Integrated Services Digital Network (ISDN) 


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Integrated Services Digital Network (ISDN)



ISDN services can be carried over existing telephone network infrastructure to terminal adapters (TAs) in the client machine. A common I S D N interface standard has a digital communications line consisting of three independent channels: two Bearer (B) channels, each at 64Kbit/s, and one Data (D) channel at 16Kbit/s. The D channel is used to carry signaling and supervisory information to the network, while the B channels carry the data and can be linked to provide a 128Kbit/s data channel.

Wireless connections

The wireless alternatives come in two forms: satellite and cellular. Satellite systems require the use of a modem to maintain the upload. Downstream bandwidth is provided via a dedicated satellite dish, connector hardware and proprietary software. Cellular systems use assigned radio frequencies and are based around a network of transmitters that are arranged in a cellular network, much like cellular mobile phone systems.

The cable alternative

Cable companies can also offer affordable broadband services over copper coaxial or fibre infrastructure networks. The connection is shared by several customers on a branch, so actual connection rates are variable, unlike ISD N and DSL.

Digital Subscriber Line (DSL)

DSL technology capitalises on the existing network of copper infrastructure, but allows digital signals to be carried rather than analogue. It allows the full bandwidth of the copper twisted-pair telephone cabling to be utilized.

With splitter-based services, the DSL signal is pulled out from the phone line as it enters your premises and is wired separately to a DSL modem. This involves additional hardware and installation by the service provider at the customer site. The shielded option involves no installation, but the telephone company's equipment and some of your equipment might need upgrading.

With Asymmetric Digital Subscriber Line (ADSL), most of the duplex bandwidth is devoted to the downstream direction, with only a small proportion of bandwidth being available for upstream. Much Internet traffic through the client's connection, such as Web browsing, downloads and video streaming, needs high downstream bandwidth, but user requests and responses are less significant and therefore require less on the upstream. In addition, a small proportion of the downstream bandwidth can be devoted to voice rather than data, allowing you to hold phone conversations without requiring a separate line.

DSL-based services are a very low-cost option when compared to other solutions offering similar bandwidth, so they can be made available to the customer at extremely competitive prices.

II. Re-read the text to find the answers to these questions.

Match the terms in Table A with the statements in Table B.

Table A

a ISDN

b TA

c Data channel

d Bearer channel

e DSL

f Splitter-based services

g ADSL

Table B

i DSL system that separates the digital signals from the analogue signals

ii Digital channel used to carry ISDN signalling and supervisory information to the network

iii Device installed on a PC to allow it to receive ISDN signals

iv Integrated Services Digital Network

v Asymmetric Digital Subscriber Line

vi Digital channel used to carry ISDN data

vii Digital Subscriber Line

2. Mark the following statements as True or False:

3.a

ISDN can only operate over a special digital telephone line,

b

Two ISDN channels can be combined to give the user double the bandwidth,

c.

Computers connected to a satellite system do not need a modem,

d

Cellular networks work in a similar way to mobile phone systems,

e. DSL systems require a special digital telephone line,

f DSL systems use analogue signals.

g You need a separate line to hold normal phone conversations on an ADSL system.

Text 12

I. Find the answers to these questions in the following text.

1 What does data encryption provide?

a privacy b integrity с authentication

2 A message encrypted with the recipient's public key can only be decrypted with:

a the sender's private key

b the sender's public key

c the recipient's private key

3 What system is commonly used for encryption?

4 What is the opposite of 'encrypt'?

5 A message-digest function is used to:

a authenticate a user

b create a MAC

c encrypt a message

3. What information does a digital certificate give to a client?

Safe Data Transfer

Secure transactions across the Internet have three goals. First, the two parties engaging in a transaction (say, an email or a business purchase) don't want a third party to be able to read their transmission. Some form of data encryption is necessary to prevent this. Second, the receiver of the message should be able to detect whether someone has tampered with it in transit. This calls for a message-integrity scheme. Finally, both parties must know that they're communicating with each other, not an impostor. This is done with user authentication.

Today's data encryption methods rely on a technique called public-key cryptography. Everyone using a public-key system has a public key and a private key. Messages are encrypted and decrypted with these keys. A message encrypted with your public key can only be decrypted by a system that knows your private key. For the system to work, two parties engaging in a secure transaction must know each other's public keys. Private keys, however, are closely guarded secrets known only to their owners. When I want to send you an encrypted message, I use your public key to turn my message into gibberish. I know that only you can turn the gibberish back into the original message, because only you know your private key. Public- key cryptography also works in reverse - that is, only your public key can decipher your private key's encryption.

To make a message tamper-proof (providing message integrity), the sender runs each message through a message-digest function.

 

This function within an application produces a number called a message-authentication code (MAC). The system works because it's almost impossible for an altered message to have the same MAC as another message. Also, you can't take a MAC and turn it back into the original message.

The software being used for a given exchange produces a MAC for a message before it's encrypted. Next, it encrypts the MAC with the sender's private key. It then encrypts both the message and the encrypted MAC with the recipient's public key and sends the message.

When the recipient gets the message and decrypts it, they also get an encrypted MAC. The software takes the message and runs it through the same message-digest function that the sender used and creates its own MAC. Then it decrypts the sender's MAC. If the two are the same, then the message hasn't been tampered with.

The dynamics of the Web dictate that a user-authentication system must exist. This can be done using digital certificates.

A server authenticates itself to a client by sending an unencrypted ASCII-based digital certificate. A digital certificate contains information about the company operating the server, including the server's public key. Thedigital certificate is 'signed' by a trusted digital-certificate issuer, which means that the issuer has investigated the company operating the server and believes it to be legitimate. If the client trusts the issuer, then it can trust theserver. The issuer 'signs' the certificate by generating a MAC for it, then encrypts the MAC with the issuer's private key. If the client trusts the issuer, then it already knows the issuer's public key. The dynamics and standards of secure transactions will change, but the three basic tenets of secure transactions will remain the same, if you understand the basics, then you're already three steps ahead of everyone else.

II. Re-read the text to find the answers to these questions.



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