LAYERED TASKS
We use the concept of layers in our daily
life. As an example, let us consider two friends who communicate through postal
mail the process of sending a letter to a friend would be complex if there were
no services available from the post office.
Sender, Receiver, and Carrier
At the Sender Site
Let us first describe, in order, the
activities that take place at the sender site.
• Higher layer. The sender writes the letter,
inserts the letter in an envelope, writes the sender and receiver addresses,
and drops the letter in a mailbox.
• Middle layer. The letter is picked up by a
letter carrier and delivered to the post office.
• Lower layer. The letter is sorted at the
post office; a carrier transports the letter.
0n the Way: The letter is then on its way to
the recipient. On the way to the recipient's local post office, the letter may
actually go through a central office. In addition, it may be transported by
truck, train, airplane, boat, or a combination of these.
At the Receiver Site
• Lower layer. The carrier transports the
letter to the post office.
• Middle layer. The letter is sorted and
delivered to the recipient's mailbox.
• Higher layer. The receiver picks up the
letter, opens the envelope, and reads it.
Reference Models:
In computer networks, reference models give a conceptual framework that standardizes communication between heterogeneous networks.
•OSI Model
•TCP/IP Protocol Suite
- OSI stands for Open Systems Interconnection
- Created by International Standards Organization (ISO)
- Was created as a framework and reference model to explain how different networking technologies work together and interact
- It is not a standard that networking protocols must follow
- Each layer has specific functions it is responsible for
- All layers work together in the correct order to move data around a network
Application Layer: Contains all services or protocols needed by application software or operating system to communicate on the network
Examples
–Firefox web browser uses HTTP (Hyper-Text Transport Protocol)
–E-mail program may use POP3 (Post Office Protocol version 3) to read e-mails and SMTP (Simple Mail Transport Protocol) to send e-mails
•Handles three primary tasks:
Translation , Compression , Encryption
Session Layer
- Responsible for managing the dialog between networked devices
- Establishes, manages, and terminates connections
- Provides duplex, half-duplex, or simplex communications between devices
- Provides procedures for establishing checkpoints, adjournment, termination, and restart or recovery procedures
- Takes data from higher levels of OSI Model and breaks it into segments that can be sent to lower-level layers for data transmission
- Conversely, reassembles data segments into data that higher-level protocols and applications can use
- Also puts segments in correct order (called sequencing ) so they can be reassembled in correct order at destination
- Concerned with the reliability of the transport of sent data
- May use a connection-oriented protocol such as TCP to ensure destination received segments
- May use a connectionless protocol such as UDP to send segments without assurance of delivery
- Uses port addressing
- Responsible for moving packets (data) from one end of the network to the other, called end-to-end communications
- Requires logical addresses such as IP addresses
- Device example: Router
- Routing is the ability of various network devices and their related software to move data packets from source to destination
- Is responsible for moving frames from node to node or computer to computer
- Can move frames from one adjacent computer to another, cannot move frames across routers
- Encapsulation = frame
- Requires MAC address or physical address
- Protocols defined include Ethernet Protocol and Point-to-Point Protocol (PPP)
- Device example: Switch,Bridge
- Two sublayers: Logical Link Control (LLC) and the Media Access Control (MAC)
- Data Link layer addressing, flow control, address notification, error control
- Determines which computer has access to the network media at any given time
- Determines where one frame ends and the next one starts, called frame
- synchronization
- Deals with all aspects of physically moving data from one computer to the next
- Converts data from the upper layers into 1s and 0s for transmission over media
- Defines how data is encoded onto the media to transmit the data
- Defined on this layer: Cable standards, wireless standards, and fiber optic standards.
- Copper wiring, fiber optic cable, radio frequencies, anything that can be used to transmit data is defined on the Physical layer of the OSI Model
- Device example: Hub, Repeater
- Used to transmit data
An exchange using the OSI model
A protocol suite is a large number of related protocols that work together to allow networked computers to communicate
Typical protocols:
- FTP – File Transfer Protocol
- For file transfer
- Telnet – Remote terminal protocol
- For remote login on any other computer on the network
- SMTP – Simple Mail Transfer Protocol
- For mail transfer
- HTTP – Hypertext Transfer Protocol
- For Web browsing
•TCP is a connection-oriented protocol
- Does not mean it has a physical connection between sender and receiver
- TCP provides the function to allow a connection virtually exists – also called virtual circuit
- Dividing a chunk of data into segments
- Reassembly segments into the original chunk
- Provide further the functions such as reordering and data resend
- Offering a reliable byte-stream delivery service
- Functions the same as the Transport layer in OSI
- Synchronize source and destination computers to set up the session between the respective computers
Host-to-network layer
The Host-to-network layer is the lowest layer of the TCP/IP reference model. It combines the link layer and the physical layer of the ISO/OSI model. At this layer, data is transferred between adjacent network nodes in a WAN or between nodes on the same LAN.
OSI Model Vs TCP/IP Model:
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