GPRS for 2 Hours Training
Transcript of GPRS for 2 Hours Training
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Agenda
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Introduction
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GPRS/EGPRS
GPRS A feature that makes it possible to send packet data over the GSM network GPRS uses two different coding schemes, CS1&2
CS3&4 A new feature should be supported and implemented in BSC ands BTS CS3&4 are only supported in downlink
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Coding Schemes
Coding
Info.bits
Max. data rate per TS (kbps) 8.0
C/I (dB) ~6
CS 1 CS 2 CS 3 CS 4
160
240 288
12.0 14.4
~9 ~12
400
20.0
~17
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GSM and GPRS Technology which permits mobile data communication using packet switching techniques GSM allows circuit switched (CS) data transfer Data transfer on a dedicated channel (connection oriented) Connection setup procedure needed as in modem Subscriber charged according to time of connection TS is held for duration of connection - waste of resources
GPRS designed as an extension to digital cellular networks Connectionless packet switched (PS) data service Standardized by ETSI Radio resources shared between CS and PS data New terminals are required7
GPRS Ciphering
GSM Ciphering: Between BTS & MS Just for voice
GPRS Ciphering: Between SGSN & MS For signaling & user data because of no separated link for signaling and user data on Abis8
What is EDGE ? EDGE is the next step in the evolution of GPRS. EDGE is a method to increase the data rates on the radio link for GSM The main advantage of EDGE is that it will offer higher data rates without fundamentally changing the hardware infrastructure Uses 8-PSK modulation in good conditions Increase throughput by 3x (8-PSK 3 bits/symbol vs GMSK 1 bit/symbol)
Fall back to GMSK modulation when far from the base station
New handsets / terminal equipment; additional hardware in the BTS Core network and the rest remains the same TDMA (Time Division Multiple Access) frame structure 200kHz carrier bandwidth allows cell plans to remain
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EDGE as Capacity Multiplier
voice
voice
voice
voice
voice
GPRS
GPRS
GPRS
Standard GSM Transceiver
voice
voice
voice
voice
voice
Free TS
Free TS
EDGE
EDGE Transceiver
Increased Capacity10
EDGE is based on GPRS
Internet
GPRS
MS
SGSN SGSN 64 EDAPEDGE TRU
GGSN
BTS
BSC/PCU
EDGE
MS
New Modulation
GPRS = EDGE
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(E)GPRS Basic Technical Parameters
GPRS Modulation Symbol rate Modulation bit rate Radio data rate per time slot User data rate per time slot User data rate (8 time slots) Radio data rate (8 TSs) GMSK 270 ksym/s 270 kb/s 22.8 kb/s 20kb/s (CS4) 160kb/s (182.4kb/s)
EDGE 8-PSK / GMSK 270 ksym/s 810 kb/s 69.2 kb/s 59,2 kb/s (MCS9) 473,6kb/s (553.6kb/s)
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EDGE modulation technique
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Standardized Improvement: EGPRS Coding Schemeskbps 60 54.4 50 59.2
40 44.8 30 20.0 14.4 12.0 10 0 CS1 CS2 CS3 CS4 8.0 8.8 MCS1 MCS2 MCS3 MCS4 MCS5 MCS6 MCS7 MCS8 MCS9 11.2 14.8 17.6 22.4 29.6
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GPRS GMSK modulation
EGPRS 8PSK modulation
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Standardized Improvement: Packet Transmission
GPRS
EGPRS
Re-transmission with another CS not possible
Re-transmission with another MCS possible
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Standardized Improvement: EGPRS Coding SchemesChannel Coding Schemes Throughput / TS Family
MCS1 MCS2 MCS3 MCS4 MCS5 MCS6 MCS7 MCS8 MCS9
8.8 11.2 14.8 17.6 22.4 29.6 44.8 54.4 59.2
C B A C B8PSK GMSK
A B A A
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Payload FormatMCS-3 Fa mily A 37 octets 37 octets MCS-6 MCS-9 MCS-3 34+3 octets Fa mily A padding 34+3 octets 37 octets 37 octets
MCS-6 34 octets 34 octets 34 octets 34 octets
MCS-8
MCS-2 Fa mily B 28 octets 28 octets MCS-5 MCS-7 MCS-1 Fa mily C 22 octets 22 octets 28 octets 28 octets
MCS-4
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MS classes of operation
Class A mode of operation allows an MS to have a circuit switched connection at the same time as it is involved in a packet transfer.
Class B mode of operation allows a MS to be attached to both circuit switched and packet switched connections, but it can not use both services at the same time. However, MS that is involved in a packet transfer can receive a page for circuit switched traffic. The MS can then suspend the packet transfer for the duration of the circuit switched connection and afterwards resume the packet transfer. This requires the Gs interface between the MSC and SGSN to be present.
