CCNA ITN: Module 6

data link layer

prepares network data for transmission via NIC-to-NIC communication

OSI Layer 2

enables upper layers to access media, encapsulates data into frames, manages access, and error checks

without data link layer

network layer protocols would need to account for every media type

IEEE 802 data link sublayers

Logical Link Control and Media Access Control

LLC sublayer

communicates between networking software and hardware

MAC sublayer

controls media access, handles addressing and framing, depends on media type and topology

MAC resolution

enables communication by resolving different network environments

serial links

direct device-to-device, may not require MAC-level access control

data transitions

routers re-encapsulate data link frames at each hop for new media

network access standards

defined by IETF, IEEE, ITU, ISO, ANSI

physical topology

layout of devices and cables (e.g., star, point-to-point)

logical topology

path data follows through a network

WAN topologies

point-to-point, hub and spoke, mesh, and hybrid

LAN topologies

star, extended star, bus, and ring

half-duplex

devices transmit and receive, but not simultaneously

full-duplex

devices transmit and receive simultaneously (default for Ethernet switches)

multiaccess networks

multiple devices can access media (Ethernet LANs, WLANs)

contention-based networks

use CSMA/CD or CSMA/CA for access control

CSMA/CD

legacy Ethernet collision detection method for wired LANs

CSMA/CA

wireless collision avoidance method for WLANs

controlled-based networks

each node has a designated time to transmit

legacy controlled access

token ring, ARCNET

modern Ethernet

full-duplex, no access method needed

frame

pdu of the data link layer, created by encapsulating Layer 3 packet

frame header

contains start flag, addressing, type, and control info

frame trailer

contains error detection (CRC) and stop flag

physical addresses

Layer 2 addresses unique to each device, not tied to network location

Layer 2 addresses

used for local delivery only, no meaning beyond local network

moving networks

physical address remains the same even if device changes networks

routers

forward packets by IP address, re-encapsulate in new frames

Ethernet

used in wired LANs

WLAN

wireless communication using IEEE 802.11

WAN protocols

PPP, HDLC, Frame Relay, ATM, X.25

protocol choice

depends on logical topology and physical media

LAN bandwidth

high for many hosts, not cost-effective for WANs