OsI Model by palaniappan13

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									OSI MODEL
 An ISO (International standard Organization) that covers all aspects of 




network communications is the Open System Interconnection (OSI) model. An open system is a model that allows any two different systems to communicate regardless of their underlying architecture (hardware or software). The OSI model is not a protocol; it is model for understanding and designing a network architecture that is flexible, robust and interoperable. The OSI model is a layered framework for the design of network systems that allows for communication across all types of computer systems. The OSI model is built of seven ordered layers

The OSI 7-layer Model
• OSI - Open Systems Interconnection • Defined in 1984 and become an international standard
All Away Pizza Sausage Throw Not Do Please

Seem To Need Data Processing

Functions:  Transmits raw bit stream over physical cable Defines cables, cards, and physical aspects Defines NIC attachments to hardware, how cable is attached to NIC Defines techniques to transfer bit stream to cable

Protocols: •IEEE 802 •IEEE 802.2 •ISO 2110 •ISDN

2.Data Link(data frames to bits)
 Turns packets into raw bits 100101 and at the receiving end turns bits

into packets.  Handles data frames between the Network and Physical layers  The receiving end packages raw data from the Physical layer into data frames for delivery to the Network layer  Responsible for error-free transfer of frames to other computer via the Physical Layer This layer defines the methods used to transmit and receive data on the network. It consists of the wiring, the devices use to connect the NIC to the wiring, the signaling involved to transmit / receive data and the ability to detect signaling errors on the network media

Logical Link Control error correction and flow control  manages link control and defines SAPs  802.1 OSI Model  802.2 Logical Link Control Media Access Control communicates with the adapter card  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

3.Network Layer (addressing; routing)
 Translates logical network address and names to their physical address

(e.g. computer name ==> MAC address)  Responsible for  addressing  determining routes for sending  managing network problems such as packet switching, data congestion and routing  If router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data  Think of this layer stamping the addresses on each train car


4.Transport Layer (packets; flow control & error-handling)
 Additional connection below the session layer

 Manages the flow control of data between parties across the network
 Divides streams of data into chunks or packets; the transport layer of    

the receiving computer reassembles the message from packets “Train" is a good analogy => the data is divided into identical units Provides error-checking to guarantee error-free data delivery, with on losses or duplications Provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free Provides flow control and error-handling

 NWLink  NetBIOS / NetBEUI  ATP

5.Session Layer (syncs and sessions)
 Establishes, maintains and ends sessions across the network

 Responsible for name recognition (identification) so only the

designated parties can participate in the session  Provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted  Manages who can transmit data at a certain time and for how long  Examples are interactive login and file transfer connections, the session would connect and re-connect if there was an interruption; recognize names in sessions and register names in history

 NetBIOS Name Pipes  Mail Slots


6.Presentation Layer(Translation)
 Translates from application to network format and vice-versa  all different formats from all sources are made into a common uniform

format that the rest of the OSI model can understand  responsible for protocol conversion, character conversion, data encryption / decryption, expanding graphics commands, data compression  sets standards for different systems to provide seamless communication from multiple protocol stacks  not always implemented in a network protocol

7.Application Layer (User Interface)
 Used for applications specifically written to run over the    

network Allows access to network services that support applications; Directly represents the services that directly support user applications Handles network access, flow control and error recovery Example apps are file transfer, e-mail, NetBIOSbased applications


Summary of layers

Network Protocols
What is network protocol ? A network protocol defines rules and conventions for communication between network devices. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets. How Network Protocols Are Implemented Modern operating systems like Microsoft Windows contain builtin services or daemons that implement support for some network protocols. Applications like Web browsers contain software libraries that support the high level protocols necessary for that application to function. For some lower level TCP/IP and routing protocols, support is implemented in directly hardware (silicon chipsets) for improved performance.

Types of Network Protocols
 The most common network protocols are:

 Ethernet
 Local Talk  Token Ring  FDDI

 Here is some common-used

network symbols to draw different kinds of network protocols.

 The Ethernet protocol is by far the most widely used. Ethernet uses an





access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally effect the speed of transmission on the network

 The Ethernet protocol allows for linear bus, star, or tree topologies. Data

can be transmitted over wireless access points, twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps up to 1000 Mbps.