Class C mode of operation allows an MS only to be attached to one service at a time. An MS that only supports GPRS and not circuit switched traffic will always work in class C mode of operation.
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Network operation modes (1)
Network operation mode I: The network uses combined procedures. The MS needs only to monitor one common control channel, the CS common control channel or the PS common control channel.This mode requires the optional Gs interface between MSC and SGSN.
Network operation mode II: The network does not use combined procedures. All common control signaling, both for CS and PS connections, is performed over the CS common control channel. The Gs interface should not be present.
Network operation mode III: The network does not use combined procedures. All common control signaling for PS connections is performed over the PS common control channel, and all common control signaling for CS connections is performed over the CS common control channel. This require a class A or B MS to listen on two common control channels. The Gs interface should not be present.
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GPRS Network Structure
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GPRS
How to upgrade a GSM network to GPRS? 1. For the BSS software upgrade hardware upgrade (PCU) 2. New GPRS support nodes (SGSN GGSN)
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GPRS: General Packet Radio Service
Circuit SwitchedUm Abis
SCP BSC & PCUA PSTN
TDM
BTS SIM FR HLR AUC
Packet Switched Core
Gb
IPGn Gi
Internet Corporate
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GPRS Structure
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Packet Control Unit PCU : is responsible for the GPRS/EGPRS radio resource management in BSS Radio interface Handling the Medium Access Control (MAC) Handling Radio Link Control (RLC) layers Gb interface Handling the BSS GPRS Protocol (BSSGP) Handling Network Service (NS) layers
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Functions of SGSN? The SGSN is the MSC of the GPRS network At least one SGSN per GPRS network SGSN has the following functions: Protocol conversion between IP backbone and BSS protocols Authentication of GPRS users Mobility management of GPRS enabled MS Routing of data to the relevant GGSN Interaction with the NSS (MSC/VLR, HLR, EIR) via SS7 network Collection of charging data records pertaining to GPRS calls Collection of traffic statistics
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Functions of GGSN? GGSN acts as a router to the external network GGSN has the following functions: Routes mobile-destined packets coming from external networks to the relevant SGSN Routes packets originating from a mobile to the correct external Network Interfaces to external IP networks Collects charging data and traffic statistics Allocates dynamic or static IP addresses to mobiles either by itself or with the help of a DHCP or a RADIUS server26
Border Gateway (BG) Necessary to interconnect operators' GPRS backbone networks to support roaming Provides a direct tunnel between different operators' GPRS networks (rather than transferring via the public Internet) Roaming is not supported in GPRS Release 1HPLMN SGSNMMS Center
BG
PLMN BG
PLMN GGSNWAP
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Charging Gateway (CG)
GPRS charging data records (CDR) are generated by SGSNs and GGSNs in the network SGSN and GGSN transfer CDR using GTP The Charging Gateway collects all this data together processes it passes it on to the Billing System
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Lawful Interception Gateway (LIG) Lawful interception is an action based on the law, which is performed by the GPRS network It provides information for a Law Enforcement Agency (LEA) about some pre-defined target subscriber Information could include : data sent and received by the interception target location information subscriber information etc.
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GPRS Interface
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GPRS interfaces
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GPRS Interface The most important open interfaces in the GPRS network are: Gb - SGSN to BSS Gn - between GSNs (GTP) Gr - between SGSN and HLR (MAP) Gs - SGSN to MSC (BSSAP+) Gi - GGSN to external data networks Gf - SGSN and the EIR (MAP) Gd - SGSN and the GMSC (SMSC) Gp - between GSNs of different PLMNs
The user packets are transported encapsulated using the GPRS Tunnelling Protocol (GTP) over the GPRS backbone. The backbone is an IP network.
