Business Data Communications and
Networking Ch 4 Definitions
LLC - ANS-Logical link control sublayer. It's the data link layer's connection to the network layer
above it.
Is responsible for communicationg with the network layer software and for taking the network
layer PDU and surrounding it with a data link PDU. At the receiving computer, the sublayer
removes the data link PDU and passes the message to the network layer.
MAC sublayer - ANS-Media Access Control Sublayer/protocol that defines how packets are sent
to a LAN. It mainly controls when computers transmit through contention and controlled access.
The second sublayer of the data link layer. Also controls the physical hardware.
Contention - ANS-Computers wait until the circuit is free and then transmit whenever they have
data to send. Used in Ethernet LANs often.
Controlled access - ANS-The wireless access point controls the circuit and determines which
clients can transmit at what time. Two techniques: access requests and polling.
Access request - ANS-Client computers that want to transmit a message will send a request to
the device that is controlling the circuit.
The controlling device gives permission for one computer at a time to transmit.
Polling - ANS-The process of sending a signal to a client computer that gives it permission to
transmit. The clients store all messages that need to be transmitted.
Sometimes the controlling device polls the client to see if it has data to send and if it has data it
will send it.
Roll call polling - ANS-The controller works through a list of clients (first polling client 1, then 2
then 3 etc). Still involves asking the client if it has a message to send.
Hub polling/Token passing - ANS-One device starts the poll and passes it to the next computer
on the multipoint circuit. Then it sends its message and passes the poll to the next computer on
the circuit.
Error rate - ANS-The pattern of errors that can be expected on the circuits.
, Burst error - ANS-More than 1 data bit is changed by the error causing condition. Errors are not
uniformly distributed in time.
Line noise - ANS-Undesirable electrical signals introduced by equipment or natural distrubances
that weakens the signal performance..
Several types: white, impulse, cross-talk, echo, attenuation, and intermodulation.
White noise/Gaussian - ANS-A hiss or static. Is caused by thermal agitation of electron and can
not be prevented completly. To fix it: increase the signal to noise ratio.
Impulse noise - ANS-Is the primary source of errors in data communications. It's a crackling
noise that can last as long as 1/100 of a second. Sources of this noise include: voltage changes,
lightning flashes, fluorescent lights, and poor connections in circuits.
Cross-Talk - ANS-Happens when you can hear other conversations in the background. Occurs
when one circuit picks up signals in another.
What causes it? Increases with increased communication distance, proximity of the two wires,
increased signal strength, and higher frequency signals. Also due to wet or damp weather.
Echoes - ANS-The result of poor connections that cause the signal to reflect back to the
transmitting equipment. It causes errors when it is strong enough to be heard.
Attenuation - ANS-Is the loss of power a signal suffer as it travels from the sending computer to
the receiving computer. As the medium absorbs power, the signal becomes weaker. It increases
as frequency increases or as the diameter of the wire decreases.
Intermodulation noise - ANS-Is a special type of cross-talk. The signals from two circuits
combine to form a new signal that falls into a frequency band reserved for another signal. A
maladjusted modem may cause this noise.
Error prevention - ANS-Several ways to prevent errors. One is shielding (protecting wires by
covering them with an insulating coating). Another one is to move the cables away from sources
of noise. Changing multiplexing techniques can fix Cross-talk and intermodulation noise.
Amplifiers - ANS-Takes the incoming signal, increases its strength, and resends it
on the next section of the circuit. Usually used on analog circuits.
Repeaters - ANS-Usually used on digital circuits. Receives the incoming signal, translates it into
a digital message, and resends the message.
Error-Detection - ANS-Is where you send extra data with each message. The extra data is
added to each message by the data link layer of the sender on the basis of some math formula.
Networking Ch 4 Definitions
LLC - ANS-Logical link control sublayer. It's the data link layer's connection to the network layer
above it.
Is responsible for communicationg with the network layer software and for taking the network
layer PDU and surrounding it with a data link PDU. At the receiving computer, the sublayer
removes the data link PDU and passes the message to the network layer.
MAC sublayer - ANS-Media Access Control Sublayer/protocol that defines how packets are sent
to a LAN. It mainly controls when computers transmit through contention and controlled access.
The second sublayer of the data link layer. Also controls the physical hardware.
Contention - ANS-Computers wait until the circuit is free and then transmit whenever they have
data to send. Used in Ethernet LANs often.
Controlled access - ANS-The wireless access point controls the circuit and determines which
clients can transmit at what time. Two techniques: access requests and polling.
Access request - ANS-Client computers that want to transmit a message will send a request to
the device that is controlling the circuit.
The controlling device gives permission for one computer at a time to transmit.
Polling - ANS-The process of sending a signal to a client computer that gives it permission to
transmit. The clients store all messages that need to be transmitted.
Sometimes the controlling device polls the client to see if it has data to send and if it has data it
will send it.
Roll call polling - ANS-The controller works through a list of clients (first polling client 1, then 2
then 3 etc). Still involves asking the client if it has a message to send.
Hub polling/Token passing - ANS-One device starts the poll and passes it to the next computer
on the multipoint circuit. Then it sends its message and passes the poll to the next computer on
the circuit.
Error rate - ANS-The pattern of errors that can be expected on the circuits.
, Burst error - ANS-More than 1 data bit is changed by the error causing condition. Errors are not
uniformly distributed in time.
Line noise - ANS-Undesirable electrical signals introduced by equipment or natural distrubances
that weakens the signal performance..
Several types: white, impulse, cross-talk, echo, attenuation, and intermodulation.
White noise/Gaussian - ANS-A hiss or static. Is caused by thermal agitation of electron and can
not be prevented completly. To fix it: increase the signal to noise ratio.
Impulse noise - ANS-Is the primary source of errors in data communications. It's a crackling
noise that can last as long as 1/100 of a second. Sources of this noise include: voltage changes,
lightning flashes, fluorescent lights, and poor connections in circuits.
Cross-Talk - ANS-Happens when you can hear other conversations in the background. Occurs
when one circuit picks up signals in another.
What causes it? Increases with increased communication distance, proximity of the two wires,
increased signal strength, and higher frequency signals. Also due to wet or damp weather.
Echoes - ANS-The result of poor connections that cause the signal to reflect back to the
transmitting equipment. It causes errors when it is strong enough to be heard.
Attenuation - ANS-Is the loss of power a signal suffer as it travels from the sending computer to
the receiving computer. As the medium absorbs power, the signal becomes weaker. It increases
as frequency increases or as the diameter of the wire decreases.
Intermodulation noise - ANS-Is a special type of cross-talk. The signals from two circuits
combine to form a new signal that falls into a frequency band reserved for another signal. A
maladjusted modem may cause this noise.
Error prevention - ANS-Several ways to prevent errors. One is shielding (protecting wires by
covering them with an insulating coating). Another one is to move the cables away from sources
of noise. Changing multiplexing techniques can fix Cross-talk and intermodulation noise.
Amplifiers - ANS-Takes the incoming signal, increases its strength, and resends it
on the next section of the circuit. Usually used on analog circuits.
Repeaters - ANS-Usually used on digital circuits. Receives the incoming signal, translates it into
a digital message, and resends the message.
Error-Detection - ANS-Is where you send extra data with each message. The extra data is
added to each message by the data link layer of the sender on the basis of some math formula.
