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Showing posts from August, 2009

AntiVirus software

These are stand-alone programs that will protect your computer from viruses, worms and some trojans, and allow you to clean it should you become infected.

• Norton Antivirus ( Trial / Buy
• Mcafee Virus Scan ( Trial / Buy
• PC-Cillin ( Trial / Buy
• Anti-Viral Toolkit Pro ( Trial / Buy
• AVG Anti-Virus ( Free / Buy

Note that all of these websites (except have areas that will conduct free online virus scans on your machine; however, only Trend Micro’s “House Call” page ( will actually clean your machine if it finds any. In any case, these online scans are only band-aid solutions for disinfecting your machine after the fact – you should still be purchasing a realtime Anti-Virus software package (such as those listed above) to prevent viruses from getting into your system in the first place.

Update your virus/trojan definitions on a r…

Viruses, Worms, Trojans Preventative Solutions

Use common sense - don’t visit, or download programs from, people/websites you don’t know or trust.Don’t open “spam” or unknown emails or attachments – delete them instead.Check with your friend over the phone if you receive an email from them with
an attachment you weren’t expecting – verify they did intend to send you the
email/attachment, to make sure it wasn’t a worm sending it instead (many
viruses/worms/trojans are sent from people unaware they’ve been infected).Obtain the latest patches for Windows, Internet Explorer and Outlook Express
to plug the many security holes and vulnerable exploits found in these programs.Un-hide file extensions. This addresses the problem where Windows by default
likes to hide the extensions of filenames; ie "susie.jpg" shows up as just "susie".
The danger is that "susie.jpg.exe", which is an executable program, would be
shown as "susie.jpg" which many would mistake for just a picture. This is
potentially very dangero…

Virus Suspicious

Computer Virus Suspicious File Types. Be wary if any of these show up in your email

•.ade: Microsoft Access projectextension
• .adp: Microsoft Access project
• .bas: Microsoft Visual Basic class module
• .bat: Batch file
• .chm: Compiled HTML Help file
• .cmd: Microsoft Windows NT Command script
• .com: Microsoft MS-DOS program
• .cpl: Control Panel extension
• .crt: Security certificate
• .exe: Program
• .hlp: Help file
• .hta: HTML program
• .inf: Setup Information
• .ins: Internet Naming Service
• .isp: Internet Communication settings
• .js: JScript file
• .jse: Jscript Encoded Script file
• .lnk: Shortcut
• .mdb: Microsoft Access program
• .mde: Microsoft Access MDE database
• .msc: Microsoft Common Console document
• .msi: Microsoft Windows Installer package
• .msp: Microsoft Windows Installer patch
• .mst: Microsoft Visual Test source files
• .pcd: Photo CD image, Microsoft Visual compiled script
• .pif: Shortcut to MS-DOS program
• .reg: Registration entries
• .scr: Screen saver
• .sct: Windows Script Comp…

Computer Virus How you get infected?

In each case, viruses/worms/trojans can all be spread in the guise of literally ANYTHING people find desirable, such as a free game, movie, song, etc. Victims typically downloaded the file from a website, via file sharing programs such as KaZaa, over instant messaging (such as MSN), or by just carelessly opening some email attachment without thinking. Although these programs can also be exchanged on floppy disks, today the Internet has taken precedent as the distribution medium of choice.

• Viruses, worms and trojans often masquerade under files with various extensions, the most common of which are .exe (executable program – beware!), .com, .vbs, .pif, and .js. Be very suspicious if files with the following extensions arrive in your email without you expecting them, as they could contain a virus, worm or trojan.

• Macro viruses may arrive in the form of Microsoft Word (.doc), Microsoft Excel (.xls), and Microsoft Powerpoint (.ppt) files. Also, be wary of .htm and .html files; because th…

Trojan Types

Again, just as with viruses and worms, there are many types of Trojan programs - the most popular being “Back Orifice”, “Netbus” and “SubSeven”. Depending on the type of trojan installed and the motives of someone who gains access to an infected machine, the results can be disastrous: trojans can allow someone to see what you are seeing (on their screen), to transfer files from your computer to theirs, to delete your files and crash your computer, to use keyloggers to track (log) what you type in order to steal passwords/bank account/credit card info, to open and close CD-ROM drives, take control of your mouse, turn the machine and monitor on/off, and much more.

Almost all trojans will attempt to open a “port” (metaphorically, an open port is like an open door to a house) to broadcast the presence of an infected machine to “port scanners” (people looking for open ports on infected machines, to break into). Some trojans are also programmed to establish a direct connection to a specific …

What is computer Trojan

In today's computer world, a Trojan horse is often defined as a program that seemingly does one thing, but its true function is hidden in order to fool you. For example, you download what appears to be a movie or music file, but when you click to open it, you unleash a program that could do any number of things. Trojans usually operate silently, in the background - the most common purpose for them is that they can allow someone to gain access and control your computer over the Internet, and use it for whatever purposes they wish, often without your knowledge.

