Page 193 - CISSO_Prep_ Guide

that are responsible for the routing of high-speed internet
communications. The router builds a routing table using routing
protocols such as Border Gateway Protocol (the first routers
were called gateways) and learns how best to direct traffic
towards its destination. In many ways, a router is like a traffic
circle used to handle vehicular traffic. Traffic enters the circle
and then routes off on the road that is best towards the ultimate
destination. The intelligence of the routing decision is similar to
the decision made by a driver - to take the shortest route, the
fastest route, the route with least construction delays, etc. So
also, a router may direct traffic over different routes depending
on traffic volumes and congestion.

Just like on the road, some traffic has labels that indicate it has a
priority, similar to emergency vehicles that are permitted to
bypass other traffic that is waiting to be routed. The label, like
the siren on an emergency vehicle, indicates that this packet
should be processed quickly and not be held back, waiting for
other packets to be processed. The delay in processing is called
latency, and this would severely affect the quality of some types
of traffic, such as Voice over IP (VoIP). The labeling of the
traffic allows for the provision of Quality of Service (QoS) and
Class of Service (CoS) traffic management. QoS is usually used
to provide guaranteed bandwidth for traffic volumes, and CoS is
used to grant priority to certain packets over others (for
example, voice packets over regular data packets). This also
prevents the jitter that would affect data traffic from impacting
voice communications. Jitter is the variation in the arrival time
of a packet due to different routing, latency, etc.