Junos® OS For Dummies®, 2nd Edition

Knowing What Pieces a COS Configuration Requires

The basic idea behind CoS is that you examine traffic entering your network to determine what type of traffic it is. Once you know the type of traffic (voice traffic, data traffic, traffic tied to a particular customer, and so on), you can mark that traffic at the packet layer accordingly. As those packets flow through your network, each router can then identify the traffic and make decisions on how to handle it based on its type. In this manner, all of your delay-sensitive traffic can be forwarded faster, or your critical traffic may be less likely to be dropped in times of congestion.

CoS is how you control jitter and delay in your network. In this chapter, we explain exactly what jitter and delay are, beginning with the following definitions:

• Jitter is the variation in delay over time. The primary contributor to jitter is the variability of queuing/scheduling delay over time.
• Propagation delay is the time it takes for signals to traverse a link — basically the speed of light.
• Switching delay is the time difference between receiving a packet on an incoming interface and the queuing of the packet in the scheduler of its outbound interface.
• Serialization delay is the time taken to clock a packet onto a link.
• Scheduling/queuing delay is the time difference between enqueueing the packet of the outbound interface scheduler and the start of clocking the packet onto the outbound link.

Figure 15-1 shows the different components that ...