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Wireless LAN MAN

In the wireless LAN marketplace there are two main solutions, using infra-red or radio waves. None of these solutions pretends to offer mobility - they merely offer a slightly more flexible way of coupling your desktop computer to the LAN. In some cases this wireless flexibility can be very desirable - laptop users do not need to continually plug and
unplug network cables, they can simply move into the covered area and start working. In some older buildings, pulling cable to every desktop can be expensive, so using a

wireless solution can save money. Of course, the usual drawbacks apply - the bit rate is much lower than that offered by other LAN technologies. Typically IR and RF based LANs offer a bit
rate of about 4 Mbps. Compared with Ethernet, which is over 25 years old and yet manages 10 Mbps, that can be a considerable drawback. IR based nets are also further limited by the fact that their connections must be "line-of-sight" based. This is not a problem for RF based LANs.

An interesting solution to the MAN question has been developed by an American company
called Metricom. They've developed a wireless system that breaks away from the usual
wireless network topology that we saw. Instead of building a hierarchical system consisting of base stations, serving nodes and a backbone, Metricom build a semiintelligent mesh consisting of small boxes that sit on top of street light and utility poles. One or more of these pole top boxes is equipped with a fixed connection to the global Internet. When a user sends data it goes to the nearest pole top box. This box then independently routes the information to a box that has a fixed connection. All this routing is done automatically by the meshed network, there is no human intervention and no need for hand crafted routing tables.

Metricom's system currently offers a bit rate of 38 kbps, a speed they hope to increase to 56 kbps some time in 1999.

They offer a public Internet access service in three American urban areas, the Bay Area in California, Seattle and Washington DC. Metricom's solution is also being used by several campus based networks and by Sun Microsystems (among others) to cover their corporate campus. The system uses the license free portion of the RF spectrum and utilizes a patented frequency hopping technique to provide security and maintain good link quality. The pole top boxes are positioned every 400 to 800 meters and take approximately 5 minutes to install. Metricom believe that their system has almost unlimited scalability, and could cover the whole of the states if necessary.


Assignment Lanka Populer Posts ever

Token passing

In Token Ring another access method is used, called Token Passing.

With Token Passing, a small message, called a token, constantly circulates around the ring. If the token is marked as free, the host that receives the ”free” token can transmit its data and mark the token as busy. All the hosts along the ring receive the data and the busy token, until the host that sent the original message sets the token free again.

Right now the token is with host B, and it is marked free. Let’s say that host B wants to send data to host D. B sets the token to busy and adds its data. The host C has now received the token and the data, but since C is not the receiver it just passes on the token and the data into the ring.

The host D receives the data and sets the token to copied. The token and the data are then
passed on into the ring.

The host A just passes on the token and the data. The host B sees that the data has been received in a proper way by D. B therefore deletes the data and sets the token …

Network operating systems

After the physical network the NOS, Network Operating System, is the most important part in a PC network. The network operating system is responsible for the levels 3 to 7 in the OSI model. This means that NOS is independent of the type of network. It works in the same manner on an Ethernet as a Token Ring.

There are two types of NOS. The first type is called "peer to peer” which means that no dedicated server is needed. The other type is server based and is used for larger, more demanding networks.

There are many vendors with their own types of NOS on the market.

The leading systems for "peer to peer” based NOS are NetWare Lite and Windows for Workgroups.

The leading systems for server based NOS are Novell NetWare, Windows NT and OS/2 LAN Server.


CSMA/CD stands for "Carrier Sense Multiple Access with Collision Detect”. CSMA/CD
is a random control access method.

The CSMA/CD access method is used as the access control method in Ethernet and is defined in a standard from IEEE. The CSMA/CD algorithm is quite simple and the efficiency for an ordinary Ethernet is about 65%. This means that the effective bandwidth for a 10 Mbps Ethernet is about 6.5 Mbps. The rest is lost, mainly due to collisions.

Before one host will transmit it must ”listen” on the medium whether or not another host is
transmitting. If the medium is ”quiet” the host can send its data. The term "Carrier Sense" indicates that a host listens before it transmits.

"Multiple Access" means that many hosts can be connected to the network and all hosts
have the same right to transmit.

With CSMA/CD, it occasionally happens that two hosts send their packets at the same
time. This will make a collision on the network. The information about the collision is

The History of Local Area Networks, LAN

In the mid 70's Robert Metcalf and David Boggs at Xerox experimented with communication
between the computers. This became the first implementation of Ethernet.

In 1982, the second version of Ethernet was implemented by Digital, Intel and Xerox. This is
the version of Ethernet that is still in use today.

