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2.2 Explain Data Link Layer Protocols and Media Access Control : - Coggle…
2.2 Explain Data Link Layer Protocols and Media Access Control :
Message Segmentation
:fire: Large streams of data are divided into smaller, more manageable pieces to send over the network.
:fire: many different conversations can be interleaved on the network, called multiplexing
:fire: Each piece must be labeled.
:fire: If part of the message fails to make it to the destination, only the missing pieces need to be retransmitted.
Protocol Data Units
:silhouettes: As application data is passed down the protocol stack, information is added at each level. This is known as the encapsulation process.
:silhouettes: The form that the data takes at each layer is known as a Protocol Data Unit (PDU).
Data - application layer PDU
Segment – Transport layer PDU
Packet – Network layer PDU
Frame – Data Link Layer PDU
Bits – Physical Layer PDU
Encapsulation Example
The encapsulation process works from top to bottom:
:star: Data is divided into segments.
:star: The TCP segment is encapsulated in the IP Packet.
:star: The IP packet is encapsulated in the Ethernet Frame.
De-encapsulation
The de-encapsulation process works from bottom to top.
De-encapsulation is the process used by a receiving device to remove one or more of the protocol headers.
:check: The data is de- encapsulated as it moves up the stack toward the end-user application.
Network Addresses
Network layer source and destination addresses - Responsible for delivering the IP packet from the original source to the final destination.
:warning: Source IP address - The IP address of the sending device, the original source of the packet.
:warning: Destination IP address - The IP address of the receiving device, the final destination of the packet.
Data Link Addresses
The purpose of the data link address is to deliver the data link frame from one network interface to another network interface on the same network.
As the IP packet travels from source to destination it is encapsulated in a new data link frame when it is forwarded by each router.
Devices on the Same Network
The network layer addresses, or IP addresses, indicate the original source and final destination.
Network portion – The left-most part of the address indicates which network the IP address is a member of.
Host portion – The remaining part of the address identifies a specific device on the network.
The data link frame which uses MAC addressing, is sent directly to the receiving device
Source MAC address - address of sending device.
Destination MAC address – address of receiving device.
Devices on a Remote Network
:checkered_flag: Sending to a remote network - the source and destination IP addresses represent hosts on different networks
:checkered_flag: The data link frame cannot be sent directly to the remote destination host. Therefore the frame is sent to the default gateway (nearest router interface).
:checkered_flag: The router removes the received Layer 2 information and adds new data link information before forwarding out the exit interface.
Data Link
Data link layer is divided into two sublayers:
:star: Logical Link Control (LLC)
Communicates with the network layer.
Identifies which network layer protocol is being used for the frame.
Allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media.
:star: Media Access Control (MAC)
Defines the media access processes performed by the hardware.
Provides data link layer addressing and access to various network technologies.
Communicates with Ethernet to send and receive frames over copper or fiber-optic cable.
Communicates with wireless technologies such as Wi-Fi and Bluetooth.
Media Access Control
Physical networks can consist of different types of physical media such as copper wires, optical fibers, and wireless consisting of electromagnetic signals, radio and microwave frequencies, and satellite links.
Providing Access to Media
At each hop along the path, a router:
:snowflake: Accepts a frame from a medium
:snowflake: De-encapsulates the frame
:snowflake: Re-encapsulates the packet into a new frame
:snowflake: Forwards the new frame appropriate to the medium of that segment
Data Link Layer Standards
Engineering organizations that define open standards and protocols that apply to the network access layer include:
:beer_mugs: Institute of Electrical and Electronics Engineers (IEEE)
:beer_mugs: International Telecommunication Union (ITU)
:beer_mugs: International Organization for Standardization (ISO)
:beer_mugs: American National Standards Institute (ANSI)
Media Access Control
Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway.
Physical topology
Refers to the physical connections and identifies how end devices and infrastructure devices such as routers, switches, and wireless access points are interconnected.
Logical Topology
-Refers to the way a network transfers frames from one node to the next. These logical signal paths are defined by data link layer protocols.
Point-to-Point
- Permanent link between two endpoints.
Hub and Spoke
- A central site interconnects branch sites using point-to-point links.
Mesh
- Provides high availability, but requires that every end system be interconnected to every other system. Administrative and physical costs can be significant.
Physical Point-to-Point Topology
:tada: Frames are placed on the media by the node at one end and taken from the media by the node at the other end of the point-to- point circuit.
Logical Point-to-Point Topology
:lock: End nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices.
Logical Point-to-Point Topology
:soccer: End nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices.
:soccer: However, the use of physical devices in the network does not affect the logical topology.
:soccer: The logical connection between nodes forms what is called a virtual circuit.
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Physical LAN Topologies
Star
:point_right::skin-tone-2: End devices are connected to a central intermediate device. Use Ethernet switches.
Extended Star
:point_right::skin-tone-2: Additional Ethernet switches interconnect other star topologies.
Bus
:point_right::skin-tone-2: Used in legacy networks. All end systems are chained to each other and terminated in some form on each end. Switches are not required to interconnect the end devices. Bus topologies using coax cables were used in legacy Ethernet networks because it was inexpensive and easy to set up.
