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PCU houses L2 functions (RLC/MAC/Scheduling) [23.060:12.8].
Network architecture:
23.002, Section 5, Fig 1b
23.060, Section 5.4, Diagram 2/2a
GPRS Protocol layers:
23.060, Section 5.6.1, Diagrams 4/4a has GPRS User plane protocol layers.
23.060, Section 5.6.3.1. Diagram 7 has GPRS Control plane protocol layers.
24.007, Section 5.2, Diagrams 5.3/5.6 gives inside view GSM/GPRS Protocol layers for MS.
PDCH (Packet Data Channel) uses 52-multiframe structure.
In one multi-frame – 52 bursts or slots, 12 radio blocks - each 4 bursts, 2 PTCCH slots, and 2 idle frames (slots) [45.001:5.3].
GPRS presence is indicated by "System Information 13" (sent over GSM BCCH).
It will have either GPRS specific information or pointer to PBCCH (GPRS BCCH) [44.018:10.5.2.37b].
PBCCH, if present, will broadcast Packet System Information (types).
Packet Data Channels (logical) [43.064:4]
Packet Broadcast Control Channel (PBCCH) DL
Packet Common Control Channel (PCCCH)
Packet Random Access Channel (PRACH) UL
Packet Paging Channel (PPCH) DL
Packet Access Grant Channel (PAGCH) DL
Packet Timing advance Control Channel, uplink (PTCCH/U)
Packet Timing advance Control Channel, downlink (PTCCH/D)
Packet Data Traffic Channel (PDTCH) DL/UL
Packet Associated Control Channel (PACCH) DL/UL
Mapping above logical channels to PDCH (radio blocks)
43.064, Section 5 – provides overall description
45.002, Section 6, along with Section 7 – provides details
GPRS scheduler has a concept of TBF, Temporary Block Flow, to achieve data transfer.
Data implies LLC PDUs containing NAS signaling or User packet data.
Idea is UL/DL allocations continue till TBF is active.
TBF is identified by TFI.
MS multiplexing is achieved via another identifier TLLI.
TFI is at RLC/MAC level whereas TLLI is at LLC level (so known till SGSN).
Refer 44.060, Section 5.2 to know about TBF, TFI, and USF, along with below sections:
7.1 TBF establishment initiated by the mobile station on PCCCH
7.2 TBF establishment initiated by the network on PCCCH
RLC/MAC control blocks play a role of signaling layer in DL/UL allocations.
Gist of allocation procedure (assuming MS is GPRS attached, a PDP Context is active, and MS is in packet idle mode) [44.060:5,7,10].
DL
MS keeps reading RLC/MAC control blocks
If it gets a RLC/MAC control block with a certain message providing information about DL assignment, it uses the allocation to receive the data.
UL
MS keeps reading “RLC/MAC control blocks” sent in Radio blocks in PDCH bursts (which are not marked for PPCH or PBCCH).
RLC/MAC control blocks contain a field, USF.
If MS finds a control block with USF=7(free), MS sends a certain message in next UL RLC/MAC control block requesting UL allocation.
Remember 3 slot difference in UL versus DL.
MS again keeps reading RLC/MAC control blocks.
If it gets a RLC/MAC control block with a certain message providing information about UL assignment, it uses the allocation send the data.
Allocation procedure in bit more detail:
DL
MS will continue to receive RLC/MAC Control blocks, looking for PACKET DL ASSIGNMENT message with its TLLI (PAGCH reception).
PACKET DL ASSIGNMENT will contain allocation i.e. TFI, ARFCN, and Time slot#.
Now MS will look for RLC/MAC data blocks with above TFI in a specified carrier and time slot.
UL
LLC would like to send PDUs (MS side).
MS looks for RLC/MAC Control block with USF=7.
If USF=7 is received, next radio block in UL is free.
MS sends Packet Channel Request.
Packet Channel Request could be for One-phase, Two-phase, or Short Access.
MS may request one phase/short access, Network may decide to go for Two-phase.
One phase/Short access:
Network sends Packet Uplink Assignment with Dynamic allocation.
Dynamic allocation carry USF#. MS wait for RLC/MAC block with the given USF#.
Once USF# is seen, MS uses next UL Radio Block to transmit LLC PDU in RLC/MAC Data block with TLLI (for contention resolution).
This access is always in RLC Ack mode.
Network sends Packet UL Ack with TLLI (for contention resolution at MS).
Two phase access:
Network sends Packet Uplink Assignment with Single Block allocation.
MS uses above Single Block allocation to send Packet Resource Request.
This contain TLLI (for contention resolution) and MS's multi-slot (capability) class.
Network sends Packet Uplink Assignment with Dynamic allocation and TLLI (for contention resolution at MS).
Dynamic allocation carry USF#. MS wait for RLC/MAC block with the given USF#.
