LAYERED TASKS
 At the Sender Site
 The activities that take place at the sender site.
 Higher layer. The sender writes the letter, inserts the letter in an envelope, writes the sender and receiver
addresses, and drops the letter in a mailbox.
 Middle layer. The letter is picked up by a letter carrier and delivered to the post office.
 Lower layer. The letter is sorted at the post office; a carrier transports the letter.
 On the Way
 The letter is then on its way to the recipient. On the way to the recipient's local post office,
the letter may actually go through a central office. In addition, it may be transported by truck,
train, airplane, boat, or a combination of these.
 At the Receiver Site
 Lower layer. The carrier transports the letter to the post office.
 Middle layer. The letter is sorted and delivered to the recipient's mailbox.
 Higher layer. The receiver picks up the letter, opens the envelope, and reads it.

OSI Model
 The model
 Functions of the layers

OSI Model
Layered Architecture
 The OSI model is composed of
seven ordered layers:
 physical (layer 1), data link (layer
2), network (layer 3), transport
(layer 4), session (layer 5),
presentation (layer 6), and
application (layer 7).
 The layers involved when a
message is sent from device A to
device B.
 As the message travels from A to
B, it may pass through many
intermediate nodes.
 These intermediate nodes usually
involve only the first three layers

OSI Layers
The passing of
the data and
network
information down
through the
layers of the
sending device
and back up
through the
layers of the
receiving device
is made possible
by an interface
between each
pair of adjacent

An Exchange Using the OSI Model
At each layer, a
header, or possibly a
trailer, can be added
to the data unit.
Commonly, the trailer
is added only at layer
2.
When the formatted
data unit passes
through the physical
layer (layer 1), it is
changed into an
electromagnetic
signal and

PHYSICAL LAYER -
FUNCTIONALITIES
Physical Layer coordinates the functions required to carry a bit stream over a
physical medium. It deals with the mechanical and electrical specifications of
the interface and transmission medium.
 Physical characteristics of interfaces and medium
 Representation of bits
 Data rate
 Synchronization of bits
 Line configuration
 Physical topology
 Transmission mode

DATA LINK LAYER -
FUNCTIONALITIES
The data link layer transforms the physical layer, a raw
transmission facility, to a reliable link. It makes the physical
layer appear error-free to the upper layer (network layer).
 Framing
 Physical Addressing
 Flow control
 Error control
 Access control

NETWORK LAYER -
FUNCTIONALITIES
The network layer is responsible for the source-to-destination
delivery of a packet, possibly across multiple networks
(links). Whereas the data link layer oversees the delivery of
the packet between two systems on the same network
(links), the network layer ensures that each packet gets from
its point of origin to its final destination.
 Logical Addressing
 Routing

TRANSPORT LAYER -
FUNCTIONALITIES
The transport layer is responsible for process-to-process delivery of the
entire message. A process is an application program running on a host.
Whereas the network layer oversees source-to-destination delivery of
individual packets, it does not recognize any relationship between those
packets.
 Service point addressing
 Segmentation and reassembly
 Connection control
 Error control
 Flow control

SESSION LAYER -
FUNCTIONALITIES
The services provided by the first three layers (physical, data
link, and network) are not sufficient for some processes. The
session layer is the network dialog controller. It establishes,
maintains, and synchronizes the interaction among
communicating systems.
 Dialogue control
 synchronization

PRESENTATION LAYER -
FUNCTIONALITIES
The presentation layer is concerned with the syntax and
semantics of the information exchanged between two
systems.
 Translation
 Encryption
 Compression

APPLICATION LAYER -
FUNCTIONALITIES
The application layer enables the user, whether human or
software, to access the network. It provides user interfaces
and support for services such as electronic mail, remote file
access and transfer, shared database management, and
other types of distributed information services.
 Network virtual terminal
 File transfer
 Mail services
 Directory services

TCP IP PROTOCOL SUITE
TCPIIP protocol suite is
made of five layers: physical,
data link, network, transport,
and application. The first four
layers provide physical
standards, network
interfaces, internetworking,
and transport functions that
correspond to the first four
layers of the OSI model.
The three topmost layers in
the OSI model, however, are
represented in TCPIIP by a
single layer called the
application layer

TCP/IP Protocol Architecture
Model
OSI Ref.
Layer
No.
OSI Layer
Equivalent
TCP/IP Layer TCP/IP Protocol Examples
5,6,7 Application, session,
presentation
Application NFS, NIS, DNS, LDAP, telnet,
ftp, rlogin, rsh, rcp, RIP, RDISC,
SNMP, and others
4 Transport Transport TCP, UDP, SCTP
3 Network Internet IPv4, IPv6, ARP, ICMP
2 Data link Data link PPP, IEEE 802.2
1 Physical Physical
network
Ethernet (IEEE 802.3), Token
Ring, RS-232, FDDI, and others

