NETWORKING TUTORIALS
------------------INTRO---------------------
Local Area networks (LAN’s).
A LAN supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. A LAN is useful for sharing resources like files, printers, games or other applications. A LAN in turn often connects to other LANs, and to the Internet or other WAN’s.
LAN networks are generally inexpensive to buy, and usually are very easy to set up. Specialized operating system software may be used to configure a LAN. In fact most versions of Microsoft Windows provide a software package called Internet Connection Sharing (ICS) that supports controlled access to LAN resources.
Wide Area Networks (WAN’s).
A WAN network is generally used to interconnect lots of LAN’s together. The Internet can be thought of as a bunch of LANs interconnected by WANs. An average packet will run across a company's local Ethernet (LAN), up an ISDN or leased line or PPP link (WAN) to an Internet Service Provider. The ISP has Ethernet too (LAN) that transports the packet to the right router for delivery to a cross-country provider (WAN). The packet begins bouncing from one LAN site to another over WAN links.
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If a WAN is set up across two different countries, then the company would have to lease the lines through different countries which can become expensive. A possible way around this is the use of a VPN. The picture below shows how WAN’s and LAN’s interact with the internet:
Virtual Private Network (VPN’)
Basically, a VPN is a private network that uses a public network (usually the Internet) to connect remote sites or users together. Instead of using a dedicated, real-world connection such as leased line, a VPN uses "virtual" connections routed through the Internet from the company's private network to the remote site or employee. It reduces operational costs that a traditional WAN would have, and also because it is private they are usually more secure. For a great article on how VPN’s work please visit:
http://computer.howstuffworks.com/vpn.htm
Peer to Peer
In a peer to peer network (or abbreviated to P2P) every computer acts as both a client and a server, as there is no dedicated server like in a client/server network. This is a good networking solution when there are only a few computers in close proximity. Often there are security risks as people setting permissions for shared resources often do not get it right. Subsequently they are often not used by large businesses, but are very popular for small home run LAN networks.
Peer to peer networks have really hit off on the internet recently with the emergence of file sharing services like winmx, bearshare and Kazaa. What this means is that individual users connect to each other directly, without need for a central point of management. There are obvious security issues with this; mainly that another user is browsing your shared files, so you have to ensure that there is no sensitive information kept in there.
Client/Server
Client/server describes the relationship between two computer programs in which one program, the client, makes a service request from another program, the server, which fulfills the request. A good example that I came across was this one:
“For example, to check your bank account from your computer, a client program in your computer forwards your request to a server program at the bank. That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve your account balance. The balance is returned back to the bank data client, who in turn serves it back to the client in your personal computer, which displays the information for you.”
The client/server model has become one of the central ideas of network computing. Most business applications being written today use the client/server model. So does the Internet's main program, TCP/IP.
In the usual client/server model, one server, sometimes called a daemon, is activated and awaits client requests. Generally a server program allows multiple client programs to access and use its services at the same time. Both client programs and server programs are often part of a larger program or application. Relative to the Internet, your web browser is a client program that requests services from a Web server (which technically is called a Hypertext Transport Protocol or HTTP server) in another computer on the net. Similarly, your computer with TCP/IP installed allows you to make client requests for files from File Transfer Protocol (FTP) servers in other computers on the Internet.
Security and permissions can be managed by one or more administrators which cuts down on the problem of computer illiterates from messing with permissions. This type of network also allows for convenient backup services, reduces network traffic and provides a host of other services that come with the network operating system (NOS).
----------------NETWORK TOPOLOGIES-----------------
In networking, the term topology refers to the layout of connected devices on a network. This article introduces the standard topologies of computer networking.
1) BUS
This topology is old school and essentially has each of the computers on the network daisy-chained to each other, using the peer to peer system. It is configured by connecting a "T-connector" to the network adapter and then connecting cables to the T-connectors on the computers on the right and left. At both ends of the chain the network must be terminated with a 50 ohm impedance terminator. Bus networks use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium, that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.
The advantage of using a set up like this is that it is simple to set up, as well as cheap. However as all the computers are linked together a failure can cause all the computers on the network to fail, and it can be difficult to troubleshoot where the problem came from.
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As you can see if computer #1 sends a packet to computer #4, it must pass through computers #2 and #3, creating excess traffic. If there was a problem with computer#2 or #3 it would not be clear to computer #4 which one it was without checking, and in a larger network this can be a problem.
2) STAR
The star is probably the most commonly used topology today. It uses twisted pair cabling and requires that all devices are connected to a hub. The advantage of this is that the hub gives centralized monitoring to the network, and a failure to one computer will not affect another one. However if the hub was ever to fail, then every computer in the network would go down, so subsequently these have to be very secure.
