Application Layer 
The Application Layer (the seventh
layer) of the OSI Model provides network services, which are closest
to the user. Such programs as Internet Explorer, Netscape Communicator, Eudora
Pro, and other end-user application software. This layer establishes
communication with intended partners, synchronizes agreement on
procedures for error recovery and control of data integrity.
The protocols, which function on this
layer, are Server Message Block (SMB) and Network Control Program (NCP).
Services, which provide network access,
include:
- Telnet and File Transfer Protocol
(FTP)
- Trivial File Transfer Protocol (TFTP)
- Network File System (NFS)
- Simple Network Management Protocol (SNMP)
- Simple Mail Transfer Protocol (SMTP)
- Hyper Text Transfer Protocol (HTTP)
Devices function up to this layer
include hosts and gateways.
Presentation Layer 
The Presentation Layer ensures that the
information that the application layer of one system sends out
is readable by the application layer of another system. If necessary,
the presentation layer translates between multiple data formats by
using a common format.
The Presentation Layer also provides
data encryption to ensure protection as data journeys through the
network. When the encrypted data is received, it decrypts and formats
the message before passing along to the Application Layer.
Protocols include NCP. Data formats include ASCII, EBCDIC, encrypted, jpeg, gif, mpeg,
quicktime, flash, wav, avi, and mp3.
Devices functioning up to this layer
include hosts and gateways.
Session Layer 
The session layer establishes, manages,
terminates sessions between two communicating hosts, and provides its
services to the presentation layer. It also synchronizes dialogue
between the two hosts and manages their data exchange. The Session
Layer also offers provisions for efficient data transfer, class of
service, security authorization, and exception reporting of session
layer, presentation layer, and application layer problems.
Three types of dialogs used in the
Session Layer are simplex, half-duplex, and full-duplex. A simplex
dialog allows information to flow from one device to another without
requiring a reply transmission.
Half-duplex, which is also known as a
two-way alternate (TWA) transmission, allows data to flow in two
directions from one device to another; however, each device cannot
send a transmission until the previous signal has been completely
received. When one device sends a transmission and requires the
destination device to respond, the destination device must wait until
the initial transmission is complete before it can send its response.
Full-duplex, which is also known as a
two-way simultaneous (TWS) transmission, allows devices to send data
to another device without having to wait until the wire is clear. When
a device transmits a signal, the destination device does have not to
wait until the signal is complete to send a reply to the source
device. Full-duplex enables two-way traffic to occur
simultaneously during one communication session. A telephone is an example of
full-duplex.
Protocols include Network File System (NFS),
Structured Query Language (SQL), Remote Procedure Call (RPC), X-Window
System, AppleTalk Session Protocol (ASP), and Digital Network
Architecture Session Control Protocol (DNA SCP).
Devices functioning up to this layer
include hosts and gateways.
Transport Layer 
The Transport Layer segments data from
the sending host's system and reassembles the data into a data stream
on the receiving host's system. The boundary between the session layer
and the transport layer can be thought of as the boundary between
media-layer protocols and host-layer protocols. Whereas the
application, presentation, and session layers are concerned with
application issues, the lower three layers are concerned with data
transport issues.
The transport layer attempts to provide
a data transport service that shields the upper layers from transport
implementation details. Specifically, the main concern of the
transport layer includes issues such as how reliable transport between
two hosts is accomplished. In providing communication service, the
transport layer establishes, maintains, and properly terminates
virtual circuits. In providing reliable service, transport error
detection-and-recovery and information flow controls are used.
When the Transport Layer receives data
from the upper layers, it breaks up the information into segments
(smaller pieces) to be sent through the lower levels of the OSI Model
and then to the destination device.
Protocols used in this layer are:
- Sequenced Package Exchange (SPX)
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
- NetBIOS Extended User Interface
(NetBEUI)
Services used at this layer use TCP to
provide connection-oriented communication with error free delivery and
UDP to provide connectionless communications without guaranteed packet
delivery (unreliable delivery).