Once USF# is seen, MS uses next UL Radio Block to transmit LLC PDU in RLC/MAC Data block (without TLLI).
Note that DL/UL allocations procedures depend on GMM states.
Please below sections from 44.060 for more details:
Section 7.1.2: Single phase access
Section 7.1.3: Two phase access
Section 11.2: RLC/MAC Control messages
Table 11.2.1 contains the list of these messages and their section numbers
Section 8.1: MAC procedure for transfer of RLC data blocks
Section 9.3: Operation during RLC data block transfer
Flags (in RLC/MAC control block) to explore further: Payload type, USF, USF granularity, RRBP, CV, TFI, FBI
GMM states are like RR states in LTE and 5G NR.
GMM_READY state indicates that TLLI or LLC Connection is alive.
DL PDU transmission can simply be done by PACKET DL ASSIGNMENT message by RLC/MAC level.
GMM_STANDBY indicates that LLC Connection is *not* present.
In such case, DL PDU transmission will require Paging.
Spec 23.060, Section 6.1.1 gives more and clean details on GMM states.
There are two aspects of Timing advance - Initial (UL synchronisation) and Updates in connected mode.
GSM
44.004, Section 6.1, 7.1.1
45.010, Section 5.4, 5.6.1, 6.5.1
GPRS
Initial TA (for UL TBF and DL TBF), Polling
Continuous Timing Advance Update procedure
AB in PTCCH/U (one burst on allocated subchannel, 16 subchannels)
TA update over PTCCH/D (4 bursts, 2 in 52 multi-frame)
TA update contains TA for all subchannels
Specification references:
"Top level" - 43.064, Section 6.5.7
45.010, Section 5.4, 5.6.2, 6.5.2
43.064, Section 5.4
45.002, Section 6.3.2.2.2, 6.3.2.3.2, 7/Table 6
44.060, Section 7.1.2.5, 7.1.3.5, 7.2.1.1
44.004, Section 7.8
ARQ (Automatic Repeat Request) is part of RLC (unlike in LTE/5G NR, where it is part of MAC/L1).
RLC Acknowledged mode indicates ARQ.
ARQ (type 1) is retransmission of erroneous RLC data blocks.
ARQ mechanism is based on sliding window method.
RLC mode is specific to TBF (cannot be changed during TBF xfer).
One phase UL access is always in RLC Ack mode.
RLC_MODE bit in PACKET DOWNLINK ASSIGNMENT and PACKET UPLINK ASSIGNMENT.
There is no RLC_MODE bit in PACKET RESOURCE REQUEST as mentioned by 44.060 section 9.3.0.
PACKET UL ACK/NACK goes from Network to MS.
PACKET DL ACK/NACK goes from MS to Network.
ACK/NACK has bitmap.
Section references from 44.060:
Section 7.1 (look for "RLC mode" and "RLC_MODE")
Section 9.1
Section 9.1.2
Section 9.1.3
Section 9.1.4.1
Section 9.1.9.1
Section 9.3.2.0, 9.3.2.2, 9.3.2.3
Section 11.2.6, 11.2.28, 12.3
Section 11.2.7
ARQ article
Power Control for GPRS:
43.064, Section 6.5.8
45.008, Section 10.2, Annex B
EGPRS (Enhanced GPRS) is part of EDGE, "Enhanced Data rates for GSM Evolution".
Top level description: Overview of 3GPP Release 99, Section 4.4.1.
Main features:
GSMK + 8-PSK
MCS (Modulation and Coding Scheme)
Incremental Redundancy (ARQ Type II)
Impacts/Additions:
Channel (PDTCH) coding (MCS), Training sequences
Modulation
Blind detection of modulation (at receiver)
IR processing
MCS families (Link adaptation)
Changes to TBF establishment
Changes in RLC/MAC data block headers
Changes to ARQ procedures (e.g. Window size)
Link to EGPRS course content (course supposedly conducted by Gunnar Heine earlier) is here (it gives list of points that can be explored to learn about EGPRS in detail).
How do cell advertise its EGPRS capability ?
SI13 > GPRS Cell options > EGPRS_PACKET_CHANNEL_REQUEST bit
Specification references:
44.018:3.5.2.1.2 Initiation of the packet access procedure: channel request
44.060:12.24 GPRS Cell Options
New message - EGPRS Packet Channel Request [44.060:11.2.5a]
It could be sent on RACH or PRACH.
New training sequences to indicate 8 PSK capability.
8 PSK is mandatory for MS, so 8 PSK capability in DL only or DL/UL both.
Determining MCS (for EGPRS):
1) Modulation detection with rotated training sequence
1a) Effectively same training sequence as GSM, compare 45.002 "Table 5.2.3a TSC Set 1 - GMSK" with "Table 5.2.3f TSC Set 1 – 8PSK".