Fast Ethernet
 To allow for an increased speed of transmission, the Ethernet protocol has

developed a new standard that supports 100 Mbps. This is commonly called Fast Ethernet.  Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet is becoming common in schools that have been recently wired.

Local Talk
 Local Talk is a network protocol that was developed by Apple

Computer, Inc. for Macintosh computers. The method used by Local Talk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so.  Local Talk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-topeer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established.  The Local Talk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of Local Talk is speed. Its speed of transmission is only 230 Kbps.

Token Ring
 The Token Ring protocol was developed by IBM in the mid-1980s. The

access method used involves token-passing. In Token Ring, the computers are connected so that the signal travels around the network from one computer to another in a logical ring.  A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation.  If a computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the data is captured by the receiving computer.  The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased.

 Fiber Distributed Data Interface (FDDI) is a network protocol that

is used primarily to interconnect two or more local area networks, often over large distances.  The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete ring.  A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbps.

 Asynchronous Transfer Mode (ATM) is a network protocol that

transmits data at a speed of 155 Mbps and higher. ATM works by transmitting all data in small packets of a fixed size; whereas, other protocols transfer variable length packets.  ATM supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable.  ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local area networks

Gigabit Ethernet
 The most recent development in the Ethernet standard is a protocol

that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for backbones on a network at this time.
 In the future, it will probably be used for workstation and server

connections also. It can be used with both fiber optic cabling and copper. The 1000BaseTX, the copper cable used for Gigabit Ethernet, is expected to become the formal standard in 1999.

Compare the Network Protocols

Hypertext Transfer Protocol
 Hypertext Transfer Protocol (HTTP) is an application-level protocol for

distributed, collaborative, hypermedia information systems. Its use for retrieving inter-linked resources led to the establishment of the World Wide Web.
 HTTP development was coordinated by the World Wide Web Consortium

and the Internet Engineering Task Force (IETF), culminating in the publication of a series of Requests for Comments (RFCs), most notably RFC 2616 (June 1999), which defines HTTP/1.1, the version of HTTP in common use.
 Typically, an HTTP client initiates a request. It establishes a Transmission

Control Protocol (TCP) connection to a particular port on a host (port 80 by default; see List of TCP and UDP port numbers).

Simple Mail Transfer Protocol:
 Simple Mail Transfer Protocol (SMTP) is an Internet standard for

electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 10), and last updated by RFC 5321 (2008) which describes extended SMTP (ESMTP), the protocol in widespread use today.
 While electronic mail server software uses SMTP to send and

receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) to access their mail box accounts on a mail server.

File Transfer Protocol
 File Transfer Protocol (FTP) is a network protocol used to exchange and

manipulate files over a TCP computer network, such as the Internet. An FTP client may connect to an FTP server to manipulate files on that server.
 FTP runs over TCP.[1] It defaults to listen on port 21 for incoming

connections from FTP clients. A connection to this port from the FTP Client forms the control stream on which commands are passed from the FTP client to the FTP server and on occasion from the FTP server to the FTP client.
 FTP uses out-of-band control, which means it uses a separate connection

for control and data. Thus, for the actual file transfer to take place, a different connection is required which is called the data stream. Depending on the transfer mode, the process of setting up the data stream is different. Port 21 for control (or program), port 20 for data.

Sliding Window Protocol
 Sliding Window Protocol is a bi-directional data transmission protocol

used in the data link layer (OSI model) as well as in TCP (transport layer of the OSI model). It is used to keep a record of the frame sequences sent and their respective acknowledgements received by both the users.
 In transmit flow control, sliding window is a variable-duration window

that allows a sender to transmit a specified number of data units before an acknowledgment is received or before a specified event occurs.
 An example of a sliding window is one in which, after the sender fails to

receive an acknowledgment for the first transmitted frame, the sender "slides" the window, i.e. resets the window, and sends a second frame. This process is repeated for the specified number of times before the sender interrupts transmission. Sliding window is sometimes (loosely) called acknowledgment delay period.

 Transmission Control Protocol  The Transmission Control Protocol (TCP) is one of the core protocols of

the Internet Protocol Suite. TCP was one of the two original components, with Internet Protocol (IP), of the suite, so that the entire suite is commonly referred to as TCP/IP.  Whereas IP handles lower-level transmissions from computer to computer as a message makes its way across the Internet, TCP operates at a higher level, concerned only with the two end systems, for example, a Web browser and a Web server. In particular, TCP provides reliable, ordered delivery of a stream of bytes from a program on one computer to another program on another computer.  Besides the Web, other common applications of TCP include e-mail and file transfer. Among its other management tasks, TCP controls message size, the rate at which messages are exchanged, and network traffic congestion.

