OSI Model - Pioneer Pacific College

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1
-
Application

File Transfer

File Management

Message Handling (e
-
mail)


2
-
Presentation

Formats data for network

Converts data to a language that the network can
handle.

May Encrypts/decrypts

(Codes and decodes graphics and file format
information)


3
-
Sess
ion

(traffic cop)

Establish and maintain link (the session)

Maintain synchronization

Reestablish lost connections

Sets terms of communications



Who talks first



How long each can talk

Often
Terminal
-
to
-
Mainframe
connection

ISP
-
home

4
-
Transport

(most import
ant layer)

Flow Control


selects a灰p潰oiate rate

pe煵enci湧


reassem扬es 灡c步ts in c潲rect 潲摥r

pen摳 ACh⁷hen 摡ta⁩s c潲rect

pen摳 re煵ests when 摡ta is c潮tains err潲s

pen摩ng will retransmit in ACh潴 received

jay 扲ea欠large 灡c步ts int漠smaller

潮es

qrans灯pt layer services:



(TCP in TCP/IP)



(SPX (Sequence Packet Exchange
-

in
IPX/SPX)


5
-
Network

Routs data
-

Selects Best path

Translates network to physical address

May segment/reassemble if necessary

Routers

6
-
Data Link

Divides data into frames

for transmission

Adds to frame:



Destination address



Source address



Control Information

(data)



Error Checking Information

Bridges,
switches

7
-
Physical


Nic, cables



OSI Model

Open System Interconnect

Greek for Equal

Pneumonic

All

People

Seem

To

Need

Da
ta

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Processing

OSI Model

Upper Layers

Gateways


ear摷are 潲 p潦tware 潰orate in these three layers



The most common is software running on a computer.

Application
Layer

Interfaces with network services ( applications)

Services Include:



File



Print



Message



Etc.

Presentation
Layer

Formats the data



Character sets are converted



Data is encrypted



Data may be compressed



Usually handles the redirection of data streams


Most standard computer systems use the American Standard Code for Information
Interchange (ASCI
I). Mainframe computers (and some IBM networking systems) use the
Extended Binary Coded Decimal Interchange Code (EBCDIC). The two are totally
different. Protocols at the Presentation layer can translate between the two.

Session Layer

Maintains the sessio
n

Establishes, Synchronizes, Maintains and Ends a Session

Does:



Security Authentication



Connection ID Establishment



Data Transfers (Checkpoints)



Acknowledgment



Connection Release

OSI

Middle Layers

Transport Layer

Flow Control



Handles Errors



Acknowledges
(ACK) receipt of data



Resizes messages into smaller or larger segments



(reassembles the messages back to original size)



Handles logical address/name resolution


Establishes one of two types of Connection Services:



Connection
-
oriented

o

Uses acknowledgements
and responses to establish and ensure virtual
connection is maintained

o

TCP is a connection
-
oriented protocol (TCP/IP stack)



Used by FTP and HTTP (File Transport Protocol) (HyperText
Transport Protocol)

o

SPX is a connection
-
oriented protocol (IPX/SPX stack)



Connectionless

o

No error or flow control


less 潶erhea搠


faster

o

UDP is a connectionless protocol (User Datagram Protocol
-

part of TCP/IP
stack)

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Used by TFTP and NFS (Trivial File Transport Protocol and Network
File System)

o

IPX is a connectionless protoco
l

o

NetBEUI is a connectionless protocol


Address/Name Resolution



Translates logical address to logical name



DNS (TCP/IP stack protocol) provides cross
-
platform Address/Name resolution.
(Domain Name Services)



NDS Database


(k潶el airect潲y pervices) 灲潶i摥
s a摤dessLname res潬ution at the
qrans灯pt iayer in res灯pse t漠oigher level kap airect潲y oe煵ester

ketw潲欠iayer

iayer P

C潮tr潬s i潧ical ketw潲欠A摤dessing an搠o潵ti湧


Logical Network Addressing



Uses protocol centric addressing (i.e. TCP
\
IP or IPX)



