1. Ct1403 Lecture#2: DATA LINK LAYER
Asma AlOsaimi

2. Objectives In this chapter, you will learn to:
Describe the operation of the Ethernet sublayers.
Identify the major fields of the Ethernet frame.
Describe the purpose and characteristics of the Ethernet MAC address.
ARP
Arp issues

3. Ethernet Operation Ethernet
Ethernet standards –
Most widely used LAN technology
Operates in the data link layer (Layer 2 protocols) and the physical layer (Layer 1 technologies)
Supports data bandwidths of 10, 100, 1000, 10,000, 40,000, and 100,000 Mbps (100 Gbps)
Define and Two separate sub layers of the data link layer to operate - Logical link control (LLC) and the MAC sublayers
Family of networking technologies that are defined in the IEEE LLC and standards-MAC+Physical
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4. Ethernet Operation LLC and MAC Sublayers
Handles communication between software layers and hardware layers
NIC driver
MAC
Implemented by hardware, typically in the computer NIC
Two primary responsibilities:
Data encapsulation
Frame delimiting- synchronization
Addressing – MAC
Error detection - CRC
Media access control – collision conrtol
Control of frame placement on and off media
Media recovery
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5. Media access control There are two types.
Controlled access control– no collision
Contention based access control – detect and clear collision

6. Contention-Based Access
Characteristics
Contention-Based Technologies
Stations can transmit at any time
Collision exist
There are mechanisms to resolve contention for the media
CSMA/CD for Ethernet networks
CSMA/CA for wireless networks
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7. Controlled Access Characteristics Controlled Access Technologies
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Characteristics
Controlled Access Technologies
Only one station can transmit at a time
Devices wishing to transmit must wait their turn
No collisions
May use a token passing method
Token Ring (IEEE 802.5)
Fiber Distributed Data Interface (FDDI)

8. Media Access Control Contention-Based Technologies
The two commonly used methods are:
CSMA/Collision Detection
The device monitors the media for the presence of a data signal
If a data signal is absent, indicating that the media is free, the device transmits the data
If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later
While Ethernet networks are designed with CSMA/CD technology(full duplex) , with today’s intermediate devices, collisions do not occur and the processes utilized by CSMA/CD are really unnecessary
Wireless connections in a LAN environment still have to take collisions into account
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9. Frame Transmission (CSMA/CD)

10. Frame Transmission (CSMA/CD)

11. Frame reception (CSMA/CD)

12. Frame reception (CSMA/CD)

13. Ethernet Operation Media Access Control
Contention-Based Technologies
The two commonly used methods are:
CSMA/Collision Avoidance (CSMA/CA) media access method
Device examines the media for the presence of data signal - if the media is free, the device sends a notification across the media of its intent to use it
The device then sends the data.
Used by wireless networking technologies
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14. Data Link Layer Layer 2 Standards
Section 4.3.3

15. Layer 2 Standards Data Link Layer Standards
Standard organization
Networking Standards
IEEE
802.2: Logical Link Control (LLC)
802.3: Ethernet
802.4: Token bus
802.5: Token passing
802.11: Wireless LAN (WLAN) & Mesh (Wi-Fi certification)
802.15: Bluetooth
802.16: WiMax
ITU-T
G.992: ADSL
G G.8199: MPLS over Transport aspects
Q.921: ISDN
Q.922: Frame Relay
ISO
HDLC (High Level Data Link Control)
ISO 9314: FDDI Media Access Control (MAC)
ANSI
X3T9.5 and X3T12: Fiber Distributed Data Interface (FDDI)
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16. Ethernet Encapsulation
Early versions of Ethernet were relatively slow at 10 Mbps
Now operate at 10 Gigabits per second and faster
Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent
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Ethernet II is the Ethernet frame format used in TCP/IP networks.

17. Data Link Frame Ethernet Frame
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18. Data Link Frame Point-to-Point Protocol Frame
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19. Data Link Frame 802.11 Wireless Frame
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20. Ethernet Operation MAC Address: Ethernet Identity
Layer 2 Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits
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21. Ethernet MAC MAC Address Representations
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22. Ethernet MAC Unicast MAC Address
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23. Ethernet MAC Broadcast MAC Address
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24. Ethernet MAC Multicast MAC Address
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Multicast MAC address is a special value that begins with E in hexadecimal
Range of IPV4 multicast addresses is to

25. MAC address IP address This address does not change
MAC and IP MAC and IP
MAC address
This address does not change
Similar to the name of a person
Known as physical address because physically assigned to the host NIC
IP address
Similar to the address of a person
Based on where the host is actually located
Known as a logical address because assigned logically
Assigned to each host by a network administrator
Both the physical MAC and logical IP addresses are required for a computer to communicate just like both the name and address of a person are required to send a letter
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26. Ethernet MAC End-to-End Connectivity, MAC, and IP
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27. ARP – Address Resolution protocol
ARP relies on certain types of Ethernet broadcast messages and Ethernet unicast messages, called ARP requests and ARP replies.
The ARP protocol provides two basic functions:
Resolving IPv4 addresses to MAC addresses
Maintaining a table of mappings

39. Entries in the ARP table are time stamped.
static map entries can be entered in an ARP table, but this is rarely done. Static ARP table entries do not expire over time and must be manually removed.

40. ARP Issues HOW ARP can create a problem

41. ARP Issues Mitigating ARP Problems

42. Recourses:
L: Rehab AlFallaj, lecture notes
Cisco slides