Send and Wait Problem:

With the "Send and Wait" problem, the sender must wait for an acknowledgement from the intended receiver before starting to transmit another packet.   If the packet or ack is lost (no acknowledgement arrives), then the sender must wait for a timer to expire before resending.  

(a) What happens if the timer is set too short?

If the timer is set too short, the sender will resend the packet one or more times which will cause delays.   For example, a sender sends a packet and the timer expires before it receives an ack from the receiver.   The sender sends another copy of the data unit. When the sender receives the ack from the first packet, it must then detect the duplicate data units and discard them.   The sequence number of the data unit is included in the acknowledgement so that no combination of lost transmissions will lead to confusion about what data unit has arrived safely.   But this overhead will cause delays.

(b) What happens if the timer is set too long?

Delays can also occur if the timer is set too long.   When the packet or ack is lost, the sender will wait until its long timer expires before resending, thus causing delays.   There are no delays when no packets or acks are lost.