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THE_OSI_MODEL.ppt
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TEC5313
Advanced Telecommunications
Eastern Illinois University

The OSI Model

By: Dr. Rigoberto Chinchilla

Network Design Goals

  • Connectivity: Everybody regardless of “status” or localization must be connected

  • Reliable performance: Is critical that the organization can operate competitively without interruptions

Robustness of the connection

Few errors in transmission

Network Design Goals

  • Management Control: The network must have troubleshooting specialized tools for network analysis.

These tools are key to the installation, performance and maintenance of the network

  • Scalability:

The ability to grow the network easily +

The fact that growth does not affect efficiency (It does not make sense to grow easily when growing increments problems)

The Layered Model

  • For a complex, multivendor internetwork to operate , its devices must be able to communicate with each other

  • The networking industry uses a model: The OSI model- that provides guidelines for efficient telecommunications

  • The separation of networking functions is called LAYERING

A general model of Communications

  • Learning about Networking is EASIER when you start with theory and concepts and THEN move on to more concrete aspects of implementation/practice.

  • As network PROFESSIONALS, you need to learn of how networks communicate BEFORE designing, building, and maintaining network

Complexity

  • The network communication process is complex :

The data , in form of ELECTRIC signals, MUST travel across media (The modem plays an important role !)

The destination ID should be correct, The Sender ID should be correct and the PROTOCOLS used in each layer should be perfectly understood on both ends

The data needs to be converted BACK to its original form in order to be understood by the recipient.

Chunking is the way to go !

  • A message (right before to be sent) is “chunked” in many pieces in order to handle the whole message better

The first “chunking” occurs at OSI level 5

The chunks at this level are called “dialog units”. These are “virtual chunks” divided logically by content (i.e. a bank transaction should be though as a UNIT, because must be have integrity and must be complete)

So in level 5 we are not “chunking” information by the size in bits (as in the next levels) but by the message unity

The “chunks” from level 5 (Dialog Units) are sent to OSI level 4 in order to be further classified/tagged and rebuild and are called “segments” once in level 4.

Chunking is the way to go !

  • Once in level 4 , the dialog units are divided in many pieces (based on a maximum number of bits) to prepare them for transmission.

Each piece in level 4 (Called “Segment”) is “prepared” for transmission by

Specifying the protocol to be used (i.e. TCP or UDP)

The type of error correction algorithms to be used

Sequence of each segment (because it must be reassembled once reach the other side).

The PORT at which the segment is directed etc. (port in level 4 context means application like http, telnet, ping etc.)

Once ready is sent to level 3

Chunking is the way to go !

  • Beware that AFTER the dialog units are “chunked” to form Segments , these segments GROW in size because all of the above information has been added.
  • At level 3 We will have to ADD more information to the SEGMENT , therefore on level 3 a PACKET will be build which will be BIGGER than the segment because we need to add more information.
  • The generic NAME of the chunks is PDUs (protocol data units)

SEGMENT : is the level 4 PDU

PACKET: Is the LEVEL 3 PDU

Chunking is the way to go !

  • THE PROCESS OF ADDING MORE INFORMATION WHILE THE “CHUNK” IS MOVING DOWN FROM LEVEL 4 THEN TO LEVEL 3 THEN TO LEVEL 2 IS CALLED

“ENCAPSULATION”

Encapsulation

  • The first reason what is called “encapsulation” is because the new information added is appended at the beginning and at the end of the chunk (typically) when moving from level 4 to level 3 to level 2 to level 1. Giving the sense you are adding extra stuff on both sides.

  • The second reason is called “encapsulation” is because the information in level 4 is NOT understood in level 3 , and the “stuff” send to level 2 from level 3 is NOT understood by level 2 !

  • So EACH level handles the information coming from the upper level as “data” …. level 3 does not have idea that level 4 added some extra stuff and handle ALL what level 4 sends as “data”

Encapsulation

  • Because “stuff” (important information) is passed/added from one level to another level and the next level does not understand what is passed from the previous level , the generic unit that is passed is known as “PDU” Protocol data unit.

For Example from level 3 to level 2 a PDU is passed (in this case the PDU passed from level 3 to level 2 is a packet)

Chunking is the way to go !

The segments are sent to OSI level 3 to be further processed and are called “Packets” or “Datagrams”

In level 3 , MORE stuff have to be added like

IP address of Origin

IP address of destination

Packet sequence

Length of the packet (they could be of different length)

Some Flags (bits either in the “0” or “1” position) to indicate special conditions (like Urgency or priority)

Pretty much the Routing information is added in level 3 .

Chunking is the way to go !