Computer Worm Types

There are many types of worm programs, many of which are quite insidious - they can compromise the security of an infected machine and leave it vulnerable to future attacks, collect passwords and other confidential information (which can then be automatically emailed to other people), or be programmed to delete files or deface websites.

Worms are particularly well-known for scanning through a person’s computer for email addresses, and then propagating themselves to the addresses found. Some worms will also send a file from your computer to every person they propagate to, which could be disastrous if the attached file is confidential/personal information.

Computer Worms

Worms are programs that once run, take advantage of a computer’s ability to send and receive information. They use this ability to propagate themselves automatically (usually through email) over a network such as the Internet, and cause massive congestion (slow response time, server overloads) in the process. They can also do more malicious acts, and slow down your machine.

Computer Virus types

Boot Sector Virus: replaces or implants itself in the boot sector - an area of the hard drive (or any other disk) accessed when you first turn on your computer. This kind of virus can prevent you from being able to boot your hard disk/computer.File Virus: infects applications. These executables then spread the virus by infecting associated documents and other applications whenever they're opened or run.Macro Virus: Written using a simplified macro programming language, these virusesaffect Microsoft Office applications, such as Word (.doc) and Excel (.xls). A document infected with a macro virus generally modifies a pre-existing, commonly used command (such as Save) to trigger its payload upon execution of that command.Multipartite Virus: infects both files and the boot sector - a double whammy that can
reinfect your system dozens of times before it's caught.Polymorphic Virus: changes code whenever it passes to another machine; in theory these
viruses should be more difficult for…

Viruses, Worms, Trojans What are they?

Simply put, viruses are (primarily) destructive computer programs created by someone that once run, attempt to destroy the data (files) on your computer. A virus spreads when an infected program is run (executing the virus code), which in turn infects more files on the same machine. This usually happens silently and without your knowledge until its too late. In general, viruses have 1) an infection phase where they reproduce widely, 2) an attack/trigger phase (such as a certain date or time) which causesthem to 3) deliver their “payload”, and do whatever damage they have been programmed to do (if any).

Spyware What is it?

Spyware is any application that collects information about your computer activities and then sends that information to another individual or company without your knowledge or permission. Spyware often arrives bundled with freeware (free) or shareware (trial) programs, through email or instant messenger, as an Active X install, or by someone with access to your computer. Once on your drive, spyware secretly installs itself and goes to work. Spyware can be difficult to detect, and difficult (if not impossible) for the average user to remove.

Spyware can:
• Track your online surfing habits, profile your shopping preferences, gather personal information (age, sex, etc, possibly credit card info, PIN numbers)

• Send your email address to the company/person that made the spyware; that company/person can now send spam to your email account.

• Decrease your connection speed/hog your internet connection by sending information about you and your computer to the company/person that made the spyware


VIRUS Announcement

You should be alert during the next few days. Do not open any message with an attachment entitled 'POSTCARD FROM BEJING', regardless of who sent it to you. It is a virus which opens A POSTCARD IMAGE, which 'burns' the whole hard disc C of your computer. This virus will be received from someone who has your e-mail address in his/her contact list. This is the reason why you need to send this e-mail to all your contacts.. It is better to receive this message 25 times than to receive the virus and open it..

If you receive a mail called 'POSTCARD FROM BEJING,' even though sent to you by a friend, do not open it! Shut down your computer immediately.

This is the worst virus announced by CNN. It has been classified by Microsoft as the most destructive virus ever. This virus was discovered by McAfee yesterday, and there is no repair yet for this kind of virus. This virus simply destroys the Zero Sector of the Hard Disc, where the vital information is kept.More…

Free WiFi

While commercial services attempt to move existing business models to Wi-Fi, many
groups, communities, cities, and individuals have set up free Wi-Fi networks, often adopting a common peering agreement in order that networks can openly share with each other. Free wireless mesh networks are often considered the future of the Internet.

Many municipalities have joined with local community groups to help expand free Wi-Fi networks (see Mu-Fi). Some community groups have built their Wi-Fi networks entirely based on volunteer efforts and donations.

