Internet Protocol

The Internet Protocol (IP ) treats all traffic equally. Packets may be discarded on network congestion or may be re-ordered as they transit through the network. Traffic load-balancing may distribute traffic arriving on a single link over multiple output links. Route diversity causes unpredictable packet arrival times and extra delay due the need for packet re-sequencing at the destination. Higher-level data protocols such as TCP are assumed to accommodate transmission errors, jitter, and packet resequencing. Because of these characteristics, the IP protocol is called a "best-effort" protocol and by itself cannot guarantee quality of service because the service degrades as the network load of increases.

IP routers

Follow these guidelines in implementing IP routers:

• Layer 3 IP routed switches should be used rather than pure layer 3 IP routers

Layer 3 IP routed switches can significantly reduce the forwarding delay for real-time traffic by using switching for low-latency traffic.

• Implement multiple traffic class queues
• Implement DiffServ traffic marking
• Voice and signalling traffic should be handled with the highest priority
• 802.3af phantom power supply should be provided to power telephones
• Deactivate traffic load balancing

This can cause transfer delays and increase jitter.

• Deactivate UDP packet filtering

This may prevent the forwarding of RTP voice packets.

• Packet fragmentation should be activated for non-real-time traffic on the node outputs with high link occupancy

IP router networks

Follow these guidelines in designing IP router networks:

• The number of traversed IP routers for VoIP packets between any two telephones should be minimized.

The target should be less than four in a campus environment. The delay through pure IP routers is in general very high since they store complete packets before they are forwarded.

• Layer 2 Ethernet switches and routed-switches should be preferred in VoIP networks, not pure IP routers.

Layer-3 routed-switches (for low latency traffic) directly forward frames/packets as soon as the destination address is captured, the routing table content is read, and the destined output link is free. This significantly minimizes the voice packet delay.

• DiffServ provisioning should assign voice signaling and bearer traffic the highest priority in the whole network.
• The public Internet should not be used, because in most cases it does not meet the network performance requirements for VoIP.

Dynamic Host Configuration Protocol service

Follow these guidelines for Dynamic Host Configuration Protocol (DHCP):

• Preferably, all IP telephone addresses on a LAN should be assigned by a highly reliable DHCP service
• Other processing intensive applications should not be running on the same server running the DHCP service

Running other applications on the same server as the DHCP service may severely impact the performance of the DHCP by causing time-outs.