V.90  Modem Technology

								

																																March 12, 1999

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Table of Contents

V.90 Modem Technology............................................................................4

	Background......................................................................................	4
	How V.90 Exploits The Telephone Network..................................	5
	Matching V.90 Modem Signals to Network Characteristics..........8
	V.90 Performance Expectations......................................................9
V.90 Information on the Web....................................................................10

								
V.90 Modem Technology
Background

Prior to announcements of 56K modem activities in September, 1996, by Rockwell Semiconductor Systems and US Robotics, V.34 modem technology with data rates up to 33.6 Kbps in each direction was widely considered to be at the theoretical data rate limit for public telephone network modem connections.  V.34 modems viewed the telephone network as purely analog, even though significant segments of each connection made use of digital transmission and switching equipment.   The key to achieving a significant increase in data rate with 56K modems is an awareness by the modem of the digital nature of the network. With that awareness, a 56K modem is able to align its transmission signals with specific network characteristics in order to minimize the distortion to those signals.   This directly corresponds to a lower noise level in the modem receiver, enabling the receiver to operate at higher data rates.

As 56K technology began to develop, two distinctly different implementations were introduced to the marketplace.   US Robotics developed x2 technology, while Rockwell Semiconductor Systems was joined by Lucent Technologies to develop K56flex technology.   IBM obtained a license to the x2 technology, and developed its own implementation of x2  technology in the Thinkpad ACP modem.   Numerous 56K modem products were developed using each of these two proprietary technologies, but customer acceptance was limited due to concerns about inter-operability.    The problem was that a 56K modem could only take advantage of the higher speeds offered by 56K technology if it was connected to another modem which implemented the same technology.   If an individual subscriber bought a K56flex modem for his pc, but his Internet service provider used US Robotics x2 modems, connections would be limited to V.34 speeds.   The same was true for an individual with an x2 modem dialing into a service provider based on K56flex technology.   Fortunately, the industry recognized early on, that competing proprietary technologies would severely restrict market growth.   The ITU (International Telecommunications Union), which has established recognized worldwide standards for previous modem technology (e.g. V.32, V.32 bis, V.34), provided a forum for converging the two proprietary technologies into a unified modem standard which was accepted by all parties involved.    This standard was officially issued as V.90 by the ITU in September of 1998.     

How V.90 Exploits The Telephone Network 

Figure 1 illustrates a typical V.34 modem connection.   As far as the modems are concerned, the connection between the two modems consists of a local loop of standard telephone cable at each end, interconnected by the telephone network.   The telephone network is often viewed as a cloud, since most subscribers aren’t aware of the details of how a call is handled within the network.  The only network cloud characteristics of interest to the modems are typically the quality of the cable terminations (which determine signal echo components) and the overall delay through the cloud.

Even as the industry moved toward digital (T-1 or ISDN) connections between the network and service provider modems, thus eliminating one of two local loop cables,  the modem’s view of the connection was unchanged.   Even though V.34 modems could not take full advantage of it, the migration toward digital connections to server modems was building the infrastructure required to support future 56K modems.   Figure 2 illustrates a typical modem connection through the telephone network to a digitally-attached service provider modem. 

To understand how a V.90 modem achieves a performance advantage over V.34 modem, one must first look at the nature of the digital portion of the telephone network.   Within the network, all analog signals (whether voice or modem) must be digitized.   This process involves taking amplitude samples of the analog signals at a rate of 8000 samples per second.   The sampling rate is based on the range of frequencies which the telephone network must faithfully transport, and is a universal timing rate throughout the network.   While analog signals consist of a continuous range of signal amplitudes, the telephone network allocates only 8 bits to represent each amplitude sample, limiting the digital result to one of 256 signal levels.   The difference between the analog signal level at the time it is sampled and the closest available digital level can essentially be represented as noise added to the desired signal each time the network converts an analog signal into the corresponding digital representation.   This quantization noise is one of the limiting factors in V.34 modem performance.    

