No Hype 56k Site

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HOT First head-to-head review of X2 and K56flex modems in PC Week of June 2, 1997.

  • How 56K-bps Modems Stack Up.
    • Reports 56k modems show "as much as 76 percent increase in throughput" v. conventional v.34 modems in lab tests (not the real world).
    • Compares Supra, Hayes, Motorola, and US Robotics 56k models.
    • Notes 56k modems may connect at one speed, then fall back to a slower speed for data transmission.
    • Projects "real-world connections for these modems will probably be in the 42K-bps-to-46K-bps range."
    • Neither protocol seems better than the other.
  • Why 56K-bps Modems Aren't Always That Speedy.
    • Discusses limitations of phone lines, particularly residential ones, that may limit throughput to 33.6.
  • The High Cost of Handling High-Speed Traffic.
    • Discusses limitations of 56k for remote access.

HOT Infoworld reports E-Tech plans modems supporting both X2 and K56flex standards.

According to the hype, 56k modems are the greatest thing since Zip drives. In some respects, they are. They're ideal for surfing the world wide web, but perhaps less than ideal for other uses, as detailed below.

The problem begins with competing standards. There are two independent and mutually exclusive 56k protocols: one (X2) from US Robotics, the other (K56flex) from Lucent, Rockwell and others. X2 modems and K56flex modems can't talk to each other faster than 33.6kbps.

Don't even think about buying either type of 56k modem until you know which protocol your ISP will support. If possible, hold out until there is a genuine standard for 56k modems. And if you can't wait, make sure you buy a modem that can be readily upgraded to the final standard.


  1. Not all phone lines will support 56k modems.
  2. FCC says you can only run at 53k.
  3. Real world connection speeds are under 56k (but often better than 33.6).
  4. 56k speed is for downloads only; uploads limited to 33.6k.
  5. Upload speed is inversely proportional to download speed.
  6. 56k modems don't talk to each other at 56k.
  7. 56k may be impractical for remote access.
  8. Latency -- why some aspects of 56k are no faster than with your current modem.
  9. Data buffering and compression -- two more factors to slow things down.
  10. Good things about 56k modems.

The Hype

1 56k modems will not work everywhere. US Robotics has set up a BBS where you can test your line for X2 support by dialing 888-USR-X2-4U from a terminal program and logging in as LINE TEST. Perhaps some vendor will do the same with the K56flex standard, although the USR test should apply.

If your phone line fails the test, you cannot use a 56k modem.

2 In the United States, the FCC places a ceiling of 53k on your modem. While the modems can theoretically work at 56k, they are legally and practically constrained to operate 5% slower than this.

3 While a 56k modem may offer a download improvement compared with 28.8 and 33.6 modems, they are significantly more expensive today. In various reviews, I have read of typical download speeds in the 35-50k range. At the low end, it's barely better than 33.6; at the top end, it's very fast but your upload speed suffers grievously (see point 5). And, as with all protocols faster than 14.4, throughput is very much dependent on line conditions.

US Robotics' Mindspring Test shows 4.3% of connections at 49.2k or faster, 61.4% at 44-48k, the remaining 34.3% connecting between 29.3k and 42.6k. At the same time, remember that most 28.8k and 33.6k modems rarely, if ever, achieve their rated speed.

4 These modems don't offer similar performance in both directions. In fact, they can only achieve their best throughput when you are downloading files. Since this is what most of us do most of the time, that may not be a huge negative factor. The 56k modems are capable of uploading at 28.8 or 33.6kbps. And the receiver modem has to be connected to a digital line to surpass 33.6k performance.

5 To add insult to injury, the maximum total throughput (upload plus download) is limited to roughly 64kbps, just as it is on 33.6 modems. This is an asynchronous protocol. This means that a 48k download connection leaves only 16k of bandwidth for uploads. The faster your download, the slower your upload!