  • The Packets are then sent to OSI level 2 to be further processed (adding MORE stuff) and are called “frames”
  • The PACKET is THEN FILLED with more stuff like

ETHERNET addresses (sender and receiver)

FRAME ERROR algorithms etc.

  • In summary FRAMES are build so the information can travel on SWITCHES for long distances and/or in Local Area Networks (LANS)

Chunking is the way to go !

  • Finally the “frames” have to be reprocessed to form BITS ! First in the form of “1” and “0’s” in SOFTWARE form THEN formed in ELECTRICAL or OPTICAL pulses can be put in to the media using the right modulation technique, Coding /decoding etc.
  • The Frames then are ultimately decomposed in bits and then transformed to voltage, electricity, light etc. so they can travel as bits over the media.

Chunking

  • A One page word document for example of 5000 bytes (~40,000 bits) is typically divided in many chunks of 1500 bits

  • Each little chunk is processed by adding a sequential number, a destination an origin and some other information, so EACH chunk will grow from lets say 1500 bits to a bigger amount of bits when going down through the OSI levels (Upon the protocol used)

Chunks of Information are the key !

  • Before data can be send across networks as electrical/optical pulses, it first MUST be broken in manageable chunks
  • The data that travel across the network is not the original information, instead is The original information codified !
  • The GENERIC and TECHNICAL NAME of a CHUNK at any level is called “PDU” or Protocol Data Unit.

Why all of this ?

  • Why not to send the info without all these “complications”?
  • Well……. believe or not the “Explosion” in telecommunications technologies of the last three decades has been due to this “chunking model” ! Combined with

Huge amounts of BW (better and more optical fiber)

Powerful processors, better coding platforms cheap memory and smaller electronic components.

Why all of this ?

  • Well … the “chunks” (packets if routers are used or Frames and cells if switches are used) MIGHT NOT (typically they don’t ) travel over the same path and they do not even reach the destination in the sequential order they were created! (that is why they have to have a sequence number)

  • The above has proven to be the MOST efficient way we know to send/receive information

Until someone else figures out the new telecommunication generation, so far NOBODY has figured out a better or more efficient way !

Advantages of “chunking”

  • Computers on networks can take turns sending “chunks”, and one computer with a large amount of data to transmit will not monopolize the network’s bandwidth

  • If network is disrupted and a chunk is lost, only that small amount of data, rather than the entire file, must be retransmitted

  • Typically, “chunks” can take different paths to the destination (even if they come from the same source!). So if one path becomes congested they can take an alternative one

What is a protocol ?

  • Is a common set of rules and languages that should be followed by the parties involved

  • In Telecommunications means that the networks should restrict themselves to follow the “rules” and the “control signals” (language)

  • PROTOCOL: “ A set of rules that determine the format and transmission of data”

  • There are different protocols in telecommunications depending of the function they are performing

Who rules the protocols ?

  • OSI (International Organization for Standardization)

TCP

IP

DECNET, SNA

  • IETF (Internet Engineering task force)

Internet drafts (how the internet should evolve, behaves etc)

  • RFC

Implementations of Domain Name Servers (DNS)

Specifications on Network Address Translations (NAT)

  • IEEE

The 802 Project (created February of 1980) this regulates the 802.x protocols

THE OSI 7 layer model (Each layer has its own protocols)

Why a layered model ?

  • To divide the interrelated aspect of network operation into less complex elements

  • To define standards interfaces for “plug and play” compatibility and multivendor integration

  • To enable engineers to specialize design and development effort on modular functions

Why use a layered model?

  • To promote symmetry in the different internetwork modular functions so they interoperate

  • To prevent changes in one area from affecting other areas so each area can evolve more quickly

  • To divide the complexity of internetworking into discrete, more easily learned operation subsets

The “peer to peer” principle

  • “Each layer of a transmitting system uses its own protocol to communicate with its peer layer in the receiving system”

  • Each layer’s protocol exchanges information, called protocol data units (PDU’s), between layers

The encapsulation principle

  • Each layer of the OSI model depends on the service function of the layer below it

  • To provide service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field

  • The lower layer can add whatever headers and trailers it will use to perform its functions

7-Layer OSI Reference Model

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

Network

Layer

Application

Application

Data Link

Layer

Physical

Layer

Network

Layer

Data Link

Layer

Physical

Layer

Communicating End Systems

One or More Network Nodes

End-to-End Protocols

THE ROUTER way to communicate between TWO different networks

NOTICE from the PREVIOUS SLIDE how the ROUTER WORKS:

FIRST: It takes the Ones and zeroes in electrical form and convert them to a software form (LEVEL 1)

SECOND : It REBUILDS the FRAME (LEVEL 2)

THIRD: Takes away the stuff added in level 2 until finds the PACKET

FOUR: Reads from the PACKET (Among other things) WHERE it has to send it, Meaning which port of the router has to output that information.

