Lecture # 02
Network Models
Course Instructor:
Engr. Sana Ziafat
Protocol
 Set of rules that govern data communication
 Protocol Defines:
What is communicated
2. How it is communicated
3. When it is communicated
1.
Protocol
 protocols define format, order of msgs sent and
received among network entities, and actions taken on
msg transmission, receipt
a human protocol and a computer network protocol:
Hi
Hi
Got the
time?
2:00
time
TCP connection
req.
TCP connection
reply.
Get http://gaia.cs.umass.edu/index.htm
<file>
Key Elements of PROTOCOL
 Syntax – format or structure of data
 Semantics – meaning of each section of bits
 Timing – when and how fast data should be sent
What’s a protocol?
human protocols:
 “I have a question”
 introductions
… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
 machines rather than humans
 all communication activity in Internet governed by protocols
Layering & Protocol Stacks
Layered Tasks
An example from the everyday life
Hierarchy?
Services
Why layered communication?
 To reduce complexity of communication task by
splitting it into several layered small tasks
 Functionality of the layers can be changed as long as
the service provided to the layer above stays
unchanged
 makes easier maintenance & updating
 Each layer has its own task
 Each layer has its own protocol
Reference Models
 OSI reference model
 TCP/IP
OSI Reference model
 Open System Interconnection
 7 layers
1. Create a layer when different abstraction is needed
2. Each layer performs a well define function
3. Functions of the layers chosen taking internationally
standardized protocols
4. Number of layers – large enough to avoid complexity
Seven layers of the OSI model
Peer-to-peer Processes
 Layer x on one machine communicates with layer x on another
machine - called Peer-to-Peer Processes.
 Interfaces between Layers
Each interface defines what information and services a layer
must provide for the layer above it.
Well defined interfaces and layer functions provide
modularity to a network
 Organizations of the layers
 Network support layers : Layers 1, 2, 3
 User support layer : Layer 5, 6, 7
 It allows interoperability among unrelated software systems
 Transport layer (Layer 4) : links the two subgroups
The interaction between layers in the OSI model
Exchange using OSI Model
OSI Layers
Physical layer
physical
connection
Transporting bits from one end node to the next
 Duties performed by Physical layer are:
1.
2.
3.
4.
5.
6.
7.
Physical characteristics of interfaces and media
Representation of bits
Data rate
Synchronization of bits
Topology
Line configuration
Transmission mode
Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Hardware equipment
 Network adapter
 Repeater
 Network hub
 Modem
Data Link layer
logical
connection
Transporting frames from one end node to the next one
- framing
- physical addressing
- flow control
- error control
- access control
Data Link layer
- hop-to-hop delivery-
Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
 Network Devices:
-Bridge, Switch, ISDN Router, Intelligent
Hub, NIC, Advanced Cable Tester
Network Layer

The network layer is responsible for the delivery of
individual packets from the source host to the destination
host.
Network layer

End-to-End packet delivery


Needed when 2 devices are attached to
different networks


From the original source to a destination
What is the network definition here?
Main duties:
1.
2.
Logical addressing
Routing
Source to destination delivery
Data Link
Network
layer
Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
 Router works as the post office and network layer
stamps the letters (data) for the specific destinations.
 Protocols: These protocols work on the network layer
IP, ICMP, ARP, RIP, OSI, IPX and OSPF.
 Network Devices: Network devices including Router,
Frame Relay device and ATM switch devices work on
the network layer.
Transport layer
 Process-to-Process delivery of the entire message
◦ From the original source to a destination
 Needed when several processes (running programs)
active at the same time
 Note: What is a process
 Main tasks:
◦
◦
◦
◦
◦
Service Point addressing
Segmentation and reassembly
Connection control
Flow control
Error control
Transport Layer
 The transport layer is responsible for the delivery
of a message from one process to another.
Transport layer -an example of a reliable
delivery -
Note
The transport layer is responsible for the delivery
of a message from one process to another.
Note
The session layer is responsible for dialog
control and synchronization.
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Session Layer
 The session layer allows a system to add check points or
synchronization points.
 This layer allow two systems to enter in to dialog
Presentation Layer
 Presentation layer is concerned with syntax and
semantics of information exchanged between two
systems.
 Provides translation , encryption and compression to
data
 It is a best layer for cryptography.
Note
The presentation layer is responsible for translation,
compression, and encryption.
Application Layer

The application layer is responsible for providing
services to the user.
 Services provided by Application layer:
- File transfer, Access
- Mail services
Application layer
 Enables user to access the network
 Provides services to a user
 E-mail
 Remote file access and transfer (Telnet, FTP)
 Access to WWW (HTTP)
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Note
The application layer is responsible for
providing services to the user.
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 Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET,
TFTP, BOOTP and SMB protocol are operated on the
application layer.
OSI Reference Model
 A convenient aid for remembering the OSI layer
names is to use the first letter of each word in the
phrase:
 All People Seem To Need Data Processing
Summary of layers and protocols
 Low-level protocols define the electrical and physical standards to
be observed, bit- and byte-ordering and the transmission and
error detection and correction of the bit stream
 High-level protocols deal with the data formatting, including the
syntax of messages, the terminal to computer dialogue, character
sets, sequencing of messages
TCP/IP Protocol
45
TCP/IP Vs OSI Model
Four Level of Addresses
47
Relationship of Layers & Addresses in TCP/IP
48
Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
49
Example 1
In Figure on next slide a node with physical address 10
sends a frame to a node with physical address 87. The
two nodes are connected by a link (bus topology LAN).
As the figure shows, the computer with physical address
10 is the sender, and the computer with physical address
87 is the receiver.
Figure Physical addresses
Example 2
As we will see in later lectures, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
Example 3
Figure on next slide shows a part of an internet with two
routers connecting three LANs. Each device (computer
or router) has a pair of addresses (logical and physical)
for each connection. In this case, each computer is
connected to only one link and therefore has only one
pair of addresses. Each router, however, is connected to
three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each
connection.
Figure IP addresses
Example 4
Figure shows two computers communicating via the
Internet. The sending computer is running three
processes at this time with port addresses a, b, and c. The
receiving computer is running two processes at this time
with port addresses j and k. Process a in the sending
computer needs to communicate with process j in the
receiving computer. Note that although physical
addresses change from hop to hop, logical and port
addresses remain the same from the source to
destination.
Figure Port addresses
Example 5
As we will see in later chapters, a port address is a 16-bit
address represented by one decimal number as shown.
753
A 16-bit port address represented
as one single number.
Readings
 Chapter 1 (B. A Forouzan)
 Section 1.4
 Chapter 2 (B. A Forouzan)
 Section 2.1, 2.2,2.3, 2.4, 2.5
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Q&A