For more information, see wireless community network, where there is also a list of the free Wi-Fi networks one can find around the globe. OLSR is one of the protocols used to set up free networks. Some networks use static routing; others rely completely on OSPF. Wireless Leiden developed their own routing software under the name LVrouteD for community wi-fi networks that consist of a completely wireless backbone. Most networks rely heavily on open …

Wifi Universal

Another business model seems to be making its way into the news. The idea is that users
will share their bandwidth through their personal wireless routers, which are supplied with specific software. An example is FON, a Spanish start-up created in November 2005. It aims to become the largest network of hotspots in the world by the end of 2006 with 70000 access points. The users are divided into three categories: linus share Internet access for free; bills sell their personal bandwidth; and aliens buy access from bills. Thus the system can be described as a peer-to-peer sharing service, which we usually relate to software.

Although FON has received some financial support by companies like Google and Skype, it remains to be seen whether the idea can actually work. There are three main challenges for this service at the moment. The first is that it needs much media and community attention first in order to get through the phase of "early adoption" and into the mainstream. Then co…

WiFi Commercial

Commercial Wi-Fi services are available in places such as Internet cafes, coffee houses, hotels and airports around the world (commonly called Wi-Fi-cafés), although coverage is patchy in comparison with cellular.


• T-Mobile provides HotSpots in many partner retail locations including many Starbucks, Borders Books, and a variety of hotels and airports.

• a Columbia Rural Electric Association subsidiary offers 2.4 GHz Wi-Fi service across a 3,700 mi² (9,500 km²) region within Walla Walla and Columbia counties in Washington and Umatilla County, Oregon.

• WiSE Technologies provides commercial hotspots for airports, universities, and independent cafes in the US;

• Boingo Wireless has over 45,000 hotspots worldwide, including most major airports in the U.S.

• restaurant chain Panera Bread provides free Wi-Fi access at its restaurants.

• Other large hotspot providers include Wayport, iPass, and iBahn.

• There are also a number of aggregators of Wi-Fi, the main one being BOZII, they allow use…

WiFi cellular

Some argue that Wi-Fi and related consumer technologies hold the key to replacing
cellular telephone networks such as GSM. Some obstacles to this happening in the near future are missing roaming and authentication features (see 802.1x, SIM cards and RADIUS), the narrowness of the available spectrum and the limited range of Wi-Fi. It is more likely that WiMax will compete with other cellular phone protocols such as GSM, UMTS or CDMA. However, Wi-Fi is ideal for VoIP applications e.g. in a corporate LAN or SOHO environment. Early adopters were already available in the late '90s, though not until 2005 did the market explode. Companies such as Zyxel, UT Starcomm, Sony, Samsung, Hitachi and many more are offering VoIP Wi-Fi phones for reasonable prices.

In 2005, low-latency broadband ISPs started offering VoIP services to their customers. Since calling via VoIP is free or low-cost, VoIP enabled ISPs have the potential to open up the VoIP market. GSM phones with integrated Wi-Fi & VoI…

Range Extender

A wireless range extender (or wireless repeater) can increase the range of an existing
wireless network by being strategically placed in locations where a wireless signal is sufficiently strong and near by locations that have poor to no signal strength. An example location would be at the corner of an L shaped corridor, where the access point is at the end of one leg and a strong signal is desired at the end of the other leg. Another example would be 75% of the way between the access point and the edge of its useable signal. This would effectively increase the range by 75%.

Wireless ethernet Bridge

A wireless ethernet bridge connects a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes.

WiFi Devices

Wireless Access Point (WAP)

A wireless access point (AP) connects a group of wireless stations to an adjacent wired
local area network (LAN). An access point is similar to an ethernet hub, but instead of
relaying LAN data only to other LAN stations, an access point can relay wireless data to
all other compatible wireless devices as well as to a single (usually) connected LAN
device, in most cases an ethernet hub or switch, allowing wireless devices to
communicate with any other device on the LAN.

WiFi works

The typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant influence on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at least 1 Mbit/s. Based on the settings (e.g. the SSID), the client may decide whether to connect to an AP. Also the firmware running on the client Wi-Fi card is of influence. Say two APs of the same SSID are in range of the client, the firmware may decide based on signal strength to which of the two APs it will connect. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially b…

Wifi Origin

Despite the similarity between the terms "Wi-Fi" and "Hi-Fi", statements reportedly [3] made by Phil Belanger of the Wi-Fi Alliance contradict the popular conclusion that "Wi- Fi" stands for "Wireless Fidelity".

According to Mr. Belanger, the Interbrand Corporation developed the brand "Wi-Fi" for the Wi-Fi Alliance to use to describe WLAN products that are based on the IEEE 802.11 standards. In Mr. Belanger's words, "Wi-Fi and the yin yang style logo were invented by Interbrand. We (the founding members of the Wireless Ethernet Compatibility Alliance, now called the Wi-Fi Alliance) hired Interbrand to come up with the name and logo that we could use for our interoperability seal and marketing efforts. We needed something that was a little catchier than 'IEEE 802.11b Direct Sequence'."

