Specifications

Ethernet LAN Standards 3-3
The Open Systems Interconnect (OSI) Model
• LAYER SIX: Presentation Layer–The presentation layer deals with data
translation and code conversion between devices with different data
formats (e.g., ASCII to EBCDIC). This layer also handles translation
between differing device types and file formats, as well as data
encryption and decrypting services.
• LAYER FIVE: Session Layer–The session layer manages the
communication dialogue (the “session”) between two communicating
devices. The session layer establishes rules for initiating and
terminating communications between devices and provides error
recovery as well. If an error or communications failure is detected, the
session layer retransmits data to complete the communications process.
The session layer requests a certain level of service from the transport
layer such as one way transmission that doesn’t require a reply, or a two
way conversation that requires a lot of monitoring and feedback.
• LAYER FOUR: Transport Layer–The transport layer deals with the
optimization of data transfer from source to destination by managing
network data flow and implementing the quality of service requested
by the session layer. The transport layer determines the packet size
requirements based on the amount of data to be sent and the maximum
packet size allowed on the communications media. If the data to be sent
is larger than the maximum packet size allowed on the network, the
transport layer is responsible for dividing the data into acceptable sizes
and sequencing each packet for transmission. During the dividing and
sequencing process, this layer adds information such as sequence
number and error control information to the data portion of the packet.
When receiving data from the network layer, the transport layer
ensures that the data is received in order and checks for duplicate and
lost frames. If data is received out of order, which is possible in a
larger, routed network, the transport layer correctly orders the data
and passes the data up to the session layer for additional processing. A
popular protocol that uses the transport layer is Transmission Control
Protocol (TCP) used in TCP/IP.
• LAYER THREE: Network Layer–The network layer accepts data from
the transport layer and adds the appropriate information to the packet
to provide proper network routing and some level of error control. Data
is formatted for the appropriate communications method such as local
area network, wide area network such as T1, or packet switched
technology such as X.25. A popular protocol that uses the network layer
is the Internet Protocol (IP) used by TCP/IP.