See benchmarks from Windows Source for more details. Add the best upload and download speeds for the barely compressible self-extracting archive in each test (33.3k, 37.3k, 48k, and 49.3k connections): they never add to more than 63.2kbps. These benchmarks show the X2 modem doesn't achieve upload speeds equal to a 28.8 modem at any of the tested upload speeds. This may or may not apply to K56flex.

If you are hosting a web site, this can be a strong argument against adding a 56k modem. Outside users would be limited to whatever bandwidth is left after you establish a connection with your ISP.

6 Not only are the X2 and K56flex standards incompatible with each other, they cannot talk to other modems of the same protocol at 56k either. Both protocols require a complementary modem on the receiving end, one that can upload at 56k and download at whatever bandwidth remains. This means you cannot connect to another computer at 56k unless that computer has a receiver modem. Even if you have a set of 56k modems, remember that you'll be running up to 53k in one direction and under 33.6k in the other. Finally, the receiver modem must have a digital connection to the phone company.

7 If you wanted to use a 56k modem for remote access, read point 6 again. No dice. First, the 56k connection requires a local phone connection with no more than one digital converter. Second, the required 56k receiver modems are not readily available to users. Although receiver modems are being made, manufacturers want to get these into the hands of ISPs first, dealers (local or mail order) are unlikely to stock them, and they are more expensive (US Robotics' sells for about $450). Third, the whole issue of download speed reducing upload speed (point 5) can make remote access with a 56k modem less efficient than with a 33.6 modem.

8 Some aspects of 56k modems are no faster than on the 33.6, 14.4, or even 2400bps modem you may already be using.

Although your modem can move lots of data in a second, there is a delay between your computer deciding to send data and the modem beginning to transmit that data to the receiver.

To understand this in depth, I recommend It's the Latency, Stupid by Stuart Cheshire. He contends that what we call modem speed would better be called capacity. A 14.4 modem has the capacity to transmit 14,400 bits per second; this does not mean it will always do so. In part, this is because of the overhead (latency) of modem handshaking ("I'm ready to transmit -- are you ready to receive? Yes? Okay, here comes the data."), which is rooted in protocols going back to the earliest data modems.

In real world tests, the following latencies are typical (most results from Cheshire):

In the language of seconds, an ethernet link is established in under 1/250 sec., cable modem in about 1/125 sec., ISDN in roughly 1/40 sec. But your modem regardless of rated speed takes 1/6 to 1/9 sec. to establish a connection. Even if your modem could send 53,000 bits per second (FCC mandated maximum), at least one-ninth of a second is wasted per file. Assuming a 53k connection, a 56k modem will process no more than 47,100 bits during the first second of transmission. If your browser is downloading three or four files concurrently (which is not unusual), even more time is spent establishing communication.

It is quite likely that manufacturers could reduce the latency of analog modems, but it might come at the expense of compatibility with older hardware. Unless that happens, there's nothing you can do about bad latency other than switch to ISDN, a cable modem, or ADSL (when it becomes available).

9 Another bottleneck is the way modems usually process data: one packet at a time. The modem waits until it receives a specified number of bits, then sends them. If you use compression, there is an added delay while the modem compresses or decompresses the data. This reduces throughput, since this data is sent in batches rather than as a continuous stream of bits.

Here's what happens. Let's say you're using an external 56k modem on a 115kbps serial port and packet size is 1000 bits. The modem must receive all this data before it will process them. This takes 9 ms. Now the modem transmits the data, which takes at least 19 ms (assuming 53k connection). Fortunately, most modems have large enough buffers to transmit one packet while receiving the next one. Still, to send 1000 bits of data via modem will take at least 138 ms (latency plus input plus transmission). Throughput is 7.2kbps.

This assumes no compression. If your modem compresses data at 2:1, it will take 17 ms to receive 2000 bits (enough data to compress into 1000 bits). Add 1 millisecond for compression. (Cheshire notes that the compression/decompression chips in modems are far less powerful than the CPUs in today's computers. Thus, it would make more sense to compress the data before sending it to your modem and turn off the modem's compression.) Add 19ms to send the data. You've sent 2000 bits of data in perhaps 147 ms. Throughput is 13.6kbps.

