ARASH HABIBI LASHKARI
PUBLICATIONS
  Qualifications and Certificates
  Technical Committee Member
  Professional Experience
  Research & Developme
  Awards and Honors
  In the News
  Pulications
  Teaching
  Supervising
  Workshops
  Auxiliary Activities
Introduction to Computer Networks - Network Basics:
Computer Network
A computer network is a collection of computers connected to each other. The network allows computers to communicate with each other and share the resources and information. A computer network, also referred to as just a network, consists of two or more computers, and typically other devices as well (such as printers, modems, switches and routers), that are linked together so that they can communicate with each other and thereby exchange commands and share resources and data.
LANs (Local Area Networks)
A network is any collection of independent computers that communicate with one another over a shared network medium. LANs are networks usually confined to a geographic area, such as a single building or a college campus. LANs can be small, linking as few as three computers, but often link hundreds of computers used by thousands of people. The development of standard networking protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organizations.
WANs (Wide Area Networks)
Often a network is located in multiple physical places. Wide area networking combines multiple LANs that are geographically separate. This is accomplished by connecting the different LANs using services such as dedicated leased phone lines, dial-up phone lines (both synchronous and asynchronous), satellite links, and data packet carrier services. Wide area networking can be as simple as a modem and remote access server for employees to dial into, or it can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the expense of sending data sent over vast distances.
MAN (Metropolitan Area Networks)
A Metropolitan Area Network (MAN) is a relatively new class of network, it serves a role similar to an ISP, but for corporate users with large LANs. There are three important features which discriminate MANs from LANs or WANs:
1. The network size falls intermediate between LANs and WANs. A MAN typically covers an area of between 5 and 50 km diameter. Many MANs cover an area the size of a city.
2. A MAN (like a WAN) is not generally owned by a single organisation. The MAN, its communications links and equipment are generally owned by either a consortium of users or by a single network provider who sells the service to the users. This level of service provided to each user must therefore be negotiated with the MAN operator, and some performance guarantees are normally specified.
3. A MAN often acts as a high speed network to allow sharing of regional resources (similar to a large LAN). It is also frequently used to provide a shared connection to other networks using a link to a WAN.
Internetwork
A Internetworking involves connecting two or more distinct computer networks or network segments via a common routing technology. The result is called an internetwork (often shortened to internet). Two or more networks or network segments connected using devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
-Intranet
-Extranet
-Internet
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.
Intranet
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.
Extranet
An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
Internet
The Internet, sometimes called simply "the Net," is a worldwide system of computer networks - a network of networks in which users at any one computer can, if they have permission, get information from any other computer (and sometimes talk directly to users at other computers). It was conceived by the Advanced Research Projects Agency (ARPA) of the U.S. government in 1969 and was first known as the ARPANET. The original aim was to create a network that would allow users of a research computer at one university to be able to "talk to" research computers at other universities. A side benefit of ARPANet's design was that, because messages could be routed or rerouted in more than one direction, the network could continue to function even if parts of it were destroyed in the event of a military attack or other disaster.
Today, the Internet is a public, cooperative, and self-sustaining facility accessible to hundreds of millions of people worldwide. Physically, the Internet uses a portion of the total resources of the currently existing public telecommunication networks. Technically, what distinguishes the Internet is its use of a set of protocols called TCP/IP (for Transmission Control Protocol/Internet Protocol). Two recent adaptations of Internet technology, the intranet and the extranet, also make use of the TCP/IP protocol.
Ethernet
Ethernet is the most popular physical layer LAN technology in use today. Other LAN types include Token Ring, Fast Ethernet, Fiber Distributed Data Interface (FDDI), Asynchronous Transfer Mode (ATM) and LocalTalk. Ethernet is popular because it strikes a good balance between speed, cost and ease of installation. These benefits, combined with wide acceptance in the computer marketplace and the ability to support virtually all popular network protocols, make Ethernet an ideal networking technology for most computer users today. The Institute for Electrical and Electronic Engineers (IEEE) defines the Ethernet standard as IEEE Standard 802.3. This standard defines rules for configuring an Ethernet network as well as specifying how elements in an Ethernet network interact with one another. By adhering to the IEEE standard, network equipment and network protocols can communicate efficiently.
Fast Ethernet
For Ethernet networks that need higher transmission speeds, the Fast Ethernet standard (IEEE 802.3u) has been established. This standard raises the Ethernet speed limit from 10 Megabits per second (Mbps) to 100 Mbps with only minimal changes to the existing cable structure. There are three types of Fast Ethernet: 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable. The 100BASE-TX standard has become the most popular due to its close compatibility with the 10BASE-T Ethernet standard. For the network manager, the incorporation of Fast Ethernet into an existing configuration presents a host of decisions. Managers must determine the number of users in each site on the network that need the higher throughput, decide which segments of the backbone need to be reconfigured specifically for 100BASE-T and then choose the necessary hardware to connect the 100BASE-T segments with existing 10BASE-T segments. Gigabit Ethernet is a future technology that promises a migration path beyond Fast Ethernet so the next generation of networks will support even higher data transfer speeds.
