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Layer 7
- Application
- PDU (Protocol Data Unit): Data
- Software applications that provide services such as DNS, FTP, DHCP, SNMP, Telnet, and HTTP.
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Layer 6
- Presentation
- PDU (Protocol Data Unit): Data
- Encrypt and encode (or represent) data in a standard format such as SSL, ASCII, MIME, shells and redirectors operate at this layer.
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Layer 5
- Session
- PDU (Protocol Data Unit): Data
- Set up and tear down sessions between hosts. RPC, NetBIOS, SQL connections, and API operate at this layer.
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Layer 4
- Transport
- PDU (Protocol Data Unit): Segments
- Data Encode by the uppder layers is broken into segments that receive either TCP or UDP headers. These protocols deal with the flow of the data and sequence of the segments.
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Layer 3
- Network
- PDU (Protocol Data Unit): Packets
- The layer 4 segments and encapsulated into IP packets. This layer deals with the path selection (routing) of the data over the network. IPv6 and NAT operate at this level.
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Layer 2
- Data Link
- PDU (Protocol Data Unit): Frames
- Layer 3 packets are encapsulated into frames with headers using MAC addresses to identify source and destination. Ethernet, wireless local-area network (WLAN), Asynchronous Transfer Mode (ATM), and Point to Point Protocol (PPP) operate at this layer.
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Layer 1
- Physical
- PDU (Protocol Data Unit): Bits
- The layer 2 frams are encoded into bits, represented as electrical signals, light waves, or radio waves, and are transmitted over cables (copper or fiber-optic) or the air (radio waves).
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Protocol Data Units
PDU Acronym
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Data, Segments, Packets, Frames, Bits
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