Application – The application layer provides network services to user applications. For example, the NFS user interface can be mapped to this layer of the model. Other examples would include: file transfer, electronic mail, remote access, client/server processes, information location, and network management.

Presentation-The presentation layer provides code formatting and conversion services. If necessary, the presentation layer translates between multiple data representation formats for text, data, audio, video, and graphics. The presentation layer negotiates data transfer syntax for the application layer. Data encryption would be a function handled at the presentation layer.

Session-The session layer establishes, manages, and terminates communication sessions between applications. Essentially, the session layer coordinates service requests and responses that occur when applications communicate between different hosts. For example, the session layer would establish duplex (full or half), define and group formatted data, and offer some session recovery or checkpoint mechanism between the applications coordinated between the hosts.

Transport- The transport layer defines end-to-end connectivity between host applications. Transport services include the following functions. Segment upper-layer applications, Establish end-to-end operations, send segments from one end host to another end host ensuring data integrity through the use of checksum calculations and flow control mechanisms, and ensure data reliability (optional) wherein users can request reliable data transport. The transport layer utilizes multiplexing. The transport layer can be connectionless or connection-oriented. TCP is connection-oriented whereas udp is not.

Network-The network layer determines the best way to move data from one place to another. It manages device addressing and tracks the location of devices on the network. The router operates at this layer. IP is an example of a protocol that provides the functionality defined at this layer. The determination of best path is done by: path determination, communicating path information, and addressing.

Data-link-The data link layer provides data transport across a physical link. To do so, the data link handles the following operations: physical addressing, network topology, line discipline, error notification, orderly access to the physical medium, and optionally flow control. The IEEE divided the data-link into 2 sub-layers. Namely, LLC and MAC.

LLC- The LLC sublayer manages communications between devices over a single link of a network. It defines the fields that allow multiple higher-layer protocols to share use of the data link.

MAC- the MAC sublayer provides access to the LAN medium in an orderly manner. The MAC address is a 48-bit (12 hexadecimal digits) number. The first 3 bytes (6 hex) contain a manufacturer identification code.

Physical-The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating the physical link between the systems. This would include: voltage levels, data rates, maximum transmission distances, and physical connectors.

Connection-oriented sessions- Both sending and receiving application programs inform their respective operating systems that a connection will be initiated. With TCP, an initial synchronize request (SYN) is sent from the client. Server will send acknowledgement (ACK) and it’s own SYN. Client will respond with an ACK and SYN after which data transfer will begin. Flow control and congestion avoidance is also used at the transport layer for connection-oriented services. Congestion can come about if a single computer is sending data at rates which exceed what the network can transfer and/or if multiple computers are sending data at acceptable rates, however the sum of their data exceeds what can be transferred over the network. In a connection-oriented session, a "not ready" indicator can be sent to the sender. Upon receiving the notification, the sender would hold data until it received a "ready" indicator.

Connection-oriented services also utilize flow control with windowing. Windowing allows the sender to send multiple (up to window size) data segments before receiving an ACK from receiving computer.

Hardware addresses are used to get a packet from one local device to another local device. Logical addresses are used to get a packet end-to-end through an internetwork.

• To divide the interrelated aspects of network operations into less complex elements.
• To define standard interfaces for "plug-and-play" compatibility and multivendor integration.
• To enable engineers to specialize design and development efforts on modular functions enabling new applications and services to be deployed without redesigning each lower layer.
• To promote symmetry in the different internetwork modular functions so they interoperate.
• To divide the complexity of internetworking into discrete, more easily learned operation subsets.

• Build the data so that it can traverse the network.. Upper layers (7-5).
• Package data for end-to-end support (transport layer). Add a transport-layer header.
• Append network address in header. Put data into a packet or datagram. Network layer.
• Append local address in data-link header. Packets are put into frames. Frame type must match the data-link type. Data-link layer.
• Convert to bits for transmission. Physical layer.

Flow control is achieved through windowing (or sliding-windows) described above.

• Path determination- to enable path determination, the routing services provide: routing table initialization and maintenance, routing update processes and protocols, routing domains and address specifications, route metric assignment and control.
• Communicating path information- Path information contains the names of all the networks that must be crossed along the path. By using consistent end-to-end addressing to represent the path of media connections, the network layer can find a path without using broadcasts which invoke unnecessary process overhead, and waste capacity on any devices or links that do not need to receive the broadcast.
• Addressing: Network and Host. Routing processes on internetworks typically are concerned only with the network portion of an address. Internetworking devices can have more than one network address, and must have a different address for each network-layer protocol supported by a particular device.