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Computer Communication & Networks
Lecture # 02
Nadeem Majeed Choudhary
[email protected]
Layering & Protocol Stacks
Reference Models
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
OSI reference model
TCP/IP
OSI Reference model


1.
2.
3.
4.
Open System Interconnection
7 layers
Crate a layer when different abstraction is needed
Each layer performs a well define function
Functions of the layers chosen taking internationally
standardized protocols
Number of layers – large enough to avoid
complexity
Seven layers of the OSI model
Exchange using 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
OSI Layers
Physical layer
physical
connection
Transporting bits from one end node to the next
- type of the transmission media (twisted-pair, coax, optical fiber, air)
- bit representation (voltage levels of logical values)
- data rate (speed)
- synchronization of bits (time synchronization)
- deals with the optical, mechanical and electrical features
- transmission modes ( half duplex, full duplex)
Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

Protocols:
- Modems
- Optical Cables, Connectors

Network Devices:
- Hubs, Repeaters, and Amplifier.
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-
Data Link layer
- example-
Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
Data Link layer

Network Devices:
-Bridge, Switch, ISDN Router, Intelligent
Hub, NIC, Advanced Cable Tester
Data Link layer

Protocols: Logical Link Control
• error correction and flow control
• manages link control and defines SAPs
Media Access Control
• controls the type of media being used:
802.3 CSMA/CD (Ethernet)
802.4 Token Bus (ARCnet)
802.5 Token Ring
802.12 Demand Priority
Network Layer

The network layer is responsible for
the delivery of individual packets from
the source host to the destination host.
Network layer
Not a message

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.
3.
4.
Logical addressing
Routing
Switching
Congestion control and QoS
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Source to destination delivery
Data Link
Network
layer
21
Network layer
- example -
Network layer addresses
Data Link layer addresses
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Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
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Network Layer

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

Main tasks:





Port addressing
Segmentation and reassembly
Congestion control
Flow control
Error control
25
Transport Layer
 The transport layer is responsible for the delivery
of a message from one process to another.
Transport Layer
27
Transport layer -an example of a reliable delivery -
28
Note
The transport layer is responsible for the delivery
of a message from one process to another.
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Transport Layer


Protocols: These protocols work on the
transport layer TCP, UDP, SPX, NETBIOS,
ATP and NWLINK.
Network Devices: The Brouter, Gateway and
Cable tester work on the transport layer.
Note
The session layer is responsible for dialog
control and synchronization.
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Session Layer

The session layer is responsible for
dialog control and synchronization.
Presentation Layer
Presentation Layer

Presentation layer is a best layer for
cryptography.
Note
The presentation layer is responsible for translation,
compression, and encryption.
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Application Layer

The application layer is responsible
for providing services to the user.
Application Layer

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)
38
Note
The application layer is responsible for
providing services to the user.
39
Application Layer

Protocols: FTP, DNS, SNMP, SMTP,
FINGER, TELNET, TFTP, BOOTP and SMB
protocol are operated on the application
layer.

Network Devices: Gateway network device is
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
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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
42
TCP/IP Protocol
43
TCP/IP Vs OSI Model
44
Four Level of Addresses
45
Relationship of Layers & Addresses in TCP/IP
46
Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
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Example 2.1
In Figure 2.19 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 2.19 Physical addresses
Example 2.2
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 2.3
Figure 2.20 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 2.20 IP addresses
Example 2.4
Figure 2.21 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 2.21 Port addresses
Example 2.5
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 2 (B. A Forouzan)

Section 2.2,2.3, 2.4, 2.5
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Q&A