The Wi-Fi Alliance themselves invoked the term "Wireless Fidelity" with the marketing of a tag line, "The Standard for…

WiFi History

Wi-Fi uses both single carrier direct-sequence spread spectrum radio technology, part of the larger family of spread spectrum systems and multi-carrier OFDM (Orthogonal Frequency Division Multiplexing) radio technology. Unlicensed spread spectrum was first authorized by the Federal Communications Commission in 1985 and these FCC regulations were later copied with some changes in many other countries enabling use of this technology in all major countries. These regulations then enabled the development of Wi-Fi, its onetime competitor HomeRF, and Bluetooth.

The precursor to Wi-Fi was invented in 1991 by NCR Corporation/AT&T (later Lucent & Agere Systems) in Nieuwegein, the Netherlands. It was initially intended for cashier systems; the first wireless products were brought on the market under the name Wave LAN with speeds of 1 Mbit/s to 2 Mbit/s. Vic Hayes, who was the primary inventor of Wi-Fi and has been named the 'father of Wi-Fi,' was involved in designing standards s…

About Wifi

Wi-Fi, also, WiFi, Wi-fi or wifi, is a brand originally licensed by the Wi-Fi Alliance to describe the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications.

Wi-Fi was developed to be used for mobile computing devices, such as laptops, in LANs, but is now increasingly used for more applications, including Internet and VoIP phone access, gaming, and basic connectivity of consumer electronics such as televisions and DVD players, or digital cameras. There are even more standards in development that will allow Wi-Fi to be used by cars in highways in support of an Intelligent Transportation System to increase safety, gather statistics, and enable mobile commerce IEEE 802.11p.

A person with a Wi-Fi device, such as a computer, telephone, or personal digital assistant (PDA) can connect to the Internet when in proximity of an access point. The region covered by one or several access points is called a hotspot. Hotspots can range from a single room t…

User Base

In the future there will be other VLAN solutions. One promissing solution, that many poeple wait for, is user based VLAN. When a user logs on a particular host, the user identity is anylized by the switch and then the host becomes a part of a particular VLAN. For example, the user Bob logs on the host P in the picture with his own user ID and password. The switch A decides that Bob belongs to VLAN 1.

Protocol based

Protocol based VLAN means that a host belongs to a particular VLAN based on which
protocol it uses for communication. For example, the host P in the picture is a Netware
client which normally uses IPX protocol, which means that it belongs to IPX VLAN.

Mac based

Mac based VLAN means that a host belongs to a particular VLAN based on which MAC
address the host has. MAC based VLAN is independent of which physical switch port the
host is connected to. For example, the host P in the picture has the MAC address 00-10-4-
B-62-1E-A4, which means that host P belongs to VLAN 1, as can be seen in the left table.
As you can see the same MAC address of the host P is also in the table for switch B. This
means that if we connect host P to any port of switch B, the host P will still belong to

Port based

There are a number of different sollutions to create VLAN. Port based solution means that a
host belongs to a particular VLAN based on which physical port in the switch the host is
connected to. For example, the host P in the picture is connected to port 4 of switch A,
which means that host P belongs to VLAN 1, as can be seen in the left table.


A switch makes it possible to configure something called VLAN. A VLAN, which stands for Virtual Local Area Network, is a logical LAN consisting of a group of hosts. One physical LAN can be divided into several VLANs. A VLAN can be configured by one or several switches, which makes it possible to be geographically distributed but having a logical presence. Users of the same VLAN can communicate with each other at LAN speeds and with no router latency.

There are different solutions for communication between VLANs, but the most common way is to use a router. The router is sometimes integrated in the switch.

LAN Switching

Fast Forward
Fast forward or cut-through switching is the fastest way of forwarding packets thorough a
switch. The switch forwards the packets as soon as the switch is able to determine the
destination MAC address. Although this generally reduces network latency, fast forward
switching doesn't verify the checksum and consequently allows bad packets to pass, which
can reduce the available bandwidth. In fast forward switching the sending direction is never
established which means that two hosts can send to each other simultaneously which will
lead to a collision.

Store and Forward
In Store and forward switching the switch waits until the entire packet is received before
sending it to the destination. This lets the switch verify the packet's checksum and eliminate
the possibility of forwarding bad packets. While the packet is stored in the buffer of the
switch, the transmission direction is established, which means that no collisions can occur.
A disadvantage with store and forward switching …

LAN Components

A hub is a commonly used device for connecting hosts to each other, using bus or ring topology. Each host is attached to a hub via a port. When a hub receives a signal on one port it transmits that signal to all other ports. Many hubs also regenerate and amplify weak signals before re-transmitting them.

