–The presentation layer provides a variety of coding and conversion functions
that are applied to application layer data. These functions ensure that
information sent from the application layer of one system will be readable by
the application layer of another system. Examples of presentation layer coding
and conversion schemes include ASCII, EBCDIC, JPEG, GIF, TIFF, MPEG, QuickTime,
various encryption methods, and other similar coding formats.
–The session layer establishes, manages, maintains, and terminates
communication sessions between applications. Communication sessions consist of
service requests and service responses that occur between applications located
in different network devices. Some examples of session layer implementations
include Remote Procedure Call (RPC), Zone Information Protocol (ZIP), and
Session Control Protocol (SCP).
Transport – The transport layer segments and reassembles data into data streams. It is also responsible for both reliable and unreliable end-to-end data transmission. Transport layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery. Some examples of transport layer implementations include Transmission Control Protocol (TCP), Name Binding Protocol (NBP), and OSI transport protocols (SPX).
Network –The network layer uses logical addressing to provide routing and related functions that allow multiple data links to be combined into an internetwork. The network layer supports both connection-oriented and connectionless service from higher-layer protocols. Network layer protocols are typically routing protocols. However, other types of protocols, such as the Internet Protocol (IP), are implemented at the network layer as well. Routers reside here at the network layer. Some common routing protocols include Border Gateway Protocol (BGP), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP). Packets and datagrams are sent across this layer of the OSI model (IPX).
– The physical layer defines the electrical, mechanical, procedural, and
functional specifications for activating, maintaining, and deactivating the
physical link between communicating network systems.
Physical layer specifications define such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and the physical connectors to be used. Physical layer implementations can be categorized as either LAN or WAN specifications. Some common LAN physical layer implementations include Ethernet/IEEE 802.3, Fast Ethernet, FDDI, and Token Ring/IEEE 802.5. Some common WAN physical layer implementations include High-Speed Serial Interface (HSSI), SMDS Interface Protocol (SIP), and X.21bis.