Q. 1. What is a 56k leased line?
Q. 2. What is a 56k modem?
Q. 3. How does the speed of a 56k leased line differ from the speed of a 56k modem connection?
Q. 4. How does the quality of a 56k leased line differ from the quality of a 56k modem connection?
Q. 5. What accounts for the higher cost of a 56k. leased line?
Q. 6. A DSL line is much higher speed. Why should I even consider a 56k leased line?
Q. 1. What is a 56k leased line?
A 56k leased line is a type of digital leased line, available from your local phone company and similar to a T1 line, which is capable of carrying digital data between two remote locations at a fixed transmit and receive speed of 56,000 bps. The telco guarantees the quality and speed of the line. Like analog leased lines and T1 lines, it connects a customer site to a single remote destination (unlike a typical, "circuit switched" phone line, which can connect to any one of millions of other phone lines). The connection is always available, without the need to dial or wait for connection to be established.
A 56k modem (such as a V.90 modem) is a high-speed variant of the traditional modem, which converts digital data into audible sounds that can travel over an ordinary analog voice line, and can be converted back into digital data at the remote end. V.90 modems operate at speeds of up to 53,333 bps on the receive side, and up to 33,600 bps on the transmit side.
Q. 3. How does the speed of a 56k leased line differ from the speed of a 56k modem connection?
Data on a 56k leased line always travels at an exact speed of 56,000 bps in both directions. Despite the name, 56k modems are restricted, in the best case, to speeds of 53k bps on the receive side, and 33k bps on the transmit side. In practice, the actual speed of a modem depends heavily on line conditions. In some cases, 56k modems will be unable to connect at speeds above 26,400 bps. This is especially true in rural areas, in areas with older phone equipment, and in newer housing developments.
In contrast, a 56k leased line always operates at a rate of 56,000 bps in both directions.
Q.4. How does the quality of a 56k leased line differ from the quality of a 56k modem connection?
When you order a 56k leased line from the telco, they're making a commitment to provide a telephone line provisioned to successfully operate at 56,000 bps at all times. Referred to as a "conditioned" line, a 56k leased line comes a higher standard of maintenance and service when transmission problems arise.
In contrast, modems operate over voice lines, and telcos are only obligated to provide voice grade service. (In some states, telcos are in fact obligated by law to maintain voice lines of sufficient quality to run modems at minimum speeds of 14.4k or 9600 bps; but no faster.) Customers experiencing transmission problems with modems typically get no cooperation from the telco. Moreover, in a significant number of cases, voice grade lines will have characteristics that will prevent modems from getting anywhere near 53k bps, and a modem user has no formal recourse when stuck with a low quality line.
The consequence of this is that with, it is exceedingly optimistic to rely on a modem being able to receive at speeds above 50 kbps. And, of course, the transmit side is limited to 33 kbps. A 56k leased line, however, is guaranteed by the telco to operate at 56,000 bps in both directions.
Q. 5. What accounts for the higher cost of a 56k leased line?
The bulk of the expense of the 56k leased line is paid to the local telephone company. Because it is a "conditioned line", the telephone company also provides a higher level of service - they will ensure that your line will operate at full speed, and will be responsive when you have transmission problems. Since 56k modems operate over voice grade lines, telcos will provide little, if any, assistance, unless the problems perceptibly affect voice conversations.
Given that a 56k leased line is always connected, typically without per minute charges, the cost may compare favorably to keeping a modem line up 24 hours a day.
Q. 6. A DSL line is much higher speed. Why should I even consider a 56k leased line?
If DSL is available in your area, and your line qualifies, DSL may be the best choice for you. However, DSL is not yet universally available, and even in locations where it is available, some customers may have lines that are too long to qualify for DSL (>18,000 ft). A 56k leased line is most attractive for customers that are unable to obtain DSL or ISDN lines.
Q. 7. Can you explain the technical factors particular to 56k modem connections that degrade transmission speed?
One major reason why 56k modems rarely live up to their name is because they are designed to extract every spare ounce of speed possible from lines that do not efficiently carry digital data, and were once thought to be fundamentally incapable of anything faster than 9600 bps. Consequently, a lot of things have to go in your favor in order to reach the top speeds. In not a few cases, you may never be able to achieve high speeds from a 56k modem.
Another major reason is due to inhospitable line characteristics. Sometimes, these characteristics are accidental, such as the long local loop lengths that are very common in the New England area. In other cases, voice grade lines are installed with devices such as load coils that are designed to enhance voice quality, but which are detrimental to modem transmissions. Finally, in an increasing number of cases, telcos have installed equipment on voice grade lines to permit increased phone line density, which do not have a perceptible impact on voice quality, but which severely degrade modem speeds. (Examples of such equipment include PairGain devices and universal subscriber line concentrators.)
Modems will usually shift speeds several times during a connection, according to line conditions; and in not a few cases, the speed reported by the modem on connect will drop quickly and never be reattained.
Q. 8. Can you provide a technical explanation why 56k leased lines are able to achieve more reliable speeds and superior quality?
The design of the telephone system is based on digital channels which carry 56,000 bps of bandwidth (called a DS 0). Each voice line and each 56k leased line is allotted a single channel within the telephone network. Voice lines are digitized at the end of the local loop; this digitization converts your voice, which it receives as an analog signal, into a 56,000 bps stream of data. At the other end, it is converted back into an audible analog signal. Modems work by converting the telco customer's digital data to and from an audible analog form that can pass through the voice grade line. Unfortunately, the process is inherently inefficient and susceptible to damaging distortion; moreover, substantial complexity is required to compensate for telco equipment operating under the assumption that is carrying a voice call. To use an analogy, trying to send data across a voice grade line is like attempting to swim a river in the upstream direction. Consequently, modems can never utilize the whole 56,000 bps allocated to the connection's channel.
In contrast, in a 56k leased line scenario, the customer premises equipment (analogous to a modem) transmit and receives digital data directly in the format used internally to the telephone network. Without the analog to digital conversions required over a voice line, no bandwidth is wasted. As the phone line is provisioned by the telco to be used for digital data, there is no bandwidth expended compensating for telco equipment mistakenly "improving" the signal for the human ear, nor are complex protocols and algorithms required to probe the line condition and recover from distortion. Finally, as digital data is more resilient in the face of the ordinary distortions to which every local loop is susceptible, the transmission itself is more reliable. Therefore, a 56k leased line connection operates as smoothly and efficiently as possible.