A switch is a multiport device that handles routing between different hosts based on their MAC addresses. A switch ”learns” MAC addresses from the hosts that are connected to the switch, and stores them in an internal table. When two hosts communicate with each other, the switch creates a temporary connection path between them. This means that only two hosts will hear each other and not like the hub where everyone hears everything. For example if host A and host B have a conversation with each other, then host C and D can also communicate at the same time without any disturbance from host A or B. There is also possibility for one host to broadcast, which means that the packets will be t…


ATM, which stands for Asynchronous Transfer Mode is a ”de facto standard” developed
by the ATM Forum and is a switching method of communication, which can be used in both
LANs and WANs.

ATM specifications are being written to ensure that ATM smoothly integrates numerous
existing network technologies.

Today, in many instances, separate networks are used to carry voice, data and video
information, mostly because these traffic types have different characteristics. For instance,
data traffic tends to be "bursty" while voice and video tend to be more "continuous".
With ATM, separate networks will not be required. ATM is the only technology which
from the beginning, was designed to accommodate the simultaneous transmission of data,
voice and video.

ATM is available at various speeds but the most commonly used are 25, 155 and 622 Mbps.

Fiber Data

FDDI stands for Fiber Distributed Data Interface. FDDI standard was developed by ANSI,
the American National Standards Institute. It is based on the use of double optical fiber cable
and provides for a token-passing ring configuration, operating at 100 Mbps.
FDDI is being developed to deal with the requirements of high-speed LANs, MANs and
backbone networks. Since FDDI consists of two fiber rings, primary and secondary ring,
there is good redundancy and high availability. Normally traffic only flows on the primary
ring, but if the primary ring is broken then the secondary ring is used.

Token Ring

Token Ring is mainly used to connect equipment from IBM and Novell.
In this picture you have two environments, IBM and Novell, co-existing on a single Token
Ring. Although these two environments cannot communicate with each other in this
configuration, they can still use the same Token Ring.

Token Ring Novell

A normal way for an organization to go from mainframes to more modern computers is to
use the existing Token Ring network but to change the earlier IBM devices to personal

Novell was one of the first to see this market and they are using Token Ring to connect their
servers and clients together.

In the picture you can see a typical configuration with different types of personal computers
working as Novell clients and servers.

IBM implementation

In this picture we see an IBM implementation of Token Ring. An IBM mainframe 3090
cannot directly communicate with the Token Ring. To do that it needs an NCP which is a
dedicated computer that only handles the communication between the mainframe and the
Token Ring network.

The users sitting on terminals can access the data from the mainframe through a terminal
server. There can be several thousand terminals connected to a mainframe.

Another possibility is to use mini computers such as AS/400. These mini computers can be
accessed by directly connected terminals as in the picture, or from a terminal server.

Token ring Network

Token Ring was introduced by IBM in 1987 and became their main architecture. The
standard for Token Ring from IEEE came in 1989.

Token Ring is physical star and logical ring topology. This means that you connect the
computers physically in a star configuration to the hub, but the computers still pass the
access rights with help of a token in a ring.

The bandwidths used in Token Ring are 4 or 16 Mbps.


Ethernet can be used to connect equipment from different vendors. Different protocols can
also be used at the same time on Ethernet. For example Novell’s IPX/SPX can be used
together with TCP/IP. Almost all modern computers, printers and network components can
connect to Ethernet.

In this picture you have three environments, Novell, SUN and Digital, co-existing at the
same time on a single Ethernet. Although these three environments cannot communicate with
each other in this configuration, they can still use the same Ethernet.

Sun microsystems

Sun microsystems was one of the earliest manufacturers of UNIX workstations. Sun had an
early vision that ”The network is the computer”. SUN is using Ethernet and TCP/IP as a
strategic platform. Since every UNIX workstation and UNIX server comes with an Ethernet
card and TCP/IP software, it is ready for direct connection to the network.
For the PC market, SUN has developed PCNFS software, so that a PC can communicate
with SUN equipment.

Ethernet Digital

Ethernet, as defined in IEEE 802.3 standard, can use both star and bus topology with bandwidths between 10 and 100 Mbps. Ethernet is today the most common technique used
in Local Area Networks.

Digital uses Ethernet for communication between their products. This picture represents an
early implementation by Digital. You can see that Vax computers can be accessed by VT220
terminals, through a terminal server.

LAN Technologies

This diagram shows what has happened to the development of the two most used LAN technologies today, Ethernet and Token ring. 10 Megabits Ethernet exists in two versions. Version two as specified by Digital. Intel and Xerox, is the most commonly used version and IEEE standard 802.3 which is not so commonly used. These two versions are not compatible, because the frame format differs.

