Tuesday, July 1, 2014

Network Communications


Network Communications
How Data is packaged

At first, one might assume that data is sent as a continuous stream of ones and zeros from one computer to another. In fact, data is broken down into small, manageable packets, each wrapped with the essential information needed to get it from its source to the correct destination. This lesson introduces the concept of packets as the basic building blocks of network data communications.
After this lesson, you will be able to:
  • Define the term "packet," including its function and components.
  • Describe the contents and function of each packet component: header, data, and trailer.

The Function of Packets in Network Communications

Data usually exists as rather large files. However, networks cannot operate if computers put large amounts of data on the cable at the same time. A computer sending large amounts of data causes other computers to wait (increasing the frustration of the other users) while the data is being moved. This is not called "sharing"; it is called "monopolizing the network." There are two reasons why putting large chunks of data on the cable at one time slows down the network:
  • Large amounts of data sent as one large unit tie up the network and make timely interaction and communications impossible because one computer is flooding the cable with data.
  • The impact of retransmitting large units of data further multiplies network traffic.
These effects are minimized when the large data units are reformatted into smaller packages for better management of error correction in transmission. This way, only a small section of data is affected, and, therefore, only a small amount of data must be retransmitted, making it relatively easy to recover from the error.

Figure 1: Large continuous streams of data slow down the network
In order for many users at once to transmit data quickly and easily across the network, the data must be broken into small, manageable chunks. This way, users each get their share of access to the network. These chunks are called packets, or frames. Although the terms "packet" and "frame" are often used interchangeably, there are some differences based on the type of network. This lesson uses the term "packet," meaning "a unit of information transmitted as a whole from one device to another on a network."
NOTE


"Device" is a generic term for a computer subsystem. Printers, serial ports, and disk drives are often referred to as devices; such subsystems frequently require their own controlling software, called device drivers. Packets are the basic units of network communication. Figure 3.6 shows data that is being broken into packets. With data divided into packets, individual transmissions are speeded up so that every computer on the network has more opportunities to transmit and receive data. At the target (receiving) computer, the packets are collected and reassembled in the order of the original data.
Figure 3.6 Breaking data into packets
When the network operating system at the sending computer breaks the data into packets, it adds special control information to each frame. This makes it possible to:
  • Send the original, disassembled data in small chunks.
  • Reassemble the data in the proper order when it reaches its destination.
  • Check the data for errors after it has been reassembled.

Packet Structure

Packets can contain several types of data including:
  • Information, such as messages or files.
  • Certain types of computer control data and commands, such as service requests.
  • Session control codes, such as error correction that indicate the need for a retransmission.

Packet Components

All packets have certain components in common. These include:
  • A source address that identifies the sending computer.
  • The data that is intended for transmission.
  • A destination address that identifies the recipient.
  • Instructions that tell network components how to pass the data along.
  • Information that tells the receiving computer how to connect the packet to other packets in order to reassemble the complete data package.
  • Error-checking information to ensure that the data arrives intact.
Figure 3.7 shows these packet components grouped into three sections: header, data, and trailer.
http://pluto.ksi.edu/~cyh/cis370/ebook/images/F03xx07.JPG
Figure 3.7 Packet components
Header
The header includes:
  • An alert signal to indicate that the packet is being transmitted.
  • The source address.
  • The destination address.
  • Clock information to synchronize transmission.
Data
This describes the actual data being sent. This part of the packet varies in size, depending on the network. The data section on most networks varies from 512 bytes—or 0.5 kilobytes (KB)—to 4 KB.
Because most original data strings are much longer than 4k, data must be broken into chunks small enough to be put into packets. It takes many packets to complete the transmission of a large file.
Trailer

The exact content of the trailer varies depending on the communication method, or protocol. However, the trailer usually contains an error-checking component called a cyclical redundancy check (CRC). The CRC is a number produced by a mathematical calculation on the packet at its source. When the packet arrives at its destination, the calculation is made again. If the results of both calculations are the same, this indicates that the data in the packet has remained stable. If the calculation at the destination differs from the calculation at the source, this means the data has changed during the transmission. In that case, the CRC routine signals the source computer to retransmit the data.     