Third scenario: your computer compresses the data at 2:1. This should take far less than 1ms since today's CPUs are so powerful. Add 9 ms to send the compressed data to your modem 19 ms to transmit, and 110ms for latency, and you've send 2000 bits of data in 139 ms. Final throughput is 14.4kbps.

Let's assume a typical web page is 8KB in size and contains 8 small graphics. HTML is highly compressible, since it is plain ASCII text.

Small art files on the web average 1KB (8000 bits) in size. Most are GIFs or JPEGs, which are already compressed. Except in rare instances, the compression routines in your modem will do nothing to speed their transmission and may actually slow it. The following table shows transmission time in seconds and throughput in kbps for 8KB of source code, for a single 1KB graphic, and to download an entire page with 8 images. Numbers assume a 110 ms latency and 115kbps serial connection to the modem. (56k assumes 53kbps connection, which tests have shown is unlikely to occur in the real world.)

Real world results will vary with line condition and serial port speed. Many older computers have serial ports that top out between 19.2 and 57.6kbps. Serial port should be set to at least twice modem speed (115k for a 56k modem). More on this topic on the web.



27.1 / 2.4k
4.6 / 14.0k
2.4 / 27.3k
2.1 / 31.6k
1.4 / 48.3k


3.5 / 2.3k
0.7 / 11.9k
0.4 / 20.3k
0.4 / 22.6k
0.3 / 29.9k

Page + GIFs

55.4 / 2.3k
10.1 / 12.8k
6.0 / 23.1k
5.0 / 26.2k
3.5 / 36.7k

The following table assumes 2:1 compression of HTML source code, but no compression (or overhead) for GIFs.



13.6 / 4.8k
2.4 / 27.3k
1.3 / 51.8k
1.1 / 59.4k
0.7 / 87.8k

Page + GIFs

41.9 / 3.1k
7.9 / 16.4k
4.5 / 28.9k
4.0 / 32.4k
2.9 / 44.3k

The following table assumes 3:1 compression of HTML source code, but no compression (or overhead) for GIFs. Note that for the 56k modem, the assumed 115k serial port becomes a bottleneck. This is barely faster than 2:1 compression, but only because 115k is a bit more than twice the maximum 53k speed of the modem. A faster serial port or internal modem would overcome this throughput restriction.



9.1 / 7.1k
1.6 / 39.8k
0.9 / 73.9k
0.8 / 84.2k
0.7 / 94.9k

Page + GIFs

37.4 / 3.5k
7.1 / 18.2k
4.1 / 31.6k
3.7 / 35.3k
2.9 / 45.1k

Some interesting facts emerge.

10 Good things about 56k modems.

  1. They may offer better performance than your current modem over the same line. This is especially true if you are running 14.4 or slower.
  2. They are capable of downloading faster than 33.6kbps if you use the same kind of 56k modem as your ISP.
  3. They are reasonably priced for cutting edge technology.
  4. This may be the fastest modem technology that won't require a dedicated phone line (like ISDN), multiple phone lines (just being announced), or a special receiver at the phone company's switching station (ADSL). Then again, they said the same thing about 9600 and 33.6.
  5. Most can be upgraded to the 56k standard when it is established.


  1. If you have a 14.4 or slower modem, you're overdue to upgrade. Buying a 56k modem makes sense. Be sure you buy an upgradable one using the protocol your ISP supports. If your ISP doesn't plan to support 56k modems until a standard emerges, you still have a good 33.6k modem until then. (In fact, you'll probably have better upload speeds when connecting to 28.8 and 33.6 modems.)
  2. If you have a 28.8 or 33.6 modem, I recommend against buying a 56k modem at this time. Wait until the two factions have established a standard. While you wait, prices will probably drop.
  3. If you need to buy a new modem, buy an upgradable 56k one using the protocol supported by your ISP.

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This page and site ©1997 by Daniel Knight. Last updated June 6, 1997.