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CHAPTER 2.2 Explain Data Link Layer Protocols and Media Access Control. -…
CHAPTER 2.2 Explain Data Link Layer Protocols and Media Access Control.
Data Encapsulation
Message Segmentation
:!:Large streams of data are
divided into smaller.
:check: By sending smaller pieces, many different conversations can be interleaved on the network, called
multiplexing.
:check: Each Piece must be labeled.
:check: If part of the message fails to make it to the destination, only the missing pieces need to be retransmitted.
Protocol Data Units
:!: As application data is passed down the protocol stack, information is added at each level. This is known as the
Encapsulation
process.
The form that the data takes at each layer is known as a Protocol Data Unit :check:
◦ 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 work process works from top to bottom:
◦ Data is divided into segments. :check:
◦ The TCP segment is encapsulated in the IP Packet. :check:
◦ The IP packet is encapsulated in the Ethernet Frame. :check:
De-encapsulation
:!:The de-encapsulation work process works from bottom to top:
:CHECK:Process used by a receiving device to remove one or more of protocol headers.
◦ The data is de-encapsulation as it moves up the stack toward the end-user application.
Data Acess
Network Addresses
Responsible for delivering the IP packet from the original source to the final destination. :<3:
◦ Source IP address - The IP address of the sending device, the original source of the packet. :star:
◦ Destination IP address - The IP address of the receiving device, the final destination of the packet.:star:
Data Link Addresses
To deliver the data link frame from one network interface to another network interface on the same network. :<3:
◦ 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. :star:
Devices on the Same Network
The network layer addresses, or IP addresses, indicate the original source and final destination. :<3:
◦ Network portion – The left-most part of the address indicates which network the IP address is a member of. :star:
◦ Host portion – The remaining part of the address identifies a specific device on the network. :star:
:checkered_flag: 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
Sending to a remote network - the source and destination IP addresses represent hosts on different networks.
◦ 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). :star:
◦ The router removes the received Layer 2 information and adds new data link information before forwarding out the exit interface. :star:
Purpose of the Data Link Layer
The Data Link Layer
LAYER 2 DATA LINK ADDRESSES
Data Link Sublayers
Data link layer is divided into two
sublayers:
◦ Identifies which network layer protocol is being used for the frame.
◦ Communicates with the network layer.
◦ Logical Link Control (LLC)
◦ Media Access Control (MAC)
◦ Defines the media access processes performed
by the hardware.
◦ Communicates with wireless technologies such
as Wi-Fi and Bluetooth.
Media Access Control
As packets travel from the source host to the destination host, they travel over different physical networks.
Physical networks can consist of different types of physical media such as copper wires, consisting of electromagnetic signals, radio and microwave frequencies, and satellite links.
Providing Access To Media
At each hop along the path, a router:
• Accepts a frame from a medium
• De-encapsulates the frame
• Re-encapsulates the packet into a new frame
• Forwards the new frame appropriate to the medium of that segment.
Data Link Layer Standards
Engineering organizations that define open standards andprotocols that apply to the network access layer unclude: :red_flag:
◦ Institute of Electrical and Electronics Engineers (IEEE)
◦ International Telecommunication Union (ITU)
◦ International Organization for Standardization (ISO)
◦ American National Standards Institute (ANSI)
Topologies
Controlling Access to the Media
Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway. :<3:
The absence of any media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles. :<3:
However, not all roads and entrances are the same. Traffic can enter the road by merging, by waiting for its turn at a stop sign, or by obeying signal lights. A driver follows a different set of rules for each type of entrance. :<3:
PHYSICAL TOPOLOGY
Physical topology
- Refers to the physical connections and identifies hoe end devices and
infrastructure devices such as
routers, switches, and wireless access points are interconnected.
LOGICAL TOPOLOGY
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.
WAN Topologies
Common Physical WAN Topologies
Point-to-Point
- Permanent link between two endpoints. :check:
Hub and Spoke
- A central site interconnects branch sites using point-to-point links. :check:
Mesh
- Provides high availability, but requires that every end system be interconnected to every other system. Administrative and physical costs can be significant. :check:
Physical Point-to-Point Topology
Frames
are placed on 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
• End nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices.
• However, the use of physical devices in the network does not affect the logical topology.
• The logical connection between nodes forms what is called a virtual circuit.
Physical LAN Topologies
Star - End devices are connected to a central intermediate device. Use Ethernet switches.
