QSpiders - Day1 Network Basics

© 1999, Cisco Systems, Inc.www.cisco.com
Networking Basics

1-2CSE: Networking Fundamentals—Internetworking © 1999, Cisco Systems, Inc.www.cisco.com
The OSI Model
• OSI Layer is meant for Networking
manufacturers and developers to
provide them a standard based on
which they can make their products.
• All OSI Layers are independent from
each other, which makes introducing
changes easier as no other layers are
effected.
• Ease of Troubleshooting.

© 1999, Cisco Systems, Inc.www.cisco.com
The Layered Model
© 1999, Cisco Systems, Inc. www.cisco.com

Why a Layered Network Model?
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
• Reduces complexity (one big
problem to seven smaller
ones)
• Standardizes interfaces
• Facilitates modular
engineering
• Assures interoperable
technology
• Accelerates evolution
• Simplifies teaching and
learning

Devices Function at Layers
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
NIC Card
Hub

Host Layers
7 Application
6 Presentation
5 Session
4 Transport
Network
3 Data Link
1
Host layers: Provide
accurate data delivery
between computers}
Physical

Media Layers
7 Application
6 Presentation
5 Session
4
Transport
3 Network
2 Data Link
1 Physical
Host layers: Provide
accurate data delivery
between computers
Media layers: Control
physical delivery of messages
over the network
}
}

Layer Functions
Provides network services to
application processes (such as
electronic mail, file transfer, and
terminal emulation)
7 Application

Layer Functions
Network services to applications
• Ensures data is readable by
receiving system
• Format of data
• Data structures
• Negotiates data transfer
syntax for application layer
7 Application
6 Presentation Data representation

Layer Functions
Inter-host communication
• Establishes, manages, and
terminates sessions between
applications
7 Application
6 Presentation
5 Session
Data representation

Layer Functions
7 Application
6 Presentation
5 Session
Transport4
Data representation
End-to-end connection reliability
• Concerned with data transport
issues between hosts
• Data transport reliability
• Establishes, maintains, and
terminates virtual circuits
• Fault detection and recovery
• Information flow control

Layer Functions
7 Application
6 Presentation
5 Session
Transport4
Network3
Data representation
Addresses and best path
• Provides connectivity and path
selection between two end
systems
• Domain of routing

Layer Functions
7 Application
6 Presentation
5 Session
Transport4
Network3
Data Link2
Data representation
Access to media
• Provides reliable transfer of data
across media
• Physical addressing, network
topology, error notification, flow
control

Layer Functions
7 Application
6 Presentation
5 Session
Transport4
Network3
Data Link2
Physical1
Data representation
Access to media
Binary transmission
• Wires, connectors, voltages,
data rates

7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
Host A
Peer-to-Peer Communications
Application
Presentation
Session
Transport
Network
Data Link
PhysicalBits
Frames
Packets
Segments
Host B

Application Layer
• 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, ie. EBCDIC(Extended Binary code
Decimal Interchange Code) 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.

Presentation Layer
ASCII
EBCDIC
Encrypted
• Text
• Data
login:
PICT
TIFF
JPEG
GIF
• Graphics
• Visual images
• Sound
• Video
MPEG
QuickTime
MIDI
• Provides code formatting and
conversion for applications

Session Layer
– Session Establishment
•Establishes a session between two
devices before actual transmission of
data.
– Dialog Control
•Simplex
•Half Duplex
•Full Duplex

Session Layer
• Network File System (NFS)
• Structured Query Language (SQL)
• Remote-Procedure Call (RPC)
• X Window System
• AppleTalk Session Protocol (ASP)
• DEC Session Control Protocol (SCP)
Service Request
Service Reply
• Coordinates applications as
they interact on different hosts

Transport Layer
• Segments upper-layer applications
• Establishes an end-to-end connection
• Sends segments from one end host to another
• Optionally, ensures data reliability

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

Socket
• Socket is a software component and points to a particular service
running on a particular node.
• Structure of a socket
– IP Address + Port Address
• Each service has a unique Port address
• Max. Port Addresses can be 65,536
• Port address 1-1023 is reserved for specific Services like
– WWW - 80
– FTP - 21
– SMTP - 25
• Port Addresses are reserved for standardization purpose.

