Channel Mapping in Call Processing

This note will talk about channel mapping mainly between logical channel and physical channels in LTE, and how those mappings are applied at each stages of signaling in call processing.

Big Picture First

Depending on which level you are working on in UE development/Test procedure, the amount of knowledge you need to know would be different. But I think there are a couple of big pictures that may help almost anybody working in full protocol stack.

First big picture I would like to introduce is the channel mapping as shown below. Just try to pick any RRC messages and try to follow the arrow for the message. If you read those pages about MAC and RLC, it will remind you of a lot of detailed information.

Overall Sequence and Layer Mapping

Following is a sequence diagram showing not only the message but also basic configurations of each layer. More detailed description of each layer in the context of full protocol stack will be explained in "Full Stack" section.

Just read through this sequence whenever you have time until you can duplicate the sequence without looking into this again. This can be a good framework for your study and good guide for troubleshooting.

Step	Direction	Channel	Message
1	UE <---- NW	BCCH -> BCH, No MAC Header, RLC TM, No PDCP	MIB
2	UE <---- NW	BCCH -> DL SCH, No MAC Header, RLC TM, No PDCP	SIBs
3	UE ----> NW	PRACH -> RACH, No MAC Header, No RLC, No PDCP	PRACH Preamble
4	UE <---- NW	RACH -> DL SCH, No MAC Header, No RLC, No PDCP	RACH Response
5	UE ----> NW	UL SCH -> UL CCCH MAC Header, No RLC, No PDCP	RRC Connection Request
6	UE <---- NW	DL CCCH -> DL SCH MAC Header, No RLC, No PDCP	RRC Connection Setup
7	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : RRC Connection Setup Complete + NAS : Attach Request (ESM:PDN Connectivity Request)
8	UE <---- NW	DL DCCH -> DL SCH MAC Header, RLC AM, PDCP	RRC : dlInformationTransfer EMM : Authentication Request
9	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : ulInformationTransfer EMM : Authentication Response
10	UE <---- NW	DL DCCH -> DL SCH MAC Header, RLC AM, PDCP	RRC : dlInformationTransfer EMM : Security Mode Command
11	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : ulInformationTransfer EMM : Security Mode Complete
12	UE <---- NW	DL DCCH -> DL SCH MAC Header, RLC AM, PDCP	RRC : Security Mode Command
13	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : Security Mode Complete
14	UE <---- NW	DL DCCH -> DL SCH MAC Header, RLC AM, PDCP	RRC : RRC Connection Reconfiguration EMM : Attach Accept ESM : Activate Default EPS Bearer Context Request
15	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : RRC Connection Reconfiguration Complete
16	UE ----> NW	UL SCH -> UL DCCH MAC Header, RLC AM, PDCP	RRC : ulDirectTransfer EMM : Attach Complete ESM : Activate Default EPS Bearer Context Accept
17	UE <---- NW	DL DCCH -> DL SCH MAC Header, RLC AM, PDCP	RRC : RRC Connection Release

Downlink Channel Map

The diagram you saw above a kind of message flow(event diagram) in time sequence. The diagram shown below is not a time based, but it shows the channel mapping (or data flow across the full protocol stack). Pick one of the message from the diagram shown above and try to find right route for this digram and see how much details you can add.

For example, if you picked the message "RRC Connection Setup", the start point would be "RRC Message msg4".

Following is a tabular presentation of DL Channel Map. (LCID and TrCH Number would be different depending on the network or Network Simulator)

RB	Lo CH	PDCP	RLC	Lo CH	LCID	MAC Hdr	HARQ	RNTI	Tr CH
	PCCH		TM	PCCH	N/A	NONE	NONE	NONE	PCH
	BCCH 0		TM	BCCH 0	N/A	NONE	NONE	NONE	BCH 0
	BCCH 1		TM	BCCH 1	N/A	NONE	Broadcast	SI RNTI	DL SCH 0
	RA_RES		TM	RA_RES	N/A	NONE	NONE	RA RNTI	DL SCH 1
SRB0	DL CCCH	USED	TM	DL CCCH	0	NONE	NORMAL	T-CRNTI	DL SCH 1
SRB1	DL DCCH 0	USED	AM	DL DCCH 0	1	NORMAL	NORMAL	CRNTI	DL SCH 1
SRB2	DL DCCH 1	USED	AM	DL DCCH 0	2	NORMAL	NORMAL	CRNTI	DL SCH 1
DRB 0	DL DTCH0	USED	UM/AM	DL DTCH0	3	NORMAL	NORMAL	CRNTI	DL SCH 1
DRB 1	DL DTCH1	USED	UM/AM	DL DTCH1	4	NORMAL	NORMAL	CRNTI	DL SCH 1
DRB 2	DL DTCH2	USED	UM/AM	DL DTCH2	5	NORMAL	NORMAL	CRNTI	DL SCH 1

Uplink Channel Map

Following is a tabular presentation of DL Channel Map. (LCID and TrCH Number would be different depending on the network or Network Simulator)

