Issue 6.0 NEC SL11006 - 14 Network Design ConsiderationsNot all network hardware supports QoS and each manufacturer has their ownmethods of implementing QoS. The explanations below are as generic as possible.The installer/maintainer of the data network should be familiar with the QoScharacteristics of their equipment and should be able to configure the equipmentaccordingly.Quality of Service is commonly used to describe the actual implementation ofprioritization on network hardware. This prioritization (at Layer 2 and Layer 3 of theOSI model) is described in Figure 6-1 Layer 2 Diagram (802.1Q) on page 6-5.6.1 PrioritizationWhen data is transmitted through a network, bottlenecks can occur causingthe available bandwidth to be reduced or the data to increase. This impactsthe packet delivery.Consider data communication between the two computers shown in thediagram Figure 6-1 Layer 2 Diagram (802.1Q). The Hosts can transmit dataat 100 Mbps. When a packet from Host A, destined for Host B, reaches therouter, the available bandwidth is reduced to 256Kbps and the packet flowmust be reduced. Figure 6-3 Network Bottleneck Example shows a diagramof this condition.For this example, each end of the network has only.one host Typically, manyhosts are sending data over the narrow bandwidth. The routers buffer packetsand transmit them over the WAN lines as efficiently as possible. When thisoccurs, certain packets are dropped by the router and some packets aredelayed.Figure 6-3 Network Bottleneck Example256KbpsPrivate Circuit(Leased Line)Data Switch RouterHost ARouter Data SwitchHost B100Mbps100Mbps100Mbps100Mbps