Du
plicate addresses often render both users down

o

With Win9X first user is OK second user will be off line



Duplicate TCP/IP addresses are more common than IPX conflicts

o

IPX uses the unique MAC address as part of its address



IPX addresses have two portions

o

Net
work portion

o

Node portion



For IPX (8:12


塘塘塘塘:u塘u塘塘塘塘)

o

Network portion is an 8 digit hex number


qhe
fmu ketw潲欠A摤dess



Randomly assigned by installation program or manually by SYSOP

o

Node portion is the MAC address



For TCP/IP

o

A dotted decimal n
otation is used (XXX.XXX.XXX.XXX)

o

4 three digit decimal numbers (or four octets) (decimal #s 1
-
254)


Routing



Routers create an internetwork out of network segments.



Each routers contain a Routing Table that permits the router to pass data with
specific add
resses to a specific port



Routing tables may be either Static or Dynamic

o

Static Routing Tables are created and maintained manually by the
administrator. (Time
-
intensive, expensive) In NT use the ROUTE command

o

Dynamic Routing uses discovery protocols to ide
ntify segments in the
internetwork and to update other routing tables


m潳t 灯灵lar r潵ting
techn潬潧o



Older Route Discovery Protocols use RIP (Routing Information
Protocol) a Distance Vector Route Discovery method



RIP for TCP/IP



RIP for IPX



Distance Ve
ctor Route Discovery method broadcast the routing table
every minute


receivi湧 r潵ters a摤⁡‱⁴漠the h潰oc潵nt an搠then
re扲潡摣asts it⸠Creates high netw潲欠潶erhea搮



Link State Route Discovery method is more efficient.

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Sends the table multicast (not b
roadcast) every five minutes



Other routers only respond with changes to the table (a if
there is a change)



Current Link State Route Discovery protocols:

o

OSPF for TCP/IP (Open Shortest Path First )

o

NLSP for IPX (NetWare Link Services Protocol)


Routable Pro
tocols:



Protocol

Route Discovery Protocol



IPX


RIP or NLSP



TCP/IP

RIP or OSPF



XNS


RIP

Nonroutable Protocols:



NetBEUI


On a workstation a
Default Gateway

is the address that packets are sent to if the
address is not located on its segment


Network Layer De
vices



Router



Connects segments into an internetwork



Connects LAN to the Internet



Facilitates communications between segments by selecting best route



Can connect dissimilar lower level networks (Ethernet & Token Ring)



Can provide firewall services and packe
t filtering



Brouter



Designed to bridge dissimilar networks


n潴 very effecti癥


n潴 use搠much



Layer 3 Switch



Does everything that a Layer 2 Switch does plus basic routing for VPNs


Most routing protocols function at this layer



Translates logical names in
to physical addresses



Prioritizes data transfer (puts AV data ahead of e
-
mail)



(Prioritization is know as Quality of Service (QoS)



Routs data from source to destination



Builds and tears down packets

OSI Model

Lower Layers

Data Link Layer

Layer 2

Controls
:



Controls logical network topology

o

Ethernet

o

Token Ring

o

Etc.



Method of media access

o

Contention

o

Polling

o

Token Passing



Controls physical addressing



Controls transmission connection and synchronization

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Sending Data:



Accepts data frame from Network layer and
reformats it to match the network
logical typology (Ethernet, Token
-
ring, etc).



Adds Data Link header and trailer to each new data frame



Manages the flow of data to the Physical layer



(The Physical layer will reject a data frame if the network is busy.)



Li
stens of an ACK for each data frame it sends and re
-
sends it if an ACK is not
received.


Receiving Data:



Accept raw data frames from physical layer



Checks destination address on each frame received



Discards all data frames not addressed to it



Performs an e
rror check on accepted data frames

o

Send ACK if the data frame is OK

o

If it is bad, it does not send ACK and discards the data frame



(The send site will re
-
sends a frame if the ACK is not received)



Removes Data Link header and trailer from the data frame



Rep
ackages the data frame as required and passes it to Network layer


IEEE 802 divided the Data Link Layer into two Sublayers

(Institute of Electrical and Electronics Engineers)