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GPRS Radio Channel
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Packet Data Channel (PDCH) Physical channel dedicated to packet data traffic is called a PDCH (Packet Data Channel) PDCH use spare traffic channels (TCH) in a cell Each PDCH is shared by multiple MS and network Each PDCH may have a number of logical channels
PDCH carry GPRS data and control signaling PDCH classified into (details later) PCCCH (Packet Common Control Channels) PBCCH (Packet Broadcast Control Channels) PDTCH (Packet Data Traffic Channels) PACCH (Packet Associated Control Channels)34
Logical Channels in PS (GPRS)Packet Broadcast Control Channel
PBCCHPacket Common Control Channel
Logical ChannelsPDCCHPacket Dedicated Control Channel
Packet Traffic Channel
Traffic PCCCH
PDTCHPacket Data Traffic Channel (UL/DL)
PPCHPacket Paging Channel (DL)
PAGCHPacket Access Grant Channel (DL)
PNCH
PTCCH/D
PRACHPacket Random Access Channel (UL)
Packet Notification Packet Timing Channel (DL) advance Control Channel (DL)
PTCCH/UPacket Timing advance Control Channel (UL)
PACCHPacket Associated Control Channel (UL/DL)
Available with MASTER PDCH
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Broadcast channels
GSM FCCH Frequency Correction Channel SCH Synchronization Channel BCCH Broadcast Control Channel
GPRS PBCCH Packet Broadcast Control Channel Broadcasts packet data specific System Information messages MS continuously monitors this GSM BCCH can also be used
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Common Control channels GSM PCH (Paging Channel) RACH (Random Access Channel) AGCH (Access Grant Channel)
GPRS PPCH (Packet Paging Channel) Can be used for paging both CS & PS services GSM PCH can also be used
PRACH (Packet Random Access Channel) Used for uplink channel reservation & to obtain TA GSM RACH can also be used
PAGCH (Packet Access Grant Channel) Used for resource assignment during packet transfer establishment phase GSM AGCH can also be used
PNCH (Packet Notification Channel) Downlink only channel used for PTM-M notifications to a group of MS before PTM-M packet transfer Only in GPRS Phase 2
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Dedicated channels GSM SDCCH (Standalone Dedicated Control Channel) SACCH (Slow Associated Control Channel) FACCH (Fast Associated Control Channel) TCH (EFR/FR/HR Traffic Channel)
GPRS PACCH (Packet Associated Control Channel) Bi-directional dedicated channel for transferring ack./power control or resource assignment/reassignment Messages
PDTCH (Packet Data Traffic Channel) Bi-directional Corresponds to the resource allocated to a single MS on one physical channel for user data transmission
PTCCH (Packet Timing advance Control Ch.) Uplink dedicated (for transmission of random access bursts) Downlink common (for transmission of timing advance information to several MSs).38
GPRS Logical channels
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1.Possibility Cell without MPDCH BCCH Broadcast for CS and GPRS CCCH Signaling for CS and GPRS
PCH PS Connection CS
BTS
RACH Request for CS PS
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2.Possibility Cell with MPDCH First configured dedicated PDCH becomes MPDCH PBCCH PCCCH Traffic PDCH
CCCH BCCH PCH PPCH -> PBCCH, PCCCH on Channel X, TS Y CS PS Connection RACH PRACH Request for PS CS
BTS
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GPRS Air-Interface Layer
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GPRS Protocol Stack: Gb and Um InterfaceApplication Relay
IP/X.25Network
IP/X.25 SNDCPRelay
SNDCP LLCData Link
GTP
GTP
LLC RLC MAC GSM rfBSS Um Gb
UDP/TCP
UDP/TCP
RLC MAC
BSSGP NW serv L1bis
BSSGP NW serv L1bisSGSN
IP L2 L1
IP L2 L1GGSN Gn Gi 43
Physic
GSM rfMT
Layers of the GPRS air interface Physical layer MAC (Medium Access Control) layer RLC (Radio Link Layer) layer
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PDU, radio block, and bursts
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Radio Blocks(456 - M) bits MS M- bits
Coding Bits Header
INFO BitsRLC/MAC Block
Application
Network 456 Bits / Radio Block
LLC RLC MAC GSM RFRadio Blocks overMAC
BTS
BSCGb
SGSNLLC layerUSER DATA
GSM RF
PCU
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Physical Layer
Physical Layer : Lowest layer of GPRS protocol stack Primary function is to provide services for information transfer over a physical channel Physical Layer is split into two sub-layers Physical RF layer Modulation of RF signals at the transmitter GMSK for GPRS (1 symbol per bit) 8 PSK for EGPRS (1 symbol per 3 bits) Demodulation of RF signals at the receiver
Physical Link sublayer47
Physical Layer Physical Layer : Physical RF layer Physical Link sublayer Framing: Placement of data into bursts, frames, radio blocks, etc. Data coding for maximising the data throughput Detection and correction of errors due to noise in the medium Procedures for detecting congestion on the air interface Procedures for synchronising MS and network Procedures for monitoring and evaluation of radio link quality Procedures for cell (re-)selection Transmitter power control48
TBF TBF is : The transmission of packets to or from a certain MS The correspondence to a CS call setup is an assignment of an uplink or a downlink TBF for a packet transfer. An MS can have a TBF in one direction or one in each direction. Each TBF is identified by a Temporary Flow Identity (TFI) At assignment of a TBF, the MS is informed of which timeslot(s) to use and its TFI.
