company not have the re- and either Gigabit Ethernet or 100base-TX/FX links sources to implement and maintain network manage- connect the access layer. Redundancy is important at this ment, outsource companies are springing up to assist layer, but not as important as in the core. with this need. The access layer connects servers and workstations. Switches at this layer are smaller, usually 24-48 ports. Common issues Desktop computers and workstations are usually connected at 10 Mbps (or 100 Mbps) and servers are connected at 100 Mbps (or 1 Gbps). Limited redundancy Common network designs that can severely impact net- is used. Some QoS and security features can be imple- work performance when using VoIP include the following: mented here. Using a flat, non-hierarchical network: This can re- For VoIP to work well, WAN links should be properly sults in bottlenecks as all traffic must flow across sized with sufficient bandwidth for voice and data the uplinks (at maximum 1 Gbps) versus traversing traffic. Each voice call uses between 6.3 kbps and switch fabric (up to 256 Gbps). The greater the 80 kbps, depending on the desired codec, quality, and number of small switches (layers), the greater the header compression used. Interoffice bandwidth de- number of uplinks and the lower the bandwidth for mands can be sized using traditional phone metrics such an individual connection. Under a network of this as average call volume, peak volume, and average call type, voice performance can quickly degrade to an length. unacceptable level. Quality of Service also becomes increasingly impor- Multiple subnets on a VLAN: A network of this type tant with WAN circuits. In this case, QoS can be taken to can have issues with broadcasts, multicasts, and mean classification and prioritization of voice traffic. routing protocol updates. It can greatly impact voice Voice traffic should be given absolute priority through performance and complicate troubleshooting issues. the WAN, and if links are not properly sized or queuing Hub-based netw