56-kbps Modem Technology VS IDSN



CSE 588 - Network Systems

May 1, 1997





It is sometimes hard to believe that the Internet phenomenon has exploded so rapidly. With the exponentially increasing popularity of the WWW and the Internet, more and more people are getting online, checking e-mail, and surfing the Web. All o f this creates a need for speed -- a faster connection to the Internet, so that less time is spent waiting for text and pictures to be downloaded to your browser computer.


There is a multitude of types of connections that can be used to link up to the Internet: from 28.8-kbps / 36.6-kbps V.34 modems, to frame relay, to IDSN, to T-1 connections. In general, the faster the connection, the more expensive it is to set up and to keep connected. Recently a new technology has been invented that attempts to provide the speed of a frame relay or IDSN connection (56-kbps) at price of a normal modem. This paper will take a look at the new 56-kbps modem technology, and compare it wi th existing IDSN technology.



What is 56-kbps Modem Technology?


56-kbps modem technology is based on a new transmission scheme that takes advantage of the fact that the majority of the public switched telephone network (PSTN) is digital, and that the only part that is analog is the portion that connects you r home to the telephone company’s central office. Existing 28.8 and 36.6-kbps modems assumed that the whole network was analog, and converted analog data to digital data (ADC) when transmitting signals across the PSTN, and back from digital to analog (DAC ) when receiving signals from the PSTN. This occurs on both the client and server modems. The V.34 modems utilized compression schemes to optimize the conversations, but ultimately, noise and corruption occurs in the conversion s, forcing the transmission rates to reach a maximum of 36.6-kbps. (See v.34 Modem Architecture figure below).

The new 56-kbps modem transmission scheme, however, bypasses one leg of the digital and analog conversation. The server modem is assumed to be directly connected digitally to the PSTN, and therefore does not need any conversions whatsoever. The only conversation that takes place is at the client end, where the modem is still connected through a analog connected. (See 56K Architecture figure below.) The removal of one of the conversion loops reduces that amount of noise and corruption, and theref ore allows higher transmission rates.

However, there are several caveats and requirements that must be mentioned about the 56-kbps technology:


The last point brings up an intriguing issue. At this time, there are two major companies that are developing two competing and incompatible versions of 56-kbps modems: U.S. Robotics’ X2 modems, and Rockwell and Lucent’s K56Flex modems. Neither version has been set as the 56-kbps "standard," though many ISPs and modem manufactures have already taken sides. Nevertheless, as of yet, a X2 modem will not communicate at all with a K56flex modem at 56-kbps speeds whatsoever.


A standard may be in the near future, however. A number of industry leaders have banded together to form the Open 56K Forum, which is a "industry-wide coalition dedicated to achieving widespread implementation of 56-kbps analog modem technology.&q uot; They will conduct trials of interoperable 56-kbps modems, and strive to reach a standard. A date and time has not yet been set for the meeting.



What is IDSN?


IDSN stands for Integrated Serviced Digital Network, and is a standardized design for a completely digital communications network. It creates a digital connection between yourself and the telephone network, replacing the original analog connect ion to the PSTN (removing the analog / digital conversions completely, and thereby increase the transmission speeds). However, the connection still runs over the existing copper telephone wiring that connects to the PSTN. There are several types of IDSN, but for the purposes of this paper, we will concentrate on Basic Rate ISDN (BRI). BRI divides the telephone line into 3 channels: 2 "B" channels, and 1 "D" channel (See IDSN Architecture figure below).

The B channels are used to transmit data, while the D channel is used for administering the connection - carrying call setup and user data. The B channels carry data at a rate of 56-kbps each, and the D channel transfers at a rate of 16-kbps. In some areas an "out-of-band" D channels is used, so that the B channels can transmit at a full 64-kbps rate. With both B channels active, one can make two phone calls simultaneously, or transfer data at a 112-kbps rate (128-kbps if using an out- of-band D channel).



Comparison of 56-kbps Modems with ISDN


We will take a look at both 56-kbps modem and IDSN technologies, and compare them based on their transmission speed (real and theoretical), latency, ease of setup, cost, and availability.


Transmission Speed


In order to get a complete picture of the differences in transmission speed between 56-kbps modems and IDSN, we have to look at both the uplink and downlink speeds, and both the theoretical and actual speeds. Data for the actual transmission speeds was taken from tests performed by ZDnet and Cnet on U.S. Robotics X2 Sportster modem. According to Cnet, testing the actual transmission speed proved to be fairly difficult, since the transmission speeds varied greatly depending on the noise sensitivity on t he phone lines. The uplink speeds varied from 18.01-kbps to 22.73-kbps, while the downlink speeds varied from 38.04-kbps to 44.8-kbps. Additionally, in order to compare 56-kbps modems with a 2-channel IDSN, I have extrapolated the performance of two simul taneous multi-tasked 56-kbps modems.


Connection Type

Downlink Speed (Theoretical)

Downlink Speed (Actual)

Uplink Speed (Theoretical)

Uplink Speed (Actual)

28.8-kbps Modem





36.6-kbps Modem





56-kbps Modem






2 56-kbps Modems (Extrapolated)






IDSN (1 channel)





IDSN (2 channels)






* Cnet test data (http://www.cnet.com/Content/Reviews/Hands/030397/robotics.html)

** ZDnet test data (http://www.zdnet.com/wsources/content/970312/56k_review.html)


None of the connection types, for uplink or downlink, reached their "theoretical" maximum transmission speeds. Both the 28.8-kbps and 36.6-kbps connections came reasonably close to their theoretical speeds, as did the 1-channel ISDN connection. In addi tion, the 2-channel ISDN connection and the 56-kbps connection were significantly slower than their theoretical speeds.