Model
 Physical Network Layer
 The physical network layer
specifies the characteristics of
the hardware to be used for the
network.
 For example, physical network
layer specifies the physical
characteristics of the
communications media.
 The physical layer of TCP/IP
describes hardware standards
such as IEEE 802.3,
 the specification for Ethernet
network media, and RS-232,
 the specification for standard pin
connectors.
 Data-Link Layer
 The data-link layer identifies
the network protocol type of
the packet, in this instance
TCP/IP.
 The data-link layer also
provides error control and
“framing.”
 Examples of data-link layer
protocols are Ethernet IEEE
802.2 framing and Point-to-
Point Protocol (PPP) framing.

Model
 Internet Layer
 The Internet layer, also
known as the network layer or
IP layer, accepts and
delivers packets for the
network.
 This layer includes the
powerful
 Internet Protocol (IP),
 Address Resolution Protocol
(ARP), and
 Internet Control Message
Protocol (ICMP).
 IP Protocol
 The IP protocol and its associated routing protocols are
possibly the most significant of the entire TCP/IP suite. IP is
responsible for the following:
 IP addressing – The IP addressing conventions are part
of the IP protocol. Designing an IPv4 Addressing Scheme
introduces IPv4 addressing and IPv6 Addressing Overview
introduces IPv6 addressing.
 Host-to-host communications – IP determines the
path a packet must take, based on the receiving system's IP
address.
 Packet formatting – IP assembles packets into units
that are known as datagrams. Datagrams are fully
described in Internet Layer: Where Packets Are Prepared
for Delivery.
 Fragmentation – If a packet is too large for transmission
over the network media, IP on the sending system breaks
the packet into smaller fragments. IP on the receiving
system then reconstructs the fragments into the original

Model
 Internet Layer
 ARP Protocol
 The Address Resolution Protocol
(ARP) conceptually exists
between the data-link and
Internet layers.
 ARP assists IP in directing
datagrams to the appropriate
receiving system by mapping
Ethernet addresses (48 bits
long) to known IP
addresses (32 bits long).
 ICMP Protocol
 The Internet Control Message
Protocol (ICMP) detects and
reports network error
conditions. ICMP reports on the
following:
 Dropped packets – Packets that
arrive too fast to be processed
 Connectivity failure – A
destination system cannot be
reached
 Redirection – Redirecting a
sending system to use another
router

Model
 Transport Layer
 The TCP/IP transport layer ensures
that packets arrive in sequence
and without error, by swapping
acknowledgments of data
reception, and retransmitting
lost packets.
 This type of communication is known
as end-to-end.
 Transport layer protocols at this level
are Transmission Control Protocol
(TCP), User Datagram Protocol
(UDP), and Stream Control
Transmission Protocol (SCTP). TCP
and SCTP provide reliable, end-to-end
service. UDP provides unreliable
datagram service.
 TCP Protocol
 TCP enables applications to communicate with each other as
though they were connected by a physical circuit. TCP sends
data in a form that appears to be transmitted in a character-by-
character fashion, rather than as discrete packets. This
transmission consists of the following:
 Starting point, which opens the connection
 Entire transmission in byte order
 Ending point, which closes the connection.
 TCP attaches a header onto the transmitted
data. This header contains many parameters
that help processes on the sending system
connect to peer processes on the receiving
system.
 TCP confirms that a packet has reached its
destination by establishing an end-to-end
connection between sending and receiving

Model
 SCTP Protocol
 SCTP is a reliable, connection-oriented
transport layer protocol that provides the
same services to applications that are
available from TCP.
 Moreover, SCTP can support connections
between systems that have more than one
address, or multihomed.
 The SCTP connection between sending and
receiving system is called an association.
 Data in the association is organized in
chunks. Because SCTP supports
multihoming, certain applications, particularly
applications used by the telecommunications
industry, need to run over SCTP, rather than
 UDP Protocol
 UDP provides datagram
delivery service.
 UDP does not verify
connections between
receiving and sending
hosts. Because UDP
eliminates the processes
of establishing and
verifying connections,
applications that send
small amounts of data use
UDP.

Addressing
 4 levels
 Physical(link
address)
 Logical(IP)
 Port address
 Specific
address(email
Address)

2. UNIT I-NETWORK MODELS for studying computer networks