FIVE: ONCE it knows where to send the packet, it has to REBUILD the FRAME to its original state (it might change the ETEHERNET ADDRESS)

SIX : It has to decompose the FRAME in bits again and THEN put the info in

Electrical/optical form to travel through the media toward another router.

Encapsulation steps

  • Build the data

  • Package data from end-to-end transport

  • Append network address in header

  • Append local address in the data-link header

  • Convert to bits for transmission.

OSI layers and encapsulation

Application Processes

The Upper layers : Application Layer(7)

  • The application layer (Layer 7) of the OSI model is the closest (interface) to the user

  • It provides network services, such as file access printing, to the user’s application :TELNET and HTTP

  • Rather than provide services to other OSI layers, provides services to applications OUTSIDE the OSI model

The Upper layers : Application Layer(7)

  • The application layer decides:

What “applications” need services ?

Word processing programs

Bank terminal programs

Spreadsheet programs

Web pages and databases

LEVEL 7 is called the “HUMAN-MACHINE” Interface level

Application layer

  • The application layer is the closest to the end user when interacting with software applications such as sending and receiving e-mail over a network

  • The application layer deals with data packets from client-server applications, domain name servers and network applications by examining the following elements:

Client/server

Domain name system

E-mail

Telnet

FTP, TFTP, HTTP

Layer 7 Responsibilities

  • Synchronizing cooperating applications between the user and the OSI model.

  • Establishing agreement on procedures for error recovery between the USER and the layer 7

  • What are “Layer 8 problems” : IS a Code between telecomm-computer people to describe that the problem is the human in front of the machine. (kind of joke )

OSI Upper layers

Layer 6: Presentation Layer

  • Layer 6 presents data in a form that the receiving device can understand what the sending device is saying.

Two people talking different languages will not work !

  • Functions:

Data Formatting

Data Encryption

Data Compression

Presentation Layer Tasks

  • Determines how graphic images, sound, and movies are presented
  • Provides encryption of data
  • Compresses text
  • Converts graphic images into bit streams so that they can be transmitted across a network

Presentation Layer Functions and Standards (layer 6)

Presentation Layer Functions

Layer 6

  • Imagine two dissimilar systems, the first system uses Extended Binary Coded Decimal Interchange Code (EBCDIC) to represent characters on the screen.

  • The Second system uses ASCII for the same function.

  • Layer 6 provides the translation between these two different types of codes

File Formats used in layer 6

File formats

  • ASCII(7 bits code character) Contains simple character data and lack of any sophisticated formatting commands, such a boldface or underline (i.e. NOTEPAD)

  • EBCDIC is an 8 bit code character used in mainframes

Pictures, Music, and Movies

Data Encryption (Layer 6)

An encryption key is used to encrypt the data at its source and then to decrypt the data at its destination.

Encrypted data is called cipher text; unencrypted text is called clear text.

Data Compression (Layer 6)

Presentation layer (6)

  • The presentation layer provides code formatting and conversion services

  • Code formatting ensures that applications have a meaningful information to process

  • If necessary, the presentation layer translates between multiple data representation formats for text, data, audio, video, and graphics

Session Layer (5)

  • The session layer establishes , manages, and terminates communication sessions between applications.

  • Essentially, the session layer coordinates service request and responses that occurs when applications communicate between different hosts.

Session Layer (5)

  • As an Example a BANK transaction has to be treated in very specific DIALOG UNITS , it generally decompose the “message” (bank transaction) in very specific parts that has to be EXACTLY completed (re-assembled) by the application.

Layer 5: dialog control

  • The session layer establishes, manages, and terminates sessions between APPLICATION layer protocols:

Starting

Stopping

Synchronizing/Re-synchronizing

  • The session layer coordinates applications as they interact on two communication hosts

Dialog Separation

Layer 5: dialog control

  • Communication between two computers involves many mini-conversations, thus ensuring that the two computers can communicate effectively

  • One requirement of theses mini-conversations is that each host plays dual roles:

Requesting service, like a client

Replying with service, like a server

  • Determining which role they are playing at any given moment is part of dialog control

Layer 5: dialog separation

  • Dialog separation is the ORDERLY initiation, termination, and management of communication in which a transaction is tracked through completion

  • A transaction is defined as an atomic or indivisible unit of work- That is, it is a unit of work that either must fully complete or must be rolled back to some known state, as through it never began.