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3) RING
The ring topology looks the same as the star, except that it uses special hubs and Ethernet adapters. In a ring network, every device has exactly two neighbors for communication purposes. There are two cable paths that exist, but messages can only travel in one direction. A failure in any cable or device breaks the loop and can take down the entire network. These types of networks used to be found in schools and universities, as all have equal access to the network. However again it is difficult to troubleshoot and any failures may affect all the users.
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4) MESH
Mesh topologies are combinations of the above and are common on very large networks. For example, a star bus network has hubs connected in a row (like a bus network) and has computers connected to each hub.
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. Some WANs, like the Internet, employ mesh routing, and it can often be used in larger University style networking.
5) TREE
Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub ports) alone.
----------------------LAN CONNECTIVITY DEVICES------------------
1) REPEATERS
Repeaters are used due to the problem of weakening electrical signals; this applies for both wire, and wireless networks. Electrical signals traveling through copper wires weaken due to the wire's electrical resistance. This effect limits the lengths of the cable that can be used, but a repeater overcomes this limit, when there is a need to connect two computers at a larger distance. This is because a repeater is connected to two cable segments and any electrical signal reaching the repeater from one segment, will be amplified and retransmitted to the other segment.
2) BRIDGES
A bridge functions the same as a repeater but also filters data traffic at a network boundary to reduce traffic problems by dividing it into two segments.
A bridge can also connect unlike network segments (i.e. token ring and Ethernet). Bridges create routing tables based on the source address. If the bridge can't find the source address it will forward the packets to all segments. Bridges operate at the data link layer (Layer 2) of the OSI model (Shown later in this paper). Bridges inspect incoming traffic and decide whether to forward or discard it.
3) ROUTERS
A router will do everything that a bridge will do and more. Routers are used in complex networks because they do not pass broadcast traffic, and are used for wired and wireless networks. A router will determine the most efficient path for a packet to take and send packets around failed segments. Technically, a wired or wireless router is a Layer 3 gateway, meaning that the wired/wireless router connects networks, and that the router operates at the network layer of the OSI model. Routers are often used in home networking to connect a LAN to the internet (WAN) By maintaining configuration information in a piece of storage called the "routing table," wired or wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers, which is an excellent security feature.
4) BROUTERS
As its name suggests, this is a mixture of a bridge and a router, and takes the best features out of both. This is because it can be configured to pass the unroutable protocols by imitating a bridge, while not passing broadcast storms by acting as a router for other protocols.
5) GATEWAYS
A network gateway is used as a connection to a mainframe or the internet. Gateways enable communications between different protocols, data types and environments. This is achieved via protocol conversion, whereby the gateway strips the protocol stack off of the packet and adds the appropriate stack for the other side. Depending on their implementation, network gateways can operate at any level of the OSI model from application protocols to low-level signaling.
-------------------OSI 7 LAYER MODEL-------------------
A way to learn and memories the OSI 7 layer model is as follows:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers.
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--------TABLE USUALLY HERE----------------
-----------------------------ACCESS METHODS-------------------------
1) CSMA/CD
This stands for "carrier-sense multiple access with collision detection" and is the method used on Ethernet networks whereby all computers on the network check the cable for traffic before attempting to transmit a packet After detecting a collision, a device waits a random delay time and then attempts to re-transmit the message. If the device detects a collision again, it waits twice as long to try to re-transmit the message. This is known as exponential back off.
2) CSMA/CA
Stands for "carrier-sense multiple access with collision avoidance". This access method prevents collisions by having computers broadcast an intent to send a packet, unlike CSMA/CD that deals with network transmissions once collisions have been detected. This broadcasting of intent to send can cause excess network traffic and slow things down, as a signal has been transmitted before any real data has been transmitted.
3) TOKEN PASSING
Token passing is the access method used by token ring networks, and is a type of Channel access method (CAM). With this method, a packet called a token (a special series of bits) is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time, which can be problematic and slow the network down.
4) DEMAND PRIORITY
This access method is used with 100VG-AnyLAN networks. The repeaters, bridges, routers or hubs search the network for requests that are waiting to be sent. If 2 or more requests are received by the network hardware at once, the data with the highest priority is sent. The advantage of this type of network is that this privilege is set by the administrator, and also all computers can send and receive at the same time.
-----------------------CONCLUSION------------------------
Hopefully this paper should give you an inside and better understanding of how networks work. There were a number of resources I used in making this paper. The networking paper on http://www.techtutorials.com was particularly useful, (in fact the OSI 7 layer table is a direct copy from there). I also used http://compnetworking.about.com/ as they have some very good definitions on their site.
If anyone wishes to distribute this tutorial then feel free, just give it credit where due, and a link to www.computer-tutorials.org
Hope you enjoyed the read
Sco