Looking again at Figure 2, and following the downstream path of a modem signal starting at the service provider modem, we see that the original modem signal is generated in a digital format,  and passed through the network without converting to an analog signal until reaching the connection to the remote subscriber’s local loop cable.   In fact, the network never passes this signal through the digitizing process which is responsible for the quantization noise previously described.   The conversion from digital format to analog signal which does take place at the network edge does not introduce quantization noise since the signal levels generated  precisely correspond to the digital codes input to the conversion process.    In fact, the only place where the downstream signal is subjected to quantization noise is at the circuit within the subscriber’s modem where the analog signal is converted to a digital format in order for the modem to process the signal.    Fortunately, today’s modems have many more levels (as many as 65,536 discrete levels in some implementations) to choose from when representing received signals in a digital format, resulting in a quantization noise level which is typically small enough that it is not a limiting factor in modem performance.   One additional issue must be dealt with to enable a V.90 modem.   The modem transmitter at the service provider must process the signal to be transmitted with full knowledge of the limitations of the 8 bit PCM representation in the network.   In other words, the transmitter can not simply generate a transmit signal using 16 bit or 32 bit resolution, and then convert the results to the proper 8 bit PCM format.   This would generate the same type of quantization noise a V.34 modem signal experiences.   Instead, the V.90 modem uses the 8 bit PCM format as an integral part of the signal generation to insure that nothing is lost due to PCM formats in the network.    

Referring again to Figure 2,  but following the upstream path starting at the subscriber’s modem toward the service provider modem, we see that the network does impose a conversion from the analog signal on the local loop into the digital format used within the network.   As described previously, this results in significant quantization noise in the upstream signal, and is a principal reason that the V.90 standard limits upstream transmission to V.34 speeds.   In fact, the basic technology for upstream transmission is the same as that used for V.34 modems (a choice which facilitated implementation of 56K modems ). 

The foregoing discussion leads to another critical issue relating to V.90 performance.   Recalling that the key feature of the connection path which enabled V.90 performance was the absence of an analog to digital conversion on the downstream signal within the network, the question is whether or not all modem users will be able to use such a connection.   Unfortunately, the answer is no.   Many telephone subscribers are connected to the network via “Subscriber Loop Carrier Circuit” systems, sometimes referred to as “SLIC” connections.   These systems convert the analog signal into the digital format out in the subscriber’s neighborhood, and route multiple connections via digital carrier signals to the telephone central office.   Unfortunately, at that point, the individual signals are reconverted to analog, to be connected to standard analog termination equipment in the central office switch.   This extra conversion results in the additive quantization noise which prevents V.90 operation and forces the modems to fall back to V.34 mode in order to connect successfully.   This type of connection is illustrated in Figure 3, and is sometimes referred to as digital discontinuity, because the digital signal path is not continuous between the subscriber’s local loop and the service provider modem.    

This scenario is also commonly experienced by the traveling modem user, since many hotels use digital PBX systems for their internal telephones, some of which connect to the Public Telephone Network via analog trunk lines as shown in Figure 4.    The net result on the signal is the same as with the SLIC connection, an extra conversion from digital to analog and back to digital which prevents V.90 from working.   In these cases, the modem will detect the digital discontinuity, and connect using V.34 technology.

Matching V.90 Modem Signals to Network Characteristics

In addition to the basic consideration of the 8 bit PCM format used to represent analog signals in the network, a  V.90 modem must deal with other issues affecting the transport of these PCM samples through the network.   These issues, commonly referred to as Digital Impairments, involve various rules for mapping one set of PCM levels or codes into another set.   These rules may be categorized into two distinct types of mapping: RBS and PAD.

RBS is an acronym for Robbed Bit Signaling which is a term describing a process by which the phone company mixes call control signaling information with data over the digital connection.  This process is used frequently in the United States and results in data transfer limitations which have to be overcome by the modem designer.   This signaling information is embedded in the least-significant bit of the 8 bit PCM samples for one out of every six samples in the sequence of samples representing the signal within the network, hence the least significant bit has been “robbed” from the modem signal to be used for call control signaling in the network.   Each T-1 link in the network can inject RBS independently of other links, and different implementations of this process can result in different translations of the modem signals.   The TIA 3857 specification suggests that up to 3 of these RBS impairments may be encountered in the network. However, actual experience has shown some cases with additional RBS impairments, as well as nonstandard RBS mappings not described in the specification.    Modem users in Europe and other A-law network countries do not have to worry about RBS impairments, and may experience somewhat higher connection data rates.
  
PADs are signal power attenuations introduced into the phone system by the telephone company via a digital mapping from one set of m-law codes to another.  PADs are introduced to protect telephone company equipment from signals coming in that might be powerful enough to damage circuitry.  There are at least 20 different types of digital PAD mappings in the US network which a modem must accommodate.  There are also cases where several of these digital PADs occur in tandem.   PADs present a different challenge to modem designers than does RBS. 