Example: A bank withdrawal

  • At the "Time Axis, t = checkpoint," the host A session layer sends a synchronization message to host B, at which time both hosts perform the following routine:

Back up the particular files

Save the network settings

Save the clock settings

Make note of the endpoint in the conversation

Dialog Separation

Layer 5 Protocols

Layer 5: The session Layer

  • The session layer allows two applications to synchronize their communications and exchange of data

  • This layer breaks the communication between two systems into dialog units and provides major and minor synchronization points during that communication

  • Example: A large distributed database transaction among multiple systems might use session layer protocols to ensure that a transaction either is completed fully or is “rolled back” to a know checkpoint on all systems

COMPUTER engineers vs Electrical Engineers

  • Until the 90’s (approx.) COMPUTER and or software ENGINEERS used to deal with the upper three levels of the OSI model. Electrical/telecomm Engineers used to deal with the LOWER four Levels.

  • MOST schools now in the world have united as “Electrical and computer Engineering Schools” because in the 2000’s BOTH engineers and technologist has to deal with the seven levels.

OSI Lower layers Typical protocols

Transport Layer

  • The transport layer defines end-to-end connectivity between host applications (not user applications)

Establish end-to-end operations, it constitutes a logical connection between the endpoints of the internetwork

Send segments from one end to another end host: As the transport layer sends its segments, it can also ensure data integrity through the use of check sum calculations on the data

Provides flow control mechanisms

Provides data reliability

Transport Layer

  • Transfers data end-to-end from process in a machine to process in another machine
  • Reliable stream transfer or quick-and-simple single-block transfer
  • Port numbers enable

virtual multiplexing

Message segmentation and reassembly

Connection setup, maintenance, and release

Transport

Layer

Network

Layer

Transport

Layer

Network

Layer

Network

Layer

Network

Layer

Communication Network

OSI layers

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Encapsulation Picture

Network Layer (Layer 3)

  • Transfers packets across multiple links and/or multiple networks

  • Addressing must scale to large networks

  • Nodes jointly execute routing algorithm to determine paths across the network

  • Forwarding transfers packet across a node

  • Congestion control to deal with traffic surges

  • Connection setup, maintenance, and teardown when connection-based

Network Layer

  • “Is in charge of decide where to go and what will be the road to take” (uses the TCP and UDP as the most “famous” protocols.)

  • This function is performed by a router

  • Routers understand an International accepted addressing scheme (INTERNET) to decide where to go.

  • Routers understand a local accepted addressing scheme too (INTRANET) to decide where to go

Router role in data communications

Routers Understand IP addressees

  • IP Address : Unique identifier of a computer in the world (If it is connected to the internet) : example: 205.218.6.5

  • IP addresses can also be used even if the computers are not connected to the internet for building private WAN and LAN networks

IP addresses and Networks

  • Each SEPARATE network MUST have a different set of IP addresses as an example
  • THE MASK (something we will explain in detail later) of a network ,tells how many IP addresses are assigned to a network

A network with 4 IP addresses has a mask 255.255.255.252

A network with 8 IP addresses has a mask of 255.255.255.248

A network with 16 IP addresses has a mask of 255.255.255.240

A network with 32 IP addresses has a mask of 255.255.255.224

  • The mask ONLY tells the maximum number of computer devices that can be connected in a network NOT how many are connected in reality
  • The maximum number of computer devices that can be connected in a network are the number of IP addresses assigned MINUS two

IP addresses and Networks

The rule of thumb (for now) will be as follows (PLEASE use this for your LAB 1 exam)

A network with 1 or 2 computer devices needs 4 IP addresses (mask 255.255.255.252)

A network with more than 2 and less or equal than 6 needs 8 IP addresses (mask 255.255.255.248)

A network with more than 6 computer devices and less or equal than 14 computer devices needs 16 addresses (mask 255.255.255.240)

A network with more than 14 computer devices and less or equal than 30 computer devices needs 32 IP addresses (mask 255.255.255.224)

Once a Portion of IP addresses has been assigned to a particular network, their IP addresses can NOT be used in a different network , even if the first network is not using them

Router role in data communications

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Network Layer

  • The most famous (and used) addressing schema is called IPv4 used by the IP protocol
  • IPv4 Uses an address and a mask

Address: 205.218.7.1 (example)

Mask: 255.255.255. 248 (Example)

  • The mask tells HOW MANY IP addresses are assigned to a network which gives and indication of how many computers are (or can be connected) within a particular network

  • Another popular protocol used to be “IPX” by Novell networks (Pretty much out of phase now)

The Data Link Layer (2)