User Datagram Protocol
 The User Datagram Protocol (UDP) is one of the core members of the

Internet Protocol Suite, the set of network protocols used for the Internet.
 With UDP, computer applications can send messages, in this case referred

to as datagrams, to other hosts on an Internet Protocol (IP) network without requiring prior communications to set up special transmission channels or data paths. UDP is sometimes called the Universal Datagram Protocol. The protocol was designed by David P. Reed in 1980 and formally defined in RFC 768.
 UDP uses a simple transmission model without implicit hand-shaking

dialogues for guaranteeing reliability, ordering, or data integrity. Thus, UDP provides an unreliable service and datagram's may arrive out of order, appear duplicated, or go missing without notice.

 UDP uses a simple transmission model without implicit hand-

shaking dialogues for guaranteeing reliability, ordering, or data integrity. Thus, UDP provides an unreliable service and datagram's may arrive out of order, appear duplicated, or go missing without notice.  UDP assumes that error checking and correction is either not necessary or performed in the application, avoiding the overhead of such processing at the network interface level. Time-sensitive applications often use UDP because dropping packets is preferable to using delayed packets.  If error correction facilities are needed at the network interface level, an application may use the Transmission Control Protocol (TCP) or Stream Control Transmission Protocol (SCTP) which are designed for this purpose.

Classful network
 Classful network is a term that is used to describe the network

architecture of the Internet until around 1993. It divided the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded by the first three bits of the address, defined a different size or type (unicast or multicast) of the network.
 Today, remnants of classful network concepts remain in practice

only in a limited scope in the default configuration parameters of some network software and hardware components (e.g. netmask), but the terms are often still heard in general discussions about network structure among network administrators.

To remain compatible with the existing IP address space and the IP packet structure, the definition of IP addresses was changed in 1981 in RFC 791 to allow unicast addresses with three different sizes of the network number field (and the associated rest field), as specified in the table below:

Class ranges:

The address ranges used for each class are given in the following table, in the standard dotted decimal notation.

IP address
 An Internet Protocol (IP) address is a numerical identification (logical

address) that is assigned to devices participating in a computer network utilizing the Internet Protocol for communication between its nodes.[1] Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6).
 The role of the IP address has been characterized as follows: "A name

indicates what we seek. An address indicates where it is. A route indicates how to get there."
 The original designers of TCP/IP defined an IP address as a 32-bit number

and this system, now named Internet Protocol Version 4 (IPv4), is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of the address space, a new addressing system (IPv6), using 128 bits for the address, was developed (RFC 1883).

 The Internet Protocol also has the task of routing data packets

between networks, and IP addresses specify the locations of the source and destination nodes in the topology of the routing system.
 For this purpose, some of the bits in an IP address are used to

designate a sub network. The number of these bits is indicated in CIDR notation, appended to the IP address, e.g.,

Subnet Mask
 A subnet allows the flow of network traffic between hosts to be

segregated based on a network configuration. By organizing hosts into logical groups, sub netting can improve network security and performance.
 Perhaps the most recognizable aspect of subletting is the subnet

mask. Like IP addresses, a subnet mask contains four bytes (32 bits) and is often written using the same "dotted-decimal" notation. For example, a very common subnet mask in its binary representation.

 11111111 11111111 11111111 00000000 is typically shown in the

equivalent, more readable form

 Applying a Subnet Mask  A subnet mask neither works like an IP address, nor does it exist

independently from them. Instead, subnet masks accompany an IP address and the two values work together. Applying the subnet mask to an IP address splits the address into two parts, an "extended network address" and a host address.
 For a subnet mask to be valid, its leftmost bits must be set to '1'. For

example, 00000000 00000000 00000000 00000000 is an invalid subnet mask because the leftmost bit is set to '0'.

 Conversely, the rightmost bits in a valid subnet mask must be set to

'0', not '1'. Therefore, 11111111 11111111 11111111 11111111 is invalid.
 All valid subnet masks contain two parts: the left side with all mask

bits set to '1' (the extended network portion) and the right side with all bits set to '0' (the host portion), such as the first example above.

Question Session

Thank You

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