A V.90 subscriber modem must learn the nature of these digital impairments during initial startup of each connection.  Then, the subscriber’s modem and the service provider modem must agree to using a common subset of the 256 PCM levels or code points which will not be affected adversely by the digital impairments.   In some cases, the combination of impairments will limit the achievable data rate for a connection.

V.90 Performance

Modem data rates achievable over the Public Telephone Network vary widely based on the location of the caller, the location being called, the path through the network for the specific call, the modem product being used by the individual PC owner, and the modem product deployed at the service provider location.  This results from the fact that the Public Network was designed to mainly transmit voice energy.  

When data calls are established, and data is transmitted across the network, the signal is very sensitive to the electrical characteristics of the telephone line, the length of this line to the central office, and the telephone company equipment imperfections.   Attenuation, phase distortion, frequency offsets, random noise, and nonlinear distortions are just some of the analog impairments which affect the quality of modem signals.   Any noise in the phone line can result in diminished connection speeds and data throughput.   This noise may be introduced by such common sources as power lines (inside or outside your home / office), other equipment connected to the same phone line (cordless phones, answering machines, phone line surge protectors, etc.) and atmospheric conditions.   These analog impairments can adversely affect the modem signals by reducing the signal to noise ratio.   You may be able to make some improvement in connection speed by using another phone extension in your home / office or by disconnecting some of the equipment you may have attached to the phone line.   Some connections experience digital discontinuity, and are forced to fall back to V.34 speeds.   In addition to these analog impairments, PADs and RBS digital impairments mentioned previously may adversely affect data rate capabilities.

There are a tremendous number of these combinations of phone line and network impairment conditions, each affecting the data connection in a different manner.  Modem products will adapt to these combinations differently because modem designers approach the process with different objectives.   Because of these differing design objectives, one type of modem might out perform all other types on a specific set of line conditions, but that same modem might not do as well as other modems on other line conditions.   The ACP modem has been optimized for performance over a wide variety of line conditions and server modem types because of the mobile nature of its typical users.   

When considering the performance of a modem, it is also important to remember that the line speed reported by the modem and/or the communications software is not always indicative of how satisfactory the connection will "feel" to the user.  The true measure of connection usability is the amount of data transferred per second for the call.  This cannot be measured strictly by the reported connect speed.  If an unreasonably high connection speed is established for current line conditions, then many errors may occur which will cause the modem to continuously retransmit data.  This will, of course, make the connection "feel" slower than one having very few errors at a lower connect speed.

IBM has designed  the Thinkpad ACP modem to be optimized for the most common line conditions identified by the Telecommunications Industry Association network model.  There also has been much work aimed at handling line conditions beyond those covered in the TIA network model.  The Thinkpad ACP modem software currently attempts to compute the highest line speed attainable on each connection that will also minimize the number of data transfer errors.

V.90 Information on the Web

	“56K Basics: How it works”
		http://www.56k.com/basics/basics.shtml

	“56K Basics: 3 things you need to use 56K”
		http://www.56k.com/basics/3things.shtml

	“56K Disadvantages: 53K limit in the U.S.”
		http://www.56k.com/cons/53k.shtml

	“What is V.90 Technology”
		http://www.v90.com/whatis.htm

	“How V.90 does its Magic”
		http://www.v90.com/v90magic.htm

	“Agreement Reached on 56K Modem Standard (ITU Press Release)”
		http://www.itu.int/newsroom/press/releases/1998/98-04.html

	“56K Modem Standard Continues to Break New Ground (ITU Press Release)”
		http://www.itu.int/newsroom/press/releases/1998/NP-3.html

	“Why you won’t get 56K”
http://www.computers.com/reviews/comparative/substory/0,29,0-14-257672-667659-1,00.html?sr.co.cri.inav.crsu667659

	“Disadvantages of 56K: Only one A/D conversion allowed”
		http://www.56k.com/cons/onead.shtml

	“Disadvantages of 56K:  Won’t work with some PBXs”
		http://www.56k.com/cons/pbx.shtml

	“Fastconnection: No Room Service After 33.6Kbps”
		http://www.zdnet.com/pcmag/pclabs/nettools/1714/sb1.html