IDT6.4/3 - Data-Link Layer Overview

IDT6.4/3 - Data-Link Layer

• The data link layer prepares network data for the physical network.
• It sits between the network layer and the physical layer in the OSI model.
• It enables upper layers to access the media, which the upper layer isn't aware of.
• It accepts Layer 3 packets (e.g., IPv4 or IPv6) and encapsulates them into Layer 2 frames.
• It controls how data is placed and received on the media, including the transmission and receipt of frames between endpoints.
• The data link layer directs encapsulated Layer 3 packets to the appropriate upper-layer protocol.
• It detects and rejects corrupt frames.
• A PDU (protocol data unit) at the data link layer is called a frame.
• A node is a device on a network.
• The media are the physical means used to carry data signals.
• A network is two or more devices connected to a common medium.

Data-Link Layer Purpose

• The Data Link layer is responsible for communications between end-device network interface cards.
• The layer allows upper-layer protocols to access physical-layer media.
• It encapsulates Layer 3 packets (IPv4 and IPv6) into Layer 2 frames.
• It performs error detection and rejects corrupt frames.

Data Link Layer Terms

• Frame: A PDU (Protocol Data Unit) at the Data Link layer
• Node: A device on a network
• Media: The physical means used to carry data signals
• Network: Two or more devices connected to a common medium

Data Link Layer Sublayers

• Logical Link Control (LLC): Communicates between network software and device hardware at upper and lower layers.
• Media Access Control (MAC): Responsible for data encapsulation and media access control. IEEE 802.3 (Ethernet), 802.11 (Wireless LAN), 802.15 (Wireless PAN).

Data Link Layer Protocols

• Data link layer protocols are defined by engineering organizations (IEEE, ITU, ISO, ANSI) for protocols such as Ethernet, WLAN, and Wireless PAN.

Data Link Frame - Frame Fields

• Frame has three parts: Header, Data, Trailer.
• Fields in the header and trailer vary according to data-link layer protocol.
• Amount of control information in the frame varies based on access control and logical topology.
• Frame Start and Stop: Identifies beginning and end of the frame.
• Addressing: Identifies source and destination nodes.
• Type: Identifies encapsulated Layer 3 protocol.
• Control: Identifies flow control services.
• Data: Contains the frame payload.
• Error Detection: Used for determining transmission errors/corruptions.

Start Frame of Preamble

• The Start of Frame fields are used for synchronization between sending and receiving devices.
• The first few bytes tell receivers to get ready to receive a new frame.

Addressing

• Destination MAC Address Field: Identifies where the frame needs to go.
• Source MAC Address Field: Identifies the originating NIC or interface of the frame.

Ethernet MAC Addresses

• A device sending a message will include Source MAC address and Destination MAC address in the Ethernet Header.
• When a NIC receives a frame, it checks the destination MAC address against its own physical MAC address in RAM.
• If there is a match, the frame is passed up OSI layers for de-encapsulation; otherwise, the frame is discarded.
• Ethernet NICs accept frames if the destination MAC is broadcast or multicast.

Transmission types at the Data Link Layer

• Unicast: Frame is sent to a single destination MAC address.
• Multicast: A single stream of frames is sent to multiple devices.
• Broadcast: A one-to-all transmission to all devices on the network.

MAC Address Table

• MAC Address Table determines where to forward data on a LAN.
• It maps each switch port to the MAC address of the device connected.
• This optimizes traffic forwarding, only forwarding to the port associated with the destination MAC address.

Switch Learning and Forwarding

• Every frame entering the switch is checked for new source MAC address information.
• If the source MAC address is not in the switch's table, it is added, and the incoming port is associated with the MAC address.
• If the address exists on the table, on a different port, the switch treats it as a new entry.
• Entries are removed from the table after 5 minutes by default.

Packet Forwarding

• The device uses the MAC address table to forward packets.
• In unicast mode, the switch forwards the packet to the interface matching the destination MAC address.
• Broadcast mode: The packet is sent out all ports (except the incoming port) to all devices on the LAN..

Sending the Frame to the Default Gateway

• The devices in the network will use switches to forward the data or frame to the default gateway when the destination is outside the local network.
• The default gateway is typically a router.
• When a device needs to send data to another network, it sends the frame to the default gateway, which then forwards the frame to the appropriate destination.

Data Link Layer Functions

• Line discipline: Determines who should send data.
• Flow Control: Regulates the amount of data sent to prevent overwhelming a slow receiver.
• Error Control: Detects and corrects errors during transmission.

Line Discipline

• Line Discipline is a functionality of the Data Link layer that coordinates link systems and determines which device can send data and when.
• Methods include ENQ/ACK (enquiry/acknowledgement) and poll/select.
• Using ENQ/ACK, the transmitter sends an ENQ to the receiver requesting permission to send data.
• If the receiver is ready, it responds with an ACK, allowing the transmitter to send data.
• If the receiver isn't ready, it sends a NACK, and the transmitter waits and retries.
• Following an error, the transmission will be restarted.

Flow Control

• Flow control regulates the amount of data sent to prevent a fast sender from overwhelming a slow receiver.
• Methods include Stop-and-Wait, which sends one frame at a time and waits for acknowledgement. Sliding window, which sends multiple frames and tracks the unacknowledged frames.

Error Detection

• Errors occur when receiver and sender data don't match.
• Noise in transmission can modify the binary bits ('0' to '1' or '1' to '0') during transmission.
• Error detecting codes, including parity bits and frame check sequences (FCS) are used to detect errors in the transmitted message/frame.

ARP (Address Resolution Protocol)

• ARP is used to determine the MAC address of a device given its IP address on the same local network.
• ARP provides two basic functions: (1) Resolving IPv4 addresses to MAC addresses; and (2) Maintaining a table of IPv4 to MAC address mappings.
• ARP request: Sent when a device needs a MAC address, and doesn't have one in its table.
• The request is encapsulated in an Ethernet frame with a broadcast destination MAC address to all devices on the local network, requesting the requested MAC address.
• The device having the requested IP address responds with its MAC address, updating the sending device's ARP table.