Fast Ethernet, which is specified in IEEE 802.3u, offers 100 Mbps. Fast Ethernet is a modern version of Ethernet and is often used in LAN backbone networks today (that is 1999), but is still not so commonly used for clients.

Gigabit Ethernet over fiber, is specified in IEEE 802.3z, offers 1000 Mbps. Gigabit Ethernet is not so common today (that is 1999). Gigabit Ethernet is only used in LAN backbone networks because it is expensive and there is not any need today for so high bandwidth to clients.

Gigabit Ethernet over twisted pair cable, is specified in IEEE 802.3ab, offers 1000 Mbps. This standard is not fu…


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

Access Methods

A characteristic common to all Local Area Networks is that multiple hosts have to share access to a single physical transmission medium. Several methods can be employed to control the sharing of access to the transmission medium. The various access control methods can be characterized by where in the network the transmission control function is performed. An access method can use following forms of transmission control:

1. Random control
With random control any host can transmit and permission is not required. A host may
check the medium to see if it is free before beginning to transmit.

2. Distributed control
With distributed control only one host at a time has the right to transmit and that right is
passed from host to host. This is usually done by passing on a small piece of data called a
token. The host that has the token, is the one that has the right to transmit.


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…


The topology of a network concerns the physical configuration of the devices and the cables
that connect them.

Three principle topologies are used for local area networks:

1. Bus network
On the bus network all connected hosts are sharing the same cable. All the hosts must use
the same communication speed and every host ”hears” all traffic on the cable.

2. Ring network
In the ring topology all hosts are connected into a ring. Every host in the ring receives all data
that is passing. If the data has another destination address, the host will re-transmit the data
into the ring. The data will continue to travel in this way until it reaches the destination host.

3. Star network
A star configuration includes a central controller which could be a hub or a switch. Every
host is directly connected to a port on the central controller.

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.


With some broadband systems, data signals travel from a computer to a satellite and are then beamed to the ISP where the request is processed. The main advantage of satellite broadband is that it is available to nearly anyone who has an unobstructed view of the southern sky, since satellites orbit the Earth near the Equator. Rural customers who may not have access to other broadband technology can usually receive service via satellite.

Satellite Satellite broadband has several limitations including upload and download delays, inclement weather disruptions, physical obstruction concerns, and costs to purchase, maintain and operate the necessary equipment. Due to the distance from the computer to the satellite, there is a delay of a half-second or more between information sent and data received. Inclement weather can increase the delay or disrupt satellite service altogether. Objects such as trees and buildings can severely restrict or prevent reception of satellite signals.

T Lines

T-1 Lines A T-1 line is a dedicated line supporting data rates up to 1.544 Mbps - using 24 individual channels with each supporting 64 Kbps. An individual channel can carry voice or data traffi c, while a customer switching unit/ digital switching unit (CSU/DSU) is necessary toconnect the channel to the four wires that carry the information. The CSU/DSU sends the data signal to
the router which connects it to a server that may send it to the Internet via other servers. Lines
Repeaters must be in place every 6,000 feet or less to help prevent data signal degeneration (as diagrammed below).

Telephone companies, in general, allow customers to buy individual channels in increments of 56 Kbps (8 Kbps per channel is used for data management). A full T-1 connection can theoretically accommodate 200+ users and other provider services.

T-1 lines use copper wire and offer a popular option to businesses and smaller Internet service providers (ISPs) wanting to connect to the Internet and the Internet …


Cable television companies (Comcast, Insight, Brighthouse, Time Warner, etc.) are now competing
with traditional telephone services by providing service over their own networks, usually Voice over Internet Protocol (VoIP). For more information on VoIP please refer to the OUCC’s VoIP fact sheet.

Coaxial Cable If a consumer uses a cable service for broadband access, a cable modem connects the user’s personal computer to a shared network, connecting the computer to the Internet via the cable company’s main offi ce (as shown below). Cable modems adhere to industry standards known as DOCSIS (Data Over Cable Service Interface Specifi cation). These standards allow them to interact with other DOCSIS-certifi ed equipment to ensure data privacy. Cable companies can install new service to customers very quickly and easily if those customers are already using cable TV. In general, cable companies offer faster download and upload speed than traditional DSL if the network is not congested. Cable modems…


