Designing a Communications System
Increasingly, it is networks that connect us. People communicate online from everywhere. Classroom conversations turn into instant message chat sessions, and online discussions continue at school. New services are developed every day to take advantage of the network.
Rather than developing unique and separate systems for the delivery of each new service, the network industry as a whole has adopted a development framework that enables designers to understand and maintain current network platforms. At the same time, this framework is used to facilitate the development of new technologies to support future communication needs and technological improvements.Central to this development framework is the use of generally accepted models that describe the rules and functions of the network.In this you will learn about these models, as well as the standards that make networks work, and how communication occurs over a network.
A network can be as complex as devices connected through the Internet, or as simple as two computers connected directly to each other with a single cable, and anything in between. Networks can vary in size, shape and function. However, simply having a physical wired or wireless connection between end devices is not enough to allow communication. For communication to occur, devices must know “how” to communicate.
People exchange ideas using many different communication methods. However, regardless of the method chosen, all communication methods have three elements in common. The first of these elements is the source of the message or the sender. Message sources are people or electronic devices that need to send a message to other people or devices. The second element of communication is the destination, or receiver, of the message. Fate receives the message and interprets it. A third element, called a channel, consists of the means that provide the path by which the message travels from source to destination.
Communication begins with a message or information that must be sent from a source to a destination. Sending this message, either by face-to-face communication or by a network, is governed by rules called protocols. These protocols are specific to the type of communication method that occurs. In our everyday personal communication, the rules we use to communicate through one medium, such as a phone call, are not necessarily the same as the protocols for using another medium, such as sending a letter.
For example, consider two people who communicate face to face, as shown in Figure 1. Before communicating, they must agree on how to communicate. If communication uses voice, they must first agree on the language. Then when they have a message to share, they should be able to format that message in an understandable way. For example, if someone uses the English language, but a poor sentence structure, the message can easily be misunderstood. Each of these tasks describes established protocols for achieving communication. This is also true for computer communication, as shown in Figure 2.
Many different rules or protocols govern all the communication methods that exist in the world today.
Before communicating with each other, people should use established rules or agreements to govern the conversation. For example, consider Figure 1, protocols are necessary for effective communication. These rules, or protocols, must be followed for the message to be delivered and understood successfully. The protocols must take into account the following requirements:
An identified sender and receiver
Common language and grammar
Delivery speed and time
Confirmation or recognition requirements
The protocols used in network communications share many of these fundamental features. In addition to identifying the origin and destination, computer and network protocols define the details of how a message is transmitted over a network. Common computing protocols include the requirements shown in Figure 2. Each of these will be discussed in more detail.
Another rule of communication is size. When people communicate with each other, the messages that they send are usually broken into smaller parts or sentences. These sentences are limited in size to what the receiving person can process at one time, as shown in Figure 1. An individual conversation may be made up of many smaller sentences to ensure that each part of the message is received and understood. Imagine what it would be like to read this course if it all appeared as one long sentence; it would not be easy to read and comprehend.
Likewise, when a long message is sent from one host to another over a network, it is necessary to break the message into smaller pieces, as shown in Figure 2. The rules that govern the size of the pieces, or frames, communicated across the network are very strict. They can also be different, depending on the channel used. Frames that are too long or too short are not delivered.
The size restrictions of frames require the source host to break a long message into individual pieces that meet both the minimum and maximum size requirements. The long message will be sent in separate frames, with each frame containing a piece of the original message. Each frame will also have its own addressing information. At the receiving host, the individual pieces of the message are reconstructed into the original message.
These are the rules of engagement for message timing.
Access method determines when someone is able to send a message. If two people talk at the same time, a collision of information occurs and it is necessary for the two to back off and start again, as shown in the animation. Likewise, it is necessary for computers to define an access method. Hosts on a network need an access method to know when to begin sending messages and how to respond when collisions occur.
Timing also affects how much information can be sent and the speed that it can be delivered. If one person speaks too quickly, it is difficult for the other person to hear and understand the message. In network communication, source and destination hosts use flow control methods to negotiate correct timing for successful communication.
If a person asks a question and does not hear a response within an acceptable amount of time, the person assumes that no answer is coming and reacts accordingly. The person may repeat the question, or may go on with the conversation. Hosts on the network also have rules that specify how long to wait for responses and what action to take if a response timeout occurs.