• This is where users communicate to the computer.
• This is where communication between two users are established.
• This is a point where user or application interfaces with the protocols to
gain access to the network.
• Examples are WWW, Telnet, FTP, TFTP, E-mail, SNMP, DNS
Presentation Layer
• Tasks like Translation, Encryption, decryption, compression,decompression are associated with this layer.
• It receives the data in native format & converts in standard format or receives data in standard format and converts in native format, i.e.. EBCDIC to ASCII.
• It is mainly responsible for how the data is to be presented to the Application Layer.
• Examples are PICT, TIFF, JPEG, MIDI, MPEG, GIFF etc.
• Translation at Presentation Layer :
• Bit - How many bits
• Byte - Little Indent to Big Indent
• Character - ASCII to EBCDIC
• File Syntax - LFN to SFN
• Encryption at Presentation Layer
• Encryption means scrambling bits in order to provide
security to data sent over the Network.
• Different Technology which is used for encryption are:
• RSA
• DSA
Session Layer
• The Session layer is responsible for setting up, managing, and then
tearing down sessions between Presentation layer entities.
• It coordinates communication between systems, and serves to
organize their communication by offering three different modes:
simplex, half duplex, and full duplex.
• The Session layer basically keeps different applications’ data
separate from other applications’ data.
• RPC, SQL, NFS and NetBIOS are examples of Session Layer.
Transport Layer
• Transport Layer never actually transports the data but only prepares for
transporting.
• Uses Socket to define the services running on a particular
node, the data is associated with.
• Responsible for the following :
• Segmentation
• End-to-end Communication
• Flow Control
• Error Control
• Multiplexing of Applications
• TCP, UDP and SPX work at this layer
Network Layer
• It is responsible for communicating Networks
• It recognizes Networks with the help of Network Addresses
• Network Address is a logical address like IP Address or IPX Address
• It is common for a group of computers
• It works only with Network IDs and has got nothing to do with host Ids.
• Path determination or Routing is performed at this layer.
• Router works at this layer.
Data Link Layer
• It uniquely identifies each device in the Network.
• It translates data from Network Layer into bits for the Physical layer to transmit.
• It formats the messages into Data Frames
• Adds a customized header containing Source and Destination hardware address
• This layer works with Frames
• This layer is logically divided in two sub-layers:
• LLC (Logical Link Control)
• MAC (Media Access Control)
Devices at Data Link Layer
• Devices that works at this layer are
• LAN Card
• Bridges etc.
• The MAC Sublayer maintains addresses that enable messages to
be sent and received by particular devices across a network.
• These addresses, called physical device addresses, data-link
addresses, hardware addresses, or MAC Addresses, are unique
addresses associated with the networking hardware in a
computer.
• The address is burned into the Network Interface Card (NIC) at
the time of manufacturing
Physical Layer
• Electrical and Mechanical settings are provided at this layer.
• Transmits data in the form of bits.
• This layer communicates directly with actual
communication media.
• At this layer DCE & DTE are identified
• DCE (Data Circuit-Terminating Equipment)
• Located at Service Provider’s side
• DTE (Data Terminal Equipment)
• The attached device at customer’ Place eg. Modem
• Services available to a DTE is most often accessed via a
Modem or Channel Service Unit (CSU) Data Service Unit
(DSU).
• Hubs & REPEATERS are working at this layer.
• Max. troubleshooting occurs at this layer
