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# __Network +__
# __OSI Model__
## __OSI Model__
## __Layer 1__ Bites
* Physical Layer
* It is the connects like Cat 6 RJ45 and fiber.
* It is the connents like Cat 6 RJ45 and fiber.
* Its the on and off on the cables.
---
## __Layer 2__ - Frames
## __Layer 2__ Frames
* ### Data Link Layer.
* Packagers data int frames and transmits those frames on the network.
* Layer 2 devices view networks __logically__.
* Layer 2 devices view nteworks __logically__.
* ### MAC Address
* Media Access Control
* Physical addressing system of a device which operates on the logical topology.
* Physical addressing system of a device which operates on the logial topology.
* Uses a 48-bit address assigned to a network interface card.
* __EX:__ `D2:51:F1:3A:34:65`
* `D2:51:F1` is the vendor code
* `D2:51:F1` is the vedor code
* ### Logical Link Control
* Provides connection services and allows acknowledgment of receipt of message.
* Provides connection services and allows acknowledgemnt of receipt of message.
* Is the basic form of flow control.
* Provides basic error control functions.
* Provides basic error control functios.
* Uses a check sum.
* ### How communication synchronized.
* Isochronous
* Network devices use a common reference a common reference clock source and create time slots for transmission.
* Netwok devices use a common reference a common refernce clock source and create time slots for transmission.
* Synchronous
* Network devices agree on clocking method to indicate beginning and end of frames and can use control characters.
* Asynchronous
* Network devices reference their own internal clocks and use start and stop bits
* Asynchonous
* Network devices reference thier own internal clocks and use start and stop bits
* ### What Devices are layer 2
* network cards
* Bridges
* Briges
* Switch
* MAC Address
---
## __Layer 3__ Packets
* ### Network Layer
* Forwards traffic (routing) with logical address.
* __IP address__ IPv4 IPv6
* Logical addressing
* Ip Addressing IPv4 and IPv6
* Switching (Routing)
* Route discovery and selection
* Route discovery and selction
* Connection Services
* Bandwidth usage
* Banwidth usage
* Multiplexing
* ### How should data be __forward__ or __routed__
* Packet Switching (Most common)
* Data is divided into packets and then forwarded to is IP address.
* Circuit Switching
* Dedicated communication link is established between two devices.
* It uses one path when the connect established and the whole time the connect is there.
* Dedicated communication link is establed between two devices.
* It uses one path when the connect establed and the whole time the connect is there.
* Message Switching
* Data is divided into messages which may be stored and then forwarded.
* ### Route Discovery and Selection
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---
## __Layer 4__ - Segments
## __Layer 4__ Segments
* ### Transport Layer
* TCP Transmission Control Protocol
* Connection-oriented protocol that is a reliable way to transport segments across the network
* Will ask for an acknowledgement.
* Will ask for an acknowledgement
* ![TCP](../pics/tcp.png)
* This is a Three-way handshake.
* UDP User Datagram Protocol
* Connectionless protocol that is an unreliable way to transport segments across the network
* If it dropped, Sender is unaware
* TCP vs UDP
* ![TCP vs UDP](../pics/tcp_vs_udp.png)
* Windowing
* Allows the clients to adjust the amount opf data in eash segment.
* Allows you to send more or fewer for eash segment.
* Buffering
* Occurs When devices allocate memory to store segments if bandwidth isn't readily avaiable
---
## __Layer 5__ - Session
* Session Layer
* Keeps conversations separate to prevent intermingling of data
* Setup
* Checking of user credentaials and assigning numbers to sessions to help identify them.
* Some random number that would be assigned.
* Maintain
* Transfer Data (back and forth)
* Reestablish connedtion
* Acknowlegde receipt
* Tear down
* Ending of a session after the transfer is done or when the other party disconnects.

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