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The application layer is leveraged by end-user software such as web browsers and email clients. At this layer, protocols allow software to send and receive information; presenting meaningful data to users.
Protocol Data Unit: data
Example protocols: HTTP, FTP, SMTP, POP3, IMAP, SSH, Telnet, DNS
The presentation layer prepares data for the application layer. It defines how two devices should encode, encrypt, and compress data to ensure that data is received correctly on the other end. This layer takes data transmitted by the application layer; preparing it for transmission over the session layer.
Protocol Data Unit: data
Example protocols: SSL, TLS, AFP, XDR
The session layer creates communication channels (sessions) between devices. It ensures that those sessions stay open and functional while data is transferred, and closes them when communication ends. This layer can set checkpoints during a data transfer so that devices can resume that transfer in the case of interruption.
Protocol Data Unit: data
Example protocols: NetBIOS, SMPP, RPC, SDP, SMB
The transport layer breaks the data transferred in the session layer into segments. It reassembles the segments on the receiving end into data that can be used by the session layer. This layer also carries out flow control and error control.
Protocol Data Unit: segment
Example protocols: TCP, UDP, SPX
The network layer has two main functions. The first function is breaking up segments into network packets, which are reassembled on the receiving end. The second function is routing packets by discovering the best path across a physical network. This layer uses network addresses to route packets to a destination node.
Protocol Data Unit: packet
Example protocols: IP, ICMP, ARP, NAT
The data link layer establishes and terminates a connection between two physically-connected nodes on a network. Packets are broken in up into frames, which are sent from source to destination. This layer is composed of two sublayers. Logical Link Control (LLC) identifies network protocols, performs error checking, and synchronizes frames. Media Access Control (MAC) uses MAC addresses to connect devices and define permissions to transmit and receive data.
Protocol Data Unit: frame
Example protocols: LLC, MAC, Ethernet, Wi-Fi, VLAN
The physical layer is responsible for the physical cable or wireless connection between network nodes. This layer is responsible for transmission of raw data bits, while taking care of bit rate control.
Protocol Data Unit: bit
Example protocols: USB, DSL, Ethernet, Wi-Fi, ISDN