Extended Star - Additional Ethernet switches interconnect other star topologies.
Bus - Consists of one continuous length of cabling (trunk) and a terminating resistor (terminator) at each end
Ring- Each networked device is connected to two others, like points on a circle.
Half and Full Duplex
Half-Duplex Communication
◦ Both devices can transmit and receive on the media but cannot do so simultaneously. :fire:
◦ Used in legacy bus topologies and with Ethernet hubs. :fire:
◦ WLANs also operate in half-duplex. :fire:
Full-Duplex Communication
◦ Both devices can transmit and receive on the media at the same time. :lock:
◦ Data link layer assumes that the media avaible for the media is transmission for both nodes at any time. :lock:
◦ 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. :lock:
LAN Topologies
Media Access Control Methods
Contention-Based Access
◦ Nodes operate in half- duplex.
. ◦ Compete for the use of the medium.
◦ Only one device can send at a time.
Controlled Access
◦ Each node has its own time to use the medium.
◦ Legacy Token Ring LANs are an example.
Contention-based Access - CSMA/CD
Carrier Sense Multiple Access/Collision
Detection (CSMA/CD) process in used in half-duplex Ethernet LANs.
If two devices transmit at the same time, a collision will occur.
◦ Both devices will detect the collision on the
network.
◦ Data sent by both devices will be corrupted and will be send.
CSMA/CA
Uses a method to detect if the media is clear.
Does not detect collisions but attempts to avoid them by waiting before transmitting.
NOTE:
Using switches do not use a contention-based system because the switch and the host NIC operate in full-duplex mode.
Data Link Frame
The Frame
Each frame type has three basic parts:
◦ Header
◦ Data
◦ Trailer
Frame Fields
Frame start and stop indicator flags - Identifies the beginning and end limits of the frame.
Addressing
- Indicates the source and destination nodes.
Type
- Identifies the Layer 3 protocol in the data field.
Control
- Identifies special flow control services such as QoS.
Data
- Contains the frame payload (i.e., packet header, segment header, and the data).
Layer 2 Addresses
:check: Each data link frame contains the source data link address of the NIC card sending the frame.
:check: The destination data link address of the NIC card
receiving the frame
LAN and WAN Frames
Layer 2 protocol used for a topology is determined by the technology.
Data link layer protocols include:
◦ Ethernet
◦ 802.11 Wireless
◦ Point-to-Point Protocol (PPP)
◦ HDLC
◦ Frame Relay
Ethernet MAC Address
MAC Addresses and Hexadecimal
An
Ethernet MAC address
is a 48-bit binary value expressed
as 12 hexadecimal digits (4 bits per hexadecimal digit)
Hexadecimal is used to represent Ethernet MAC addresses and IP Version 6 addresses.
• Hexadecimal is a base sixteen system using the numbers 0 to 9 and the letters A to F.
• It is easier to express a value as a single hexadecimal digit than as four binary bits.
• Hexadecimal is usually represented in text by the value preceded by 0x (E.g., 0x73).
Convert the decimal or hexadecimal value to binary, and then to convert the binary value to either decimal or hexadecimal as needed.
MAC Addresses: Ethernet Identity
◦ The MAC address rules are established by IEEE.
◦ The IEEE assigns the vendor a 3-byte (24-bit) code, called the Organizationally Unique Identifier (OUI)
Created to identify the actual source
and destination :<3:
IEEE requires a vendor to follow two simple rules:
• All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes.
• All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.
Frame Processing
Referred to as a burned-in address (BIA)
meaning the address is encoded into the ROM chip permanently.
When a device is forwarding a message to an Ethernet network, it attaches header information to the frame.
The header information contains the source and destination MAC address.
MAC Address Representations
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.
Depending on the device and the operating system, you will see various representations of MAC addresses.
Unicast MAC Address
Many network protocols, such as DHCP and ARP, use broadcasts.
A broadcast packet contains a destination IPv4 address that has all ones (1s) in the host.
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).
Multicast MAC Address
allow a source device to send a packet to a group of device.
IPv6 multicast addresses begin with FF00::/8
Multicast MAC address that begins with 01-00-5E in Hexadecimal.
MAC and IP
Destination on Same Network
Two primary addresses assigned to a device on an Ethernet LAN:
◦ Physical address (the Ethernet MAC address).
◦ Logical address (the IP address).
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 address of 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.