  • It provides the transit of data across a physical link

  • It typically work with FRAMES

  • The data link layer is concerned with physical addresses (i.e. Ethernet card or NIC card)

“0055.2EA4.8883.4231” (Ethernet address)

  • It figures out when to send BITS to the media (through a modem !) , controls error detection at the frame level

Data Link Layer

  • Transfers frames across direct connections
  • Groups bits into frames
  • Detection of bit errors; Retransmission of frames
  • Activation, maintenance, & deactivation of data link connections
  • Medium access control for local area networks
  • Flow control

Data Link

Layer

Physical

Layer

Data Link

Layer

Physical

Layer

frames

bits

Data Link Layer devices

  • The devices which process frames are called SWITCHES , because they can understand the CONTENTS of the frames and move them accordingly.
  • Switches have MULTIPLE ports. A switch which just one port is called a “Bridge”.
  • HUBS preceded switches they send frames to all devices of a network (broadcast) meaning the HUB only amplifies and transmit blindly (WITHOUT knowing the content of the Frame) Hubs are considered level 1 devices (like an amplifier /transmitter)

The Physical layer (1)

  • Defines the electrical and mechanical specifications of the communication equipment

  • Defines the rules for activating, maintaining and deactivating the PHYSICAL devices in telecomm systems

  • Works with electricity and hard devices like modems, Multiplexer, satellites etc.

Physical Layer

  • Transfers bits across link
  • Definition & specification of the physical aspects of a communications link

Mechanical: cable, plugs, pins...

Electrical/optical: modulation, signal strength, voltage levels, bit times, …

functional/procedural: how to activate, maintain, and deactivate physical links…

  • Twisted-pair cable, coaxial cable optical fiber, radio, infrared, …

What Kind of “Stuff” is done in LAYER 1 ?

  • MODULATION
  • DEMODULATION
  • AMPLIFICATION
  • Equalization (for distortion)
  • MULTIPLEXING
  • CODING (or ENCODING)
  • MEDIA: Antennas, Optical Fiber, Coaxial Cable, Copper Cable etc.)

De-Encapsulation (at the final destination)

  • When the remote device receives a sequence of BITS at LEVEL 1 and passes them to the data link layer

  • At level 2, The data link layer reassembles the FRAME

  • The FRAME is sent to level 3 and opened to find the packet.

  • The PACKET is open again to find the segment
  • And so forth ……

OSI layers and encapsulation

The TCP/IP model (Internet Model)

  • The INTERNET model was created originally by the department of defense (DoD) in the 70’s just with four layers

Application

Transport

Internet

Network

The TCP/IP original model

The TCP/IP model (Internet Model)

  • In the DoD Internet model an “APPLICATION” means a NETWORK application like FTP, TFTP,DNS etc.
  • In the OSI model context an APPLICATION means modern software at the level 7

So we have to be careful of the context and meaning of “application”

Emphasis in TEC 5313: Levels 1,2,3,4

Problems with Using IP Addresses

The Domain Name system

  • So far thousands of top-level domains exists on the Internet

.us United states

.uk United Kingdom

.edu  Education

.org  Non-profit sites

The DNS server

  • The DNS server is a device on a network that responds to request from clients to translate a domain name into the associated IP address

  • If a local DNS server is capable of translating a domain name into its associated IP address, it does so and returns the result to the client

  • If it can not translate the address, it passes the request up to the next higher- level DNS server on the system, which then tries to translate the address

  • If the DNS at this level is not capable to returns positive results to the clients sends a request to the next higher level server and so on

  • If the name can not be found , then it is considered an error and an error message is returned

Network Applications

  • DNS Function

  • E-Mail Messages

  • We select a network application based on the type of network we need to accomplish

  • Each application program type is associated with its own application protocol

WWW using HTTP (Chrome, explorer)

Remote access programs using TELNET

E-mail Programs supporting the POP3 application layer protocol

Application Layer Examples

  • Telnet – provides the capability to remotely access another computer

  • File Transfer Protocol – download or upload files

  • Hypertext Transfer Protocol – works with the World Wide Web

Network Security Services

  • Integrity Service: information received from network has not been altered during transmission

  • Authentication Service: the receiver can authenticate that information came from purported sender

  • Privacy Service: information is readable only by intended recipient

  • In applications that require network security, integrity & authentication essential; privacy not always justified

Why is important the OSI model ?

  • In a few words and very personal words (I have been involved in this are for 30 years)

I have never know a good data telecom engineer who does not understand the OSI model

I have known hundreds of people working in telecommunications , most of the ones who struggle to do a good job, do NOT understand the OSI model

NO understanding of the OSI model  Probably you will not progress too much in this field