Signalling Radio Bearer 5 (SRB5)

SRB - 5 has been introduced in order to support quick changes in Multi-mode AMR or Wideband AMR data rate

SRB - 5 exist in UL & DL with the following characteristics:

• RB Id, can be any number between (5 - 31)  - SRB#5 need not have RB - ID 5, it can be any RB ID allocated by UTRAN
• UL RLC Mode = TM - DCCH is mapped to TM RLC
• DL RLC Mode = TM - DCCH is mapped to TM RLC
• TB Size  = 3 bits - currently only 3 – bit message is defined
• TBS =  0x3, 1x3
• It is mapped to a DCH TrCH different from DCH of SRB 1 to 4.

Until now only one message has been defined in standard i.e. "Transport Format Combination Control" which is send by UTRAN on TM RLC mode.

Transport Format Combination Control (TFCC):

When this message is sent on TM RLC Mode, i.e. on SRB#5, this message has special handling and encoding:

• This message is NOT ASN.1 encoded
• This message is encoded tabular format.
• Currently only 3 – bit tabular format is defined as below:

3	2	1	Transport Format Combination Sub Set Identity value
0	0	0	0
0	0	1	1
-	-	-	-
1	1	1	7

• There is no response to this message
• If UE receives this message and the TFC Subset id is out of range or more than the subsets that UE has then UE ignores this message and behave as if no such message is received.

 Typical Scenario to use SRB#5:

1. CS call is established for Multi – Mode AMR (12.2/7.95/5.9/4.75 kbps)

2. Typical RB Setup message would include:
1. Complete TFCS list for all  data rates 12.2/7.95/5.9/4.75 kbps
2. TFC Subset for specific data rate 12.2 / 7.95 / 5.9 / 4.75 kbps (four subsets)

3. In order to change the data rate to (say 5.9 kbps), UTRAN would send  TFCC message on RLC – TM mode only indicating TFC Subset to be uses, as in our example it would be “2”, indicating third TFC subset to be used

4. UE RRC would receive this message on TM – RLC; no ASN.1 decoding is needed for this message.

5. RRC should configure it lower layers as per the given TFC Subset in the received 3 – bit message