Ring
:point_right::skin-tone-2: End systems are connected to their respective neighbor forming a ring. Unlike the bus topology, the ring does not need to be terminated. Ring topologies were used in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.
Half and Full Duplex
Half-Duplex Communication
:ear_of_rice: Both devices can transmit and receive on the media but cannot do so simultaneously.
:ear_of_rice: Used in legacy bus topologies and with Ethernet hubs.
:ear_of_rice: WLANs also operate in half-duplex.
Full-Duplex Communication
:icecream: Both devices can transmit and receive on the media at the same time.
:icecream: Data link layer assumes that the media is available for transmission for both nodes at any time.
:icecream: Ethernet switches operate in full-duplex mode by default, but can operate in half-duplex if connecting to a device such as an Ethernet hub.
Media Access Control Methods
Contention-Based Access
:ice_cream: Nodes operate in half- duplex.
:ice_cream: Compete for the use of the medium.
:ice_cream: Only one device can send at a time.
Controlled Access
:+1::skin-tone-2: Each node has its own time to use the medium.
:+1::skin-tone-2: Legacy Token Ring LANs are an example
Contention-based Access
Carrier Sense Multiple Access/Collision Detection (CSMA/CD) process is used in half- duplex Ethernet LANs.
:arrow_right: If two devices transmit at the same time, a collision will occur.
:arrow_right: Both devices will detect the collision on the network.
:arrow_right: Data sent by both devices will be corrupted and will need to be resent.
CSMA/CA
:jack_o_lantern: Uses a method to detect if the media is clear.
.
:jack_o_lantern: Does not detect collisions but attempts to avoid them by waiting before transmitting.
The Frame
Each frame type has three basic parts:
:champagne: Header
:champagne: Data
:champagne: Trailer
Frame Fields
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Ethernet MAC Adresses
MAC Addresses and Hexadecimal
An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (4 bits per hexadecimal digit).
:checkered_flag: Hexadecimal is used to represent Ethernet MAC addresses and IP Version 6 addresses.
:checkered_flag: Hexadecimal is a base sixteen system using the numbers 0 to 9 and the letters A to F.
:checkered_flag: It is easier to express a value as a single hexadecimal digit than as four binary bits.
:checkered_flag: Hexadecimal is usually represented in text by the value preceded by 0x (E.g., 0x73).
:checkered_flag: Convert the decimal or hexadecimal value to binary, and then to convert the binary value to either decimal or hexadecimal as needed.
Ethernet Identity
MAC addresses were created to identify the actual source and destination.
:writing_hand::skin-tone-2: The MAC address rules are established by IEEE.
:writing_hand::skin-tone-2: The IEEE assigns the vendor a 3-byte (24-bit) code, called the Organizationally Unique Identifier (OUI).
IEEE requires a vendor to follow two simple rules:
:writing_hand::skin-tone-2: All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes.
:writing_hand::skin-tone-2: All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.
Frame Processing
The MAC address is often referred to as a burned-in address (BIA) meaning the address is encoded into the ROM chip permanently. When the computer starts up, the first thing the NIC does is copy the MAC address from ROM into RAM.
:beer_mugs: When a device is forwarding a message to an Ethernet network, it attaches header information to the frame.
:beer_mugs: The header information contains the source and destination MAC address.
MAC Address Representations
:check: Use the ipconfig /all command on a Windows host to identify the MAC address of an Ethernet adapter. On a MAC or Linux host, the ifconfig command is used.
:check: Depending on the device and the operating system, you will see various representations of MAC addresses
Unicast MAC Address
:recycle: A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to a single destination device.
:recycle: For a unicast packet to be sent and received, a destination IP address must be in the IP packet header and a corresponding destination MAC address must also be present in the Ethernet frame header.
Multicast MAC Address
Multicast addresses allow a source device to send a packet to a group of devices.
:explode: Devices in a multicast group are assigned a multicast group IP address in the range of 224.0.0.0 to 239.255.255.255 (IPv6 multicast addresses begin with FF00::/8).
:explode: The multicast IP address requires a corresponding multicast MAC address that begins with 01-00-5E in hexadecimal.
Broadcast MAC Address
:sunny: Many network protocols, such as DHCP and ARP, use broadcasts.
:sunny: A broadcast packet contains a destination IPv4 address that has all ones (1s) in the host portion indicating that all hosts on that local network will receive and process the packet.
:sunny: When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address of FF-FF-FF-FF-FF- FF in hexadecimal (48 ones in binary).
Destination on Same Network
There are two primary addresses assigned to a device on an Ethernet LAN:
● Physical address (the Ethernet MAC address)
● Logical address (the IP address)
As an example, PC-A sends an IP packet to the file server on the same network. The Layer 2 Ethernet frame contains:
● Destination MAC address
● Source MAC address
The Layer 3 IP packet contains:
● Source IP address
● Destination IP address
Destination on Remote Network
When the destination IP address is on a remote network, the destination MAC address will be the address of the host’s default gateway.
In the figure, PC-A is sending an IP packet to a web server on a remote network.
★The destination IP address is that of the File Server.
★The destination MAC address is that of Ethernet interface of R1.
