A computer network or data network is a telecommunications network which allows computers to exchange data. In computer networks, networked computing devices pass data to each other along network links (data connections). Data is transferred in the form of packets. The connections between nodes are established using either cable media or wireless media. The best-known computer network is the Internet. Network computer devices that originate, route and terminate the data are called network nodes. Nodes can include hosts such as personal computers, phones, servers as well as networking hardware. Two such devices are said to be networked together when one device is able to exchange information with the other device, whether or not they have a direct connection to each other. Computer networks differ in the transmission media used to carry their signals, the communications protocols to organize network traffic, the network’s size, topology and organizational intent. In most cases, communications protocols are layered on (i.e. work using) other more specific or more general communications protocols, except for the physical layer that directly deals with the transmission media. Computer networks support applications such as access to the World Wide Web, shared use of application and storage servers, printers, and fax machines, and use of email and instant messaging applications.

Rbcafe » Network

Identify a Scam

Identify a scam

How do I identify a scam message ?

The first 3 necessity for a person is food, shelter and clothing. But now internet is the 4th important necessity in people’s life. The usage of internet is widely used for seeking numbers to all kind of information. But the biggest disadvantage is that you might get caught by scam mails. The scam mailsare sent by the scam companies which are listed on the internet. Try avoiding this website which can be harmful for your computers. To avoid this kind of scam messages download a fee version of anti-scam software from the internet. These stop the scam mail to enter your inbox.

The scam companies can send you a message which has an interesting subject like how to avoid cancer? Or let say about earning money in 15 days. The most usual scams found are the Nigerian. These letters contains generally of transferring money. For example a person claimed to be a widower and asked to transfer money to your bank account. It is better if you don’t give them any personal information. You will never know when you will be bankrupt. These types of letters are otherwise known as 419 scams. How will you find out if the person is a scammer or not? Here are some points which will help you in identifying the scammer.

Clues :

– Scammer never uses his original name or any other personal identification.
– They always write in capital letter and write to you in formal method like referring you with Mr. or miss in each letter.
– There are always a friendless words like my dear or greeting to you in the letter.
– The body of the letter is mostly filled with financial term like lottery, funds etc.
– You can identify the scammer from their e-mail address.
– They register the e-mail address in a formal name for example This shows that they are fake address.

Some of the popular messages are listed below :

Gain lot of money by working from your place. There is no such company who allows you to work from your place. Another type of message is you have won a price from so and so company which you have never heard of. It is highly dangerous if you give your account data to a stranger. Never trust an unknown person who might offer you a job with higher salary. There are many companies who literally steal the money from you via internet. It is safer if you don’t open an unknown scam messages. There is a possibility of hacking your computer through these scam messages. So to stay on a safer side you can download free trial or buy the original anti-scam software which can block all the scam messages.


Rbcafe » Network



whois – Internet domain name and network number directory service


whois [-adgprR] [-h host] name …


Whois looks up records in the databases maintained by several Network In-
formation Centers (NICs).

The options are as follows:

-a Use the American Registry for Internet Numbers (ARIN) database.
It contains network numbers used in those parts of the world cov-
ered neither by APNIC nor by RIPE.

-d Use the US Department of Defense database. It contains points of
contact for subdomains of .MIL.

-g Use the US non-military federal government database, which con-
tains points of contact for subdomains of .GOV.

-h host
Use the specified host instead of the default NIC (whois.inter- Either a host name or an IP address may be specified.

-p Use the Asia/Pacific Network Information Center (APNIC) database.
It contains network numbers used in East Asia, Australia, New
Zealand, and the Pacific islands.

-r Use the R’eseaux IP Europ’eens (RIPE) database. It contains net-
work numbers and domain contact information for Europe.

-R Use the Russia Network Information Center (RIPN) database. It
contains network numbers and domain contact information for sub-
domains of .RU.

The operands specified to whois are concatenated together (separated by
white-space) and presented to the whois server.

The default action, unless directed otherwise with a special name, is to
do a very broad search, looking for matches to name in all types of
records and most fields (name, nicknames, hostname, net address, etc.) in
the database. For more information as to what name operands have special
meaning, and how to guide the search, use the special name « help ».


Ken Harrenstien, and Vic White, NICNAME/WHOIS, 1 March 1982, RFC 812.


The whois command appeared in 4.3BSD.


Rbcafe » Network


Asymmetric Digital Subscriber Line


ADSL has the distinguishing characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically. Providers usually market ADSL as a service for people to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the « download » from the Internet but not needing to run servers that would require bandwidth in the other direction.

For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Lite is sometimes used for the slower versions.

Note that distances are only approximations. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (eg. non-copper cabling, cable diameter).

A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 kilometers (8000 feet). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts these rates to up to 25 Mbit/s for spans of less than 1.5 kilometers (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e. up to 50Mbit/s for two lines, etc), as well as options in power management and seamless rate adaption – changing the data rate used without requiring to resynchronise.

Because of the relatively low data-rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as web traffic; ATM runs widely over ADSL technology to ensure that this remains a possibility.

ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to connect a Web server.

How ADSL works

On the wire

ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. It is, however, possible to alter this frequency division, but it will cause issues with crosstalk.

Frequency plan for ADSL

Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, resulting in reduced throughput on the ADSL connection.There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.

ADSL can use any of a variety of modulation techniques, CAP was the de facto standard for xDSL deployments up until 1996, deployed in 90 percent of xDSL installs. Now it is deprecated in favour of DMT/OFDM modulation schemes.OFDM is used in ADSL connections that follow the G.DMT (ITU G.922.1) standard. (Annex A refers to ADSL-over-POTS).It is worth noting that in contrast to the modulation schemes that baseband technologies like Gigabit Ethernet use, ADSL uses primarily analog modulation schemes, so the ‘D’ in ADSL is a misnomer — ADSL is simply a very fast analog connection (using PPPoE or PPPoA) with much higher symbol rates and much faster handshaking between modems.

ADSL standards

Additionally, the non-Annex ADSL2 and ADSL2+ support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL usage.The downstream and upstream rates displayed are theoretical maximums. Note also that because DSLAM and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+


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