RB	Lo CH	PDCP	RLC	Lo CH	LCID	MAC Hdr	HARQ	RNTI	Tr CH
	RA_PRE		TM	RA_PRE	N/A	NONE	NONE	NONE	UL SCH 0
SRB0	UL CCCH	USED	TM	UL CCCH	0	NONE	NORMAL	T-CRNTI	UL SCH 0
SRB1	UL DCCH 0	USED	AM	UL DCCH 0	1	NORMAL	NORMAL	CRNTI	UL SCH 0
SRB2	UL DCCH 1	USED	AM	UL DCCH 0	2	NORMAL	NORMAL	CRNTI	UL SCH 0
DRB 0	UL DTCH0	USED	UM/AM	UL DTCH0	3	NORMAL	NORMAL	CRNTI	UL SCH 0
DRB 1	UL DTCH1	USED	UM/AM	UL DTCH1	4	NORMAL	NORMAL	CRNTI	UL SCH 0
DRB 2	UL DTCH2	USED	UM/AM	UL DTCH2	5	NORMAL	NORMAL	CRNTI	UL SCH 0

Channel Mapping Table throughout Call Processing

This is only an example case and Mapping (especiall LoCH No) can vary depending on situations. The point is that it will be really helpful for your troubleshooting or test case creation if you create this kind of table for your case.

Message	RB	Lo CH	LoCH No	LCID
MIB	-	BCCH	0	-
SIB 1	-	BCCH	1	-
SIB 2	-	BCCH	1	-
RRC : PRACH Preamble	-	-	-	-
RRC : RACH Response	-	-	-	-
RRC : RRC Connection Request	SRB0	UL CCCH	0	0
RRC : RRC Connection Setup	SRB0	DL CCCH	0	0
RRC : RRC Connection Setup Complete + NAS : Attach Request + ESM : PDN Connectivity Request	SRB1	UL DCCH	0	1
RRC : DL Information Transfer + NAS : Authentication Request	SRB1	DL DCCH	0	1
RRC : UL Information Transfer + NAS : Authentication Response	SRB1	UL DCCH	0	1
RRC : DL Information Transfer + NAS : Security Mode Command	SRB1	DL DCCH	0	1
RRC : UL Information Transfer + NAS : Security Mode Complete	SRB1	UL DCCH	0	1
RRC : Security Mode Command	SRB1	DL DCCH	0	1
RRC : Security Mode Complete	SRB1	UL DCCH	0	1
RRC : RRC Connection Reconfiguration	SRB1	DL DCCH	0	1
RRC : RRC Connection Reconfiguration Complete	SRB1	UL DCCH	0	1
RRC : UL InformationTransfer + NAS : Attach Complete + NAS : Activate Default EPS Bearer	SRB2	UL DCCH	1	2
RRC : UL Information Transfer + ESM : PDN Connectivity Request	SRB2	UL DCCH	1	2

Note : Refer to TS 36.331 - 9.1.1 Logical channel configurations

Cell Configuration and Channel Configuration during Call Processing

Following sequence is an example of signaling messages during the initial attach procedure and internal configurations that would happen within eNB. These internal configuration is just an example and it would vary depending on eNB hardware vendor.

Config 1) Activate Cell Physicall Layer

1) MIB

Config 2) Activate PHY, MAC, RLC for SIB Transmission (BCCH-DL DSCH)

2) SIB 1

3) SIB 2

Config 3) Configure PHY, MAC for PRACH Reception and RACH Response Transmission

4) RRC : PRACH Preamble

5) RRC : RACH Response

Config 3) Configure PHY, MAC, RLC for Msg3 (RRC Connection Request) Reception (UL-CCCH)

6) RRC : RRC Connection Request

Config 4) Configure MAC, RLC, PDCH for DL DCCH, UL DCCH

7) RRC : RRC Connection Setup

8) RRC : RRC Connection Setup Complete + NAS : Attach Request + ESM : PDN Connectivity Request

9) RRC : DL Information Transfer + NAS : Authentication Request

10) RRC : UL Information Transfer + NAS : Authentication Response

11) RRC : DL Information Transfer + NAS : Security Mode Command

12) RRC : UL Information Transfer + NAS : Security Mode Complete

Config 5) Configure PDCP for Integrity, Ciphering (We may disable Integiry/Ciphering for some test environment)

13) RRC : Security Mode Command

14) RRC : Security Mode Complete

15) RRC : RRC Connection Reconfiguration + NAS : Attach Accept + NAS : Activate Default EPS Bearer Context Req

Config 6) Configure MAC, RLC, PDCP for DL/UL DTCH+DCCH

16) RRC : RRC Connection Reconfiguration Complete + NAS : Attach Complete + NAS : Activate Default EPS Bearer Context Accept

17) RRC : RRC Connection Release

Config 7) Deactivate all the channels related to DCCH, DTCH

Config 8) Activate channels for PCCH

< MO or MT call > : In MT call, Paging should be sent.

Config 9) Configure PHY, MAC for PRACH Reception and RACH Response Transmission

18) RRC : PRACH Preamble

19) RRC : RACH Response

Config 10) Configure PHY, MAC, RLC for Msg3 (RRC Connection Request) Reception (UL-CCCH)

20) RRC : RRC Connection Request

Config 11) Configure MAC, RLC, PDCH for DL DCCH, UL DCCH

21) RRC : RRC Connection Setup

22) RRC : RRC Connection Setup Complete + NAS : Service Request

23) RRC : Security Mode Command

24) RRC : Security Mode Complete

25) RRC : RRC Connection Reconfiguration + NAS : Activate Dedicated EPS Bearer Context Request

Config 12) Configure MAC, RLC, PDCP for DL/UL DTCH+DCCH

26) RRC : RRC Connection Reconfiguration Complete + NAS : Activate Dedicated EPS Bearer Context Accept

27) RRC : RRC Connection Release