Specifications

Glossary 915
Crossover cable*
A special cable used for networking two computers without the use of a hub.
Crossover cables may also be required for connecting a cable or DSL modem
to a wireless gateway or access point. Instead of the signals transferring in
parallel paths from one set of plugs to another, the signals “crossover.” If an
eight-wire cable was being used, for instance, the signal would start on pin
one at one end of the cable and end up on pin eight at the other end. They
“cross-over” from one side to the other.
CSMA-CA*
CSMA/CA is the principle medium access method employed by IEEE 802.11
Wireless LANs. It is a “listen before talk” method of minimizing (but not
eliminating) collisions caused by simultaneous transmission by multiple
radios. IEEE 802.11 states collision avoidance method rather than collision
detection must be used, because the standard employs half duplex radios—
radios capable of transmission or reception—but not both simultaneously.
Unlike conventional wired Ethernet nodes, a Wireless LAN station cannot
detect a collision while transmitting. If a collision occurs, the transmitting
station will not receive an ACKnowledge packet from the intended receive
station. For this reason, ACK packets have a higher priority than all other
network traffic. After completion of a data transmission, the receive station
will begin transmission of the ACK packet before any other node can begin
transmitting a new data packet. All other stations must wait a longer pseudo
randomized period of time before transmitting. If an ACK packet is not
received, the transmitting station will wait for a subsequent opportunity to
retry transmission.
CSMA/CD*
A method of managing traffic and reducing noise on an Ethernet network. A
network device transmits data after detecting that a channel is available.
However, if two devices transmit data simultaneously, the sending devices
detect a collision and retransmit after a random time delay.
DC power module*
Modules that convert AC power to DC. Depending on manufacturer and
product, these modules can range from typical “wall wart” transformers that
plug into a wall socket and provide DC power via a tiny plug to larger,
enterprise-level Power Over Ethernet systems that inject DC power into the
Ethernet cables connecting access points.