OSI Data Link Layer Flashcards
- Allows the upper layers to access the media using techniques such as framing.
- Controls how data is placed onto the media and is received from the media using techniques such as media access control (MAC) and error detection.
Data Link Layer 2 Basic Services
Data Link Layer 2 Basic Services
- Allows the upper layers to access the media using techniques such as framing.
- Controls how data is placed onto the media and is received from the media using techniques such as media access control (MAC) and error detection.
At each hop the received frame is examined for errors, if an error is found in a frame, then the frame is ____
Discarded
The technique used for getting the frame on and off media is called_____
the media access method or MAC Method
Control information can describe the following
- Which nodes are in communication with each other
- When communication between individual nodes begins and ends
- If errors occurred while the nodes communicated
- Which nodes will communicate next
The data link layer frame includes the following elements:
Data, Header, Trailer
The packet from the network layer
____ link layer addressing is contained within the frame header and specifies the frame destination node on the local network.
Data
The control information, such as addressing, located at the beginning of the PDU
The frame ____ contains the control information specified by the data link layer protocol for the specific logical topology and media used.
Header
The control information added to the end of the PDU
Data link layer protocols add a ____ to the end of each frame.
Trailer
Typical field types include:
- Start and Stop Indicator Fields
- Addressing or naming fields
- Type fields
- Quality
- Data Field
The beginning and end limits of the frame
Start and stop indicator fields
The destination and/or source devices
Addressing or naming fields
The type of PDU contained in the frame
Type field
The control fields
Quality
The frame payload (network layer packet)
Data field
the data link layer is embodied as a physical entity, such as an Ethernet ______, which inserts into the system bus of a computer and makes the connection between running software processes on the computer and the physical media.
network interface card (NIC)
The two common LAN sublayers are as follows:
Logical Link Control (LLC)
MAC
places information in the frame that identifies which network layer protocol is being used for the frame.
Logical Link Control (LLC)
provides data link layer addressing and delimiting of data according to the physical signalling requirements of the medium and the type of data link layer protocol in use.
MAC
Engineering organisations that define open standards and protocols that apply to the data link layer include:
- International Organisation for Standardisation
- Institute of Electrical and Electronics Engineers (IEEE)
- American National Standards Institute (ANSI)
- International Telecommunication Union (ITU)
- Network devices take turns, in sequence, to access the
medium. - Also known as scheduled access or deterministic
Controlled Access for Shared Media
- Allow any device to try to access the medium whenever it
has data to send. - Sometimes referred to as nondeterministic
Contention-Based Access for Shared Media
in Contention-Based Access, Uses a _____ process
to first detect whether the media is carrying a signal
carrier sense multiple access (CSMA)
CSMA is usually implemented in conjunction with a method for resolving the media contention.
The two commonly used methods are:
CSMA/Collision Detect (CSMA/CD)
CSMA/Collision Avoid (CSMA/CA)
The device monitors the media for the presence of a data signal.
CSMA/Collision Detect (CSMA/CD)
The device examines the media for the presence of a data signal.
CSMA/Collision Avoid (CSMA/CA)
In a _____, the nodes do not have to
share the media with other hosts or determine whether
a frame is destined for that node.
point-to-point topology
In _______, Ethernet has established
arbitration rules for resolving conflicts arising from
instances when more than one station attempts to
transmit at the same time.
half-duplex communication
In ____, both devices can transmit
and receive on the media at the same time.
full-duplex communication
The ____ is an arrangement of the nodes and the physical connections between them.
physical topology
A_____ is the way a network transfers frames from one node to the next.
logical topology
The logical and physical topologies typically used in networks:
- Point-to-point
- Multiaccess
- Ring
- Connects two nodes directly.
- MAC protocol can be very simple.
- All frames on the media can only travel to or from the two nodes.
Point-to-Point Topology
- Enables a number of nodes to communicate by using the same shared media.
- The MAC methods used are CSMA/CD or CSMA/CA.
Multiaccess Topology
- Each node in turn receives a frame.
- Uses a controlled MAC technique called token passing.