Limitation Ms can have one TBF each direction UL/DL therefore MS can at most two active TBFs No. of TSL per TBF depends on CS load Multi-slot MS QoS
Max no. of TBF (UL+DL) per TSL = 16 Max no. of TBF in UL per TSL = 7
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Media Access Control (MAC)
Medium Access Control (MAC) layer operates above Physical Link layer Connection orientedConnections are called Temporary Block Flows (TBF) Logical unidirectional connection between two MAC entities Allocated resources on PDCH(s) One PDCH can accomodate multiple TBFs Temporary Flow Identity (TFI) is unique among concurrent TBFs in the same direction Global_TFI to each station
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Radio Link Layer Transfer of Logical Link Control layer PDUs (LLC-PDU) to the MAC layer Segmentation and re-assembly of LLC-PDUs into RLC Data Blocks Backward Error Correction (BEC) procedures for selective retransmission of uncorrectable code words in the acknowledged mode of transmission Transmission of code words based on channel conditions, i.e link adaptation Storing soft values of the erroneous RLC Data Blocks and combining them with the retransmitted RLC Data blocks51
GPRS Framing
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GPRS framing
Framing hierarchy Bursts Radio blocks Frames Multiframes Superframes Hyperframes53
GSM Multi-Frame1 hyper-frame = 2048 super-frames = 2,715,648 TDMA frames =3 hours 28 minutes 53 seconds 760 microseconds)0 1 2 3 4 5
...
2043 2044 2045 2046 2047
1 super-frame = 1326 TDMA frames (6.12 seconds) = 51 (26-frame) multi-frames or 26 (51-frame) multi-frames
0 0
1
2 1
3
......
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48
49 24
50 25
0
1
...
24 25
0
1
...
49 50
1 (26-frame) multi-frame = 26 TDMA frames (120 ms)
1 (51-frame) multi-frame = 51 TDMA frames (235.38 ms)
0
1
2
3
4
5
6
7
1TDMA frame = 8 timeslots (120/26 =~ 4.615 ms)
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GPRS Protocols Layers
IP Packet
Header
User data
Higher layer
LLC PDU
Header
Information field
Tail
LLC layer
1500 bytes
Radio Blocks
USF
RLC RLC BCS Header Information
RLC ... USF Header
RLC BCS Information
RLC/MAC layer
20-50 bytes
RLCNormal Normal Normal Normal Burst Burst Burst Burst
8PSK
Physical layer
4 114 bits
By EGPRS affected layerB0 B1 B2 X B3 B4 B5 X B6 B7 B8 X B9 B10 B11 X
Multiframe structure, 52 TDMA frames
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GSM Burst
1 TDMA frame = 8 timeslots
0
1
2
3
4
5
6
7
1 timeslot = 156.25 bit durations (15/26 =~ 0.577 ms) (1 bit duration 48/13 =~ 3.69 micro sec)
Normal Burst (NB)Frequency correction burst (FB)
TB 3 TB 3 TB 3 TB 3
Encrypted bits 57
flag 1
Training sequence 26
flag 1
Encrypted bits 57
TB 3 TB 3
GP 8.25 GP 8.25 GP 8.25
Fixed bits
142
Synchronization burst (SB)
Encrypted bits 39
Synchronization sequence 64
Encrypted bits 39
TB 3
Access burst (AB)
Synchronization sequence 41 Mixed bits 58
Encrypted bits 36
TB 3
GP 68.25
Dummy burst (DB)
TB 3
Training sequence 26
Mixed bits 58
TB 3
GP 8.25
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GPRS Radio Block
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MAC block segmentation into bursts
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Multiframe Structure on PDCH
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GPRS MultiframeTS0 TS1 TS2 TS3Radio Block0
TS4
TS5
TS7
TS8
Radio Block2
1 Radio Block = 4 Bursts = 456 (4*2*57) info bits
Radio Block3 Radio Block4 Radio Block5 Radio Block6 Radio Block7 Radio Block8 Radio Block9 Radio Block10 Radio Block11
PTCCH
Idle
GPRS Multi-Frame = 52 TDMA Frame
Radio Block1
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PDCH multi-frame structure
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GPRS Multiframe GSM uses a 51-multiframe structure GPRS uses a 52-multiframe structure Important concepts: Timeslots (TS), in which 114 bit bursts are sent (duration) Frames = Eight timeslots (8 * 114 bits)Each multiframe has 12 blocks + 2 idle frames + 2 PTCCH frames 12 *4 + 2 +2 = 52 frames
Multiframe = 52 frames (52 * 8 * 114 bits) Radio block is 4 bursts from 4 consecutive frames in the same carrier and in the same TS (456 bits)
Each uplink TS can theoretically be used by 8 MSs for data. Need for a mechanism to identify62
GPRS Radio Block Allocation
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Radio block allocation to MS On uplink, how does a MS know which PDCH and which radio block on that PDCH it may use? Assigning radio blocks of a PDCH to different MSs is done by MAC layer This process is referred to as content arbitration Content arbitration occurs only in the uplink direction. An Uplink Status Flag (USF) is used for content arbitration USF is transmitted downlink and it tells which MS may use a radio block In initial PAGCH (AGCH), MS is assigned a USF value for each PDCH MS monitors USF values in downlink transmission on assigned PDCH MS may transmit in radio blocks that have the same USF value as was allocated to it in the PAGCH message USF has 3 bits at the beginning of each radio block on the downlink So one PDCH can be used by 8 MSs at one time
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Usage of Uplink State Flag (USF)
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Radio block identification On the downlink how does MS know which radio block is destined for it? Need for an identifier of the destination of data in the downlink Downward multiplexing of radio blocks is done using Temporary Flow Identifier (TFI) What is TFI? TFI is assigned in a resource assignment message prior to transmission LLC layer frames between MS and BSS TFI is unique among concurrent processes TFI is preferable to MS identity which is a very long number TFI is included in every RLC frame header66
PDCH Allocation The PDCHs are allocated to the PCU. The PCU is then responsible for assigning channels to the different GPRS/EGPRS MS. The PDCH allocation can be done in different ways: Dedicated PDCH and Semi-dedicated PDCH are allocated and de-allocated by operator command. On-demand PDCH, serving as temporary dynamic GPRS/EGPRS resources, are allocated and released depending on the GPRS/EGPRS traffic demand.
