Introduction
A network is a combination of hardware and software that sends data from one location to another. The hardware consists of the physical equipment that carries signals from one point of the network to another. The software consists of instruction sets that make possible the services that we expect from a network.
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. Figure 1 shows the steps in this task.
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.
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.
Hierarchy: According to our analysis, there are three different activities at the sender site and another three activities at the receiver site. The task of transporting the letter between the sender and the receiver is done by the carrier. Something that is not obvious immediately is that the tasks must be done in the order given in the hierarchy. At the sender site, the letter must be written and dropped in the mailbox before being picked up by the letter carrier and delivered to the post office. At the receiver site, the letter must be dropped in the recipient mailbox before being picked up and read by the recipient.
Services: Each layer at the sending site uses the services of the layer immediately below it. The sender at the higher layer uses the services of the middle layer. The middle layer uses the services of the lower layer. The lower layer uses the services of the carrier.
The Open Systems Interconnection Model
Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|