Ring Topology
3 Parts of a Frame in Data Link Layer Protocols
Header, Data, Trailer
Indicates the beginning of the frame
Start of Frame field
Indicate the source and
destination nodes on the media
Source and Destination address fields
Indicates a particular type of
communication service for processing
Priority/Quality of Service field
Indicates the upper-layer service contained in the
frame
Type field
Used to establish a logical
connection between nodes
Logical connection control field
Used to establish the media link
Physical link control field
Used to start and stop traffic over the media
Flow control field
Congestion control field
Indicates congestion in the media
Device addresses at this layer are referred to as ______
physical
addresses
Typical frame trailer fields include:
Frame Check Sequence Fields
Stop Field
Used for error checking of the frame
contents.
Frame Check Sequence fields
Used to indicate the end of the frame. Also, can be used to
increase the size of a frame to an expected fixed size or minimum size.
Stop field
The trailer is used to determine whether the frame arrived without error.
This process is called ____.
error detection
____ is accomplished
by placing a logical or mathematical summary of the bits that comprise
the frame in the trailer.
Error detection
To ensure that the content of the received frame at the destination matches that of the frame that left the source node, a transmitting node creates a logical summary of the contents of the frame. This is known as the _____ value.
cyclic redundancy check (CRC)
The Ethernet frame field are the following:
Preamble, Destination Address, Source Address, Type, Data or payload, Frame Check Sequence (FCS)
Used to time synchronization; this also contains a delimiter to mark the
end of the timing information.
Preamble
48-bit MAC address for the destination node.
Destination Address
Value to indicate which upper-layer protocol will receive the data afte the Ethernet process is complete.
Type
This is the PDU, typically an IPv4 packet, that is to be
transported over the media.
Data or payload
A CRC value used to check for damaged frames
Frame Check Sequence (FCS)
PPP _______ requires the nodes on each end of the PPP link to ______ in order to
establish communications over a point-to-point link.
Authentication
PPP _______ reduces the size of a
data frame transmitted over a network link.
Compression
PPP _____ is the method of using multiple
data links to send the frames.
Multilink
Basic fields in a PPP frame:
Flag, Address, Control, Protocol, Data, Frame Check Sequence (FCS)
A single byte that indicates the beginning or end
of a frame.
Flag
A single byte that contains the standard PPP
broadcast address.
Address
A single byte that contains the binary sequence
00000011, which calls for transmission of user data in an
unsequenced frame.
Control
Two bytes that identify the protocol encapsulated
in the data field of the frame.
Protocol
Zero or more bytes that contain the datagram for the
protocol specified in the protocol field.
Data
Normally 16 bits (2 bytes).
By prior agreement, consenting PPP implementations can
use a 32-bit (4-byte) FCS for improved error detection.
Frame Check Sequence (FCS)
Version of 802.11 frame in use
Protocol Version
Identifies one of three functions and subfunctions of the frame:
control, data, and management
Type and Subtype
Set to 1 in data frames destined for the distribution system (devices in the
wireless structure)
To DS
Set to 1 in data frames exiting the distribution system
From DS
Set to 1 for frames that have another fragment
More Fragments
Set to 1 if the frame is a retransmission of an earlier frame
Retry
Set to 1 to indicate that a node will be in power-save mode
Power Management
Set to 1 to indicate to a node in power-save mode that more frames are
buffered for that node
More Data
Set to 1 if the frame contains WEP-encrypted
information for security
Wired Equivalent Privacy (WEP)
Set to 1 in a data type frame that uses Strictly Ordered service class (does
not need reordering)
Order
Depending on the type of frame, represents either the time, in microseconds, required to transmit the frame or an association identity (AID) for the station that transmitted the frame
Duration/ID
MAC address of the final destination node in the
network
Destination Address (DA)
MAC address of the node that initiated the frame
Source Address (SA)
MAC address that identifies the wireless device that is the
immediate recipient of the frame
Receiver Address (RA)
MAC address that identifies the wireless device that
transmitted the frame
Transmitter Address (TA)
Indicates the sequence number assigned to the frame;
retransmitted frames are identified by duplicate sequence numbers
Sequence Number
Indicates the number for each fragment of a frame
Fragment Number
Frame Body
Contains the information being transported; for data frames typically an IP packet
Contains a 32-bit cyclic redundancy check (CRC) of the frame
FCS
The ____ prepares network layer packets for
placement onto the physical media that transports data.
OSI data link layer
The wide range of data communications media requires a correspondingly wide range of____ to control data
access to these media.
data link protocols
Media access can be orderly and controlled, or it can be contention based. The ____ and ____ help determine the media access method.
logical topology, physical medium
