OSI Data Link Layer : OSI Data Link Layer Network Fundamentals – Chapter 7
Objectives : Objectives Explain the role of Data Link layer protocols in data transmission.
Describe how the Data Link layer prepares data for transmission on network media.
Describe the different types of media access control methods.
Identify several common logical network topologies and describe how the logical topology determines the media access control method for that network.
Explain the purpose of encapsulating packets into frames to facilitate media access.
Describe the Layer 2 frame structure and identify generic fields.
Explain the role of key frame header and trailer fields including addressing, QoS, type of protocol and Frame Check Sequence.
Key Terms : Key Terms NIC
MAC
LLC
CSMA
CSMA/CA or CD
Collision Half/full Duplex
Physical/Logical Topology
CRC
ARP
Deterministic/Token Passing
Data Link Layer – Accessing the Media : Data Link Layer – Accessing the Media Supports upper layers by framing data
Interacts with the physical layer by controlling access to the media
Physical addresses assignedat this layer
Varying protocols function atthis layer to support LAN orWAN technologies
Data Link Layer Protocols : Data Link Layer Protocols Different media require different Data Link layer protocols to manage media access
Network layer is isolated Remember: frames are rebuilt at each hop
Framing Prepares Data for Media : Framing Prepares Data for Media Upper layer is framed based on standards for the media
Can include: legal frame sizes, addressing, start/stop bits
Data Link Layer – Subdivided : Data Link Layer – Subdivided Logical Link Control (LLC)
Interacts with the upper layer data (packet) to frame for media
Identifies the upper layer protocols
Example: IEEE 802.2 standard
Media Access Control (MAC)
Interacts with the Physical layer to determine how a device has access
Example: IEEE 802.3 standard Is a NIC a Layer 1 or 2 device?
Standards for the Data Link Layer : Standards for the Data Link Layer
Media Access Control Techniques : Media Access Control Techniques Traffic Jam! Devices compete to put data on the wire Collisions are spikes in electrical voltage read by the NIC
Media Access Control Techniques : Media Access Control Techniques Controlled
Access method takes turns
Example: Token ring
Contention-based
Carrier Sense Multiple Access
Example: Ethernet Collisions are expected on CSMA networks
(two devices transmitting at the same time)
Duplex Signaling : Duplex Signaling For media where collisions may occur, half duplex is employed
Full duplex is used when collisions are physically (or electrically) prohibited
Logical Topologies : Logical Topologies Defines how the framing and media access occur
Independent of physical topology
Point-to-Point Topology : Point-to-Point Topology Connects two nodes (devices) directly
Employs full duplex since no other device contention
More simplistic Data Link layer protocols (adds speeds)
Virtual Circuits – other physical intermediary devices may be present but they do not contend for media
Multi-Access Topology : Multi-Access Topology Shared media – each device sees all frames
Addressing required to identify nodes
Must have methods to deal with collisions
CSMA/CD or CA
Ring Topology : Ring Topology Shared media but controlled access
Often referred to as Token Passing
Token passed around ring
Node must pickup token and use to forward data
In theory, all nodes listen but onlyone transmits
Physical topology may be differentfrom logical topology
Data Link Layer Protocols : Data Link Layer Protocols Examples: Ethernet, PPP, HDLC, Frame Relay, ATM
Frame Header - Generic : Frame Header - Generic Start bits
Address or naming scheme
Type field – what upper layer protocol is encapsulated?
Quality – control field
Data – frame payload
Framing – When are Addresses Required? : Framing – When are Addresses Required?
Frame Trailer : Frame Trailer Datalink Layer is responsible for device to device delivery
Must verify that data is good
Trailer contains:
Frame Check Sequence – validity of frame
Contains a mathematical Cyclical Redundancy Check
Field of bits indicating the end of frame
Chapter 7 Labs : Chapter 7 Labs Lab 7.5.2: Frame Examination
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