LLC
-

Logical Link Control

o

Provides error correction and flow control

o

802 Specif
ications:



802.1 Network management



802.2 Logical Link Control



MAC
-

Media Access Control (The Data Link layer physical address)

o

Has direct communications with NIC

o

Provides access control

o

802 Specifications:



802.3 CSMA/CD



802.4 Token Bus



802.5 Token Ring



80
2.12 Demand Priority


Data Link layer Devices



Bridges

o

Connects network segments

o

Restricts local traffic to local segment

o

Bridges that operate up to the LLC sublayer can connect some dissimilar
networks (ie Ethernet and Token Ring)

o

Bridges that only operate

up to the MAC layer can only connect identical
networks



Switches (AKA


iayer ㈠2witch 潲⁓witchi湧 eu戩

o

Builds a MAC Address table and routs traffic to proper port

Physical Layer

Network Hardware


qhe mhysical q潰潬潧y

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Layer 1

Translate data received f
rom the Data Link layer into signals that can be transmitted on the
medium being used. (called signal encoding)

Functional physical interface with the network



Transmission technique



Pin layout and connector type


Physical Layer Devices



Cable media (also c
alled bounded media)



NICs



Transceivers (may convert signal types


bxam灬e ㄵ
-
灩n afk Arf t漠og
-


afk


aetusche fn摵strie k潲m (derman kati潮al ptan摡r摳 扯摹)



Repeaters


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灡sses all 摡ta
an搠n潩se)



Hubs


(als漠
calle搠a⁣潮centrat潲) aulti灯rt re灥ater

o

Active (Powered) amplifies signal


exten摳 segment length

o

Passive (Typically Unpowered) reduces signal strength and segment length



MAUs


jultistation Access rnit (als漠jpAr)

o

Used for Token Ring Networks

o

Physic
al Star, Logical Ring

o

Max of 33 MAUs can be chained

5
-
4
-
3 Rule

5 Network segment


jax

㐠4e灥aters


jax

㌠P潰olate搠segments


jax

jAC⁁摤dess

Assigne搠 at⁴he cact潲y (s潭e can 扥⁣hange搠in the fiel搩

AhA:



Hardware Address



Ethernet Address



Physical Add
ress



NIC Address



Adapter Address

A 12 digit hexadecimal number (six
-
octet number)



The first 3 octets identify Manufacturer

o

The OUI


lrganizationally rni煵e f摥ntifer



The last 3 octets are unique numbers for that manfacturer

o

The Universal LAN MAC address

Media Access


CSMA/CD

Carrier Sense Multiple Access with Collision Detection



Used by Ethernet

Token Passing

Used by Token Ring



The first computer turned on creates a token



The token is then passed from computer to computer until it reaches a computer tha
t
wants to transmit data.



Only the computer that holds the token can transmit data.



It transmits the data with the token.



When the receiving computer receives the data it sends the token back to the sender.



The sender may then send more data.



When the comp
uter finishes transmitting it passes the token on to the next computer.



(Collisions are avoided)

CSMA/CA

Carrier Sense Multiple Access with Collision Avoidance

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Used by AppleTalk



The sender transmits a request to send (RTS) packet and waits for a clear to
send
(CTS) before sending.

IEEE 802
Standard



802.1

LAN/MAN Management (and Media Access Control Bridges)



802.2

Logical Link Control



802.3

CSMA/CD (802.3u includes 100BaseT)



802.4

Token Bus



802.5

Token Ring



802.6

Distributed Queue Dual Bus (DQDB) Metropoli
tan Area Network (MAN)



802.7

Broadband Local Area Networks



802.8

Fiber
-
Optic LANs and MANs



802.9

Integrated Services (IS) LAN Interface



802.10

LAN/MAN Security



802.11

Wireless LAN



802.12

Demand Priority Access Method

802.1

LAN and MAN management

MAC Bridg
es

Spanning Tree algorithms

802.2

Logical Link Control (LLC)

(Upper half of the Data Link layer


lin欠扥tween jAC⁡n搠ketw潲欠layer)


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CpjALCa


Carrier pense jultiple Access with C潬lision aetecti潮

bthernet (afu)

