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298 Chapter 10: Managing Network Performance with Queuing and Compression
FIFO
FIFO is the most basic of strategies. In essence, it is the first-come, first-served approach to data
forwarding. In FIFO, packets are transmitted in the order in which they are received.
Until recently, FIFO was the default queuing strategy for all interfaces on a router. However,
should it become necessary for the traffic to be reordered in any way, another strategy must be
invoked because FIFO gives no regard to one type of traffic over another. It simply dispatches
data as it receives it. Figure 10-3 illustrates the FIFO methodology.
Figure 10-3
FIFO
FIFO is not really queuing; it is more along the lines of buffering. The packets are routed to the
interface and stored in router memory until transmittal. The transmission order is based on the
arrival order of the first bit of the packet. Essentially, the packet's outbound buffer is selected
as soon as its outbound interface is selected.
Weighted Fair Queuing
As mentioned, FIFO is often not ideal. Fortunately, WFQ enables Telnet and other interactive
traffic to have priority over FTP and other large transfers, thus improving overall throughput.
The FTP packets get through with relatively little delay, and Telnet users see improved response
times.
In WFQ, traffic is sorted by high- and low-volume conversations. The traffic in a session is kept
within one conversation (session), and the records are handled FIFO within a particular
conversation. The lower volume interactive traffic is given a priority and flows first. The
necessary bandwidth is allocated to the interactive traffic, and the high-volume conversations
equally share whatever bandwidth is left over.
WFQ is the default on interfaces of less than 2 MB because at higher speeds, queuing is usually
not necessary. In addition, WFQ is on by default for interfaces that support it. That being said,
you should note that WFQ is not used by default on Link Access Procedure on the B channel
(LAPB) for X.25, compressed Point-to-Point Protocol (PPP), or Synchronous Data Link
Control (SDLC) interfaces.
Higher volume traffic
Low-volume traffic