A collection of computers and other devices that are connected together by communication channels for sharing information and resources is called computer network.Computer Network is :
If you link 2 or more computers together you have a Network Servers are computers that store and send data to other computers For Example an E-mail Server Clients are computers that receive and use data from servers For Example your desktop PC receiving e-mail
Uses / Benefits of Computer Network :
The main benefits or uses of computer network are:
- 2.Sharing Resources
- 3.Sharing Software
- 4.Data SharingUsing a network, different people can communicate with each other all over the world. People can communicate at very low cost via e-mail, chatting, telephone, video telephone, video conferencing, groupware, and SMS services etc.Communication :
In a computer network, resources such as, printers, scanners, fax machines and modems can be shared among different users. Suppose several personals computers and a laser printer are connected to a network. Each users can access the printer.Sharing Resources :
In a computer network, usually application programs and other software are stored on the central computer. Users connected to a network can access these programs or software.Sharing Software :
In a network environment, any authorized user can access data stored on other computers on the network. For example, on the Internet, a large number of Internet users can access same databaseData Sharing :
The Advantages(benefits) of Networking is :
Networks connect computers and the users of those computers. Individuals within a building or work group can be connected into local area networks (LANs); LANs in distant locations can be interconnected into larger wide area networks (WANs).Connectivity and Communication:
One of the most important uses of networking is to allow the sharing of data. Before networking was common, an accounting employee who wanted to prepare a report for her manager would have to produce it on his PC.Data Sharing:
Networks facilitate the sharing of hardware devices. For example, instead of giving each of 10 employees in a department an expensive color printer (or resorting to the “sneakernet” again), one printer can be placed on the network for everyone to share.Hardware Sharing:
The Internet is itself an enormous network, so whenever you access the Internet, you are using a network. The significance of the Internet on modern society is hard to exaggerate, especially for those of us in technical fields.Internet Access:
Small computer networks allow multiple users to share a single Internet connection. Special hardware devices allow the bandwidth of the connection to be easily allocated to various individuals as they need it Data Security and Management: In a business environment, a network allows the administrators to much better manage the company's critical data.Internet Access Sharing:
Under some circumstances, a network can be used to enhance the overall performance of some applications by distributing the computation tasks to various computers on the network.Performance Enhancement and Balancing:
Networks facilitate many types of games and entertainment. The Internet itself offers many sources of entertainment, of course. In addition, many multi-player games exist that operate over a local area network. Many home networks are set up for this reason, and gaming across wide area networks (including the Internet) has also become quite popular.Entertainment:
Advantages of networks :Sharing devices such as printers saves money. Site (software) licences are likely to be cheaper than buying several standalone licences. Files can easily be shared between users. Network users can communicate by email and instant messenger. Security is good - users cannot see other users' files unlike on stand-alone machines. Data is easy to backup as all the data is stored on the file server.
Disadvantages of networks :Purchasing the network cabling and file servers can be expensive. Managing a large network is complicated, requires training and a network manager usually needs to be employed. If the file server breaks down the files on the file server become inaccessible. Email might still work if it is on a separate server. The computers can still be used but are isolated. Viruses can spread to other computers throughout a computer network. There is a danger of hacking, particularly with wide area networks. Security procedures are needed to prevent such abuse, eg a firewall.
Dynamic Host Configuration Protocol (DHCP) is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway. RFCs 2131 and 2132 define DHCP as an Internet Engineering Task Force (IETF) standard based on Bootstrap Protocol (BOOTP), a protocol with which DHCP shares many implementation details. DHCP allows hosts to obtain necessary TCP/IP configuration information from a DHCP server.DHCP is :
Benefits of DHCP :
DHCP minimizes configuration errors caused by manual IP address configuration, such as typographical errors, or address conflicts caused by the assignment of an IP address to more than one computer at the same time.Reliable IP address configuration :
DHCP includes the following features to reduce network administration:Reduced network administration :
Centralized and automated TCP/IP configuration. The ability to define TCP/IP configurations from a central location. The ability to assign a full range of additional TCP/IP configuration values by means of DHCP options. The efficient handling of IP address changes for clients that must be updated frequently, such as those for portable computers that move to different locations on a wireless network. The forwarding of initial DHCP messages by using a DHCP relay agent, thus eliminating the need to have a DHCP server on every subnet.