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㠰㈮U

q潫on Bus

cact潲y Aut潭ation


k潴⁵se搠f潲 摡ta c潭mu湩cation

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rses jAr

mhysical star

i潧ical ring

㑍扰b ㄶ1扰b

Can “talk” to IMB mainframe comp
uters

Busy netw潲歳 just get sl潷


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rses twiste搠灡ir (can uses 潴her ca扬in朩 rate搠批 ty灥 (n潴 Categ潲y).

q潫on oing kfCs⁡re潲e ex灥nsive than bthernet kfCs

pignals are am灬ifie搠 an搠regenerate搠 批 each kfC  her q潫on o
ing 摥vice

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City size netw潲歳

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-

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-
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fnclu摥s:



FDDI


ci扥r aistribute搠 aata fnterface



10BaseFL

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802.9

Integrated Services (IS) LAN Interface

ISDN spec
-

Integrated Switched Digital Network

ADSL and Cable Modems have eclipsed ISDN



(They are Cheaper, Easier to Setup, and have Better Perform
ance than ISDN)

802.10

LAN/MAN Security

Spec for VPN Virtual Private Network

Using public (Internet) network for connections between LAN locations

Encrypts data for transmitting in the open

Cheaper than renting a dedicated line

802.11

Wireless LAN

802.1
2

Demand Priority Access

Developed by HP

Not supported by other manufactures (They use Fast Ethernet


with 㠰㈮㍵)

Assigns higher 扡n摷i摴h t漠frames i摥ntifie搠 with high 灲i潲ity

fm灯ptant t漠real time au摩漠an搠vi摥漠transmission

AhA:



100VG (Voice Grade
)



100VG
-
AnyLAN



100Base VG



AnyLAN



Use the OSI reference model to aid in topology decisions

Jun 7, 2001

Mike Mullins CCNA, MCP

Author's
Bio

|
E
-
Mail

|
Archive


© 2001 TechRepublic, Inc.



Knowing the seven layers of the Open Systems Interconnection (OSI) reference model and understanding where
your network devices fit in that model can prove to be a valuable asset when it

comes to adding devices to
different segments of your network and placing devices between networks. This knowledge enables you to take
control of your topology and keeps you from leaving your network design to your vendors. In this article, we’ll
explore
the most common network devices and discuss where they should reside in a good network topology.


Let’s break down the devices by layer and by their function on a network.
Figure A

provides a good overview.


Figure A


Where different devices operate on the OSI reference model


Figure A

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Layer 1, The Physical Layer: Bits

The Physical Layer provides the mechanical and electrical connections to the network. In other words, it sends
bits down a wi
re.


Repeaters

connect media segments and provide amplification and retransmission of signals with no filtering
and will propagate all errors. Use them only for connecting long segments of wire.


Hubs

connect multiple hosts to one segment of wire. All host
s share the same bandwidth. This means that there
is one large collision domain. Use them at points where you would deploy a network sensor, so the device can
see all the traffic on that portion of the network.


Layer 2, The Data Link Layer: Frames

The Dat
a Link Layer splits data into frames for sending on the physical layer and receives acknowledgement
frames. It performs error checking and retransmits frames not received correctly. It provides an error
-
free
virtual channel to the Network Layer. The Data L
ink Layer is split into an upper sublayer, Logical Link Control
(LLC), and a lower sublayer, Media Access Control (MAC).


Bridges
connect different types of networks (token ring, Ethernet, etc.), filter network traffic based on MAC
address, and remove error
s from the network. Use them to connect different types of internal networks.


Switches
, also known as Multiport Bridges, transfer data between different ports based on the destination
addresses. Each segment or port connection is its own collision domain,

but all ports are in the same broadcast
domain. Switches can be used to connect multiple ports to the same destination (i.e., multiple uplink ports), but
only one port can be active at a time. Historically, this is a hardware Layer 2 device and typically
operates in
one of three modes:



Store and Forward:

This mode copies the entire frame into memory, computes the Cyclic Redundancy
Check (CRC) for errors, and then looks up the destination MAC address and forwards the frame. This is
slow but offers the best
solution for error correction without affecting the entire backbone in
retransmission.

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Cut
-
through:

This mode reads the destination address of the frame and forwards the frame to the port
connected to that destination MAC address before the entire frame i
s seen. This is fast but provides very
little error correction and will propagate errors from one collision domain to the next.



