Download OSI 7-Layer Model

OSI 7-Layer Model
Open Systems Interconnection (OSI) model is a reference model developed by ISO
(International Organization for Standardization) in 1984, as a conceptual framework of
standards for communication in the network across different equipment and applications
by different vendors. It is now considered the primary architectural model for intercomputing and internetworking communications. Most of the network communication
protocols used today have a structure based on the OSI model. The OSI model defines the
communications process into 7 layers, which divides the tasks involved with moving
information between networked computers into seven smaller, more manageable task
groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each
layer is reasonably self-contained so that the tasks assigned to each layer can be
implemented independently. This enables the solutions offered by one layer to be updated
without adversely affecting the other layers.
The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end to end
communications between data source and destinations. Layers 3 to 1 deal with
communications between network devices.
On the other hand, the seven layers of the OSI model can be divided into two groups:
upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the
OSI model deal with application issues and generally are implemented only in software.
The highest layer, the application layer, is closest to the end user. The lower layers of the
OSI model handle data transport issues. The physical layer and the data link layer are
implemented in hardware and software. The lowest layer, the physical layer, is closest to
the physical network medium (the wires, for example) and is responsible for placing data
on the medium.
The OSI Reference Model.
The specific description for each layer is as follows:
Layer
Function
Application 
User Interface




Presentation 
Translation




Session

"syncs and
sessions"


Protocols
Network
Components
used
for
applications DNS; FTP; TFTP; Gateway
specifically written to run over BOOTP;
SNMP;RLOGIN;
the network
allows access to network SMTP; MIME; NFS;
services
that
support FINGER; TELNET;
NCP; APPC; AFP;
applications;
directly represents the services SMB
that directly support user
applications
handles network access, flow
control and error recovery
Example
apps
are
file
transfer,e-mail,
NetBIOSbased applications
Gateway
Translates from application to
network format and vice-versa
all different formats from all
Redirector
sources are made into a
common uniform format that
the rest of the OSI model can
understand
responsible
for
protocol
conversion,
character
conversion,data encryption /
decryption, expanding graphics
commands, data compression
sets standards for different
systems to provide seamless
communication from multiple
protocol stacks
not always implemented in a
network protocol
NetBIOS
Gateway
establishes, maintains and ends
Names Pipes
sessions across the network
Mail Slots
responsible
for
name
RPC
recognition (identification) so
only the designated parties can
participate in the session
provides
synchronization
services by planning check
points in the data stream => if
session fails, only data after the
most recent checkpoint need be
transmitted
 manages who can transmit data
at a certain time and for how
long
 Examples are interactive login
and file transfer connections,
the session would connect and
re-connect if there was an
interruption; recognize names
in sessions and register names
in history
Transport  additional connection below TCP, ARP, RARP;
SPX
the session layer
NWLink
 manages the flow control of
packets; flow
data between parties across the NetBIOS / NetBEUI
control & errorATP
network
handling
 divides streams of data into
chunks or packets; the transport
layer of the receiving computer
reassembles the message from
packets
 "train" is a good analogy => the
data is divided into identical
units
 provides error-checking to
guarantee
error-free
data
delivery, with on losses or
duplications
 provides acknowledgment of
successful
transmissions;
requests retransmission if some
packets don’t arrive error-free
 provides flow control and
error-handling
Network
translates logical network address IP; ARP; RARP,
and names to their physical address ICMP; RIP; OSFP;
addressing;
(e.g. computername ==> MAC
IGMP;
routing
address)
IPX
responsible for addressing
NWLink
determining routes for sending
NetBEUI
managing network problems such OSI
as packet switching, data
DDP
congestion and routing
DECnet
Gateway
Advanced
Cable Tester
Brouter
Brouter
Router
Frame Relay
Device
ATM Switch
Data Link
data frames to
bits
Physical
hardware; raw
bit stream
if router can’t send data frame as
large as the source computer sends,
the network layer compensates by
breaking the data into smaller
units. At the receiving end, the
network layer reassembles the data
think of this layer stamping the
addresses on each train car
turns packets into raw bits 100101
and at the receiving end turns bits
into packets.
handles data frames between the
Network and Physical layers
the receiving end packages raw
data from the Physical layer into
data frames for delivery to the
Network layer
responsible for error-free transfer
of frames to other computer via the
Physical Layer
this layer defines the methods used
to transmit and receive data on the
network. It consists of the wiring,
the devices use to connect the NIC
to the wiring, the signaling
involved to transmit / receive data
and the ability to detect signaling
errors on the network media
Logical Link
Control
error correction and
flow control
manages link control
and defines SAPs
802.1 OSI Model
802.2 Logical Link
Control
Media Access
Control
communicates with
the adapter card
controls the type of
media being used:
802.3 CSMA/CD
(Ethernet)
802.4 Token Bus
(ARCnet)
802.5 Token Ring
802.12 Demand
Priority
transmits raw bit stream over
IEEE 802
physical cable
IEEE 802.2
defines cables, cards, and physical ISO 2110
aspects
ISDN
defines NIC attachments to
hardware, how cable is attached to
NIC
defines techniques to transfer bit
stream to cable
Advanced
Cable Tester
Bridge
Switch
ISDN Router
Intelligent
Hub
NIC
Advanced
Cable Tester
Repeater
Multiplexer
Hubs
Passive
Active
TDR
Oscilloscope
Amplifier