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CHAPTER 2 : NETWORK ACCESS PART 2.2 - Coggle Diagram
CHAPTER 2 : NETWORK ACCESS
PART 2.2
Data Encapsulation
Message Segmentation
Large streams of data are
divided into smaller, more manageable pieces to send over the network.
◦ By sending smaller pieces,
many different conversations can be interleaved on the network, called
multiplexing
◦ Each piece must be labeled.
◦ 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 (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:
◦ Data is divided into segments.
◦ The TCP segment is encapsulated in the IP Packet.
◦ 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.
◦ The data is de- encapsulated as it moves up the stack toward the
end-user application.
Data Access
Network Addresses
Network layer source and
destination addresses - Responsible for delivering the IP packet from the
original source to the final destination.
◦ Source IP address - The IP address of the sending
device, the original source of the packet.
◦ 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
Sending to a remote network - the source and destination IP address 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).
The router removes the received Layer 2 information
and adds new data link information before forwarding
out the exit interface.
Purpose of the Data Link Layer
The Data Link Layer
(In descending order)
Application
Presentation
Session
Transport
Network
Data Link (the data link layer prepares network data for the physical network)
Physical
Data Link Sublayers
Data link layer is divided into two
sublayers:
◦ 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.
◦ 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
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, 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:
• 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 and protocols
that apply to the network access layer include:
◦ 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. 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.
However, not all roads and entrances are the same. Traffic can enter the road y 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.
Physical and Logical Topologies
8Physical topology*- Refers to the physical connections and
identifies how end devices and
infrastructure devices such as routers,
switches, and wireless access points are
interconnected.
Physical and Logical Topologies (Cont.)
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.
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
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
• 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.
LAN Topologies
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
- 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
- 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
◦ Both devices can transmit and receive on the media but cannot do so simultaneously.
◦ Used in legacy bus topologies and with
Ethernet hubs.
◦ WLANs also operate in half-duplex.
Half and Full Duplex (Cont.)
Full-Duplex Communication
◦ Both devices can transmit and receive on the media at the
same time.
◦ Data link layer assumes that the media is available for
transmission for both nodes at any time.
◦ 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
◦ Nodes operate in half- duplex.
◦ Compete for the use of the medium.
◦ Only one device can send at a time.
Media Access Control Methods (Cont.)
Controlled Access
◦ Each node has its own time to use the
medium.
◦ Legacy Token Ring LANs are an example
Contention-based Access - CSMA/CD
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: Ethernet LANs 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
Structure of the frame and the fields contained in the header and trailer depend on layer 3 protocol
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
Each data link frame contains the source data link address of the NIC card sending the frame, and 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 Addresses
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
MAC addresses were created to identify the actual source and destination.
◦ 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).
§ 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
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.
§ 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.
Ethernet MAC Addresses
Unicast MAC Address
A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to a single destination
device.
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.
Broadcast 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 portion indicating that all hosts on that local network will receive and process the packet.
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
Multicast addresses allow a source device to send a packet to a group of devices.
◦ 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).
◦ The multicast IP address requires a corresponding multicast MAC address that begins with 01-00-5E in hexadecimal.
MAC and IP
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.