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…

3G history

First generation wireless, or 1G, refers to analogue networks introduced in the mid-1980s.
Examples include advanced mobile phone service (AMPS) used in North America and total access
communications system (TACS) used in the UK. In South Africa we had the C450 mobile system
run by Telkom which was relatively expensive and took ten years to achieve ten thousand subscribers. Most 1G technologies and systems were country or region-specifi c and thus offered
limited coverage.As mobile communications grew in popularity, networks often became
overloaded, resulting in busy signals and dropped calls. The solution was second-generation
wireless, or 2G, which emerged in the early 1990s. 2G technologies were digital and offered
the much-needed capacity that 1G analogue systems did not afford. Several technologies were
widely used:

• GSM was and still is popular in Europe and Asia Pacifi c, and Latin America
• TDMA was used in the Americas and is still used in Latin America
• CDMA IS-95 or cdmaOne was used …

3G standard

3G stands for third-generation wireless technology and networks. It is based on the International Telecommunication Union (ITU) initiative for a single global wireless standard called International Mobile Telecommunications-2000 (IMT-2000). This concept of a single standard evolved into a family of fi ve 3G wireless standards. Of those fi ve, the most widely accepted are CDMA2000, WCDMA (UMTS) and TD-SCDMA. According
to the ITU and IMT-2000, a wireless standard must meet minimum bit-rate requirements to be considered 3G:

• 2 Mbps in fixed or in-building environments
• 384 kbps in pedestrian or urbanenvironments
• 144 kbps in wide area mobileenvironments
• Variable data rates in large geographic areasystems (satellite)

In addition to providing faster bit rates and greater capacity over previous-generation technologies, 3G standards excel by effectively:

• Delivering mobile data
• Offering greater network capacity
• Operating with existing second-generation technologies
• Enabling rich data appli…

3G Option

With its comparatively low power consumption and robust techniques for dealing with interference, 3G fixed wireless technology remains a very attractive competitive carrier option. Combined with Internet Protocol, 3G technology can be leveraged to support the voice and data requirements of the most demanding residential and business users. While standards groups continue to finalize mobility 3G standards for roaming and other mobile applications, an extraordinary opportunity now exists to leverage the mature, lower-level 3G physical capabilities for fixed-point
wireless. These open solutions, combined with IP-enabled networking, computing platforms offering varied API support, plus an open services gateway concept, lay the groundwork for a more comprehensive, differentiated set of residential and business services than has ever been attempted.

In sum, 3G fixed wireless technology offers the potential for consumers and telecommuters to get the bandwidth and services of sophisticated priv…

3G Advantages

Although 3G fixed implementations have not yet been finalized, there are both technical and economic advantages to the technology that seem ideal as an entry point to competitive residential services. Vendors are already developing radio transmission systems for 3G that correct some of the weaknesses of other wireless local loop technology. For example, previous WLL systems that are not based on 3G have required line-of-sight or near line-of-sight from the radio transmitter to the home or building being served. Common weather conditions such as heavy rainstorms, dense fog and blizzards can adversely affect transmission.

By contrast, fixed wireless systems based on 3G technology are designed without line-of-sight limitations or requirements. Unlike wireless local loop solutions, which required use of externally mounted antennas, a 3G-based solution can use an integrated antenna in the home terminal unit. This is a significant benefit, enabling self-installation and over-the-air service …

3G Spectrum

Although spectrum (roughly 155 MHz in the core band around 2GHz) for 3G has been allocated specifically in Europe and many other parts of the world, the US has not yet finalized a 3G spectrumplan. While the FCC has announced it will free up spectrum located in the 700 MHz band for 3G auctioning later this year, further allocations will be required to accommodate a range of competitive 3G service providers in different parts of the country. However, because of 3G’s ability to be implemented across any number of bands, operators and the FCC are working toward a solution. As yet, the FCC has not completed all spectrum considerations, and the question of whether to provide new allocations remains open. Importantly, infrastructure providers are currently working to create solutions that will deliver the same functionality as 3G without requiring the extra spectrum.

3G Required

imperative, enabling network engineers to embark on massive 3GTo achieve this level of wireless connectivity over wide areas – a continent or oceans, for example, basic core networks must be interconnected. Cooperation among industry 3G groups has been harmonization efforts to promote IMT-2000 compatibility worldwide. These efforts include developing software and hardware upgrades to core networks that prepare for highbandwidth multimedia services; as well as developing systems to harmonize two different emerging 3G CDMA operating solutions. Both will ultimately talk to each other across the global network space.

The two solutions include W-CDMA (wideband CDMA), a standard that supports fixed network speeds up to 2 MHz, and is endorsed by European standards groups and NTT DoCoMo, the largest wireless carrier in Japan, which has led the first 3G tests and commercial implementations. The second 3G operating solution is CDMA-2000, which is an evolution of the North American IS-95 CDMA sta…

3G Wireless

The IMT-2000 specification makes specific provisions for 3G Fixed Wireless Access (FWA). The International Telecommunications Union specifies that “IMT-2000 aims to exploit the potential synergy between the digital mobile telecommunications technologies being developed as part of the dramatic growth of personal telecommunications, and those rapidly evolving for Fixed
Wireless Access.” According to the ITU, this means that IMT-2000 will offer wireless access to the global telecommunications infrastructure which will serve both mobile and fixed users in both public and private networks.

