56K modems: X2, v.90, K56Flex

What are X2, K56Flex and V.90? How do they differ?

When telecommunications engineers figured out that near 56K speeds could be achieved on standard phone lines, they began working on various approaches. Rockwell Semiconductor Systems and Lucent Technologies developed a technology called K56Flex. U.S. Robotics developed their own technology and called it X-2 (pronounced X-TWO, not TIMES-TWO). While these companies developed their protocols for the same purpose (56K Download speeds), they ended up with incompatible protocols. To view U. S. Robotics "Understanding the Issues / Troubleshooting Problems in V.90 and x2" click here [Note: You will need Adobe Acrobat Reader to view it].

In response to consumer demand, the International Telecommunications Union (ITU) created a committee to draw up a standard for a unified "56K" speed protocol: V.90. V.90 encompasses the best of both worlds and all modem manufacturers have been required by market pressures to adopt it. All "56K" modems sold today are V.90 compliant. Internet Partners is a V.90 and X2 service provider. If you are unsure whether your older "56K" modem can be upgraded to V.90, contact the manufacturer. In most cases, your modem can easily be upgraded with a quick and painless "flash" software upgrade.

What can X2 / V.90 technology do for me?

Prior to 1998 and the emergence of "56K" modems, most telephony engineers believed that a 33.6Kbps connection was the limit that standard analog lines could achieve. This is still true if the phone line is mainly analog switched. V.90 exploits the introduction of purely digital switching in equipment at the telephone companies' central switching office. Instead of seeing a maximum connect speed of 33.6Kbps, you may now see an "transmit" speed of 33.6Kbps, and a "receive" speed ranging from 33.6Kbps up to 53Kbps. Browsing the Internet, downloading files, and checking mail takes half as long.

*In theory 56K products are capable of 56Kbps downloads; however, due to FCC rules which restrict power output of our modems, current download speeds are limited to 53Kbps. Actual speeds may vary depending on line conditions. An 56K-capable analog phone line and 56K-capable service provider are necessary for these high speed downloads.

What about uploads?

56K capable modems (X-2 or K56Flex) download data (receive data) at up to 56Kbps. They upload data (send data) at up to 33.6Kbps. This restriction is because the ISP "transmits" the call digitally to the telephone company, while the end-user can only "transmit" the call over analog lines.

Why does the technology not go 56K in both directions?

X2 and K56Flex Technology are asymmetric. This means that it sends and receives at different speeds to take advantage of the digital connections most ISPs (Internet service providers) have to the public phone network. Note that V.34 is also asymmetric; it can increase speed in one or the other direction, as line conditions allow. Internet Partners uses only digital ISDN PRI telephone lines.

Don't phone lines max out at about 35K? (Isn't the theoretical limit for data transmission over phone lines somewhere around 35K?) (Shannon's Law)

Yes. But this is true only if one assumes analog connections to the phone network on both ends. The actual bandwidth of a pure digital channel like ISDN is 64K. By taking advantage of the digital connections (for example, T1) that most ISPs have to the phone network, X2 provides download connect rates of up to 56K over regular phone lines.

How does it do that?

56K Technology takes advantage of a special network configuration found when an analog client connects to a digitally connected server. By eliminating one of the the analog-to-digital conversions in the downstream path, 56K modems can use the practical maximum amount of the available 64 Kbps bandwidth. This technique makes modems with 56K-capability fundamentally different from other high-speed modems. In fact, downloads could more accurately be described as encoded, rather than modulated.

Does this technology work over regular phone lines?

Yes, assuming that your telephone line only has a single analog-to-digital conversion in the "download" path. (ie: from the telephone company central office to you) Your 56K connection can provide downstream connect rates of up to 56K over regular phone lines. However, due to FCC rules which restrict the power output of our modems, current download speeds are limited to 53 Kbps.

What if I don't get 28.8 connections on my current phone line? Will this still work?

Possibly. It generally works on the vast majority of phone lines in our area at download speeds from 32K up to 53Kbps even if your usual V.34 connect speed is below 28.8K. While your results may be different, download times of files from Intnernet Partners FTP server took half as long with a 56K connection as the same file did while connected at 33.6K. Further testing also proved that using an 56K-capable modem to dial into our equipment offered a tremendous improvement over the standard 33.6 capable dial-in.

Does 56K work through PBXes?

Only if the PBX is a modern PBX and is connected to the telephone company through an ISDN PRI circuit, and is certified by it's manufacturer as V.90 compliant. Most phone systems owned by small businesses would likely not fall into this catagory. However, if your in the market for a new PBX then you should insist on 56K compliance.

Can two 56K-capable modems connect to each other at 56K?

No. They require a 56K-capable server digitally connected to the phone network. Desktop modems connected to regular phone lines will not be able to act as 56K servers. However, ISDN Terminal Adapters might be able to. For example, the US Robotics Courier I-Modem can accept V.90 calls from regular modems.

Can I upgrade my slower modem to support 56K Technology?

Maybe. Most newer mid to high-quality modems are upgradeable. Check with the manufacturer of your modem.

Are there any distance limitations regarding 56K?

While a person's distance from the telephone company's central office (CO) is one of the variables which could affect the connect speed, other variables play a role as well. In addition, there is virtually no way for the subscriber to obtain loop length or other data regarding their POTS line. While more severe loop impediments would result in a connection below 56 Kbps, you would still typically connect at a high rate (faster than V.34 speeds).

Since maximum speeds are seldom attained, what realistic speed improvements should I expect to see?

While the answer to this question is dependent upon a case-by-case analysis, you will typically see speed improvements using 56K Technology in comparison to traditional analog technologies. In fact, trials involving thousands of calls (conducted by U.S. Robotics and Lucent Technologies) in a multitude of regions have shown that a preponderance of those calls were able to achieve the faster 56K speeds.

For what it's worth, our employees have regularly observed connection speeds of 48.8Kbps from South East Portland's Hawthorne district with an external US Robotics Courier V.Everything modem.

X2 and K56Flex are different protocols than today's analog modulation schemes. These latter schemes assume an analog connection end-to-end. In contrast, this technology assumes a primarily digital connection, with an analog leg needed only to connect the subscriber modem. For that reason, customers who today cannot even achieve V.34 speeds will more than likely be able to connect via the higher 56K speeds. In addition, there are regions where the serving telephone company has declared that their subscribers will not even be able to reach V.34 speeds. Nevertheless, subscribers in these regions will typically achieve the higher 56K speeds because the telephone company has assumed in their statement an analog end-to-end connection employing the older analog modem standards.

2000 Internet Partners, Inc.
1800 NW 167th Place Suite 160 - Beaverton, Oregon 97006-8132
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