Introduction to CCNA


Definition of a Network
When you have two or more computers connected to each other, you have a network. The purpose of a network is to enable the sharing of files and information between multiple systems. The Internet could be described as a global network of networks. Computer networks can be connected through cables, such as Ethernet cables or phone lines, or wirelessly, using wireless networking cards that send and receive data through the air.

video

Advantages & Disadvantages of Networking
·         Advantages of Computer Networking
1. Easy Communication and Speed
It is very easy to communicate through a network. People can communicate efficiently using a network with a group of people. They can enjoy the benefit of emails, instant messaging, telephony, video conferencing, chat rooms, etc.
2. Ability to Share Files, Data and Information
This is one of the major advantages of networking computers. People can find and share information and data because of networking. This is beneficial for large organizations to maintain their data in an organized manner and facilitate access for desired people.
3. Sharing Hardware
Another important advantage of networking is the ability to share hardware. For an example, a printer can be shared among the users in a network so that there’s no need to have individual printers for each and every computer in the company. This will significantly reduce the cost of purchasing hardware.
4. Sharing Software
Users can share software within the network easily. Networkable versions of software are available at considerable savings compared to individually licensed version of the same software. Therefore large companies can reduce the cost of buying software by networking their computers.
5. Security
Sensitive files and programs on a network can be password protected. Then those files can only be accessed by the authorized users. This is another important advantage of networking when there are concerns about security issues. Also each and every user has their own set of privileges to prevent them accessing restricted files and programs.
6. Speed
Sharing and transferring files within networks is very rapid, depending on the type of network. This will save time while maintaining the integrity of files.
·         Disadvantages of Networking
1. Breakdowns and Possible Loss of Resources
One major disadvantage of networking is the breakdown of the whole network due to an issue of the server. Such breakdowns are frequent in networks causing losses of thousands of dollars each year. Therefore once established it is vital to maintain it properly to prevent such disastrous breakdowns. The worst scenario is such breakdowns may lead to loss of important data of the server.
2. Expensive to Build
Building a network is a serious business in many occasions, especially for large scale organizations. Cables and other hardware are very pricey to buy and replace.
3. Security Threats
Security threats are always problems with large networks. There are hackers who are trying to steal valuable data of large companies for their own benefit. So it is necessary to take utmost care to facilitate the required security measures.
4. Bandwidth Issues
In a network there are users who consume a lot more bandwidth than others. Because of this some other people may experience difficulties.
Although there are disadvantages to networking, it is a vital need in today’s environment. People need to access the Internet, communicate and share information and they can’t live without that. Therefore engineers need to find alternatives and improved technologies to overcome issues associated with networking. Therefore we can say that computer networking is always beneficial to have even if there are some drawbacks.


Network Classifications – LAN, MAN, and WAN

LAN
A local area network is a computer network that interconnects computers within a limited area such as a home, school, computer laboratory, or office building, using network media.

MAN

A metropolitan area network is a computer network in which two or more computers or communicating devices or networks which are geographically separated but in same metropolitan city and are connected to each other are said to be connected on MAN.


WAN

A wide area network (WAN) is a network that covers a broad area (i.e., any telecommunications network that links across metropolitan, regional, national or international boundaries) using leased telecommunication lines. Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations.



Network Topologies

What is a network topology? In communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology.

Bus Topology
In the bus network topology, every workstation is connected to a main cable called the bus. Therefore, in effect, each workstation is directly connected to every other workstation in the network.

Star Topology
In the star network topology, there is a central computer or server to which all the workstations are directly connected. Every workstation is indirectly connected to every other through the central computer.
Ring Topology
In the ring network topology, the workstations are connected in a closed loop configuration. Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly connected, the data passing through one or more intermediate nodes.

Mesh Topology
The mesh network topology employs either of two schemes, called full mesh and partial mesh. In the full mesh topology, each workstation is connected directly to each of the others. In the partial mesh topology, some workstations are connected to all the others, and some are connected only to those other nodes with which they exchange the most data.

