Flexible RLC PDU Size (Release 7)

Flexible RLC PDU Size was introduced in Release 7 in order to decrease RLC protocol overhead and to avoid sequence number stalling.
Flexible RLC is used with MAC-ehs in Release 7. Mac-ehs is a new feature introduced in Release 7 to increase the downlink throughput.

RLC PDU Structure before Flexible RLC Size

In previous 3GPP releases RLC PDU size was semi static, e.g. 336 bits or 656 bits. An IP packet size is 1500 octets so many RLC PDUs are required to transmit a single IP packet. The detail description about this problem is described in the following examples.

Example # 1

Assumption:

• RLC PDU size is 336 bits
• Ethernet IP Packet Size: 1500 octets
• 1 RLC PDU size = 336 bits

Maximum RLC Data (RLC SDU) size = 336 – 16 = 320 bits = 40 octets
So minimum number of RLC PDUs required to transmit one Ethernet packet = 1500/40 = 37.5 ~ 38 PDUs
For 38 PDUs header added = 76 (38 * 2 (for fixed header)) + Length Indicators + Padding (If required)

Example # 2

Assumption:

• RLC PDU size is 656 bits
• Ethernet IP Packet Size: 1500 octets
• 1 RLC PDU size = 656 bits

Maximum RLC Data (RLC SDU) size = 656 – 16 = 640 bits = 80 octets
So minimum number of RLC PDUs required to transmit one Ethernet packet = 1500/80 = 18.75 ~ 19 PDUs
For 19 PDUs header added = 38 (19 * 2 (for fixed header)) + Length Indicators + Padding (If required)

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Flexible RLC PDU Size

With the implementation of “Flexible RLC PDU Size” RLC overhead can be significantly decreased.

25.322 (4.2.1.3)

If flexible RLC PDU size is configured, the downlink AMD PDU size is variable up to the maximum RLC PDU size, and the Length Indicator size is configured by upper layers. The flexible RLC PDU size can be only configured in the downlink.

25.322 (9.2.2.9)

the length of RLC SDUs is constrained to a multiple of 8 bits
if “Flexible RLC PDU size” is configured (only applicable for downlink):
the last segment of an RLC SDU shall be concatenated with the first segment of the next RLC SDU in order to fill the data field up to the Maximum RLC PDU size. The “Length Indicator” field is used to point the borders between RLC SDUs.
if “Flexible RLC PDU size” is configured, the maximum size of the data field of the RLC PDU is 1503 octets.

CR 0342

The maximum size of the data field of the RLC PDU currently equals to the maximum MTU size of 1500 Byte used for Ethernet. However, if PDCP header needs to be included, the RLC PDU is not large enough to convey both the maximum MTU and the PDCP header. Since the PDCP header can be 0, 1 or 3 Byte, it is proposed that the maximum size of the data field should be 1503 octets.

25.322 (4.2.3)

In downlink, flexible RLC PDU size is configured, RLC SDUs are segmented if the SDU is larger than the maximum RLC PDU size. Concatenation may be performed up to the maximum RLC PDU size.

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Header Extension (HE) Enhancement in Release 7

A special value for Header Extension (HE) is introduced in Release 7.
10:The RLC AM PDU contains the last of the RLC SDU so no Length Indicator is required.

• 00: The succeeding octet contains data
• 01: The succeeding octet contains a length indicator and E bit
• 10: If “Use special value of the HE field” is configured, the succeeding octet contains data and the last octet of the PDU is the last octet of an SDU. Otherwise, this coding is reserved (PDUs with this coding will be discarded by this version of the protocol).
• 11: Reserved (PDUs with this coding will be discarded by this version of the protocol).

25.322 (11.3.2)

if the last octet of the PDU is the last octet of an SDU and there is no SDU concatenation inside the PDU, and the “use of the special value of the HE field” has been configured by higher layers, set the HE field to indicate that the last octet of the PDU is the last octet of an SDU. Set HE to 10.

NOTE 1: In downlink, if “flexible RLC PDU size” is configured, the UTRAN should segment, and if possible concatenate the RLC SDUs into AMD PDUs with a size not larger than the maximum RLC PDU size.
NOTE 2: In downlink, UTRAN can initiate the Polling function by assembling a POLL SUFI when “flexible RLC PDU size” is configured. If a POLL SUFI was assembled, UTRAN should schedule and submit to lower layer a STATUS PDU containing the POLL SUFI.

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Example

Assumptions:

• Ethernet Packet Size: 1500 Octets
• Special HE is enabled.
• Flexible RLC is set in Radio Bearer Setup

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Reference

• Radio Link Control (RLC) protocol specification (Release 7):http://www.3gpp.org/ftp/Specs/archive/25_series/25.322/25322-7a0.zip
• CR 0342: Maximum RLC PDU Size:http://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_40/docs/RP-080395.zip

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