Modified Cut
-
through:

This mode reads the first 64 bytes of the frame and then forwards the frame to
a port based on MAC destin
ation address. This is fast and efficient in error correction.


Use Layer 2 switches as your LAN subbackbone device with each host connected to a switched port. This
dedicates bandwidth and segments the lowest layer of your internal network. Switches are a
lso pretty
inexpensive.


Layer 3, The Network Layer: Datagrams/Packets

The Network Layer determines the routing of packets of data from sender to receiver. Routes can be static or
dynamic. The Network Layer provides sequencing and flow control of data, sel
ects routes, and provides quality
of service through error detection, recovery, and notification. It also segments collision and broadcast domains.
This is where a MAC or hardware address is translated into Internet Protocol (IP) addresses (or other routab
le
protocol addresses, such as IPX or AppleTalk).


Routers

are basically software
-
based packet
-
forwarding engines. The Network Layer provides information to
these devices, allowing them to base their forwarding decisions on criteria such as IP, link, or ne
twork node
availability and performance. A router’s true advantage lies in its flexibility in network protocol.


Use routers for their intended purpose. They were born to bridge networks and separate broadcast domains. This
is your “gateway” device to the
Internet or your linking device between LANs or WANs.


Switches (Layer 3)
are nothing more than wire
-
speed routers. They come in two basic models.



Port switches

decide which physical port network traffic needs to go to and direct the traffic
appropriately.
Each lane is actually a backplane segment on the switch. Because the switching is
performed locally via logic circuits and at wire speed, port switches are easier and cheaper to implement
than frame switches but give many of the same benefits.



Frame switc
hes

examine each Ethernet packet, determine which segment it came from and where it is
going, and send it on its way. These are more expensive than port switches but add a significant
performance boost to your network. They are also known as Learning Switc
hes.


Use these devices at the backbone of your network. They’re less expensive than routers and much faster. But
read on: There’s another switch that might be a better solution for your network.


Layer 4, The Transport Layer: Segments

The Transport Layer
(sometimes referred to as the Host Layer) determines how to use the Network Layer to
provide a virtual point
-
to
-
point connection. It creates and dissolves connections between hosts. The most
popular types of transport connection are TCP and UDP. TCP is a p
oint
-
to
-
point connection protocol that
delivers messages in the order in which they were sent and guarantees delivery. UDP is a connection
-
less
protocol with no guarantee of delivery. The Transport Layer is a legitimate end
-
to
-
end layer. In other words, a
program on the source machine carries on a conversation with a similar program on the destination machine.


Layer 4 Switches

are sometimes called “session switches” because they track and maintain individual sessions
from start to finish. (This doesn’t mak
e them Layer 5 Switches; they’re just “aware” of sessions.) Layer 4
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Richard L. Goldman


Switches make forwarding decisions based on session and application
-
layer information and provide load
balancing across multiple servers. Layer 4 Switches determine (through different comp
lex and weighted
algorithms) the best server of a cluster to process a service request and bind the session to that server’s IP
address until the session is terminated.


They also designate and prioritize traffic by application. Because they are “aware” at

the session level, they
have the ability to prevent unauthorized access to servers. Layer 4 Switches’ true selling point is handling all
packet processing in hardware. They utilize custom application
-
specific integrated circuits (ASICs), making
them extre
mely fast!


Substitute Layer 4 Switches for your Layer 3 Switches only if you intend to operate high
-
speed intranet
application servers with multiple 100
-
Mbit or gigabit interfaces.


What you need and what you can afford

Do not let design and implementatio
n of your network become dominated by one specific vendor. While no two
networks are exactly alike, a variety of vendors exist. So don’t settle for a device from one vendor based solely
on company name. Above Layer 2, most devices are very specific in the
protocols and configurations they
support. Know the intended purpose of your network and let budget and function dictate design.

What kind of network design tips do you have?

Do you use the OSI reference model for topology planning and troubleshooting? We

look forward to getting
your input and hearing your experiences regarding this topic. Join the discussion below or
send the editor an e
-
mail
.