Fixed wireless 3G is a converged, multimedia-driven technology that surpasses early concepts of wireless local loop which relied principally on RF and line-of-sight connections to deliver basic
POTS and narrowband data (mostly to under-served or sparsely populated areas). In fixed mode, 3G utilizes a point-to-multipoint network architecture that can transmit data and voice
simultaneously at high speeds acros…

3G Concept

While some carriers announce plans to migrate to high-speed broadband wireless in incremental steps, international standards groups are busy finalizing 3G mobile and fixed standards.

In 1998, for example, working groups at the European Telecommunications Standards Institute (ETSI) and the International Telecommunications Union (ITU) in Geneva, the
reigning worldwide standards body, assembled to evaluate no less than five competing proposals for 3G wireless networks. Some of the proposals offered backward compatibility with existing 2G
networks; others did not. Today, the 3G international standard, known as IMT-2000 (or UMTS), represents an amalgam of many international interests. Groups as diverse as the GSM Association, the Universal Wireless Communications Consortium (UWCC), ETSI, the North American CDMA Development Group, the ITU, and the Third Generation Partnership Project (3GPP) have provided technical input into the IMT-2000 specification, which is still undergoing modification.


Evolution to Wireless 3G Networks

First generation wireless referred to analog cellular transmission, which became popular in North America throughout the 1980s and early 1990s. Second generation wireless refers to the current, most common forms of digital cellular and personal communications services (PCS) primarily voice transmission technologies that utilize digital encoding and provide some low-speed, circuitswitched data for such handheld applications as phone-based email, news and stock services, and short message service (SMS). By contrast, 3G wireless is a form of sophisticated broadband transmission that in addition to handling vast amounts of voice capacity, is optimized for transmission of data and multimedia.

Development of third generation wireless air interfaces and switches has been going on intensively in universities, research centers, and wireless manufacturer settings in Europe, Japan and North America since the early 1990s. The International Telecommunications Union (ITU) released its first studies …


It is also possible to compare the two technologies with respect to the extent to which they are standardized. Broadly, it appears that the formal standards picture for 3G is perhaps more clear than for WLAN. For 3G, there is a relatively small family of internatio nally sanctioned standards, collectively referred to as WCDMA. However, there is still uncertainty as to which of these (or even if multiple ones) will be selected by service providers. In contrast, WiFi is one of the family of continuously evolving 802.11x wireless Ethernet standards, which is itself one of many WLAN technologies that are under development. Although it appears that WiFi is emerging as the market winner, there is still a substantial base of HomeRF and other open standard and proprietary technologies that are installed and continue to be sold to support WLANs. Thus, it may appear that the standards picture for WLANs is less clear than for 3G, but the market pressure to select the 802.11x family of technologi…

Support for Services

Another important difference between 3G and WiFi is their embedded support for voice services. 3G was expressly designed as an upgrade technology for wireless voice telephony networks, so voice services are an intrinsic part of 3G. In contrast, WiFi provides a lower layer data communications service that can be used as the substrate on which to layer services such as voice telephony. For example, with IP running over WiFi it is possible to support Voice-over-IP telephony. However, there is still great market uncertainty as to how voice services would be implemented and quality assured over WLAN networks.

Another potential advantage of 3G over WiFi is that 3G offers better support for secure/private communications than does WiFi. However, this distinction may be more apparent than real. First, we have only limited operational experience with how secure 3G communications are. Hackers are very ingenious and once 3G systems are operating, we will find holes that we were not previously awar…

Deployment Status

While 3G licenses have been awarded in a number of markets at a cost of billions of dollars to the licensees, we have seen only limited progress with respect to service deployment. Indeed, many of the licensees have seen their market values drop precipitously as a consequence of the high costs of obtaining the licenses, increased cost of deployment expectations, and diminished prospects for short-term revenue. The cost of obtaining the licenses contributed to the worldwide slump in the global telecommunications sector.

In contrast, we have a large installed base of WiFi networking equipment that is growing rapidly as WiFi vendors have geared up to push wireless home networks using the technology. The large installed base of WiFi provides substantial learning, scale, and scope economies to both the vendor community and end-users. The commoditization of WiFi equipment has substantially lowered prices and simplified the installation and management of WiFi networks, making it feasible for …