Network design problems

Common network design problems can severely impact network performance and VoIP quality:

• A flat, non-hierarchical network (cascading small workgroup switches together) results in bottlenecks, as all traffic must flow across the uplinks (at maximum 1Gbps) versus traversing switch fabric (up to 140 Gbps). The greater the number of small switches (layers), the greater the number of uplinks, and the lower the bandwidth for an individual connection. Under a network of this type, voice performance quickly degrades to an unacceptable level.
• Multiple subnets on a VLAN will generate issues with broadcasts, multicasts, and routing protocol updates. It should be avoided. It will greatly impact voice performance and complicate troubleshooting issues.
• Hub-based networks create some challenges for administrators. It is advisable not to link more than four 10baseT hubs or two 100baseT hubs together. Also, the collision domain, the number of ports connected by hubs without a switch or router in between, should be kept as low as possible. Finally, the effective (half-duplex) bandwidth available on a shared collision domain is approximately 35% of the total bandwidth available. In general, we recommend you use switches rather than hubs; this will create a more resilient environment for voice traffic.
• Too many access lists slow down a router. While they are appropriate for voice networks, care must be taken not to apply them to unnecessary interfaces. Traffic should be modeled beforehand, and access lists applied only to the appropriate interface in the appropriate direction, not all interfaces in all directions.
• Network Address Translation (NAT) does not coexist well with VoIP due to limitations in the H.323 VoIP standard. VoIP conversations rarely work across NAT boundaries. It is important to route voice streams around routers or firewalls running NAT or use a H.323 friendly NAT.
• Virtual Private Networks (VPN): VPNs present interesting challenges to VoIP implementations. First, the encryption used with VPNs adds significant latency to voice streams, adversely affecting the user experience. Second, VPNs generally run over the Internet. Because there is no control over QoS parameters for traffic crossing the Internet, voice quality may suffer due to excessive packet loss, delay, and jitter.