According to ETSI specification 4.60 (EN 301 349 V6.3.0)

Packet DL Ack/Nack is sent on the PACCH from the mobile station to the network to indicate the status of downlink RLC data block received and to report the channel quality of the downlink. The mobile station may optionally initiate an uplink TBF of request a temporary suspension of the downlink TBF.

TBF = Temporary Block Flow
PACCH = Packet Associated Control Channel; bi-directional, associated with a TBF
RLC = Radio Link Control
uplink = Mobile Station - > Base Station
downlink = Base Station - > Mobile Station

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