Ethernet

The classical Ethernet operation mode is referred to as half-duplex (HDX): at any given time, an Ethernet adapter can only transmit or receive data. A shared media Ethernet LAN has the following characteristics:

• Only one adapter can transmit at a time
• All stations contend for the same bandwidth
• Only one station can capture the bandwidth at any given time
• If more than one station has a packet to transmit, the medium access control (MAC) protocol will resolve the conflict and allow only one station to get access while the others wait

Ethernet LAN switches (also called layer 2 switches) isolate traffic from ports that are not involved in communication (provided they do not operate in broadcast mode). They also do not cause any shared medium collisions and can be very fast if they provide wire speed frame forwarding. Because they prevent medium sharing, they provide a much higher aggregate bandwidth. An additional feature that increases the throughput is full-duplex (FDX) operation: simultaneously receiving frames on the same input/output port pair.

Shared medium Ethernet architectures

"In a shared medium architecture, any single device producing high bandwidth traffic will likely corrupt all voice traffic."

Shared medium Ethernet hubs are still being used in networks, but shared media LAN architectures should be avoided in VoIP networks. More specifically, avoid the use of bussed architectures. Layer-1 hubs and layer-2 bridges should be avoided. Shared media hubs should be replaced with Ethernet switches. Shared media architectures permit packet collisions, and the collision rate increases when voice packets have to share the same medium with long data packets, when the occupancy of the shared medium is high, and when the network has low bandwidth.

In a shared medium architecture, any single device producing high bandwidth traffic will likely corrupt all voice traffic. Voice communication on a network initially working fine may degrade dramatically after adding such a device.

Ethernet switches

Local Area Networks make common use of Ethernet switches. For quality VoIP transmission, use Ethernet switches with the following capacities:

• 802.1p standard allows stations to expedite frames
• 802.1Q standard supports VLANs, defines tagged frame types, and provides a way to maintain priorities across LANs
• Full-duplex operation to minimize delay and increase throughput by allowing simultaneous transmission and reception of packets on ingress and egress port of the switch, thus significantly increasing the throughput as well
• Uni-directional port bandwidth of 100 Mbit/s
• 802.3af phantom power supply to power telephones (for example, Cajun P333T-PWR)

Ethernet switch networks

Ethernet switch networks should have these characteristics:

• Loops should be avoided in Ethernet switch networks.

Loops can be prevented by implementing a network topology without loops or by using the Spanning Tree Algorithm for logical loop resolution.

• The number of LAN-switches between any two endpoints should be minimized to reduce delay and jitter.

The target should be fewer than four in a campus environment. In situations where this is not possible, the number of switches should be kept to an absolute minimum.

• VoIP network traffic should be separated from data network traffic by locating each on separate VLANS