Physical Layer Flashcards
Purpose of the Data Link Layer
*Data Link Layer facilitates communication between end-device network interface cards.
*It allows upper layer protocols to access physical layer media.
*Encapsulates Layer 3 packets (IPv4 and IPv6) into Layer 2 Frames.
*Performs error detection and rejects corrupted frames.
IEEE 802 LAN/MAN Data Link Sublayers
*IEEE 802 LAN/MAN standards tailored to specific network types (Ethernet, WLAN, etc.).
*Data Link Layer consists of Logical Link Control (LLC) and Media Access Control (MAC) sublayers.
*LLC: Communicates between upper-layer software and lower-layer hardware.
*MAC: Handles data encapsulation and media access control.
Providing Access to Media
*Routers perform four Layer 2 functions along packet paths:
*Accept frame from medium.
*De-encapsulate frame to expose encapsulated packet.
*Re-encapsulate packet into new frame.
*Forward new frame on next network segment’s medium.
Data Link Layer Standards
*Data link layer protocols defined by engineering organizations.
*Examples: IEEE, ITU, ISO, ANSI.
Topologies
*Network topology: arrangement of devices and their connections.
*Physical topology: shows physical connections.
*Logical topology: identifies virtual connections via interfaces and IP addresses.
WAN Topologies
*WAN topologies: Point-to-point, Hub and spoke, Mesh.
*Point-to-point: Simplest, permanent link between endpoints.
*Hub and spoke: Central site connects branches via point-to-point links.
*Mesh: High availability, all systems connected to each other.
LAN Topologies
*LANs use star or extended star topologies for easy scalability and troubleshooting.
*Early Ethernet and Token Ring also used bus and ring topologies.
Half and Full Duplex Communication
*Half-duplex: Only one device sends/receives at a time on shared medium.
*Full-duplex: Both devices transmit/receive simultaneously.
Ethernet switches operate in full-duplex mode.
Access Control Methods
*Contention-based access: Nodes compete for medium use.
*Controlled access: Each node has designated time on medium.
*Examples: CSMA/CD (Ethernet) and CSMA/CA (Wireless LAN).
Contention-Based Access – CSMA/CD
*CSMA/CD used in legacy Ethernet.
*Half-duplex, one device sends/receives at a time.
*Collision detection: detects collisions, random retransmission.
Contention-Based Access – CSMA/CA
*CSMA/CA used in IEEE 802.11 WLANs.
*Half-duplex, one device sends/receives at a time.
*Collision avoidance: devices include transmission time, avoids collisions.
Data Link Frame
*Data link frame: encapsulates data with header and trailer.
*Frame parts: Header, Data, Trailer.
*Fields vary by data link protocol.
Frame Fields
Frame fields: Start/Stop, Addressing, Type, Control, Data, Error Detection.
Layer 2 Addresses
*Layer 2 addresses: Physical addresses, in frame header.
*Used for local delivery, updated by forwarding devices.
LAN and WAN Frames
*Data link protocol based on logical topology and physical media.
*Examples: Ethernet, 802.11 Wireless, PPP, HDLC, Frame-Relay.
*Perform media access control for specific topologies.
