CCNA-1. Chapter 7. Data link OSI level.

Data Link layer performs two basic services:

• Allows the upper layers to access different media (framing)
• Controls how data is placed onto the media and is received (media access control, error detection)

Data Link Sublayers:

• Logical Link Control (LLC):
  
  • Identify Network layer protocol
  • Frames the network layer packet
• Media Access Control (MAC):
  
  • Provides Data Link layer addressing
  • Delimiting of data according to the physical requirements (start and end of frame)
  • Regulating the placement of data frames onto the media

Network topology types:

• Physical topology  (representation of how the media is used to interconnect the devices)
• Logical topology (independent of physical layout. Define how connection between the nodes appears to the Data Link layer. LLC (Frame Type) and MAC method are choosed based on Logical topology, may differ from physical topology)

Data Link layer protocols specify the media access control methods. Media access control methods (define the processes of access the network media and transmiting frames) depend on:

• Media sharing (If and how the nodes share the media)
  
  • Shared media methods types:
    
    • Controlled (Deterministic) No collisions, Device wait for turn . MAC methods:
      
      • 
        Token passing. Recieve-> wait for token-> transmit (FDDI, Token Ring)
    • Contention-based (Non-deterministic) Collisions, Transmit at any time. Use Carrier Sense Multiple Access (CSMA) MAC methods for contention resolving:
      
      • 
        CSMA\CD - Collision Detection. Monitor media-> transmit the data (Ethernet
        
        • Carrier Sense (detects a signal on shared media)
        • Jam Signal (notify the other devices of a collision (32 bit))
        • Backoff algoritm (stop transmitting for a random time)
      • 
        CSMA\CA - Collision Avoidance. Monitor media-> send a notification-> transmit the data (Wireless LAN)
  • Non-Shared media (require little or no control placing frames onto the media, simpler rules and procedures for MAC)
• Logical Topology .
  
  • Point-to-point:
    
    • 
      Half-duplex (devices cannot simultaneously ransmit and receive on the media)
    • 
      Full-duplex (evices can transmit and receive on the media at the same time)
  • Multi-Access (Bus) :
    
    • All the nodes receive all the frames transmitted by any node in segment
    • Node determine if a frame is to be accepted by examining MAC address.
  • Ring

                                                                                       FRAME Field types:

• Frame Start (the beginning of the frame)
• Addressing (source and destination address)
• Type/Length (upper layer protocol or possibly length of frame)
• Control (Flow control services)
  
  • Priority/Quality of Service field - Indicates a particular type of communication service for processing
  • Logical connection control field - Used to establish a logical connection between nodes
  • Physical link control field - Used to establish the media link
  • Flow control field - Used to start and stop traffic over the media
  • Congestion control field - Indicates congestion in the media
• Data (Network layer packet)
• Error detection
• Frame Stop (end of the frame)

Ethernet frame:

PPP frame:

Wireless 802.11 frame:

 

• Protocol Version field - Version of 802.11 frame in use
• Type and Subtype fields - Identifies one of three functions and sub functions: control, data, and management
• To DS field - Set to 1 in data frames destined for the distribution system (devices in the wireless structure)
• From DS field - Set to 1 in data frames exiting the distribution system
• More Fragments field - Set to 1 for frames that have another fragment
• Retry field - Set to 1 if the frame is a retransmission of an earlier frame
• Power Management field - Set to 1 to indicate that a node will be in power-save mode
• More Data field - Set to 1 to indicate to a node in power-save mode that more frames are buffered for that node
• Wired Equivalent Privacy (WEP) field - Set to 1 if the frame contains WEP encrypted information for security
• Order field - Set to 1 in a data type frame that uses Strictly Ordered service class (does not need reordering)
• Duration/ID field - Depending on the type of frame, represents either the time, in microseconds, required to transmit the frame or an association identity (AID) for the station that transmitted the frame
• Destination Address (DA) field - MAC address of the final destination node in the network
• Source Address (SA) field - MAC address of the node the initiated the frame
• Receiver Address (RA) field - MAC address that identifies the device that is the immediate recipient of the frame
• Transmitter Address (TA) field - MAC address that identifies the wireless device that transmitted the frame
• Sequence Number field - Indicates the sequence number assigned to the frame; retransmitted duplicate numbers
• Fragment Number field - Indicates the number for each fragment of a frame
• Frame Body field - Contains the information being transported; for data frames, typically an IP packet
• FCS field - Contains a 32-bit cyclic redundancy check (CRC) of the frame

Upper layer standards and organizations:

• Internet Engineering Task Force (IETF) in RFCs.

Data link layer standards and organizations:

• International Organization for Standardization (ISO)
  
  • 
    HDLC (High Level Data Link Control)
• Institute of Electrical and Electronics Engineers (IEEE)
  
  • 802.1
    
    • 802.1x (port-based Network Access Control (PNAC))
  • 
    802.2 (LLC)
  • 
    802.3 (Ethernet)
    
    • 802.3ac (VLAN support added by extended maximum frame size 1518->1522 bytes)
    • 802.3ae (10Gb/s Ethernet)
  • 
    802.5 (Token Ring)
  • 
    802.11 (Wireless LAN or WLAN) - WiFi
    
    • 
      802.11a - 5 GHz frequency,speed of up to 54 Mbps.
      
      • 
        smaller coverage area
      • 
        less effective at penetrating building structures.
      • 
        not interoperable with the 802.11b and 802.11g standards
    • 
      802.11b - 2.4 GHz frequency, speed of up to 11 Mbps.
      
      • 
        longer range
      • 
        better able to penetrate building structures
      • 
        slow speed
    • 
      802.11g - 2.4 GHz frequency, speeds of up to 54 Mbps.
      
      • 
        interoperable with the 802.11b
    • 
      802.11n - 2.4 Ghz or 5 GHz. speed 100 Mbps to 210 Mbps, distance up to 70 meters.
  • 
    802.15 (Wireless Personal Area Network or WPAN) - Bluetooth (1 to 100 meters)
  • 
    802.16 (Worldwide Interoperability for Microwave Access or WiMAX) - point-to-multipoint topology  broadband access.
• American National Standards Institute (ANSI)
  
  • 
    Q922 (Frame Relay Standard)
  • 
    Q921 (ISDN Data Link Standard)
  • 
    HDLC (High Level Data Link Control)
• International Telecommunication Union (ITU)
  
  • 
    3T9.5 (MAC protocol of token ring)
  • 
    ADCCP (Advanced Data Communications Control Protocol ) functionally equivalent to HDLC

Physical layer standards (physical, electrical, mechanical properties of the media and connectors, encoding and control information techniques) and organizations:

• The International Organization for Standardization (ISO)
  
  • ISO 8877(RJ-45 connector)
• The Institute of Electrical and Electronics Engineers (IEEE) (cooper cabling categories)
• The American National Standards Institute (ANSI)
• The International Telecommunication Union (ITU)
• The Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA)
  
  • EIA-TIA 568a (wire color to pinouts)
  • EIA-TIA 568b (wire color to pinouts)
  • EIA-TIA 606A (labeling and administration)
• National telecommunications authorities such as the Federal Communication Commission (FCC) in the USA.