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============================================================================= * Forwarded by Chad Simmons (1:142/550.4) * Area : REMEMBER * From : Craig Ford, 1:106/2001 (25-Sep-96 22:57:47) * To : All * Subj : 56Kbps Modems ============================================================================= * Copied from: COMM Hello All! This is an ASCI rendering of a white paper on the new 56Kbps modems announced by Rockwell found at http://www.nb.rockwell.com/nr/modemsys/hispeed.html. Hopefully it will answer many of the questions that have been raised about the technology and its application. I have no information on availability or pricing of the devices. ============================================================================== 56 Kbps Communications Across the PSTN A new era in dial up communications - The Communications Path - Dealing with the communications path - Problems in the network - Shannon's limit - The upstream channel - Standardization - Connection limitations - Summary - Footnotes This paper describes the basics of the 56 Kbps modem technology recently announced by Rockwell Semiconductor Systems. The basic concept behind this communications technology is that the public switched telephone network (PSTN) is increasingly a digital network and not an analog network. Existing analog modems, such as V.34, view the PSTN as an analog system, even though the signals are digitized for communications throughout most of the network. Figure 1: The components of a modem connection in a digital network [-------] [ ] [ ] [-------] | MODEM | | LINEAR | 2-WIRE TWISTED | m-LAW | 64K 64K | MODEM | | |---| |----------------| |-----[delay]-----| | | DSP | | CODEC | PAIR | CODEC | | DSP | [-------] [ ] [ ] [-------] Additionally, more and more, central site modems [1] are connected to the PSTN via digital connections (T1 in the Untied States and E1 in Europe [2] ) and do not utilize a codec [3] . The modem interprets this digital stream as the representation of the modem's analog signal. Rockwell's announced 56 Kbps technology looks at the PSTN as a digital network which just happens to have an impaired section in the communications path. That impaired section is, of course, the copper wire connection between the telephone central office and the user's home, usually referred to as the analog local loop. THE COMMUNICATIONS PATH When a user at his/her home calls a central site T1 connected modem, the network situation can be represented by Figure 1, below. The user is connected to the network via a two wire twisted pair [4] copper line. At the central office, this twisted pair line is terminated by a special type of transformer, called a hybrid, which converts from two wire to four wire [5] . This four wire connection is then connected to a codec. In the United States, this codec is called a mu-law codec, named for the technique used to space the sample points (which are also called quantization levels or quantization points). In Europe, a different technique is used for spacing these points, called A-law. The mu-law codec is, in turn, connected to the digital network. The full duple x digital data, to and from the codec, is switched through the network to the central site modem DSP, allowing the central site modem DSP to communicate digitally with the mu-law codec. The mu-law codec has 255 non-uniformly spaced quantization levels which are closer together for small signal values and spread farther apart for large signal values. The modem DSP at the central site can generate any quantization point voltage on the analog line simply by sending the appropriate eight bit sample to the mu-law codec. Since the PCM codec sampling rate is 8-KHz, these voltage levels will be generated 8,000 times per second. For the modem at the user's home, the major challenge is to be able to determine which quantization point was generated by the eight bits sent by the central site modem, and to do it 8,000 times per second. To do this, the modem in the home must synchronize its sample clock to the network codec's 8-KHz clock. Clock recovery is done in existing analog modems and equivalent techniques are used to recover the network clock in this new application. Now let's look at how data is sent. Assume that the modem DSP at the Internet service provider can send only two different sample values to the mu-law codec, say the values representing the two outermost points. The two voltage levels on the analog line which result from sending these sample values can be used to represent two binary values (0 and 1). These sample values will be sent 8,000 times per second, the network clock rate. Further assume that the modem in the home can discriminate between the two voltages, 8,000 times per second. In this case, the central site modem can send data to the user at 8,000 bits per second (bps). Now let's assume that the modem DSP at the Internet service provider can send four different sample values, representing four different voltage levels. Since there will now be four different voltage levels on the analog line, we can assign two bits to each voltage level (00, 01, 10, and 11). Again, sample values will be sent 8,000 times per second. If the modem in the home can discriminate between these four different voltage levels, 8,000 times per second, then 16,000 bps can be transmitted. Table 1, following, shows how the data rate increases as more voltage levels can be transmitted and discriminated. Number of Bits per Line Rate voltage levels level (bps) ============== ======== ========== 2 1 8,000 4 2 16,000 8 3 24,000 16 4 32,000 32 5 40,000 64 6 48,000 128 7 56,000 256 8 64,000 Table 1: The relationship between the number of voltage levels on the analog l ine, the number of bits communicated per