Channels that are allocated for GPRS/EGPRS (i.e. PDCHs) are allocated in sets of maximum eight consecutive time slots, (Such a set is called a PSET) The PSET can consist of both dedicated, semi-dedicated and on-demand PDCHs. All channels in a PSET are on the same frequency or hop on the same frequency hopping set. An MS can only be assigned PDCHs from one PSET. The number of PDCHs that can be allocated in a cell is depending on the number of available TCHs and GSLs (GPRS Signalling Link).67
Dedicated PDCH A dedicated PDCH can only be used for GPRS/EGPRS traffic. Dedicated PDCHs ensure that there always are GPRS/EGPRS resources in a cell. The operator can specify up to 16 dedicated PDCHs per cell.
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Semi-Dedicated PDCH Having the advantages of the dedicated PDCH (a semi-dedicated PDCH is not possible to pre-empt by the CSD) Having the possibility to save PCU resources. It is possible to request up to 16 semi-dedicated PDCHs per cell. (The total number of dedicated PDCHs and semi-dedicated PDCHs in the cell must not exceed 16)CSD : Circuit Switched Domain69
On-demand PDCH On-demand PDCH are only allocated from the Circuit Switched Domain (CSD) when there is a need for GPRS/EGPRS traffic. An on-demand PDCH can be pre-empted by incoming CS calls in congestion situations in the cell, thus a better use of the frequency spectrum. In a cell without any CS traffic it would be possible to use all channels for GPRS/EGPRS traffic provided if there are enough PCU resource. There is however a possibility to limit the number of on-demand PDCHs.
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GPRS Cell RE/Selection
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GMM StatesGPRS Attach/Detach Ready
Idle
Ready Timer Expiry Mobile Reachable Timer Expiry Standby Packet TX/RX
MS Location Ready state Standby state Ready Idle No72
Cell Level RA Level
Idle State
The MS is turned on but not GPRS attached. The MS is "invisible" to GPRS, i.e outside the GPRS coverage area. The MS is not allocated any radio resources on a packet data physical channel. It listens to the PBCCH and PCCCH or, if those are not provided by the network, to the BCCH and the CCCH.
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Standby State
The MS is attached to the GPRS Mobility Management (MM) and sends Routing Area (RA) updates to the SGSN performs GPRS cell selection and re-selection.
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Ready State
A packet transfer is ongoing or has recently ended. A timer defines how long the MS remains in ready state after transfer. The MS performs cell update when changing cell in ready state. The MS is allocated radio resources on one or more packet data physical channels for the transfer of LLC PDUs.75
GPRS Cell Selection
For Circuit Switched MSs MS selects cell (idle mode) BSC selects cell (active mode - locating)
For Packet Switched MSs MS selects cell in both packet idle and packet transfer mode
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General Description
The MS selects the most suitable cell to camp on by calculating the value of C1, C31 and C32 for the serving cell and neighboring cells If PBCCH in s-cell: C31/C32 If no PBCCH in s-cell: C1/C2
Only cells with BSICs as indicated in the GPRS BA list are allowed to be used for cell selection ** If PBCCH exists, it will be possible to use hystereses and offsets as well as a HCS structure77
Cell change during packet transfer MS will start to read the System Information in the new cell while still hanging on the old cell. The MS makes an access in the new cell and sends a cell update message. The SGSN receives the cell update information and discovers that there was already an ongoing packet transfer. After assigning new resources in the new cell to the MS, the packet transfer is restarted.
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Combined Procedures
Combined LA & RA update. Both CS and PS paging goes via the SGSN, i.e. the MS only needs to listens to one paging channel and the paging is performed per RA.