Transport Layer—
Segments Upper-Layer Applications
Electronic
Mail
File
Transfer
Application
Presentation
Session
Segments
Data
Application
Port
Transport Data
Application
Port
Terminal
Session

Port Numbers
TCP
Port
Numbers
F
T
P
Transport
Layer
T
E
L
N
E
T
D
N
S
S
N
M
P
T
F
T
P
S
M
T
P
UDP
Application
Layer
21 23 25 53 69 161
R
I
P
520

Segmentation
• This is a mechanism wherein the data is divided into
multiple segments and sent over the network.
• By doing this different segments can use different links
for travelling across the network.
• If one segment is lost the only segment is required to be
re-sent and not the entire data.
• Once all segments reach to the destination the received
segments have to be sequenced back, which is also
done at this layer.

Transport Layer—
Sends Segments with Flow Control
Transmit
Buffer Full
Not Ready
Stop
Process
Segments
Buffer OK
ReadyGo
Resume Transmission
ReceiverSender

Flow Control
• Used while connection oriented communication
• It helps to have a control on over flow of Buffer.
• Advantages are:
– The segments delivered are acknowledged if received
– Any segment not acknowledged are retransmitted
– segments are sequenced back upon their arrival
– Congestion, Overloading and data loss are avoided
• To achieve all this it uses the technique of Sliding
window or Windowing

Transport Layer—
Establishes Connection
Synchronize
Synchronize
Acknowledge
Negotiate Connection
Receiver
Data Transfer
Connection Established
(Send Segments)
Sender

End-to-End Communication
• Connection Less Transmission
–UDP is used
–Not reliable
–Faster
• Connection Oriented Transmission
–TCP or SPX is used
–Reliable
–Slower

Connection Oriented Protocol
• These protocols relies on Acknowledgement.
• Positive acknowledgement means data has
been received.
• Negative acknowledgement means data is
lost no further data is sent till positive
acknowledgement is received.
• It is slow but Reliable.
• Eg. TCP and SPX

Transport Layer—
Reliability with Windowing
• Window Size = 1
Sender
Send 1 Receive 1
Receiver
Ack 2
Send 2 Receive 2
Ack 3
Sender
Send 1
Send 2
Receive 1
Receive 2
Receiver
• Window Size = 3
Send 3 Receive 3
Ack 4
Send 4

Transport Layer—
An Acknowledgement Technique
Sender Receiver
Send 2
Send 1
Send 3
Ack 4
Send 5
Send 4
Send 6
Ack 5
Send 5
Ack 7
1 2 3 4 5 6 7 1 2 3 4 5 6 7

Connection Less Protocol
• They do not provide acknowledgement
neither sequence numbers.
• It is faster but not reliable
• Eg. UDP

Network Layer
• It is responsible for communicating Networks
• It recognizes Networks with the help of Netwok 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.

Network Layer: Path Determination
Which Path?Which Path?
• Layer 3 functions to find the best
path through the internetwork

Network Layer: Communicate Path
1
2
3
4
5
6
7
8
9
10 11
• Addresses represent the path of media
connections

Addressing—Network and Node
Network Node
1
2
1
2
3
1
3 1
1.1
2.1
3.1
1.2
1.3
• Network address—Path part used by the router
• Node address—Specific port or device on the network

Protocol Addressing Variations
Network Node
1 1
General
Example
Network Host
10. 8.2.48
TCP/IP
Example (Mask 255.0.0.0)
Network Node
1aceb0b. 0000.0c00.6e25
Novell IPX
Example

Network Layer
Protocol Operations
• Each router provides its services to
support upper layer functions
X Y
AA
CC

Routed Versus Routing Protocol
• Routed protocol
used between
routers to direct
user traffic
Examples: IP, IPX,
AppleTalk
• Routing protocol
used only between
routers to maintain
routing tables
Examples: RIP, IGRP, OSPF

Static Versus Dynamic Routes
Uses a protocol route that a network
administrator enters into the router
Static Route
Uses a route that a network protocol
adjusts automatically for topology or
traffic changes
Dynamic Route

Static Route Example
Point-to-point or
circuit-switched
connection
“Stub” network
Only a single network
connection with no need
for routing updates
AA
BB
• Fixed route to address reflects
administrator’s knowledge

Adapting to Topology Change
AA BB
CCDD
• Can an alternate route substitute
for a failed route?

AA BB
CCDD
XX

AA BB
CCDD
XX
• Can an alternate route substitute
for a failed route?
Yes—With dynamic routing enabled

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)

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
• HUBs & REPEATERS works at this layer.
• Max. troubleshooting occurs at this layer.

QSpiders - Day1 Network Basics

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