Transcript View File

Computer Communication
& Networks
Lecture # 02
Network Models
Course Instructor:
Engr. Sana Ziafat
Set of rules that govern data communication
 Protocol Defines:
1. What is communicated
2. How it is communicated
3. When it is communicated

 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>
 Syntax
– format or structure of data
 Semantics – meaning of each section of bits
 Timing – when and how fast data should be
sent
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
An example from the everyday life
Hierarchy?
Services
 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
 OSI
reference model
 TCP/IP
 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

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
physical
connection
Transporting bits from one end node to the next
Duties performed by Physical layer are:
1. Physical characteristics of interfaces and
media
2. Representation of bits
3. Data rate
4. Synchronization of bits
5. Topology
6. Line configuration
7. Transmission mode

Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
 Network
adapter
 Repeater
 Network
 Modem
hub
logical
connection
Transporting frames from one end node to the next one
- framing
- physical addressing
- flow control
- error control
- access control
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
 The network layer is responsible for the delivery of
individual packets from the source host to the
destination host.

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
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.

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
 The transport layer is responsible for the delivery
of a message from one process to another.
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.
33
 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 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.
 The application layer is responsible for providing
services to the user.
 Services
provided by Application layer:
- File transfer, Access
- Mail services
 Enables
user to access the network
 Provides services to a user



E-mail
Remote file access and transfer (Telnet, FTP)
Access to WWW (HTTP)
40
Note
The application layer is responsible for
providing services to the user.
41
 Protocols:
FTP, DNS, SNMP, SMTP, FINGER,
TELNET, TFTP, BOOTP and SMB protocol are
operated on the application layer.
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

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
45
47
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.
 Chapter

Section 1.4
 Chapter

1 (B. A Forouzan)
2 (B. A Forouzan)
Section 2.1, 2.2,2.3, 2.4, 2.5
58