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THE OSI & TCP/IP MODELS. Tan Alam School of IT Bond University. The OSI Model. Link . Physical Layer.

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the osi tcp ip models
  • Tan Alam
  • School of IT
  • Bond University
physical layer
Physical Layer
  • Protocols at this layer generate and detect voltage (or in the case of fiber optic transmission, pulses of light) so as to transmit and receive signals carrying data.
  • The Physical layer sets the data transmission rate and monitors data error rates, though it does not provide error correction services.
  • Physical network problems, such as a severed wire and noise interference, affect the Physical layer.
data link layer
Data Link Layer

Controls communications between the Network layer and the Physical layer.

  • Frame - a structured package for moving data that includes not only the raw data, or "payload," but also
    • the current sender’s and receiver’s local network addresses (their physical address on this network) and
    • error checking and control information.
network layer
Network Layer
  • Looks at the original source and final destination Network addresses (commonly Internet address) and decides on the “next” best step to route the data from the source to the destination.
  • To do this the Network layer is responsible for passing the data from the “current source” to the “next best step destination”.
  • Network layer addresses,which reside at the Network layer of the OSI model, follow a hierarchical addressing scheme and can be assigned through operating system software.
  • Network Addresses can enable a node to be reachable via the Internet or only within their own network.
transport layer
Transport Layer
  • Primarily responsible for ensuring that data is transferred from “Process A on Node A” to “Process B on Node B” reliably, in the correct sequence, and without errors.
  • Transport protocols also handle flow control, or the method of gauging the appropriate rate of transmission based on how fast the recipient can accept data.
  • Segmentation - the process of chopping up a large message into smaller chunks (segments) so that they can move from one network to another network (networks can handle different size data units)
session layer
Session Layer
  • Session - a connection for data exchange between two parties.
  • The Session layer’s functions include:
    • Establishing and keeping alive the communications link for the duration of the session
    • Synchronizing the dialog between the two nodes
    • Determining whether communications have been cut off, and, if so, figuring out where to restart transmission
presentation layer
Presentation Layer

Serves as a translator between the application and the network.

  • At the Presentation layer, data becomes formatted in a schema that the network can understand; this format varies with the type of network used.
  • Encryption - the use of a mathematical routine to scramble data so that it can only be read by reversing the formula.
application layer
Application Layer

Provides the interface for user software to interact with the operating system and network software that enable programs to use network services.

  • Network Protocol Suites – come with application programs that enable the node to access and manage the network.
  • OSI model also has the Application program interface’s (API) (a set of instructions that allows a program to interact with the operating system) as part of the application layer.
tcp ip protocol suite compared to the osi model
TCP/IP Protocol Suite compared to the OSI Model
  • The TCP/IP suite includes the following:
    • Application layer - provides authentication and compression services, and is roughly equivalent to the Application, Presentation, and Session layers of the OSI model.
    • Transport layer - roughly corresponds to the Transport layer of the OSI model.
    • Network layer - equivalent to the Network layer of the OSI model. (sometimes called the Internetwork or Internet Layer)
    • Link layer - roughly equivalent to the Data Link and Physical layers of the OSI model.

These two are frequently listed separately as DLL (Data Link Layer) and Physical Layer (this is sometimes separated out from the other protocols).

internet protocol ip
Internet Protocol (IP)

IP belongs to the Network layer of the OSI model and to the Internetwork layer of the TCP/IP suite.

  • It provides information about how and where data should be delivered enabling it to travel through many Networks (over the Internetwork/Internet).
  • Datagram (or Packet) - the entire information passed by the Network layer to the Data Link Layer (max length 65,535bytes). This information includes the portion passed down by the Transport layer (segment) and the IP header information.
  • The IP header acts as an envelope for the segment and encapsulates it with information necessary for routers to transfer data between networks or subnets (see next slide).
  • IP is an unreliable, connectionless protocol, which means that it does not guarantee delivery of data – its main job is to route data along the best path.
hierarchy of networks
Hierarchy of Networks










transmission control protocol tcp
Transmission Control Protocol (TCP)
  • Provides reliable delivery of data by :
    • Connection-oriented – a transmission connection is established between source and destination before any data starts being sent.
    • Reliable - It provides error-checking and retransmission notification.
    • Flow control – involving the rate of data being sent and received.
    • Sequencing information – to enable large messages to be split into smaller segments.
user datagram protocol udp
User Datagram Protocol (UDP)
  • Provides unreliable delivery of data by :
    • Connection-less – no connection is established, information is just packaged and sent to the destination without checking there are available.
    • Unreliable - It provides minimal error-checking (of the single message) and each datagram is a separate message so no retransmissions are supported.
    • Flow-control and sequencing are not needed.
    • Limited size – each datagram is independent so the size of the message being carried cannot be larger then one IP packet can carry
the tcp ip application layer protocols
The TCP/IP Application Layer Protocols
  • Dynamic Host Configuration Protocol (DHCP) - an automated means of assigning a unique Network layer (or IP) address to nodes on a network.
  • File Transfer Protocol (FTP) - used to send and retrieve files between nodes.
  • Hypertext Transfer Protocol (HTTP) - protocol that enables Web browsers to issue requests to Web servers and interpret the response.
the tcp ip application layer protocols23
The TCP/IP Application Layer Protocols
  • Simple Mail Transfer Protocol (SMTP) - the protocol responsible for moving mail messages from one mail server to another over the Internet and other TCP/IP-based networks.
  • Telnet - a terminal emulation protocol used to log into remote hosts using the TCP/IP protocol suite.