Information Technology Reference
In-Depth Information
Congestion Management:
All of the QoS queuing strategies fall under this um-
brella, which are typically the primary tools you will use to implement QoS network-
wide. The queuing strategies define the rules the router should apply when
congestion occurs. For example, if a T1 WAN interface was completely saturated
with traffic, the router would begin holding packets in memory (queuing) to send
them when bandwidth is available. All the queuing strategies aim to answer that one
golden question: When there is bandwidth available, what packet goes first?
■
Congestion Avoidance:
Most QoS mechanisms engage only when congestion oc-
curs on the network. The aim of congestion avoidance tools are to drop enough pack-
ets of non-essential (or not-as-essential) traffic to the network to avoid heavy
congestion occurring in the first place.
■
Policing and Shaping:
You can think of policing as one of the few “anti-QoS”
mechanisms available (yes, you could argue this, but please don't). Rather than guar-
anteeing a certain amount of bandwidth, policing limits the amount of bandwidth
certain network traffic can use. This is useful for many of the typical “bandwidth
hogs” on the network: peer-to-peer applications, web surfing, FTP, and so on. You
can also use shaping to limit the amount of bandwidth certain network traffic can
use. It is designed for networks where the actual speed allowed is slower than the
physical speed of the interface. The difference between the two mechanisms is shap-
ing queues excess traffic (and tries to send it later), whereas policing typically drops
excess traffic.
■
Link Efficiency:
As the name implies, this final group of tools focus on delivering
the traffic in the most efficient way. For example, some low-speed links might work
better if you take the time to compress your network traffic before it is sent (com-
pression is one of the link efficiency tools).
■
Understanding QoS completely is a fairly massive undertaking, which is why Cisco has
dedicated an entire certification exam to the topic in the CCNP Voice track. At the CCNA
Voice level, Cisco has chosen to highlight two of the key QoS categories: link efficiency
mechanisms and queuing algorithms.
Link Efficiency Mechanisms
As network technology progresses and spreads around the world, links slower than T1
speed (1.544Mbps) are becoming increasingly rare. However, there are still many of these
slower (aka “speed challenged”) links in existence. There are typically two challenges fac-
ing these connections:
Lack of bandwidth makes it difficult to send the amount of data required in a timely
fashion.
■
Slower link speeds can have a significant impact on end-to-end delay due to the seri-
alization process (the amount of time it takes the router to put the packet from its
memory buffers onto the wire). On these slow links, the larger the packet, the longer
the serialization delay. For example, sending a 1500-byte packet on a 56Kbps link
adds 214ms just in serialization delay.
■
