RLC Entities:
Transparent Mode (TM) RLC Entity
As the name indicates, the TM RLC entity is transparent to the PDUs that pass through it –
no functions are performed and no RLC overhead is added. Since no overhead is added, an
RLC SDU is directly mapped to an RLC PDU and vice versa. Therefore, the use of TM RLC
is very restricted. Only RRC messages which do not need RLC configuration can utilize the
TM RLC, such as broadcast System Information (SI) messages, paging messages, and RRC
messages which are sent when no Signalling Radio Bearers (SRBs) other than SRB0 are available. TM RLC is not used for user plane data transmission in LTE.
TM RLC provides a unidirectional data transfer service – in other words, a single TM
RLC entity is configured either as a transmitting TM RLC entity or as a receiving TM RLC
entity.
Unacknowledged Mode (UM) RLC Entity
UM RLC provides a unidirectional data transfer service like TM RLC. UM RLC is
mainly utilized by delay-sensitive and error-tolerant real-time applications, especially VoIP,
and other delay-sensitive streaming services. Point-to-multipoint services such as MBMS
(Multimedia Broadcast/Multicast Service) also use UM RLC – since no feedback path is
available in the case of point-to-multipoint services, AM RLC cannot be utilized by these
services.
The main functions of UM RLC can be summarized as follows:
• Segmentation and concatenation of RLC SDUs;
• Reordering of RLC PDUs;
• Duplicate detection of RLC PDUs;
• Reassembly of RLC SDUs.
Segmentation and concatenation. The transmitting UM RLC entity performs segmentation
and/or concatenation on RLC SDUs received from upper layers, to form RLC PDUs. The
size of the RLC PDU at each transmission opportunity is decided and notified by the MAC
layer depending on the radio channel conditions and the available transmission resources;
therefore, the size of each transmitted RLC PDU can be different.
The transmitting UM RLC entity includes RLC SDUs into an RLC PDU in the order in
which they arrive at the UM RLC entity. Therefore, a single RLC PDU can contain RLC
SDUs or segments of RLC SDUs according to the following pattern:
(zero or one) SDU segment + (zero or more) SDUs + (zero or one) SDU segment.
Reordering, duplicate detection, and reassembly. When the receiving UM RLC entity
receives RLC PDUs, it first reorders them if they are received out of sequence. Out-ofsequence
reception is unavoidable due to the fact that the HARQ operation in the MAC layer
uses multiple HARQ processes (see Section 4.4). Any RLC PDUs received out of sequence
are stored in the reception buffer until all the previous RLC PDUs are received and delivered
to the upper layer.
During the reordering process, any duplicate RLC PDUs received are detected by checking
the SNs and discarded. This ensures that the upper layer receives upper layer PDUs only
once. The most common cause of receiving duplicates is HARQ ACKs for MAC PDUs being
misinterpreted as NACKs, resulting in unnecessary retransmissions of theMAC PDUs, which
causes duplication in the RLC layer.
Acknowledged Mode (AM) RLC Entity
In contrast to the other RLC transmission modes, AM RLC provides a bidirectional data
transfer service. Therefore, a single AM RLC entity is configured with the ability both to
transmit and to receive – we refer to the corresponding parts of the AM RLC entity as the
transmitting side and the receiving side respectively.
The most important feature of AM RLC is ‘retransmission’. An ARQ operation is
performed to support error-free transmission. Since transmission errors are corrected by
retransmissions, AM RLC is mainly utilized by error-sensitive and delay-tolerant non-realtime
applications. Examples of such applications include most of the interactive/background
type services, such as web browsing and file downloading. Streaming-type services also
frequently use AM RLC if the delay requirement is not too stringent. In the control
plane, RRC messages typically utilize the AM RLC in order to take advantage of RLC
acknowledgements and retransmissions to ensure reliability.
Although the AM RLC block diagram looks complicated at first glance, the transmitting
and receiving sides are similar to the UM RLC transmitting and receiving entities
respectively, except for the retransmission-related blocks. Therefore, most of the UM RLC
behaviour described in the previous section applies to AM RLC in the same manner. The
transmitting side of the AM RLC entity performs segmentation and/or concatenation of
RLC SDUs received from upper layers to form RLC PDUs together with relevant AM RLC
headers, and the receiving side of the AM RLC entity reassembles RLC SDUs from the
received RLC PDUs after HARQ reordering.
