 samples (or Prioritization/CoS should not be confused with QoS. their encoding) are transmitted, and the received sam- It is a subset of QoS. A package-delivery service provides ples are clocked out to reproduce the analog signal. IP an analogy. You can request priority delivery for a pack- networks are basically asynchronous transport devices. age. The delivery service has different levels of priority Network infrastructure CHAPTER 3.2 A:100ns TRYYISHF PT25% AV1:6.72mV Figure 3.2-12 Sample jitter testing: What we see on the scope is not the signal that carries the jitter, but the jitter inside the signal. (next day, two-day, and SO on). However, prioritization serve as a label that provides a signal to network does not guarantee the package will get there on time. It devices as to the class of service that the frame may only mean that the delivery service handles that should receive. package before handling others. To provide guaranteed Network layer prioritization The IP packet header has delivery, various procedures, schedules, and delivery a field called ToS. This field has recently been rede- mechanisms must be in place. For example, Federal Ex- fined to work with the IETF's Differentiated Ser- press has its own fleet of planes and trucks, as well as vices (Diff-Serv) strategy. Diff-Serv classifies and a computerized package tracking system. marks packets SO that they receive a specific per-hop Prioritization has been used in multi-protocol routers forwarding at network devices along a route. The ToS to give some protocols higher priority than other pro- bit is set once, based on policy information, and tocols. For example, SNA (Systems Network Architec- then read by network devices. Because IP is an ture) traffic will time-out if it is not delivered promptly, internet-working protocol, Diff-Serv works across causing retransmissions that degrade network perfor- networks, including carrier and service provider mance. Such protocols should be given high priority. A networks that support the service. Therefore