In general, NBF has no receive window, unless it detects that the remote sending computer is running a version of IBM NetBEUI which does not use network polling—for example, IBM LAN Server. NBF initiates a link with a remote computer in the same manner as IBM NetBEUI; however, NBF looks for a poll bit in received frames. If a frame is received that does not have the poll bit set, the Windows NT-based computer will wait until T2 expires before sending a frame ACK.
Note
When the poll bit is set in a received frame, NBF ignores the receive window and immediately sends an ACK.
For example, an IBM LAN Server computer is a non-polling system that may have a send window set to 1. If this the case, the registry parameter MaxIncomingFrames should be decreased from its default of 2 to 1. If not, the non-Windows NT-based computer will wait for an ACK from the Windows NT-based computer, which in this case will be sent only when the T2 time limit expires.
NBF uses a receive window based on the value of MaximumIncomingFrames in the Registry. The default value for MaximumIncomingFrames is 2, and this value does not dynamically change. It must be manually changed by changing the parameter default value in the Registry.
Note
When a Windows NT-based computer is using the MaxIncomingFrames receive window, it may not always send an acknowledgment frame after receiving exactly MaxIncomingFrames packets. This is because NBF will also wait until it receives an NDIS ProtocolReceiveComplete before sending the ACK. However, when the Windows NT-based computer receives POLL frames, it will ACK immediately (typically on return from NdisTransferData (synchronous communications) or within ProtocolTransferDataComplete (asynchronous communications).