Combined Attach : Combined GPRS and IMSI attach Results GSM and GPRS location update Needs Gs Interface
NOM (NMO) : Is a parameter in Attach Request Message that shows to SGSN which kind of attach needed If NOM = 1 Combined attach If NOM = 2 Not Combined attach
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GPRS Gb Interface Layer
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Gb Interface & Gb Protocol The Gb interface is an open interface between the PCU and the SGSN A BSC can use one or more physical links to connect to an SGSN. With SGSN in Pool a BSC can be connected to multiple SGSNs
The protocol used to provide layer 3 is BSSGP. BSSGP is a GPRS/EGPRS specific protocol. It conveys the necessary routing information to be able to transfer LLC PDUs transparently across the radio network to the Mobile Station (MS).
Layer 2 is called the NS layer. This layer is further divided into two separate layers. The upper layer is called the Network Service Control (NSC). The lower layer is called the Sub-Network Service
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Gb Interface & Gb Protocol The protocol used to provide the NSC layer is the NSC protocol. The NSC protocol provides a generic way of encapsulating BSSGP PDUs and transferring them via the Sub-Network Service
For the Sub-Network Service layer there are two alternative protocols: Frame Relay (ETSI Rel 97&98&99) Long distance between SGSN & PCU Is frame mode interface specification providing a signalling and data transfer mechanism between end-points in the network. The endpoints of the Gb interface are the BSC and the SGSN. FR shall transparently transfer NS PDUs between an SGSN and a BSC.
IP (3GPP Rel 4) Short distance between SGSN & PCU IP can instead be used for the transport. NS PDUs are transparently transferred between IP End-points (local and remote) using UDP/IP.
Gb over IP and Gb over FR is not supported simultaneously in the same BSC Gb over IP is required for SGSN in Pool If FR is used for transport, a BSC can only be connected to one SGSN If IP is used for transport and if feature SGSN in Pool is available, one BSC can be connected to multiple SGSNs.
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DTM Dual Transfer Mode (DTM) enables a user to run CS and PS services simultaneously, e.g. DTM enables support for PS services to be run in parallel with a CS call. The DTM concept is described in 3GPP Technical Specifications as a form of simplified class A operation for an MS. In DTM, the MS has a CS connection to the network and has simultaneously allocated radio resources on one or more Packet Data Channels (PDCHs). The number of timeslots that can be allocated for DTM to be used for a CS and a PS connection and the relative order and placement of the timeslots must comply with the DTM multislot class defined for the MS. DTM can only be entered in case there is an existing CS connection. If the MS is in Packet Transfer Mode when a CS connection is to be added, the packet transfer has to be temporarily halted, the CS connection is established and then the packet transfer is resumed by entering DTM.
GPRS Signalling on Main DCCH is a function that is available for DTM capable MSs that have an existing CS connection. The function makes it possible to set up PS resources by using the signalling resources from the CS connection, and to do cell update or routing area update during a CS call. Pure GPRS signalling (LLC frames) can be sent both in uplink and downlink direction using a tunneling mechanism. The messages are sent on the FACCH or SDCCH. 83
DTMDTM Assignment
Enhanced DTM in 3GPP Rel6
Dedicated call ModeCS Released PS Released
Data Transfer Mode
DTM
DTM Assignment
No call & No data
CS Released
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TLLI and SAPI TLLI shows : Which MS-SGSN relation the message belongs to TLLI Types : TLLI = PTMSI + RAI
SAPI It is added in LLC frame header It shows followings : GMM Message User Date Message SMSNote : PCU can support message up to RLC layer and upper layer messages are processed by MS and GPRS, so SAPI isnt checked by PCU
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PDPID
PDPID : Is stored in HLR One PDP ID per each users subscribed service should be assigned. Each service consists of some facilities which are served to user
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GPRS KPI
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GPRS Performance For the Packet Switched domain, it is much more difficult to define STS counters in the BSS because:1) The GPRS/EGPRS system has many layers of protocols. A session where a TBF is dropped for some reason, a retainability problem on BSS level, will normally be kept alive by TCP until a new TBF is established. 2) The GPRS/EGPRS network is a bearer for a number of different applications with different requirements. An interruption in the data transfer for a few seconds would appear as a serious performance problem to a WAP user. It would hardly be noticed by a user performing downloading of e-mails in the background.
The terms accessibility, retainability and integrity can be applied to the GPRS/EGPRS system, but only on higher layers, and by considering events, which are invisible to the BSS.
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Reference to CS Traffic
In CS domain the following benchmark types of
Key Performance Indicatorsare define
A) Accessibility (Call Success Rate) B) Retainability (Drop Call Rate) C) Integrity (Speech Quality Index)
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BSS GPRS KPIs
The following KPIs are defined in PS domain.
