Specifications

Network Configuration Concepts
219
Each LAN connection includes both a transmit pair of cables as well as a receive pair of cables.
In a full duplex Ethernet connection, data can be sent and received at the same time.
The transmit and receive pair of connections are not shared within the network device (typically
a layer 2 switch). Thus, the local phone sends 100 kbps (G.711) on the transmit pair of cables.
It also receives a similar transmission.
As in the case of TDM, both transmit and receive cables are considered a single bundle. The
device is sending data at 100 kbps. Of course, without the receive data, it isn’t possible to hold
a conversation.
Half duplex network basics
With a half duplex Ethernet connection, a number of devices can share the same data directly.
In this case, the network device doesn’t interpret the data, it simply boosts the signal and
re-sends it.
To avoid collisions in the shared-data scenario, data that is sent by one device is repeated to
all receive pairs of all connected devices. This means that when data is sent, it cannot receive
data from another device at the same time; it must wait until the next available time. The phone
still continues to send 100 kbps (G.711) of data, but must wait to receive the returned 100 kbps.
In effect, the phone still sends the same data as a phone connected with a full duplex connection,
it simply takes twice as long to send and receive data.
Summary
• A conversation requires equal amounts of data to be transmitted and received.
• The phone always sends and receives the same amount of data via a full or half duplex link.
• Full Duplex Ethernet connection: Data can be transmitted and received at the same time.
• Half Duplex Ethernet connection: Data can only be transmitted or received at separate
times, and taking twice as long to complete.
• Half Duplex connections are a less efficient means to transmit voice. Time delay is added
and bandwidth is not conserved very well using collision avoidance mechanisms.
• It appears as though a phone connected via a half duplex link takes up more bandwidth,
but in reality it takes up more time.
Conclusion: Use full duplex Ethernet connections for maximum performance. Configure any
3300 ICP network port for auto-negotiation so that the network devices can select the best
quality settings.