Ct1403 Lecture#2: DATA LINK LAYER Asma AlOsaimi . 2016

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

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 802.2 - LLC and 802.3 standards-MAC+Physical Section 5.1.1.1

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 Section 5.1.1.1

Media access control There are two types. Controlled access control– no collision Contention based access control – detect and clear collision

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 802.3 Ethernet networks CSMA/CA for 802.11 wireless networks Section 4.4.3.3

Controlled Access Characteristics Controlled Access Technologies Section 4.4.3.5 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)

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 Section 5.1.1.3

Frame Transmission (CSMA/CD)

Frame Transmission (CSMA/CD)

Frame reception (CSMA/CD)

Frame reception (CSMA/CD)

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 802.11 wireless networking technologies Section 5.1.1.3

Data Link Layer Layer 2 Standards Section 4.3.3

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.8100 - 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) Section 4.3.3.1

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 Section 5.1.2.1 Ethernet II is the Ethernet frame format used in TCP/IP networks.

Data Link Frame Ethernet Frame Section 4.4.4.6

Data Link Frame Point-to-Point Protocol Frame Section 4.4.4.7

Data Link Frame 802.11 Wireless Frame Section 4.4.4.8

Ethernet Operation MAC Address: Ethernet Identity Layer 2 Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits Section 5.1.1.4

Ethernet MAC MAC Address Representations Section 5.1.3.2

Ethernet MAC Unicast MAC Address Section 5.1.3.3

Ethernet MAC Broadcast MAC Address Section 5.1.3.4

Ethernet MAC Multicast MAC Address Section 5.1.3.5 Multicast MAC address is a special value that begins with 01-00-5E in hexadecimal Range of IPV4 multicast addresses is 224.0.0.0 to 239.255.255.255

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 Section 5.1.4.1

Ethernet MAC End-to-End Connectivity, MAC, and IP Section 5.1.4.2 http://cisco.edu.mn/CCNA_R&S_(Introduction_to_Networking)/course/module5/index.html#5.1.4.1

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

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.

ARP Issues HOW ARP can create a problem

ARP Issues Mitigating ARP Problems

Recourses: L: Rehab AlFallaj, lecture notes Cisco slides