Accessibility (GPRS Attach failure, PDP context activation failures, GPRS service access failure,TBF and CCCH Request Failure Rate )
Retainability (FTP/WAP/Web/SMTP/POP3 transfer failure, TBF pre-emption rate & Mean Time between TBF pre-emption )
QoS (GPRS Attach time, PDP Context activation time, GPRS Service access time, transfer rate, LLC Throughput & TBF load )
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Main Radio KPIs to Monitor
IP Throughput Measures the total amount of data divide by the total time taken to transmit it
IP Latency IP latency measures the delay Gb-BSS-MS-BSS-Gb
IP Transfer Interrupt Determine how often the IP transfer is interrupted
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Other Issues
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Timeslot sharingA Voice call require the GPRS TS
BCCH
TCH
TCH
TCH
TCH
GPRS GPRS
GPRS
BTS
Site with one carrier will be able to share time slots (Voice and GPRS)
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Uplink Packet Transfer Procedure TBF establishment procedure will be different depending on whether there is an MPDCH in the cell or not. If there is an MPDCH in the cell MS sends a 44.060 PACKET CHANNEL REQUEST (GPRS MS) or 44.060 EGPRS PACKET CHANNEL REQUEST (EGPRS MS) message to the PCU. If there is no MPDCH in the cell MS sends a 44.018 CHANNEL REQUEST or 44.060 EGPRS PACKET CHANNEL REQUEST (EGPRS MS) message to the PCU.
There are two main ways to allocate resources after receiving the initiating messages from the MS:1) 2) The MS is assigned resources on one or several timeslots, i.e. TBF is established (short access or one-phase access used as access type by the MS). A single timeslot is reserved for the sending of one uplink RLC block: This is used to let the MS send a Packet Resource Request message, to further specify its capabilities and/or demands. The PCU responds to this and assigns resources with a Packet Uplink Assignment to the MS, i.e. TBF is established (two-phase access used as access type by the MS).94
Uplink Packet Transfer Procedure Channel request Message (MS to BSS on RACH) Its consist of : Random reference Establishment Cause SMS Signalling Single block (2 Phase Access) One phase access
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Downlink Packet Transfer Procedure If the MS is in standby state, the downlink packet transfer is initiated by paging the MS in an RA. This is initiated by the SGSN sending a PS Paging Request message to the PCU. The PCU will then calculate which paging group the MS belongs to and send the paging request in a time slot when the MS is awake (listening). An RA consist of one or a number of cells and is identified in the standard as a subset of a Location Area (LA). In Ericsson BSS, the RA is identical to the LA. The MS responds to the page by sending any LLC PDU to the SGSN. This is done by use of the uplink packet transfer procedure. The MS is now in ready state and the SGSN can start to send LLC frames to the PCU with the cell and MS identity. When the PCU receives LLC frames from the SGSN, the PCU checks if the addressed MS is already involved in a packet transfer. If the MS already has a downlink TBF, the new LLC frame is put in the queue with the other LLC frames to that MS.
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Downlink Packet Transfer Procedure If the MS has no TBF established and no MPDCH exists, an Immediate Assignment message is sent on a timeslot that the MS listens to on the CCCH. If an MPDCH exist, a Packet Downlink Assignment message is sent on a timeslot that the MS listens to on the PCCCH. A certain time after the MS has been involved in a packet transfer it remains in non-DRX mode. That means that the MS is awake and there is no need to wait for its paging group. The message can be sent immediately. If the MS already has an uplink TBF, the PCU has to take this into consideration. The PCU will try to allocate downlink resources on the same timeslots (or at least partially) as the MS has uplink resources. This makes it possible for the MS to use both the uplink and downlink resources at the same time. The MS multislot class gives the capability of the MS regarding how many PDCH it can handle in each direction at the same time. The Packet Downlink Assignment message is sent on the control channel that is associated with the uplink transfer, the PACCH. The Packet downlink assignment message consists of a list of the channels that will be used and a TFI to address the MS.
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PDCH Reservation PDCH reservations is : the algorithm that controls which PDCHs in a cell a certain TBF should use. PDCHs are reserved for a TBF, uplink or downlink, in accordance with the GPRS/EGPRS MS multislot class, MS type (GPRS or EGPRS) and Quality of Service. The type of information transferred on the PDCH will also affect the reservation algorithm. The reservation of TBFs on PSETs are continuously supervised to check if it is possible to improve any reservations. Procedures to improve a PDCH reservation are: 1) 2) 3) Upgrade of a PDCH reservation Dynamic DL/UL PDCH reservation Re-reservation of PDCH reservation
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Changes of TBF Mode Change of TBF mode is : a procedure that makes it possible to change the TBF-mode (GPRS or EGPRS) for an ongoing TBF. This will be beneficial and in some cases a necessity in order to provide the required bandwidth for a TBF. When change of TBF mode is triggered, the TBFs connected to the MS will be released and setup again using the new TBF mode. MSs having only an UL TBF, or TBFs belonging to a DTM connection, will not be considered for TBF mode change. During the change of TBF mode procedure the transfer will be temporary suspended.99
Link Quality Control in EGPRS LQC for EGPRS : dynamically selects the most optimal Modulation and Coding Scheme (MCS) for each downlink and uplink TBF. LQC combines the two methods: Link Adaptation (LA) Link Adaptation selects the most optimal modulation and coding scheme is based on the current radio conditions. The algorithm to select the most optimal MCS is, in uplink, based on radio link quality measurements made by the BTS and in downlink the algorithm is based on radio link quality measurements made by the MS. In the Ericsson implementation this method is used both for uplink and downlink TBFs.