A 56-kbps modem, on downlink, performs significantly faster than both a 28.8 and 36.6-kbps modem, but significantly slower than a 1-channel ISDN connection. Remarkably, in comparing two 56-kbps modems (extrapolated) with a 2-channel ISDN connection, we find that both schemes are reasonably equal in terms of transmission time. The 56-kbps uplink connection speed, surprisingly, is slower than all other connection schemes, and does not even come close to the theoretical 40-kbps -transmission speed. The slowness in uplink speed is emphasized even more greatly in comparing the extrapolated performance of two 56-kbps modems versus a 2-channel ISDN connection.


Latency / Connection Time


56-kbps modems take up to one minute to handshake and connect, and are prone to static and noise. IDSN connections are made digitally and silently, and take approximately two seconds. ISDN clearly has a distinct edge over 56-kbps modems in terms of lat ency and connection time.


Ease of Setup


Both IDSN modems and 56-kbps modems support plug-and-play, so installing and setting up the modem itself is quite simple (assuming you use Windows 95! Other operating systems might be slightly more difficult.) The difficulty in setup comes in the insta llation and configuration of the physical communication line itself. For a 56-kbps modem, it is simple - merely connect the modem to the phone line, and you are set. However, for an ISDN line, you have to install a line (getting service from the telephone company can take up to several months on the waiting list), install an interface, and set up various cables and wires from the interface to your computer. 56-kbps modems clearly have the edge in this issue, and are much simpler and quicker to set up.




The 56-kbps modems cost between $150 to $300, ranging from cheaper generic models, to U.S. Robotics Sportster, which appears to be the most popular, at around $200, to the U.S. Robotics Courier at $275. Adding in about $15 per month for a second phone line, $50 to install the line, and $30 per month to your ISP for unlimited Internet access; we come to $250 for installation and hardware, and $45 per month for Internet access.


IDSN modems cost between $150 to $600, with the Motorola BitSurfr Pro being the most popular, costing approximately $300. IDSN line installation costs vary greatly depending on the service provider, but seem to average around $100. US West, who is a ma jor provider in the Seattle area, has a charge of $85 for installation. The ISDN per month charge varies even more greatly, and depends a great deal on the package you buy: a flat monthly fee with unlimited hours, or a flat monthly fee plus a extra per-mi nute charge. In addition, many providers vary the per-minute charger based on the time of day. US West provides a package with unlimited hours, with a flat monthly charge of $63.


In addition to all the ISDN charges, one still has to pay ISP setup and monthly charges. Northwest Link, an ISP provider in the Seattle Area, charges $350 to setup, and $250 a month for one channel, $350 a month for two channels. This comes out to a wh opping grand total of $735 for setup, $313 per month for one channel, and $413 for two channels!


There is a clear and dramatic difference in price:



Setup Costs

Monthly Charge

ISDN (one channel)



ISDN (two channels)



56-kbps modem




Both the setup and per-month charges for a 56-kbps modem are dramatically cheaper than the installation and per month changes for ISDN.




ISDN is widely available now, but it might take several months on the waiting list to get a connection. Similarly, to get a second phone line installed, one might need to wait for a few months as well. Finding and buying the equipment to install either ISDN or a 56-kbps modem is fairly easy as well – many mail order catalogs carry them. Finding ISP providers for ISDN is not very difficult either. However, due to the two conflicting 56-kbps modems put out by US Robotics and Rockwell, some ISPs could cho ose to support only one type of modem. Other ISPs might wait to see a standard 56-kbps modem come out before investing in one of the two technologies. Nevertheless, many ISPs have already taken sides, and are setting up test 56-kbps dial ups (America Onli ne being one of the major ISPs, and is currently using the US Robotics X2 modems).






Clearly, if one wants pure speed above all else, ISDN will be the obvious choice, for both the reasons of faster transmission speed, and for shorter latency on connect. However, 56-kbps modem technology is a cost-effective way to gain significant speed over existing V.34 modems, and is dramatically cheaper than ISDN. Additionally, the setup and installation of a 56-kbps modem is much quicker and simpler than an IDSN line. The added cost of an IDSN line seems to balance out the slower speed and easier s etup of a 56-kbps line. Two other issues seem to be the overall determining point: the slow uplink speed of 56-kbps modems, and the absence of a standard for 56-kbps modems.


If you are using your connection for purely WWW browsing, or for uplink-bandwidth non-intensive applications (telnet, email, etc.), a 56-kbps modem will increase your overall speed in general, at low cost. However, if you are running a Web server, or a ny other uplink-intensive applications, a 56-kbps modem will produce much worse performance than ISDN, and even worse than a V.34 modem


Personally, I find the lack of an industry standard and the existence of two competing and incompatible versions of 56-kbps modem technology to be the overarching deciding point of the ISDN versus 56-kbps modem argument. 56-kbps modem technology appear s to be a cost-effective alternative to ISDN, as long as one is not uplink-intensive. However, since there are two versions of the 56-kbps protocol, one has to find an ISP that has a compatible version. In addition, when the Open 56K Forum meets, and a st andard is ironed out, there is no guarantee that the standard will be backwards compatible to either of the exiting 56-kbps modems. I see 56-kbps modem technology as an emerging and developing technology, but will wait for a common 56-kbps modem standard to be adopted before investing in it.