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The TCP/IP Model
The TCP/IP suite of protocols maps to a four-layer
conceptual model that is based on the seven layer Open
System Interconnection (OSI) protocol model.
The detailed function of each layer of the Open System
Interconnection (OSI) protocol model is beyond the scope of
this topic; however, the 60-second overview is as follows:
Physical Layer – Defines the interface between the
medium and the device. This layer also transmits bits (ones
and zeros) and defines how the data is transmitted over the
physical medium. Some examples of Network Components found
at this layer are Multiplexers, Passive Hubs, Active Hubs,
Repeaters and other types of signal Amplifiers
Data Link Layer – This layer is actually divided into
two sublayers — Logical Link Control, which mainly handles
error correction and flow control, and Media Access Control,
which mainly handles the communication with the network
adapter card. Some examples of Network Components found at
the Data Link layer are Bridges, Switches and certain
Advanced Cable Testers.
Network Layer – This OSI layer is responsible for
translating logical network addresses and names such as
computernames to their MAC addresses and for addressing and
routing data packets over the network. If routers at this
layer can’t forward the data frames as large as the source
node has sent, this OSI layer will break down the data into
smaller units that the devices can handle. Some examples of
Protocols found at the Network Layer are IP, ARP, RARP,
ICMP, RIP, OSFP, IGMP, IPX, NWLink and NetBEUI. Some
examples of Network Components found at this layer are
Brouters, Routers, some types of ATM Switches and Frame
Relay hardware.
Transport Layer – The Transport Layer adds an
additional connection below the Session layer and helps
manage data flow control between nodes on the network. This
layer divides the data into packets on the sending node, and
the transport layer of the receiving node reassembles the
message from packets. The Transport Layer provides
error-checking to guarantee error-free data delivery by
requesting retransmission if some packets don’t arrive
error-free. It also sends acknowledgment of successful
transmissions back to the sending node. Some examples of
Protocols found at this layer are TCP, ARP, RARP, SPX and
NWLink. Some examples of Network Components found at the
Transport Layer are Gateways and certain types of Brouters.
Session Layer – This OSI layer, as the name implies,
establishes, maintains and ends sessions between
transmitting nodes across the network and manages which node
can transmit data at a certain time and for how long. Some
examples of Protocols found at this layer are Names Pipes,
NetBIOS Names, RPC and Mail Slots. Some examples of Network
Components found at the Session Layer are Gateways and
certain types of Proxy Servers.
Presentation Layer – The Presentation Layer
technically performs the translation of the data from the
way applications understand it to the way networks
understand it on the transmission end and then back on the
receiving node. It is responsible for protocol conversions,
data encryption / decryption, and data compression /
decompression where the network is considered. Some examples
of Network Components found at the Presentation Layer are
Gateways and certain types of Redirectors. There are no
Protocols that normally operate in this layer.
Application – The Application Layer of the OSI model
allows access to network services for applications
specifically written to run over the network, such as e-mail
and file transfer programs such as FTP. There are many
Protocols found at the Application Layer, some of which
include FTP, TFTP, BOOTP, SNMP, SMTP, TELNET, NCP, and SMB.