Token Ring
Token Ring is another form of network configuration which differs from Ethernet in that all messages are transferred in a unidirectional manner along the ring at all times. Data is transmitted in tokens, which are passed along the ring and viewed by each device. When a device sees a message addressed to it, that device copies the message and then marks that message as being read. As the message makes its way along the ring, it eventually gets back to the sender who now notes that the message was received by the intended device. The sender can then remove the message and free that token for use by others.
Protocols
Network protocols are standards that allow computers to communicate. A protocol defines how computers identify one another on a network, the form that the data should take in transit, and how this information is processed once it reaches its final destination. Protocols also define procedures for handling lost or damaged transmissions or "packets." TCP/IP (for UNIX, Windows NT, Windows 95 and other platforms), IPX (for Novell NetWare), DECnet (for networking Digital Equipment Corp. computers), AppleTalk (for Macintosh computers), and NetBIOS/NetBEUI (for LAN Manager and Windows NT networks) are the main types of network protocols in use today.
Topologies
A network topology is the geometric arrangement of nodes and cable links in a LAN, and is used in two general configurations: bus and star. These two topologies define how nodes are connected to one another. A node is an active device connected to the network, such as a computer or a printer. A node can also be a piece of networking equipment such as a hub, switch or a router. A bus topology consists of nodes linked together in a series with each node connected to a long cable or bus. Many nodes can tap into the bus and begin communication with all other nodes on that cable segment. A break anywhere in the cable will usually cause the entire segment to be inoperable until the break is repaired. Examples of bus topology include 10BASE2 and 10BASE5. 10BASE-T Ethernet and Fast Ethernet use a star topology, in which access is controlled by a central computer. Generally a computer is located at one end of the segment, and the other end is terminated in central location with a hub. Because UTP is often run in conjunction with telephone cabling, this central location can be a telephone closet or other area where it is convenient to connect the UTP segment to a backbone. The primary advantage of this type of network is reliability, for if one of these 'point-to-point' segments has a break, it will only affect the two nodes on that link. Other computer users on the network continue to operate as if that segment were nonexistent.
There are five major topology:
Bus : This is the most simplistic topology in which the nodes of the network are individually linked to up to two successive other nodes or another node and a terminating node or terminator. This is considered now an archaic topology, because of the difficulty of troubleshooting network issues (how do you know which node is causing the connection issue?), redundancy issues (if one node fails, the network as a whole can fail), the need for terminators, and the amount of traffic created (every node between A and B must receive the packet that A sends). The nodes linked in this topology are often referred to as "daisy-chained."
Bus
Ring : Similar to a bus network in that nodes are linked to each other, but dissimilar in that the ends of a ring network are not terminated because, well, there are no ends! A ring network is something like a "circular" network in which each and every node is linked to two other nodes. This shares many of the same weaknesses as the bus topology, including troubleshooting difficulty, redundancy issues, and traffic created, and also adds an additional difficulty - the difficulty of adding a node to a token ring network.
Ring
Star : This is the most typical and practical network setup. In a star network, each node maintains an individual connection to a switch, where all other nodes are connected. Traffic between two known nodes, therefore, only goes through the switch and not through other nodes. This increases theredundancy of the network (one computer faltering will not cause the network to fail), increases data privacy (unicast traffic does not travel through all nodes), and is a relatively easy-to-use setup. Disadvantages includereliance on the switch (a fail-point) and the amount of wiring necessary.
Star
Mesh : This is a rarely occurring configuration in which every node is connected to every other node; it usually occurs only in wireless networks in "ad hoc" mode, which will be discussed later; in this mode, each wireless card maintains a connection to each other wireless node it wishes to connect with, forming a "mesh" of a network. This is a relatively easy to understand option but is inefficient, requires a large amount of overhead, and is difficult to manage.
Mesh
Combined or Hybrid : This is simply a topology referring to the case wheremore than one topology is utilized. For example, you may have three token ring networks connected to a central hub, forming a star of token rings. This is one of many possibilities of a hybrid network.
Collisions
Ethernet is a shared media, so there are rules for sending packets of data to avoid conflicts and protect data integrity. Nodes determine when the network is available for sending packets. It is possible that two nodes at different locations attempt to send data at the same time. When both PCs are transferring a packet to the network at the same time, a collision will result.
Transceivers
Transceivers are used to connect nodes to the various Ethernet media. Most computers and network interface cards contain a built-in 10BASE-T or 10BASE2 transceiver, allowing them to be connected directly to Ethernet without requiring an external transceiver. Many Ethernet devices provide an AUI connector to allow the user to connect to any media type via an external transceiver. The AUI connector consists of a 15-pin D-shell type connector, female on the computer side, male on the transceiver side. Thickwire (10BASE5) cables also use transceivers to allow connections.
