2.1.2-2.1.3 Layer 2 - DataLink

Layer 2 - Data Link Layer

• Layer two runs over layer one and requires a functional layer one network for operation
• Layer two utilizes frames as a format for sending information
• It introduces MAC addresses for devices on a network
• Higher layers in the OSI model build on lower layers, adding features and capabilities
• Layer two networks are compatible with different types of layer one networks like copper, fiber, and Wi-Fi
• Focus will be on Ethernet as a common protocol for local networks

MAC Addresses

• MAC addresses uniquely identify each device on a network and are not assigned by software
• MAC addresses are hexadecimal and 48 bits long
• MAC addresses consist of two parts: the Organisationally Unique Identifier (OUI) and the Network Interface Controller (NIC) specific part
• The OUI is assigned to companies that manufacture network devices, ensuring each has a separate OUI
• These addresses are hard coded into the hardware

Frames

• Frames are used for communication at layer two and contain several components
• Preamble and Start Frame Delimiter (SFD) allow devices to identify the start of a frame
• Destination and source MAC addresses enable sending frames to specific devices or broadcasting to all devices on a local network
• EtherType specifies the layer three protocol used to encapsulate data within the frame, such as IP or the internet protocol
• The MAC header includes the destination MAC address, source MAC address, and EtherType

Payload and Encapsulation

• The payload of a frame ranges from 46 to 1500 bytes for standard frames and contains the data being sent
• Layer three protocols generally provide this data, which is indicated in the EtherType field
• Encapsulation is the process of wrapping layer three data, like an IP packet, inside an ethernet frame
• This allows layer two to deliver the data to its intended destination, where the frame will be analyzed and the layer three packet extracted for use by the layer three protocol
• Frame Check Sequence (FCS) identifies errors in the frame, allowing the destination to verify data integrity

Layer 1 vs Layer 2

• Layer two uses layer one to transmit and receive signals
• Layer two frames are transmitted over a shared physical medium by layer one, which converts the frame into voltages, RF, or light
• Layer one handles the physical transmission and reception, while layer two manages framing and other functions
• Layer one transmits raw data without understanding the frame's contents

Collisions

• Layer one provides no media access control, so devices transmit data without checking if the medium is in use
• Simultaneous transmissions can cause electrical signals to overlap and interfere with each other, resulting in a collision that corrupts data
• Layer two solves this problem by providing controlled access to the physical medium to mitigate packet collisions

Carrier Sense Multiple Access (CSMA)

• Layer two uses layer one for media access control in CSMA to check if a carrier signal can be detected.
• Layer two avoids collisions by sensing for a carrier signal.
• If no carrier is detected, the frame is passed to layer one for conversion to a physical signal across the shared medium.
• On the receiving end, layer one passes the raw bit stream to layer two.
• Layer two then reviews to ensure data is for the destination MAC address, which in turn passes the respective payload back to the game.

Layer 2+

• Layer 2 adds MAC addresses for machine-to-machine communication and media access control
• Should neither machines be able to detect carrier signals, the collision detection (CD) is required.
• From that a jam signal is sent to the devices detecting the signal, and a random back-off occurs.
• In CSMA/CD this is an essential function of Layer 2 that allows multiple decives to coexist

Additional Layer 2 information

• The OSI model consists of seven layers, each with specific functions for data transmission
• Layer 2, or the data link layer, ensures the reliable transfer of data between two nodes on the same network segment and defines access protocols for the physical network

Hubs vs Switches

• In a network where layer two protocols are utilized, to have an active network, layer one and two are jointly used
• Layer 2 uses MAC addresses while hubs forward traffic and behave in a layer 1 way
• Layer 2 enabled devices use CSMA to limit collisions
• Switches don't simply repeat anything a dumb layer one device does - it means it avoids those collisions
• Each port on the switch is its own collision domain, so the transfer only happens one port at a time, avoiding the need to forward corrupted packets.

Layer 2 Benefits

• Layer 2 adds media access control which lets devices reliably share access
• Identifiable Devices (MAC addresses)
• Collision Detection and Error Control
• Unicast Communication (one-to-one)
• Broadcast Communication (one-to-all)

Encapsulation

• Data is encapsulated in a frame which is de-encapsulated upon delivery.
• This concept will be repeated lower down the stack, with protocols building on previous protocols until the destination's Layer 7.

Switch MAC Table

• The MAC address table is learned one the first laptop on each port transfers a frame.
• From there, the switch knows which laptop and MAC address refers to the one port.
• Because of this, each port is isolated to only traffic destined for that MAC.