ECM1413 - Computers and the Internet - Network Access Layer

Network Access Layer

Objectives

  • Media Access Control Techniques
  • Frame and L2 Addressing
  • Address Resolution Protocol
  • L1 and L2 Network Devices (Hub & Switch)

Segmentation & Encapsulation

  • Network Access layer is responsible for segmentation and encapsulation of data.
  • Data flow:
    • Application layer creates data.
    • Transport layer segments the data into segments.
    • Internet layer encapsulates the segments into packets.
    • Network Access layer encapsulates the packets into frames.

Layers

  • OSI (Reference Model)
    • L1: Physical
    • L2: Data Link
      • Media Access Control (MAC) Method
      • Encapsulate a packet into a frame
      • Encoding
      • Signalling
      • Physical components
    • L3: Network
    • L4: Transport
    • L5: Session
    • L6: Presentation
    • L7: Application
  • TCP/IP (Protocol Model)
    • Network Access
    • Internet
    • Transport
    • Application

MAC Techniques: Placing Data on the Media

  • MAC for Shared Media
    • Controlled Access
    • Contention-Based Access

MAC for Shared Media: Controlled

  • “scheduled access or deterministic”
  • Only one device transmits at a time.
  • Devices must wait their turn.
  • No collisions occur.
  • Token Ring is an example.

MAC for Shared Media: Contention-based

  • “non-deterministic”
  • Devices can transmit at any time.
  • Collisions can occur, and mechanisms are needed for resolving contention.
  • CSMA/CD is used for Ethernet networks.
  • CSMA/CA is used for 802.11 wireless networks.

Different Frames: LAN Protocols

  • Ethernet (IEEE 802.2 & 802.3)
  • Wireless LAN (IEEE 802.11)
  • Token Ring (IEEE 802.5)

Different Frames: WAN Protocols

  • PPP (Point-to-Point Protocol)
  • MPLS (Multiprotocol Label Switching)
  • HDLC (High-Level Data Link Control)

Creating a Frame

  • A frame consists of a header, packet (data), and trailer.
  • Header: Contains Start, Addressing, Type, Quality Data
  • Trailer: Error Detection, Frame Stop
  • Start and Stop: Specific bit patterns denote the start and end of the frame.

LAN and WAN Frames

  • Ethernet Frame: Contains 48-bit MAC address.
  • Wireless (802.11) Frame: Contains 48-bit MAC address.
  • PPP Frame: PPP Broadcast Address

Ethernet MAC address

  • 48 bits - hexadecimal format
  • Examples:
    • Cisco MAC Address: 00-60-2F-3A-07-BC, 00:60:2F:3A:07:BC, 0060.2F3A.07BC
    • Intel MAC Address: 00-20-E0-6B-17-62, 00:20:E0:6B:17:62, 0020.E06B.1762
  • Unicast, Multicast & Broadcast (FF-FF-FF-FF-FF-FF)
  • Use ipconfig /all to view MAC address.

L2 Address: Destination on Same Network

  • How can PC1 know the MAC address of PC2?

L2 Address: Destination on Remote Network

  • How a device determines the destination MAC address?
  • ARP (for IPv4) or ICMPv6 Neighbor Discovery (for IPv6)

ARP - Step 1

  • A host (H1) needs to find the MAC address of another host (H4) with IP address 192.168.1.7.
  • H1 sends out an ARP request to learn the MAC address.

ARP - Step 2

  • The ARP request is broadcast to all hosts on the network.
  • Hosts that do not have the IP address 192.168.1.7 ignore the request.
  • Host H4, with IP address 192.168.1.7, responds with its MAC address.

ARP - Step 3

  • H1 receives the MAC address of H4 and can now forward its information.

Refresh Your Mind

  • What destination MAC address would be included in a frame sent from a source device to a destination device on the same local network?
    • b) The MAC address of the destination device.
  • What destination MAC address would be included in a frame sent from a source device to a destination device on a remote local network?
    • c) The MAC address of the local router interface.
  • What two protocols are used to determine the MAC address of a known destination device IP address (IPv4 and IPv6)?
    • a) ND
    • c) ARP
  • The destination MAC address of an Ethernet broadcast in hexadecimal is:
    • d) 48 F digits

Hubs - L1 Device

  • Hubs operate at Layer 1 (Physical Layer).
  • They simply forward any received signal to all connected devices.
  • This can lead to collisions and reduced network efficiency.

Switch - L2 Device

  • Switches operate at Layer 2 (Data Link Layer).
  • They maintain a MAC address table to forward traffic only to the intended destination.
  • This reduces collisions and improves network efficiency.
  • The switch learns MAC addresses by examining the source MAC address of incoming frames.