Devices functioning up to this layer
include hosts and gateways.
Network Layer 
The network layer is a complex layer
that provides connectivity and path selection between two host systems
that may be geographically separated. Layer 3 can be remembered as
addressing, path selection, routing, and switching.
Protocols functioning on this layer
include:
Routed Protocols
Layer 3 Protocols
- Internet Control Message
Protocol (ICMP)
- Address Resolution Protocol
(ARP)
- Reverse Address Resolution
Protocol (RARP) Routing Protocols
- Routing Information Protocol
(RIP)
- Internet Gateway Routing
Protocol (IGRP)
- Enhanced IGRP (EIGRP)
- Open Shortest Path First (OSPF)
- Exterior Gateway Protocol (EGP)
- Internet Management Group
Protocol (IGMP)
Group with Routed Protocols but label
as a Non-routable Protocol
- NetBEUI Group with Routed
Protocols
- DecNET
Services include software and hardware
addressing, packet routing between hosts and networks, resolution of
hardware and software addresses, and reports of packet delivery.
Devices functioning up to this layer
include routers and brouters.
Data Link Layer 
The Data Link Layer provides reliable
transit of data across a physical link. In so doing, the data link
layer is concerned with physical (as opposed to logical) addressing,
network topology, network access, error notification, ordered delivery
of frames, and flow control. Layer 2 can be remembered by frames and
media access control.
Ethernet CSMA/CD also operates on this
layer to determine which devices should transmit at a given time in
order to avoid collisions.
The NIC is also responsible for CSMA/CD
on Ethernet. In the case where two or more devices attempt to transmit
signals at the same time, a collision will occur. CSMA/CD instructs
the device to wait a given amount of time before transmitting another
signal to avoid another collision.
The Data Link Layer is broken down into
two sublayers by the 802 standards: Logical Link Control (LLC) and
Media Access Control (MAC). The LLC sublayer (IEEE 802.2) establishes
and maintains communication with other devices and provides
connectivity with servers when data is being transferred. LLC manages
link control and defines service access points (SAPs).
The MAC sublayer maintains a table of
physical addresses of devices. Each device is assigned and must have a
unique MAC address if the device is to participate on the network. For
example, the MAC address is similar to the individual's physical
residence address, which the post office uses to deliver snail mail.
Protocols used at this layer include
High Level Data Link Control (HDLC) for WAN connections, including
synchronous and asynchronous transmissions. The LLC protocol (IEEE
802.2) provides flow control at this layer.
Technologies which operate on this
layer include more than 18 varieties of Ethernet (specified in the
IEEE 802.3 and other standards), Token Ring (IEEE 802.5), and other
LAN technologies which rely on frames. Communications with the NIC are
also provided.
Devices functioning up to this layer
include NICs, bridges, and switches. While routers and brouters are
classified as layer 3 devices, in order to perform their functions,
they must operate on layer 1 and 2 as well.
Physical Layer 
The Physical Layer (Layer 1) defines
the electrical, mechanical, procedural, and functional specifications
for activating, maintaining, and deactivating the physical link
between end systems. Such characteristics as voltage levels, timing of
voltage changes, physical data rates, maximum transmission distances,
physical connectors, and other, similar, attributes are defined by
physical layer specifications.
The Physical Layer is responsible for
moving bits of data through physical media. Data, in the form of ones
and zeros, are turned into electrical signals, pulses of light, or
wireless signals. These signals are placed on the copper cables,
optical fibers, or emitted as wireless, using a NIC. When receiving
data from the network, the NIC turns the electrical signals, pulses of
light or wireless signals back into ones and zeros to be sent up the
hierarchy of the OSI Model.
Protocols are the cabling, signaling,
and connection standards. Services include Ethernet, Token Ring, FDDI,
and other LAN technologies. Devices, which function at this layer are
repeaters, multiport repeaters (also called hubs), media access units
(MAUs), and transceivers (transmitter/receivers, for converting one
signal type into another).
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