8. Data Link Layer Flashcards
What is data link layer responsible of?
- Communications between end-device network interface cards
What does Data Link Layer allows to access? ( 2 )
- Allows upper layer protocols to access the physical layer media and encapsulates Layer 3 packets ( Network Layer ) into Layer 2 frames
- Performs error detection and rejects corrupts frames
List out the 2 sublayers of Data Link Layer
- Logical Link Control ( LLC )
- Media Access Control ( MAC )
What stardards are specific to Ethernet, WLAN, WPAN ( Bluetooth )
- IEEE 802 LAN/MAN
What is Logical Link Control ( LLC ) responsible of?
- Communicates between the networking software at the upper layers and the device hardware at the lower layers
What is Media Access Control ( MAC ) responsible of?
- For data encapsulation and media access control
What basic layer 2 functions does router performs? ( 4 )
- Accepts a frame from the network medium
- De-encapsulates the frame to expose the encapsulated packet
- Re-encapsulates the packet into a new frame
- Forwards the new frame on the medium of the next network segment
What will packet experience when exchanging between nodes ?
- Numerous data link layers and media transitions
List out the 4 organizations for engineering organizations that define data link layer protocol
- Institute for Enlectrical and Electronic Engineers ( IEEE )
- International Telecommuncations Union ( ITU )
- International Organizations for Standardization ( ISO )
- American National Standards Institute ( ANSI )
What is a topology ?
- The arrangement and relationship of the network devices and the interconnections between them
List out the 2 types of topologies when describing networks
- Physical Topology
- Shows physical connections and how devices are interconnected
- Logical Topology
- Identifies the virtual connections between devices using device interfaces and IP addressing schemas
List out the 3 common physical WAN topologies
- Point-to-point
- Hub and Spoke
- Mesh
What is the simplest most common WAN topology?
- Point-to-point
What WAN topology consists permanet link between 2 endpoints
- Point-to-point
What WAN topology is similar to a star topology where a central site interconnects branch sites through point-to-point links
- Hub and spoke
What WAN topology provides high availability but requires every end system to be connected to every other end system?
- Mesh
How does point-to-point topology connects?
- Directy connect 2 nodes
Can the nodes share the media with other hosts in point-to-point topology?
- No
- Because all frames on the media can only travel to or from 2 nodes
- Point-to-point protoocols can be very simple
What topology that end devices usually connected to on LAN network?
- Star Topology
- Extended Star Topology
What are 2 additional topology for early ethernet and legacy token ring technologies?
- Bus Topology
- Ring Topology
What is the term Half-Duplex Communication does? What is it used for?
- Only allows one device to send or reveive at a time on a shared medium
- Used on WLAN and legacy bus topologies with Ethernet hubs
What is the term Full-Duplex Communication does? What is it used for?
- Allows both devices to simulteneously transmit and revceive on a shared medium
- Ethernet switches operate in full-duplex mode
List out 2 methods for Access Control Methods
- Contention-based access
- Controlled access
List out 2 examples that uses contention-based access
- Carrier sense multiple access with collision detection ( CSMA / CD ) as used on legacy bus-topology Ethernet
- Carrier sense multiple access with collision avoidance ( CSMA/CA ) as used on Wireless LAN
What does controlled access determine?
- Deterministic access where each node has its own time on the medium
Where does controlled access used?
- Used on legacy networks such as Token Ring and ARCNET
Where does CSMA/CD used?
- Ethernet LANs
What duplex does CSMA/CD operates at?
- Half-duplex mode
- One devices sends or receives at a time
What does CSMA/CD do?
- Uses a collision detection process to govern when a device can send and what happens if multiple device send at a same time
List out the 3 process for CSMA/CD Collision Detection Process
- Devices transmitting simultaneously will return in a signal collision on the shared media
- Devices detect the collision.
- Devices wait at a random period of the time and retransmit data.
What does CSMA/CA used in?
- IEEE 802.11 WLANs ( Wifi )
What duplex mode does CSMA/CA uses?
- Operates in Half-duplex mode where only one device sends or receives at a time
What does CSMA/CA do?
- Uses a collision avoidance process to govern when a device can send and what happens if multiple devices send at the same time
List out 2 process for CSMA/CA Collision Avoidance
- When transmitting, devices also include the time duration needed for the transmission
- Other devices on the shared medium receive the time duration information and know how long the medium will be unavailable
List out the 3 parts for the Data Link Frame
- Header
- Data
- Trailer
Will the fields of the header and trailer differ according to data link layer protocol?
- Yes
What does the amount of control information carried with in the frame depends on? ( 2 )
- Access control information
- Logical Topology
What does header have in data link frame? ( 4 )
- Frame Start
- Identifies beginning of frame
- Addressing
- Indicates source and destination nodes
- Type
- Identifies encapsulated Layer 3 protocol
- Control
- Identifies the flow control services
What does trailer have in data link frame? ( 2 )
- Error Detection
- Used for determine transmission errors
- Frame Stop
- Identifies beginning and end of frame
List out 5 protocols used in data link layer
- Ethernet
- 802.11 Wireless
- Point-to-point ( PPP )
- High-Level Data Link Control ( HDLC )
- Frame-Relay
- Each protocol performs media access control for specified logical topologies
What does Layer 2 Addresses also known as?
- Physical Address
Where does Layer 2 Addresses continained in?
- Frame Header
What are the usage of Layer 2 Addresses?
- Used only for local delivery of a frame on the link
When does Layer 2 Addresses updated
- Updated by each device that forwards the frame