introduction to osi layer
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1
Objectives
On completion of this chapter , you will beable to perform the following tasks:
Describe how data traffic is exchanged
between source and destination devicesIdentify the roles and functions of a hub,
switch, and router, and where they best fit
in the network
Select the appropriate Cisco equipment
for a given set of network requirements
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Network Structured Defined By
Hierarchy
Core Layer
DistributionLayer
AccessLayer
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Access Layer Characteristics
Access Layer
End station entry pint to the network
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Access Layer
Aggregation Point
Router traffic Broadcast / Multicast
Domains
Media Transiation
Security
Possible point for remote access
Distribution Layer
Characteristics
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Core Layer Characteristics
Core Layer
Fast transport to enterprise services
No packet manipulation
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OSI Model Overview
Application
(upper)
Layers
Reduces complexity
Standardizes interfaces
Facilitates modular
engineering
Ensures interoperabletechnology
Accelerates evolution
Simplifies teaching andlearning
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Data FlowLayers
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Application
Presentation
Session
Transport
Data Link
Network
Physical
EXAMPLES
Telnet
HTTP
ASCII, EBCDIC
JPEG
Operating System /Application AccessScheduling
Role of Application Layers
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Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Reliable Or Unreliable delivery
Error correction before retransmit
Provide logical addressing which routers use for path
determination
Combine bits into bytes and bytes into frames
Access to media using MAC addressError detection not correction
Move bits between devices
Specifies voltage , wire speed and pin-out cables
Role of Data Flow Layers
EXAMPLES
TCP
UDP
SPX
IP
IPX
802.3/802.2
HDLC
WIA/TIA-232
V.35
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Application
Presentation
Session
Transport
Network
Data Link
Physical
Data
Data
Upper layer data
Upper layer dataTCP Header
DataIP Header
LLC Header
MAC Header
0101110101001000010
Encapsulation Data
PDU
Bits
Frame
Packet
Segment
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Application
Session
Presentation
Transport
Data Link
Network
Physical
Upper Layer Data
Upper Layer Data
TCP+Upper Layer Data
IP+TCP+Upper Layer Data
MAC Header
TCP Header
LLC Hdr+IP+TCP+Upper Layer Data
LLC Header
IP Header
0101110101001000010
De-Encapsulation Data
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Physical Layer Functions
Signaling Type
Connector
Media Type
Defines
Physical Ethernet 802.3 V.35EIA/TIA-232
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Host
10 Base 2__ Thick Ehernet
10 Base 2__ Thick Ehernet
Hub
Hosts
10 Base T _ Twisted Pair
Physical Layer :Ethernet/802.3
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Physical
All devices in the same collisions domain
All devices in the same broadcast domain
Devices share the same bandwidth
Hubs operate at Physical Layer
AB C D
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Data Link Layer Functions
802.2
HDLC
Data
Link
Physical Ethernet 802.3EIA/TIA_232
V.35
Frame
Relay
Defines
Physical source and destination
address
Higher layer protocol
(Service Access Point)
associated with frame
Network topology
Frame sequencing
Flow control
Connection-oriented orconnectionless
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Data Link Layer Functions
0000.0c xx.xxxx
8
Preamble Dest Add Source Add Length Data FCS
6 6 2
Variable
4
Ethernet II uses Type here and does not use
802.2
IEEE
assigned
Vender
assigned
MAC Layer -802.3
# Bytes
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OR
Data Link
Switches and Bridges Operate
at Data Link Layer
Each segment has its own collision domain
All segments are in the same broadcast domain
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Physical
Ethernet 802.3EIA/TIA_232
V.35
Frame
Relay
Network
Data Link
IP, IPX
802.2 HD
L
C
Network Layer Functions
Interconnects multiple data
links
Defines paths through
network
Defines logical source
and destination addresses
associated with a specific
protocol
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Network Layer Functions
Network Layer End Station Packet
Source
addressIP Header
Destination
address Data
172.15.1.1
Logical
Address
Network Node
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172.16.122.204 255.255.0.0
10101100 00010000 01111010 11001100
11111111 11111111 00000000 00000000
172 16 122204
255 255 0 0
Host
Network Layer Functions(cont.)
Network
Address Mask
Binary
Address
Binary
Mask
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Routing Table
NET INT Metric
Routing Table
INTNET Metric
1
2
4
E0
S0
S0
0
0
1
12
4
S0S0
E0
1
0
0
1.1
1.2
1.0
E 0 S 0
1.32.1
2.2
4.3
S 0 E 0
4.0 4.1
4.2
Network Layer Function (cont.)
Logical addressing allows for hierarchical network
Configuration required
Uses configured information to identify paths to networks
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Routers: Operate at the
Network Layer
Broadcast control
Connects to WAN
services
Traffic management
Multicast control
Multicast Path
determination
U i R P id
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Telecommuter
Mobile User
Branch Office
Main Office
Internet
Using Routers to Provide
Remote Access
Modem or ISDN TA
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Connection-Oriented Sessions
Synchronize
Synchronize
Acknowledge
Negotiate Connection
Data Transfer
(Send Segments)
Sender Receiver
Connection EstablishedConnection EstablishedConnection Established
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