0 is replaced with 111 and 1 is replaced 001 in GSM training sequence.
This will map to +1 and -1 respectively on constellation diagram [45.004, Figure 2].
1b) 8PSK symbols are rotated by 3*π/8 [45.004:3.4 Symbol rotation]
2) Coding scheme detection
2a) Stealing bits [45.003, 5.1 subsections]
MCS 0 - 4: q(0),q(1),...,q(7) = 0,0,0,1,0,1,1,0 (same as CS-4)
MCS 5 - 6: q(0),q(1),...,q(7) = 0,0,0,0,0,0,0,0 identifies the coding scheme MCS-5 or MCS-6.
MCS 7 - 9: q(0),q(1),...,q(7) = 1,1,1,0,0,1,1,1 identifies the coding scheme MCS-7, MCS-8 or MCS-9.
2b) Header formats [44.060, 10.3a.3 subsections]
Header type 1: header for MCS-7, MCS-8 and MCS-9
Header type 2: header for MCS-6, MCS-5
Header type 3: header for MCS-4, MCS-3, MCS-2, MCS-1 and MCS-0 case
2c) CPS (Coding and Puncturing Scheme indicator) field [44.060, 10.4.8a]
Steps 1a, 1b, and 2a are done at L1 (CCU) level whereas 2b and 2c are at RLC/MAC (PCU) level.
Link Adaptation is RLC function (unlike in LTE and 5G NR).
44.060, Section 9.3.2.1,
"According to the link quality, an initial Modulation and Coding Scheme (MCS) is selected for an RLC block (see note).
For the retransmissions, the same or another MCS from the same family of MCSs may be selected. E.g. if MCS-7 is
selected for the first transmission of an RLC block, any MCS of the family B may be used for the retransmissions."
MCS family
43.064, section 6.5.5.1.2
44.060, section 8.1.1
ARQ Type II mandatory for MS and optional for BTS.
No signaling needed for DL for ARQ.
RESEGMENT bit for implicit signaling in UL for ARQ Type I or II [44.060:9.3.2.1].
Measurement and Feedback (BEP) - to do
MCS and Link adaptation for UL:
MCS is indicated in Packet UL Assignment parameter called "EGPRS Coding Command".
Link adaptation is managed with MCS Family concept (ARQ I) and ARQ II
44.060:8.1.1 Uplink RLC data block transfer,
"A RESEGMENT bit is included within each PACKET UPLINK ACK/NACK, PACKET UPLINK ASSIGNMENT …
.. For initial transmissions of new RLC blocks the channel coding commanded is applied. The RESEGMENT
bit is used to set the ARQ mode to type I or type II (incremental redundancy) for uplink TBFs when
retransmissions are needed. For retransmissions, setting the RESEGMENT bit to '1' (type I ARQ)
requires the mobile station to use an MCS within the same family as the initial transmission and
the payload may be split (refer to table 8.1.1.1). For retransmissions, setting the RESEGMENT bit to
'0' (type II ARQ) requires the mobile station to use an MCS within the same family as the initial
transmission without splitting the payload even if the network has commanded it to use MCS-1,
MCS-2 or MCS-3 for subsequent RLC blocks (refer to table 8.1.1.2), see note."
DL Measurements and its reporting (by MS):
NETWORK_CONTROL_ORDER in SI
During Packet Transfer Mode
ES/P bit in DL RLC Block
Channel Quality report in PACKET_DL_ACK/NACK
MEAN_BEP => average for 4 bursts’ BEP
BEP – Bit Error Probability
44.060
Section 5.6 Measurement reports
Section 8.1.2.2 Polling for Packet Downlink Ack/Nack
Section 8.3 Procedure for measurement report sending in Packet Transfer mode
Section 8.5 Measurement Order procedures in Packet Transfer mode
Section 9.1.8.2.1 Extended Polling
Section 11.2.6 Packet Downlink Ack/Nack
Section 12.5.3 EGPRS BEP Link Quality Measurements IE
Section 12.5.4 EGPRS Timeslot Link Quality Measurements
Section 12.5.1 EGPRS Channel Quality Report
45.008
Section 8.2.3.2 MEAN_ BEP and CV_BEP
Section 10.1.4 Network controlled Cell re-selection
TBF establishment procedures for EGPRS remains similar to GPRS with changes to:
EGPRS packet channel request (covered earlier)
EGPRS coding command (e.g. Packet UL Assignment, Immediate Assignment), check in 44.060
EGPRS window size (64 till 1024, multiples of 64) [44.060:12.5.2]
Compressed bitmap (UL ACK and EGPRS DL ACK) [44.060:9.1.10]
Extended polling (Partial bitmap for DL TBF) [44.060:9.1.8.2.1]
GSM/GPRS Rel 4 Specifications
GSM RAN refresh notes
Back to GPRS Index
Copyright © Samir Amberkar 2024
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