voltage level and the resulting line rate. DEALING WITH THE COMMUNICATIONS PATH To make this technology work over the analog loop, the modem must "equalize" the line. But this is easier said than done. Some of the problems encountered in equalizing the loop are caused by the central office codecs, which are designed for voice and not data. Also, the transformer hybrids connecting the transmit and receive paths to the loop introduce spectral nulls at DC. Some of the solutions developed by RSS engineers for these problems are being submitted as patent applications. Once these issues are dealt with, the quantization levels on the analog line are simply treated as symbols [6] in modem symbol space, in exactly the same way as combinations of amplitude an d phase are treated as symbols in an analog modem QAM space [7] . And once you're in symbol space, you can use many of the techniques already developed for traditional analog modems to improve the modem receiver's ability to discriminate between quantization levels, thereby improving communications accuracy and speed. For example, new trellis8 codes, which recognize the non-uniform spacing of the symbols, can be created and applied to allow better discrimination between the quantization levels, especially those near the origin. While not all of the existing modem coding techniques can be applied to this new communications technology, a great many can. PROBLEMS IN THE NETWORK If everything could be done perfectly, this technique would allow communications at 64 Kbps (8 bits per sample times 8,000 samples per second). However, there are a number of problems which prevent operation at this speed. First of all, in the United States, the link between the network and the central site modems can be a T1 line utilizing "robbed bit signaling" for call progress indication. Robbed bit signaling "steals" the low order sample bit in two of the samples per frame to indicate the status of an incoming (or outgoing) call. The use of this bit by the network means that the central site modem cannot always access 8 bits per sample and this reduces the achievable data rate. Additionally, the codecs in the network are not perfect. Many have a DC offset problem which limits the ability to utilize the quantization points near the origin. There may also be a significant amount of nonlinear distortion in the circuit. This further limits the achievable data rate. Finally, there is the problem of accurately determining the quantization point which was "sent" by the central site modem DSP. Since the quantization points are closer together near the origin, it is more difficult to discriminate between these points. Depending upon the channel, more or less of these points may have to be given up. Taken together, these limitations reduce the achievable data rate to about 56,0 00 bps. SHANNON'S LIMIT Shannon's limit is determined by a number of parameters but for ordinary telephone channels it is, to a large degree, determined by the channel's signal to noise ratio. Conventional modems treat the telephone network as a pure analog channel, so the analog signals generated by these modems see a PCM codec quantization distortion of about 36 dB. This distortion represents a significant impairment as data rates are increased and limits the channel to about 35 Kbps. The effects of PCM quantization distortion are avoided by using a form of amplitude modulation in which the amplitude levels are chosen to be the quantization levels of the PCM codec in the central office. The user's data is encoded into this quantization-level symbol alphabet and transmitted across the local loop in digital form. The problem then is to equalize the local loop such that the signal samples seen by the user's modem are equivalent to the quantization levels at the central office codec. This equalization problem is significantly reduced by limiting the data transmission to a single local loop. With this approach of "hooking" into the middle of the channel and avoiding one of the encoding or decoding PCM steps, the PCM quantization distortion can be treated as a deterministic impairment, and not as a random noise source, which is the case for the conventional analog modem. This raises the theoretical Shannon's limit very close to 64 Kbps, depending upon the local loop. THE UPSTREAM CHANNEL It is more difficult to equalize the upstream channel, and therefore more difficult to achieve the same high data rates as are achieved in the downstream channel. However, for Internet access, the data rate in the upstream direction is less important than downstream, since the upstream channel transmits mostly "key strokes and mouse clicks". At present, a data rate of around 30 Kbps can be attained in the upstream direction, but research continues toward increasing the rate. STANDARDIZATION Like any dial modem technology, this new technology will have the greatest value to users if it is standardized, so that products from different vendors can interoperate. RSS will be working with partners to submit the specifications for this technology to appropriate standards groups in an effort to gain international acceptance. The standards bodies are attended by some of the best minds in the modem industry so improvements should be expected by the time the technology achieves standardization. This technology provides so much value to bit starved Internet users that we expect it to be rapidly addressed by the standards bodies, especially ITU Study Group 14 which achieved the V.34 standard, and/or ANSI TR30. CONNECTION LIMITATIONS For this technology to operate, several things are required: The modems on both ends of the link must implement this new technology. During startup, the modems "identify" themselves and their capabilities to each other. Only if both