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ISDN issues

This is with reference to two articles on ISDN published in the August 2001 issue of "Network Magazine". I have the following comments and feedback.

Firstly, I'm glad that you have chosen to write on ISDN. Most technology watchers seem to have given up on ISDN and this includes journalists always in search of sensational and news making technology. It is often forgotten that while new technology is exciting, a lot of it is vapourware and quite a while away from commercial deployment. ISDN seems to be one such technology. Sandwiched between the advent of newer access technologies like cable, ADSL and Ethernet, it seems to be been written off. However, it is very much alive and kicking and offers an excellent medium for "multitasking" on data and voice connectivity at all levels (be it SOHO or be it Enterprise).

Also, a lot seems to be written on data and video since these are where the benefits are most apparent. However, one must note that ISDN is also an excellent medium for voice. For example, the capability to offer two channels over one physical line is great where there is a limitation on copper in the local loop. In PRI this is more explicit because 2 pairs of cable can carry 30 channels of 64 kbps simultaneously.

Now to my comments on your articles:
In the article on terminal adapters, I was puzzled by your statement that "many TAs cannot convert synchronous 64 kbps data into 57.6 kbps asynchronous data and that these TAs can communicate in asynchronous mode at 38.4 kbps". I don't clearly understand the context in which this statement was made or what your source was for this information. Most TAs today come with capability for async to sync PPP at 64 kbps. This means that the DTE (typically a computer) talks to the TA in an async mode through the serial port. However, when the TA connects to an ISP's remote access server (RAS) or router on the other side, it has to usually use sync PPP to communicate. In addition to async to sync PPP, some TAs also offer async connectivity in the form of X.75, V.110 or V.120. While X.75 and V.20 support 64 kbps and 128 kbps, V.110 offers various speeds up to 38.4 kbps. However, you would be hard pressed to find an ISP today who uses V.110 these days.

Please also note that all TAs, whether internal or external require power to function, especially for voice functions. Many external TAs come with analog ports for connecting traditional POTS equipment (telephone or fax), whereas "most" internal TAs are usually for data connectivity only.

"Usually" the only WAN protocols that are commonly used by TAs are PPP and ML-PPP since the TAs are usually connecting to a router. For point-to-point asynchronous connectivity between two TAs, one would use other protocols such as X.75, V.120 or V.110. Please note that the TA is usually not required to support PAP, CHAP or MD5 since these are handled by PPP.

In this article, you seemed to have overlooked the USB TA. This is a device which usually costs much less than an external TA, is smaller in size, does not require an external power supply (it is powered by the USB port on the computer). For applications where only data connectivity is required, such TAs offers an excellent and economical means to connect. Further the USB port offers better throughputs as compared to the serial port because it does not suffer from FIFO buffer problems on the UART as a serial port does. While the external TA indicative prices that you have mentioned range from Rs 10,000 to Rs 14,000, a USB TA can be available for as little as Rs 4,500, bringing it closer to the cost of a dial-up modem.

These days, one could also use a feature phone that supports data connectivity. This would also allow the simultaneous use of a data connection and a voice call.

Some comments on your article on "ISDN: An alternative to dial-up"

Please note that in India, we follow the ETSI (European Telecommunications Standard Institute) standard also known as Euro-ISDN. Under this standard, PRI offers 30B channels and 1 D channel and NOT 23B channels and 1 D channel. 23B+D is a feature of the ISDN standards in the US and some other countries.

Please also note that while error correction does occasionally reduce throughput, the real difference between analog and ISDN is that ISDN supports out-of-band call control. This means that signaling (such as call set up, etc) consumes some overhead from the 64 kbps available for a circuit switched call while ISDN has a separate D channel which is used for signaling. The B channel is free for data. These days, even the B channel can be used for packet data (called AODI: Always ON/ Dynamic ISDN).

With regards to Interface equipment, please note that under ETSI standards, the Telco is supposed to install a Network Termination Unit (often called NT-1) at the subscriber premises. This NT-1 offers an S/T interface to subscribers. Equipment should offer S/T and not U under standard conditions (as per TEC specifications). However, because of its inept handling of supply of NT-1 units, BSNL or MTNL sometimes forces subscriber to purchase their own NT-1 (without compensating them for this purchase). In order to shortcut this requirement, vendors and subscribers use equipment with the U interface. The US standard requires equipment to be equipped with the U interface.

I also could not understand how external TAs could tie up your CPU (as compared to an internal TA)?

I hope that the above points will serve to clear some of the misconceptions. Should you require further information or clarifications, please feel free to reply back.

Ketan Sanghvi
WANLANd Datacom

Information on MCSE

I highly appreciate the articles that appear in your magazine, as in India networking still is in initial stages. As a MCSE I'm always in search of good & healthy information about networking. Can you can give more information on MCSE 2000?

Also, why did you stop giving the CD with your magazine? It's not fair with readers, as you are the only publication to provide network utilities. Can you please restart your CD scheme?

Harminder Sehmi
Informative Website

I'm very happy & proud to see such an informative website for networking. Keeps your service going! With your first issue we got your CD contents on the Net itself. After that it has not been available.

Would it be possible to give links to sites and also provide the information that you carry on CD. It would be of help to me.

Kannan Sudarsanan
Caldera Linux

I am a regular reader of "Network Magazine" & I found the Caldera Linux Distributions on your free CDs very useful. Thank you very much.

Further, would it be feasible to distribute "SuSe Linux 7.2" which is very much compatible with ORACLE 9i.


Server problems

I am using Windows NT Server and over 98 clients. In addition to this we have some Windows NT standalone servers. If I am browsing other computers from Windows NT machines using network neighborhood, there is some delay in displaying those machines. Also sometimes the machines are not displayed at all.

In Windows 98 machines sometimes I am unable to find other computers in the Network Neighborhood list. But I can connect to PCs using like this \\machinename. What is the exact problem?

Mujeeb Rahman

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