Incremental Redundancy (IR) The enhanced RLC protocol in EGPRS allows the receiver to store information from previous transmissions of the same RLC data block in order to increase the probability of successful decoding. This is called Incremental Redundancy (IR). Retransmissions are likely to occur since the initial code-rate is high. The throughput will however be very high since the amount of redundancy sent will be what was actually needed and no more. In the Ericsson implementation this method is used for both uplink and downlink TBFs.
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Link Quality Control in EGPRS When configuring LQC it is possible to select the following modes: LA LA/IR LA/IR-BLER
It is possible to configure uplink and downlink separately. The IR-method is always combined with LA. The difference between LA and LA/IR is that LA/IR uses a more aggressive MCS selection and that resegmentation is not allowed in the two LA/IR modes. When running in LA/IR mode there are two variants to select from. One is the normal LA/IR-mode, the other is LA/IR combined with an additional BLER measurement. With the added BLER measurement the MCS selection algorithm becomes more stabile and the MCS selection is even more aggressive.101
Quality of Service The purpose of QoS is to differentiate access and bandwidth proportionally to the priority of the different user subscriptions. This is achieved with a QoS reservation and scheduling algorithm. In the R99 standard, new procedures are introduced to let BSS negotiate the QoS with SGSN. At activation of the PDP-Context between the MS and SGSN, a QoS level is negotiated between BSS and the SGSN. The QoS level is valid per Packet Flow Context (PFC) for an MS in BSS. In downlink the MS can have several PFCs activated in BSS, i.e. an MS can have more than one application running at the same time. In uplink there can only be one active PFC for an MS.
The inputs for the negotiation of a QoS level, are the QoS attributes and network capability. There are four main traffic classes, Background, Interactive, Streaming and Conversational where the Ericsson BSS supports Background, Interactive and Streaming:
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Quality of Service Background : The Background class is intended to be used by applications where the data is not required to arrive within a certain time, e.g. e-mail and file transfers.
Interactive : The Interactive class is intended to be used by applications where the end-user is requesting data on line and thus is expecting data to arrive quite quickly, e.g. web-browsing. It is possible to differentiate between users in the Interactive class, 3 levels of priority are available.
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Quality of Service Streaming : The Streaming class is intended to be used by applications where the end-user is requesting real time data such as video or audio clips. Introducing the support of the QoS traffic class Streaming makes it possible to fulfill Guaranteed Bit Rate (GBR) requirements for Streaming applications also in cells with high traffic load. Within traffic class Streaming there are two different priorities, Media Streaming and EIT Streaming. Media Streaming has the highest priority and is used by audio-video applications with high requirements on bandwidth. EIT Streaming is used for delay sensitive application that does not require that high bandwidth, for example applications like Push to Talk.
If Conversational is requested, it will be treated as Interactive
Ericsson BSS also supports Quality of Service for MSs and SGSNs supporting only 3GPP R97/R98 by mapping the R97/R98 specific QoS attributes onto the classes specified in R99 version of QoS.104
PDCH Allocation The total bandwidth on the channel has to be shared among all GPRS/EGPRS MSs using that channel. If a GPRS/EGPRS MS requests a channel in a cell where no GPRS/EGPRS channels can be allocated, e.g. when all channels in a cell are used as TCHs. In this case, the GPRS/EGPRS user will not get any resources, and the GPRS/EGPRS MS will find itself blocked from the system until the congestion decreases. When channels are requested due to signalling (cell update, mobility management) as few PDCHs as possible are allocated. When channels are requested for data (user data, paging response) as many PDCHs as possible according to the multi slot class and the requested coding scheme of the MS are allocated The PDCHs can be either: CS-1 to CS-2 (GPRS) capable, B-PDCH CS-1 to CS-4 (GPRS) capable, G-PDCH CS-1 to CS-4 (GPRS) and MCS-1 to MCS-9 (EGPRS) capable, E-PDCH
PDCHs can be allocated in underlaid as well as in overlaid subcells, allowing GPRS/EGPRS in underlaid and overlaid subcells105