a frame has been received correctly. Alternatively, enables the Data Link Layer to detect and correct corrupted frames. but less common, the receiver may only respond to a received frame The limited window size plus the numbering provides flow control. if it is not received correctly: negative acknowledgement. Should a receiver be unable to process frames as fast as is necess- At its most basic, the transmission process works as follows. A ary at any one point in the communication, it simply stops acknow- sender transmits a frame then waits for an acknowledgement from ledging frames it is receiving. The sender transmits as many frames the recipient. If the checksum is correct, the frame is regarded by as are allowed before it needs an acknowledgement, then stops and the receiver as correctly received and the receiver sends back an waits, giving the receiver a chance to catch up. acknowledgement. If the checksum is incorrect, the frame is re- A DTE or DCE can suspend transmission by issuing a disconnect garded as corrupted. The receiver discards that frame and simply command, acknowledged by the receiver through a UA response as waits. for set-up, after which the link can be closed. If the sender successfully receives an acknowledgement, it as- The LAPB protocol also contains a complex variety of commands sumes the frame has been correctly transferred and can send the and responses indicating reasons for retransmission, frame reject next. If nothing arrives within a set acknowledgement period, it responses and SO on. assumes the frame was not correctly received and re-transmits, Within the Data Link Layer mandate to ensure error free trans- waiting again for a response. If this is repeated a set number of mission, the CCITT X.25 1984 recommendation added a service times, the sender assumes the link is faulty. known as the Multilink Procedure (MLP). The function of MLP is There are, however, complications such as frame corruption, an to distribute packets from layer 3 across multiple physical links incorrectly rece