Network Interface Cards
Network interface cards, commonly referred to as NICs, are used to connect a PC to a network. The NIC provides a physical connection between the networking cable and the computer's internal bus. NICs come in three basic varieties: 8-bit, 16-bit, and 32-bit. The larger the number of bits that can be transferred to the NIC, the faster the NIC can transfer data to the network cable. The first thing that you need to know about network cards is that the network card has to match the network medium. The network medium refers to the type of cabling that is being used on the network.
At one time making sure that a network card matched the network medium was a really big deal, because there were a large number of competing standards in existence. For example, before you built a network and started buying network cards and cabling, you had to decide if you were going to use Ethernet, coaxal Ethernet, Token Ring, Arcnet, or one of the other networking standards of the time. In rhis days, most of the networking technologies that I mentioned above are quickly becoming extinct. Pretty much the only type of wired network used by small and medium sized businesses is Ethernet.
Network Identification Card (NIC)
Hubs
A hub is a medium used to collect signals from the input line(s) and redistribute them in various available wirings around a topology (topologies such as: Arcnet, 10base-T, 10base-F etc). Hub basically acts as signal splitter, it accepts signal through its input port and outputs it to the output ports. Some hubs help in regenerating the weak signals prior to sending them to the intended output lines, whereas some hubs help in synchronizing the data communication (in simple words, the hub not only provide the mean of interface within the network, it also provides some additional and useful features). Sometimes multiple hubs are interconnected in the network. Generally hubs are used more commonly where star topology is used.
8 port HUB
There are three main types of hubs:
Passive Hub:
Passive hubs are the ones, which does not provide any additional feature except for working just as an interface between the topology. These types of hubs do not help in rectifying/enhancing the signals they pass on in the network, in terms they do not help in enhancing the performance of the network/LAN. It is very hard to get any help from the passive hubs while troubleshooting in case if there is any fault in the hardware or the network. A passive hub simply receives signal(s) on input port(s) and broadcasts it (them) on the output port(s) without even rectifying it (them).
Active Hubs:
Active hub is a type of hub that take active participation in data communication within the network/LAN. Active hubs come with various features, such as receiving the signal (data) from the input port and storing it for sometime before forwarding it, this feature allows the hub to monitor the data it is forwarding, some hubs come with a feature that helps in transmitting data that has high priority before the data that has lower priority (this feature is very important for some applications and some type of networks), some hubs help in synchronizing data communication (by retransmitting the packets, which are not properly received at the receiving computer or by adjusting re-transmission of the data packets to compensate timing), and some active hubs come with a feature that rectify the data/signal before forwarding it in the network/LAN.
Intelligent Hubs:
Intelligent hubs add some more features to that provided by the active hubs. An intelligent hub provides all the features of a passive and an active hub; it also provides some features, which help in managing the network resources effectively and efficiently. Intelligent hubs help in improving the performance of the network/LAN that you are using. As an active hub helps in finding out where the problem persists, an intelligent hub itself finds out the problem in the network, diagnoses it and tries to rectify it without letting the problem hamper the performance of the network. Intelligent hubs provides feature that helps in determining the exact cause and exact place of the fault, this saves lot of time and energy which otherwise would have been required for finding out the exact place of fault and identifying the solution for it. Another feature of intelligent hub is that they can decide which packet goes in which output line, this helps in controlling and minimizing data traffic in the network, which results in improved performance of the network/LAN.
Switches
A device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model and therefore support any packet protocol.
Switch
Bridges
A device that connects a local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or Token Ring). You can envision a bridge as being a device that decides whether a message from you to someone else is going to the local area network in your building or to someone on the local area network in the building across the street.
Bridge
Router
a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each point-of-presence on the Internet. A router is often included as part of a network switch. For example, network 192.168.1.xxx and 192.168.2.xxx are two different networks. LAN and WAN are two different networks, too. To communicate between LAN and WAN, there is a broadband router required.
Router
--------------------------------------------------------------
Highlights
What is computer networks?
what is LAN?
what is WAN?
what is MAN?
what is Internetwork?
what is Interanet?
what is Internet?
what is Extranet?
what is Ethernet and Fast Ethernet?
what is Tranciever?
what is Collisions?
what is Network Topology?
what is Bus Topology?
what is Ring Topology?
what is Star Topology?
What is Mesh Topology?
what is Hybrid Topology?
what is Protocol?
what is Network Identification Card(NIC)?
what is HUB?
what is Switch?
what is Router?
what is Bridge?
New Brunswick, Canada     A.Habibi.L@gmail.com    A_Habibi_L@hotmail.com