L7 Flashcards
(32 cards)
What does MAC stand for?
Medium Access Control
MAC techniques describe the method for
access to the communications channel, frame
transmission and channel release for each
computer.The available bandwidth of the
channel decreases with the redundancy due to
the MAC technique.
MAC techniques manage access to the communications channel in networks.
What are the two categories of MAC techniques?
- Contention-based
- Token passing
What are the advantages of contention-based MAC techniques?
Simple implementation
- simple implementation
- small redundancy
- for a small number of computers the transfer rate is high
- for high traffic, the usage of the channel is high
What are the disadvantages of contention-based MAC techniques?
- Access time cannot be guaranteed- (is a
matter of probability, depending on the other
computers willing to transmit at the same time) - priorities cannot be established
- when the number of computers increases, the
number of collisions increases exponentially
and also the number of retransmissions needed
What is the operation principle of contention-based techniques?
Each computer may attempt transmission every
time data transmission is requested by the upper
layer (the network layer), without waiting for prior
authorization. Collisions may appear when two or
more computers start to send at the same time. In
this unwanted situation, all sending computers
must stop and then must attempt again the
transmission, at a later time.
What are the two methods to resolve collisions in contention-based MAC techniques?
- Collision detection - during
frame transmission, the sender monitors the
transmission line in order to detect different
voltages that indicate a collision; in the case of
a collision, the transmission is attempted again
after a random amount of time - Collision avoidance - the
collisions occur over the transmission of control
frames
What is an example of a MAC technique that uses collision detection?
CSMA/CD (Carrier Sense, Multiple Access with Collision Detection)
What MAC technique uses collision avoidance?
CSMA/CA (Carrier Sense, Multiple Access with Collision Avoidance)
What is the fundamental operation of token-based MAC techniques?
A special frame called token (jeton), that contains
an authorization message for the next computer
that is authorised to transmit (usually it is the
MAC address of the next computer). This token
travels the network in a certain order, usually in a
logical ring.
What are the two methods to generate a token in token-based techniques?
- centralized token where we have a central
entity that coordinates the activity by issuing in
a controlled way the tokens - distributed token where the computers are
taking turns in generating a new token
containing the MAC address of the next
computer, in la logical ring
What are the advantages of token-based MAC techniques?
- deterministic (there is a maximum time after a
computer will receive again the right to
transmit) - flexible because different options may be
implemented, including priorities - with the increase of the number of computers,
the total transfer rate increases
What are the disadvantages of token-based MAC techniques?
- complex technique, difficult to implement
- there is a constant redundancy due to the
token passing method ( even when the
computer has nothing to transmit it will still
receive the token)
What does CSMA/CD stand for?
Carrier Sense, Multiple Access with Collision Detection
What is the function of the IEEE 802 standard?
Specifies one MAC option, frame structure, and physical layer options
What are the two sublayers of the Data Link layer defined by IEEE?
- MAC (Medium Access Control)
- LLC (Logical Link Control)
What is the frame structure of IEEE 802.3/Ethernet?
Includes preamble, SFD, destination address, source address, length/type, data, PAD, and FCS
What is the purpose of the preamble in an Ethernet frame?
Bit synchronization at the receiver
What type of address is used for broadcasting in Ethernet frames?
Special broadcast address (ff ff ff ff ff ff h)
What does FCS stand for in the Ethernet frame structure?
Frame Check Sequence
What is the maximum segment length for 10BaseT Ethernet?
100m
What topology is used with 10BaseT Ethernet?
Star topology
What is the significance of the 5-4-3 rule in bus topologies?
Limits the number of segments and repeaters in the network
Fill in the blank: The MAC address is used to identify the _______ of the frame.
destination
True or False: In contention-based MAC techniques, the number of collisions increases exponentially as more computers join the network.
True