Wireless Topology
In any network setup, network topology plays an important role without which network cannot be established whatsoever. In other words, whenever two or more machines are connected to each other, a topology is automatically formed. Unlike wired networks that have multiple topologies, wireless networks have only two topologies namely Infrastructure and Ad Hoc. Since wireless networks work on unguided media, it makes sense to have limited number of topologies while configuring the wireless network setups. Moreover, since wireless networks are a bit complicated configure as compared to the wired networks, it is essential for the administrators to have complete and in-depth knowledge of wireless networking before they finally start establishing wireless networks in production environments.

Hybrid Topology

Before starting about Hybrid topology, we saw that a 
network topology is a connection of various links and nodes, communicating with each other for transfer of data.
Now let’s discuss what Hybrid Network topology is and why it finds its application in Wide Area Networks. Hybrid, as the name suggests, is mixture of two different things. Similarly in this type of topology we integrate two or more different topologies to form a resultant topology which has good points (as well as weaknesses) of all the constituent basic topologies rather than having characteristics of one specific topology. This combination of topologies is done according to the requirements of the organization.

For example, if there exist a ring topology in one office department while a bus topology in another department, connecting these two will result in Hybrid topology. Remember connecting two similar topologies cannot be termed as Hybrid topology. Star-Ring and Star-Bus networks are most common examples of hybrid network.




Basic Administration Network Models

Client/Server Network
An architecture in which the user's PC (the client) is the requesting machine and the server is the supplying machine, both of which are connected via a local area network (LAN) or a wide area network (WAN) such as the Internet. Throughout the late 1980s and early 1990s, client/server was the hot buzzword as applications were migrated from minicomputers and mainframes with input/output terminals to networks of desktop computers.

With ubiquitous access to company LANs and the Internet, almost everyone works in a client/server environment today. However, to be true client/server, both client and server must share in the business processing. To understand this principle, follow the examples below of a query to a hypothetical database of a million records, each 1,000 bytes long. Notice the amount of data flowing over the network.
Network operating systems can be based on a client/server architecture in which a server enables multiple clients to share resources.[1] Client/server network operating systems allow the network to centralize functions and applications in one or more dedicated file servers. The server is the center of the system, allowing access to resources and instituting security. The network operating system provides the mechanism to integrate all the components on a network to allow multiple users to simultaneously share the same resources regardless of physical location.[3][4]
·         Examples:
·         Novell NetWare
·         Windows Server
·         Banyan VINES
Advantages
·         Centralized servers are more stable.
·         Security is provided through the server.
·         New technology and hardware can be easily integrated into the system.
·         Servers are able to be accessed remotely from different locations and types of systems.
Disadvantages
·         Cost of buying and running a server are high.
·         Dependence on a central location for operation.
·         Requires regular maintenance and updates.


Peer-to-Peer Network
A peer-to-peer (P2P) network is a type of decentralized and distributed network architecture in which individual nodes in the network (called"peers") act as both suppliers and consumers of resources, in contrast to centralized client–server model where client nodes request access to resources provided by central servers.
Networks in which all computers have equal status are called peer-to-peer or P2P networks.
In a peer-to-peer network, tasks (such as searching for files or streaming audio/video) are shared amongst multiple interconnected peers who each make a portion of their resources (such as processing powerdisk storage or network bandwidth) directly available to other network participants, without the need for centralized coordination by servers.[1]
In a peer-to-peer network operating system users are allowed to share resources and files located on their computers and access shared resources from others. This system is not based with having a file server or centralized management source. A peer-to-peer network sets all connected computers equal; they all share the same abilities to use resources available on the network.[3]
·         Examples:
·         AppleShare used for networking connecting Apple products.
·         Windows for Workgroups used for networking peer-to-peer windows computers.



Network Operating Systems (NOS)
Network operating system refers to software that implements an operating system of some kind that is oriented to computer networking. For example, one that runs on a server and enables the server to manage data, users, groups, security, applications, and other networking functions.[1] The network operating system is designed to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks.
·         JUNOS, used in routers and switches from Juniper Networks,
·         Cisco IOS (formerly "Cisco Internetwork Operating System").
·         TiMOS, used in routers from Alcatel-Lucent
·         VRP (Versatile Routing Platform), used in routers from Huawei.