GPRS Overview

Reference:Wireless Technology, Michel Daoud Yacoub, CRC Press, 2002

Outline1. Features2. QoS parameters3. Network Architecture4. Protocol Architecture5. Data Structure6. Packet Data Channel and Logical Channels7. Media Access --- Uplink and Downlink8. Throughput performance

Oct. 2002 GPRS Overview 2

1. Features

Packet switched network on GSM infrastructure Started in 1990 for applications in road transport

telemetric and financial services IP based extending the features of GSM to

Circuit- and packet-switched services Better use of radio resources Quick setup time and access time Connection to other packet data network QoS based service Volume-based charging Point-to-point and point-to-multipoint services

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2. QoS Parameters

GPRS offers different combinations of 5 QoS parameters: Precedence priority of transmission (high, normal, low) Reliability packet loss probability Max bit rate 8 kbit/s to 2 Mbit/s Mean bit rate 0.22 bit/s to 111 kbit/s Delay spec (in Seconds) for 128 octet packets

(maximum mean, maximum 95% delay) are specified according to 4 traffic classes

Conversational (Class 1) (0.5,1.5) Streaming (Class 2) (5,25) Interactive (Class 3) (50,250) Background (Class 4) best effect

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3. Network Architecture (1)

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Network Architecture (2)

Same as GSM except that MSCs are replaced by the Serving GPRS Support Nodes (SGSN) and Gateway GSN (GGSN) pair.

One GGSN serves as a gateway to each type of data network. This is just a router with firewall for Internet connection.

SGSN connecting to the BSS is the interface to the radio access network performing: Routing of packets to the correct BSS Ciphering Authentication Session management Mobility management Logical link management

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Network Architecture (3)

MS has, in addition, a GPRS protocol stack. Packet Control Unit (PCU) is for handling

packet-switched calls, handover, radio resource configuration and channel assignment. It resides at BSC, BTS or SGSN.

MSC, GMSC, VLR, HLR, EIR and SMSC are all enhanced with GPRS subscriber information.

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4. Protocol Architecture (1)

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Protocol Architecture (2)

Follow the ISO reference All application data are encapsulated by the GPRS

tunnel protocol (GTP). GTP-PDU are routed over IP-based GPRS backbone using TCP for X.25 or UDP for IP applications. This is called GPRS tunneling.

Subnetwork Dependent Convergence Protocol (SNDCP) is the network layer between MS and SGSN. It performs multiplexing of network layer messages onto a logical connection, encryption, segmentation and compression.

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Protocol Architecture (3)

LLC layer is for maintaining logical links. RLC layer performs segmentation and reassembling of

LLC PDU. Operates in acknowledged mode (SR-ARQ) and unacknowledged mode.

MAC performs contention resolution, physical channel multiplexing and reverse-multiplexing.

Physical layer performs FEC, interleaving, congestion detection, modulation and demodulation

BSSGP (BSS GPRS Protocol) relaying QoS and routing information between BSS and GPRS

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5. Data Structure (1)

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Data Structure (2)

Typically 20%--30% protocol overhead for GPRS Application data segments to 64 kB IP PDUs. Further segment to 1.56 kB of SNDCP (Sub-net Dependent Convergence

Protocol) PDUs Then to 1.556 kB LLC PDUs Finally to 20-50 Byte MAC/RLC PDUs called blocks. Two kinds: User Data

Block and Control Block. These blocks are then attached with tail bit. Perform coding as shown in Table 6.1 Puncturing into Radio Blocks of size 456 bits Radio Blocks are segmented to four 114 bit subsegments. Each sub-segment is placed into a GSM time slot.

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Data Structure (3)

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6. Packet Data Channel (PDCH) Logical Frame

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PDCH Logical Frame (2)

Uses GSM Framing structure. 52 TDMA frames constitute a multiframe of which 48 are data frame.

The 8 TDMA slots in a frame form 8 PDCH. In each PDCH channel, there are 12 radio blocks. The set of 4 frames defines a logical frame (20 ms) for

which a radio block (or an access) can be sent. An access is uniquely addressed by the PDCH channel

number (0 to 7) and the logical frame number.

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GPRS Logical Channels

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7. Media Access (1)

Asymmetric and independent up-and-down-link channels Data transfer by means of Temporary Block Flow (TBF).

A TBF is a virtual connection between MS and BSS supporting unidirectional transfer of data on PDCH. It is labeled by TFI (Temporary Flow Identifier).

TFI is 7 bits for uplink and 5 bits for downlink. Closed-ended type (CET) TBF the amount of data is

fixed at the initial access. Open-ended type (OET) TBF amount of data transfer

is arbitrary, or until a TBF is terminated.

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Media Access (2) Access protocol reservation ALOHA Using a special access burst format for which there are

Extended tail bits (8), 3 bits for normal burst Extended sync. Bits (41), 26bits for normal burst Coded and Encrypted data (36)

15 extra synchronization bits are used because MS in an initial access may not have any timing information.

The decoded and decrypted data total 8 or 11 bits contain the reservation information: Reason of access one-phase or two-phase access request,

page response, measurement report,etc. MS class and requested radio priority Number of blocks

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Uplink Data Transfer (1)

One-phase procedure MS sends a packet channel request (PCR) to BSS over PRACH indicating one-phase. BSS responds a packet uplink assignment (PUA) message includes the information on resources to be assigned to MS. This includes carrier, time plot, USF (uplink state flag, 3 bits for addressing the MS requesting data transfer), TFI, open-or-close-ended TBF, Packet Associated Control Channel (PACCH). Data transferred by MS together with TLLI (temporary logical link identifier). BSS sends back ACK/NACK. TBF terminated by the packet control ack message.

Two-phase procedure Here PUA message gives information on a single block including slot number, TFI, power control parameters and PACCH. MS then sends a pack resource request (PRR) message. BSS responds with PUA indicating time slot, TFI, USF, TLLI. Data transfers.

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Uplink Data Transfer (2)

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Uplink Data Transfer (3)

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Downlink Data Transfer

BSS page the MS MS use the PRACH to request a downlink packet

channel. The rest is the same as uplink procedure.

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8. Throughput Performance

Nominal: CS-1 64 kbit/s CS-2 96 kbit/s CS-3 115 kbit/s CS-4 160 kbit/s

Protocol overhead reduces these rates to much smaller values Simulation in a single cell with a single carrier, excluding mobility and

handover. 8 PDCHs are considered. Typical Internet traffic from web browsing and email transfers. TCP segment

size is 536 bytes. Throughput measured:

44 kb/s for CS-1 (C/I = 12dB) 66 kb/s for CS-2 (C/I = 16dB) 75 kb/s for CS-3 (C/I = 20dB) 100 kb/s for CS-4 (C/I =24dB)

Throughput efficiency therefore is about 60-70%