modems have this capability do they try to establish a 56 Kbps connection. The central site modem pool must have a digital connection to the network, such as with a T1 or E1 line (which may or may not be ISDN). There must be no conversions of the digital signal within the network. Any conversions will prevent the modem DSP in the central site from generating the proper voltage levels on the analog line which will prevent this technique from operating. Examples of digital conversions are: (1) mu-law to A-law or vice versa (this means users will not be able to use this technology for communications between the US and Europe), (2) conversion to ADPCM, such as occurs in transatlantic submarine cables, (3) conversion to analog and back to digital somewhere along the link, or (4) other signal conversions such as ATT's TrueVoice [9] . To check if a fully digital path exists, with no conversions, the modems send a "probing" signal between themselves. If conversions are detected, the connection is established at V.34 rates. These limitations are not serious, especially for Internet access which generally involves a local telephone call. SUMMARY The 56 Kbps technique announced by Rockwell Semiconductor Systems is achieved by viewing the public switched telephone network as a digital network instead of an analog network. Overcoming the limitations of the analog loop at the customer site is not easy but can be accomplished through the use of standard modem techniques. It is easier to overcome these limitations in the "downstream" direction, resulting in an asymmetrical modem technology with higher data rates from the service provider to the user than in the return direction. The theoretical Shannon's limit for this technique is close to 64 Kbps, depending upon the local loop. Rockwell will work with partners to submit this specification to the appropriate standards bodies so that an interoperability specification can be ratified and published. This new communication technology announced by Rockwell Semiconductor Systems promises to provide a new era in dial up communications capabilities, especially for bit starved Internet power users, and another life extension for the "analog" modem. Rockwell Semiconductor Systems is the fastest growing business segment of Rockwell. Based in Newport Beach, Calif., it comprises the Multimedia Communications Division and the Wireless Communications Division. The Multimedia Communications Division is the world leader in facsimile and PC modem devices for personal communications electronics. The Wireless -!- timEd/2 1.10+ - Origin: Home of the Fidonet COMM Echo * 713-458-0237 * (1:106/2001) ============================================================================= ~NAME All Hi All , hope you are having a nice day Chad Simmons <chads@unix.collider.com> Written on Sun 29 Sep 1996 at 00:36:21. ... BEWARE - Tagline Thief in this echo --- Terminate 4.00/Pro * Origin: Chads Point via Terminate 4.0 (1:142/550.4) ─────────────── FIDO MESSAGE AREA==> TOPIC: 105 HIGH SPEED MODEM Ref: DDW00098Date: 09/29/96 From: CHAD SIMMONS Time: 12:36am \/To: ALL (Read 4 times) Subj: 2 56Kbps Modems ============================================================================= * Forwarded by Chad Simmons (1:142/550.4) * Area : REMEMBER * From : Craig Ford, 1:142/550 (25-Sep-96 22:57:47) * To : All * Subj : 2 56Kbps Modems ============================================================================= * Copied from: COMM Communications Division offers total system solutions for advanced cordless telephony and global positioning system (GPS) receiver engines and is developing products and technologies to address the Personal Communications Services (PCS) and wireless packet data markets. FOOTNOTES 1. Central site modems are those installed at a service provider, such as an Internet service provider, or at a corporation to allow many simultaneous connections for Remote LAN access. They are generally manufactured as cards containing many modems which plug into the device which provides the appropriate access. 2. A T1 line is a digital service provided by the telephone company to provision the equivalent of 24 individual voice lines. It operates at 1.544 Mbps. An E1 line is the European equivalent and provisions the equivalent of 30 individual voice lines. It operates at 2.048 Mbps. 3. Coder/decoder. The device which sits between the digital portion of the network and the analog local loop and converts between analog and digital. 4. The physical connection between the central office and the home is two individual copper wires of 24 or 26 gauge twisted about each other to minimize crosstalk. It's length depends upon the distance from the home to the central office but is normally less than 18,000 feet. 5. The network carries the two sides of a voice call in two separate channels. However, the connection to the home is only two wires. The hybrid converts between this separate channel system, referred to as four wire, and the two wires serving the home. 6. A "symbol" is an information carrying token. In this 56 Kbps technology, a symbol is a voltage level. In ordinary modem technology, a symbol is a combination of amplitude and phase. The term "symbol" was adopted after the original term, "baud", became corrupted in common usage. 7. The symbols in QAM space are created by simultaneously modulating a carrier in amplitude and phase 8. A trellis code is a technique to improve the modem receiver's ability to discriminate between two adjacent symbols. 9. However, TrueVoice can be disabled using the same techniques as are used to disable echo suppressors. Copyright (R) 1996 Rockwell International, all rights reserved ============================================================================ Regards... Craig aka: cford@ix.netcom.com