User's Manual

4/1/05 Glossary
OL-7426-02
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 plug to another, the signals cross over.
For instance, in an eight-wire crossover cable, the signal starts on pin one at one end of the cable and
ends up on pin eight at the other end. Similarly, the other wires cross over from pin two to pin seven,
pin three to pin six, and pin four to pin five.
CSMA/CA
CSMA/CA is the principle medium access method employed by IEEE 802.11 WLANs. 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 WLAN 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 Supply
A module that converts 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 (PoE) systems that inject DC power into the
Ethernet cables connecting access points.
DES
Data Encryption Standard. A cryptographic algorithm used to protect data transmitted through an
unsecured network.
DHCP
A utility that enables a server to dynamically assign IP Addresses from a predefined list and limit their
time of use so that they can be reassigned. Without DHCP, an IT Manager would have to manually
enter in all the IP Addresses of all the computers on the network. When DHCP is used, it automatically
assigns an IP Address to each computing device as it logs onto the network.
Dialup
A communication connection via the standard telephone network, or Plain Old Telephone Service
(POTS).
Digital Certificate
An electronic message used to verify a client’s identity, and which can be used to encrypt data. Used in
asymmetric public/private key encryption, in which public-key encrypted data can only be decrypted
with the private key, and vice versa.