EXP Computer S200 manual Limiting Bridge Frame Sizes, Overview

		Configuring the PathBuilder S200 Series Switch for Bridging Operation
	Limiting Bridge Frame Sizes
Overview
		Although there are valid reasons for using larger frame sizes on bridges, there are
		limiting factors that must be considered when selecting a maximum frame size.
		There are several reasons for limiting the maximum size of the frame, especially
		where bridging is done remotely across a WAN:
		• The larger the frame, the longer it takes to transmit the frame on a WAN link.
		• Increasing the frame size also causes a reduction in frame overhead.
		Conversely, the smaller the frame, the less time it takes to transmit the frame. Since a
		smaller frame cannot be forwarded by an intermediary node until it is fully received,
		a large frame cannot be forwarded for the time it takes to transmit and receive it over
		a given link. On a LAN, this time is less of an issue where the link speed is
		approximately 10 Mbps. On a WAN link, it becomes an issue because the
		transmission times for large frames become significant.

Example of Frame Figure 17 shows the effect on transit delay across a network for two cases: in one, an

Sizesend system sends a large packet as a single frame and in the other, the same large packet is sent as three smaller packets.

End

System

A

Large Packet

Total Transit

Time

End

System

A

Small Packet

Total Transit

Time

Node 1

Node 1

Node 2

Node 2

Node 3

Node 3

End

System

B

End

System

B

Figure 17. How Packet Size Affects Transit Delay

Small packets are forwarded more quickly by intermediate nodes resulting in the end system receiving several short frames in less time than a long frame. How much improvement is achieved depends on the transmission times and line speeds involved. The trade-off in this case is that even though the transit delay is reduced, the packet-per-second load is increased on all three nodes (and two end systems) involved. In this case, the factor is at least three if continuous streams of packets are involved.

Bridging 35

T0008-16F Release 5.2M