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Why UM mode in RLC protocol doesn't support re transmission.

sachin bajwan
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Unacknowledged mode in RLC is meant for delay sensitive applications such as VOIP. If retransmissions were allowed, it would have significant impacts on call quality. By the time the re-transmitted packet was received it would be very late and cause more harm than good. Instead, packets are continuously transmitted and while there may be some that are never received, it results in better overall quality.

A good reference on the 3 RLC modes can be found here: http://www.tweet4tutorial.com/rlc-mode/

Jeff
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RLC shall operate in various modes based on the type of application or requirement from the application. In other words, to accommodate various type of services (streaming , transfer of huge file, etc..), there are different modes in RLC such as TM, UM and AM.

The RLC UM mode uses sequence numbering of data blocks (PDU) for reordering and duplicate detection of RLC PDUs, but there is no mechanism for the retransmission to address specific application / service requirements. So, the RLC UM mode is used in scenarios where there is no requirement for error free delivery or retransmission is not expected (Application requiring lower delay/latency). Few examples will be that of VoIP ,UDP based flows or broadcast transmissions (Point to multipoint traffic such as Multimedia Broadcast/Multicast Service (MBMS)).

So, services that can sustain error rates or not expecting retransmission are mapped to RB having RLC in UM mode and services that need reliability(error sensitive and latency insensitive) like file transfer/ftp will get mapped to RB having RLC in AM mode.

Karthik Balaguru
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UM mode is configured by the RRC layer of protocol stack for the applications like VOLTE, VOIP etc. As mentioned by Jeff, retransmission may have the significant impacts on call quality. For more information about RLC_UM mode, refer the 3GPP Specification 36.322.