Use DHCP :Valid TCP/IP configuration parameters for all clients on the network. Valid IP addresses, maintained in a pool for assignment to clients, as well as excluded addresses. Reserved IP addresses associated with particular DHCP clients. This allows consistent assignment of a single IP address to a single DHCP client. The lease duration, or the length of time for which the IP address can be used before a lease renewal is required. A DHCP-enabled client, upon accepting a lease offer, receives: A valid IP address for the subnet to which it is connecting.
CDMA (Code-Division Multiple Access) is a channel access method used by various radio communication technologies. It is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and Wideband CDMA deliver data many times faster.CDMA is :
Characteristicsof CDMA :Ø spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. These systems were designed using spread spectrum because of its security and resistance to jamming. Ø CDMA can effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Ø CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. Ø In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Ø Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; Ø CDMA systems use the soft hand off, which is undetectable and provides a more reliable and higher quality signal.
General Specification of CDMA :Ø Rx: 869-894MHz Tx: 824-849MHz Ø 20 Channels spaced 1250kHz apart (798 users/channel) Ø QPSK/(Offset) OQPSK modulation scheme Ø 1.2288Mbps bit rate Ø IS-95 standard Ø Operates at both 800 and 1900 MHz frequency bands
Ø Efficient practical utilization of fixed frequency spectrum. Ø Flexible allocation of resources. Ø Many users of CDMA use the same frequency, TDD or FDD may be used Ø Multipath fading may be substantially reduced because of large signal bandwidth Ø No absolute limit on the number of users, Easy addition of more users. Ø Impossible for hackers to decipher the code sent Ø Better signal quality Ø No sense of handoff when changing cells Ø The CDMA channel is nominally 1.23 MHz wide. Ø CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. Ø CDMA is compatible with other cellular technologies; this allows for nationwide roaming. Ø The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes.
Ø One of the early applications for code division multiplexing is in GPS. This predates and is distinct from its use in mobile phones. Ø The Qualcomm standard IS-95, marketed as cdmaOne. Ø The Qualcomm standard IS-2000, known as CDMA2000. This standard is used by several mobile phone companies, including the Globalstar satellite phone network. Ø The UMTS 3G mobile phone standard, which uses W-CDMA. Ø CDMA has been used in the OmniTRACS satellite system for transportation logistics.
Asynchronous transfer mode (ATM) is a switching technique used by telecommunication networks that uses asynchronous time-division multiplexing to encode data into small, fixed-sized cells. This is different from Ethernet or Internet, which use variable packet sizes for data or frames. ATM is the core protocol used over the synchronous optical network (SONET) backbone of the integrated digital services network (ISDN).Asynchronous Transfer Mode (ATM) :
The features of ATM Network is :
The interface between ATM switches is called a Network to Network Interface (NNI). The interface between an ATM switch and an end user is called a User to Network Interface (UNI), which is the interface between a public ATM carrier and a customer's switch.Interface types :
ATM switches come in a variety of types and sizes. A small switch (a few gigabits) is called customer premise equipment (CPE). A large switch (100 GB or larger) is called a central office (CO) switch. A CPE and CO cannot be defined in terms of speed. They also have different MTBF rates. Attached device sevices can range from residential television sets to large computers.Switch types :
ATM networks support a variety of link speeds, from 1.544 Mbps to 622 Mbps and higher.Link speeds :
Users can attach by means of category 3 and category 5 for copper interfaces and multimode or single mode for fiber-optic interfaces.Attach media :