A receiver (104,106,116,118,126,128,130,132,134,136,1102,1202) that is operable to receive data from a transmitter (104,106,116,118,126,128,130,132,134,136,1102,1202) across a wireless link, the receiver comprising: an antenna (1142, 1144, 1146, 1205); a radio frequency unit (1132,1134,1136,1204) communicatively coupled to the antenna (1142, 1144, 1146, 1205); and at least one digital processor (1126,1128,1130,1206) coupled to the radio frequency unit (1132,1134,1136,1204) and operable to cause the receiver (104,106,116,118,126,128,130,132,134,136,1102,1202) to: receive successfully, at a physical layer operating thereon, a first physical layer frame across the wireless link from a physical layer operating on the transmitter; receive, at a link layer operating thereon, a first packet data unit extracted from the first physical layer frame; in response to receiving the first packet data unit at the link layer, determine, based on a sequence number of the first packet data unit, that a missing packet data unit with a sequence number preceding the sequence number of the first packet data unit has not been received at the link layer; based on the determination, start a timer at the link layer; in response to expiration of the timer while the missing packet data unit has still not been received, transmit, from the link layer via the physical layer a negative acknowledgement, NAK, to the transmitter, wherein the NAK identifies the missing packet data unit by its sequence number and wherein the NAK is configured to cause a retransmission of the missing packet data unit. an antenna (1142, 1144, 1146, 1205); a radio frequency unit (1132,1134,1136,1204) communicatively coupled to the antenna (1142, 1144, 1146, 1205); and at least one digital processor (1126,1128,1130,1206) coupled to the radio frequency unit (1132,1134,1136,1204) and operable to cause the receiver (104,106,116,118,126,128,130,132,134,136,1102,1202) to: receive successfully, at a physical layer operating thereon, a first physical layer frame across the wireless link from a physical layer operating on the transmitter; receive, at a link layer operating thereon, a first packet data unit extracted from the first physical layer frame; in response to receiving the first packet data unit at the link layer, determine, based on a sequence number of the first packet data unit, that a missing packet data unit with a sequence number preceding the sequence number of the first packet data unit has not been received at the link layer; based on the determination, start a timer at the link layer; in response to expiration of the timer while the missing packet data unit has still not been received, transmit, from the link layer via the physical layer a negative acknowledgement, NAK, to the transmitter, wherein the NAK identifies the missing packet data unit by its sequence number and wherein the NAK is configured to cause a retransmission of the missing packet data unit. receive successfully, at a physical layer operating thereon, a first physical layer frame across the wireless link from a physical layer operating on the transmitter; receive, at a link layer operating thereon, a first packet data unit extracted from the first physical layer frame; in response to receiving the first packet data unit at the link layer, determine, based on a sequence number of the first packet data unit, that a missing packet data unit with a sequence number preceding the sequence number of the first packet data unit has not been received at the link layer; based on the determination, start a timer at the link layer; in response to expiration of the timer while the missing packet data unit has still not been received, transmit, from the link layer via the physical layer a negative acknowledgement, NAK, to the transmitter, wherein the NAK identifies the missing packet data unit by its sequence number and wherein the NAK is configured to cause a retransmission of the missing packet data unit.