Chapter 4: Protocols and Models

 Communication Fundamentals

People exchange ideas using many different communication methods. However, all communication methods have the following three elements in common:

• Message source (sender): Message sources are people or electronic devices that need to send a message to other individuals or devices. 
• Message destination (receiver): The destination receives the message and interprets it. 
• Channel: The channel consists of the media that provide the pathway over which the message travels from source to destination. 

Rule Establishment

Protocols are necessary for effective communication and include: 

    • An identified sender and receiver 

    • Common language and grammar 

    • Speed and timing of delivery 

    • Confirmation or acknowledgment requirements

Protocols used in network communications also define: 

    • Message encoding 

    • Message delivery options 

    • Message Formatting and Encapsulation 

    • Message Timing 

    • Message Size

Message Encoding

Encoding between hosts must be in appropriate format for the medium. Messages are first converted into bits by the sending host.

Each bit is encoded into a pattern of sounds, light waves, or electrical impulses depending on the network media. The destination host receives and decodes the signals in order to interpret the message.

Message Formatting and Encapsulation

• There is an agreed format for letters and addressing letters which is required for proper delivery. 
• Putting the letter into the addressed envelope is called encapsulation. 
• Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. 
• A frame acts like an envelope providing destination address and source address.

Message Timing

Message timing is very important in network communications. Message timing includes the following:

• Flow control: This is the process of managing the rate of data transmission. Flow control defines how much information can be sent and the speed at which it can be delivered. For example, if one person speaks too quickly, it may be difficult for the receiver to hear and understand the message. In network communication, source and destination devices use network protocols to negotiate and manage the flow of information. 
• Response timeout: 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 a network use network protocols that specify how long to wait for responses and what action to take if a response timeout occurs.
• Access method: The access method determines when someone can send a message. In Figure 3-5, two people are talking at the same time, and a “collision of information” occurs. It is necessary for the two people to back off and start again. Likewise, when a device wants to transmit on a wireless LAN, it is necessary for the WLAN network interface card (NIC) to determine whether the wireless medium is available.

Message Delivery Options

A message can be delivered in different ways. Sometimes, a person wants to communicate information to a single individual. At other times, the person may need to send information to a group of people at the same time, or even to all the people in an area.

Network communication involves similar delivery options, including the following:

• Unicast: Information is transmitted to a single end device. 
• Multicast: Information is transmitted to one or more end devices. 
• Broadcast: Information is transmitted to all end devices. 

PROTOCOLS

Protocol A written specification that defines what tasks a service or device should perform. Each protocol defines messages, often in the form of headers, plus the rules and processes by which the messages are used to achieve some stated purpose

Protocol Interaction

Sending a message over a computer network typically requires the use of several protocols, each one with its own functions and format. 

PROTOCOL SUITES
PROTOCOL SUITES is a set of protocols that work together to provide comprehensive network communication services. A protocol suite can be specified by a standards organization or developed by a vendor.

Evolution of Protocol Suites

A protocol suite is a set of protocols that work together to provide comprehensive network communication services. Since the 1970s there have been several different protocol suites, some developed by standards organizations and others developed by various vendors. 

• Internet Protocol Suite or TCP/IP: This is the most common and relevant protocol suite used today. The TCP/IP protocol suite is an open standard protocol suite maintained by the Internet Engineering Task Force (IETF). 
• Open Systems Interconnection (OSI) protocols: This is a family of protocols developed jointly in 1977 by the International Organization for Standardization (ISO) and the International Telecommunications Union (ITU). The OSI protocol also included a seven-layer model called the OSI reference model. The OSI reference model categorizes the functions of its protocols. Today OSI is mainly known for its layered model. The OSI protocols have largely been replaced by TCP/IP.
• AppleTalk: Apple released this short-lived proprietary protocol suite in 1985 for Apple devices. In 1995, Apple adopted TCP/IP to replace AppleTalk.
• Novell NetWare: Novell developed this short-lived proprietary protocol suite and network operating system in 1983, using the IPX network protocol. In 1995, Novell adopted TCP/IP to replace IPX.