t Bandwidth. enter the burst range. Supporting voice adds even more considerations when The problem with the latter method, however, is that designing a network. These additional considerations the actual queuing mechanism is not always intuitive. include Even though the aggregate voice traffic throughput cannot exceed the CIR, it is possible that a voice packet Delay could be sent in the burst range. The technical workings Jitter of this are beyond the scope of this document. But simply stated, it is possible that a voice packet would Duplex. arrive right after many data packets have already been Generally speaking, these concerns dictate a hierarchical transmitted in the CIR range, such that the voice packet network consisting of at most three layers: core, VolP Technology CHAPTER 3.3 distribution, and access. Some smaller networks can percent of peak bandwidth is sufficient. Also, SLAs collapse the functions of several layers into one device. should be established with the carrier that define maxi- The core layer is the heart of the network. Its purpose mum levels of delay and frame loss, and remediation is to forward packets as quickly as possible. It should be should the agreed-to levels not be met. designed with high availability in mind. Generally, these Network management is another important area to high-availability features include redundant devices, re- consider when implementing VoIP. Because of the dundant power supplies, redundant processors, and re- stringent requirements imposed by VoIP, it is critical to dundant links. In the current era, core interconnections have an end-to-end view of the network and ways to increasingly use Gigabit Ethernet. implement QoS policies globally. Products such as HP The distribution layer links the access layer with the Open View Network Node Manager, Visibility, Concord core. It is here that QoS features and access lists are NetHealth, and MRTG will help administrators maintain applied. Generally, Gigabit Ethernet connects to the core acceptable service. Should a 