In the mid 80's the first PC-networks started to appear. Network components such as
bridges and routers were now available on the market.

The normal bandwidth of the Local Area Network today is 10 Mbps.

In the near future we will see higher bandwidths, such as 100 to 1000 Mbps.

windows vista wireless

How to connect a Vista computer to a wireless network using WLAN AutoConfig This document will serve as a guide for Microsoft Vista clients in joining a wireless network using WLAN AutoConfig service.

What is WZC or WLAN AutoConfig?

Wireless Zero Configuration (WZC) or WLAN AutoConfig is a service included with modern versions of Microsoft Windows that dynamically selects a wireless network to connect to base on a user's preferences and various settings. This can be used instead (or in the absence) of a wireless network utility from the manufacturer of a computer's wireless networking device.

To join a wireless network:

1. Open the Start menu and click Control Panel.2. In the Control Panel Home window, click View network status and tasks.

2.If you are using Classic View, double- click Network and Sharing Center.

3. Click Connect to a network.

4. Select your network from the list and click Connect.

5.If security is enabled on the network, you will be asked to type your network security…


Digital Subscriber Line (DSL)

DSL Digital Subscriber Line (DSL) provides a dedicated digital circuit between a user’s premises and the Internet through the telephone company’s central offi ce via ordinary copper telephone wires. The two primary forms of DSL are Asymmetric Digital Subscriber Line (ADSL) and Symmetric Digital Subscriber Line (SDSL). ADSL has a higher download speed (1.544 to 6.1 Mbps downstream) and a lower upload speed (16 Kbps to 1.5 Mbps). SDSL’s download and upload speeds (1.544 Mbps) are equal. SDSL does not provide voice capabilities. ADSL – which is more widely used and available – must be within 18,000 feet of the central offi ce while SDSL users must be within 12,000 feet. Some companies, however, have begun to use new technologies such as fi ber lines and/or repeaters to extend DSL capabilities up to 25,000 feet.

An ADSL modem has a “plain old telephone service” POTS) splitter and a channel separator. The POTS splitter divides the phone line into two channels (voic…

how to setup wifi with windows xp

Configuration -Wifi with Windows XP

If your computer is not currently connected to a wireless network, Windows may present the following message to let you know that it has detected one or more of them. Click on the Wireless networks detected balloon to display a list of available networks.

If your computer is already connected to a wireless network or does not present the above message, right-click on the Wireless Network Connection icon on the right of the Task Bar and then click on View Available Wireless Networks to display the list of available wireless networks.

From the Wireless Network Connection window, click on Change advanced settings under Related Tasks. Do not choose JMU-Official-Wireless from here the first time.

If you are logged in as a regular (non-Administrator) user, Windows will let you know that you cannot change some settings. Click on the OK button.

The Wireless Network Connection Properties window will come up on the General tab. Click on the Wireless Networks ta…

Hybrid topologies

Different topologies may be interconnected to form subnets to a main network. It is possible, for example, to connect a star, a hub and a ring together beneath another star topology network.

Star Topology network

Star network or the Star Topology network is a network design which Computers in a star, The topology are connected by cables to a hub. In this management of the network is made much easier (such as adding and removing devices), because of the central point. However because it is centralized more cable is required.
Because most star topologies use twisted-pair cables, the initial installation of star networks is also easier.

"Star topology diagram" 

If one computer fails the network will continue to function, but if a hub fails all computers connected to it will also be affected. Star topologies are, or are becoming the topology of choice for networks.

Star Topology Advantages:

The Cabling, Hardware per node is Cheaper than a mesh topology. (less cable, less hardware per node) In the Star Topology the Single links make the network easier to reconfigure. As we discussed above Low cable housing requirements compared to "mesh topologyAnother good thing is If one link is dam…


IEEE standards for LANs describe different types of transmission media. It could be cable,
fiber or wireless.

Cables typically come in two flavors: twisted pair cables or coaxial cables.

Twisted pair cables
A twisted-wire consists of two insulated strands of copper wire that have been braided.
Often a number of twisted-wire pairs are grouped together into a twisted pair cable. Twisted
pair cables are used both for data communication and telephony.
In the picture the twisted pair cables would typically be used in the star topology in the
middle, that is between the hub and the connected hosts.

Coaxial cables
Coaxial cables consist of a central conducting copper core that is surrounded by insulating
material. The insulation is surrounded by a second conducting layer, which can consist of
either a braided wire mesh or a solid sleeve. In the picture, the coaxial cable would typically
be used for the bus network seen on the top.

Optical fiber:
Optical fibers can be used to carry data signals in th…