ADSL
adsl modem
The distinguishing characteristic of ADSL over other forms of DSL is that the
volume of data flow is greater in one direction than the other, i.e. it is
asymmetric. Providers usually market ADSL as a service for consumers to connect
to the Internet in a relatively passive mode: able to use the higher speed
direction for the "download" from the Internet but not needing to run servers
that would require high speed in the other direction. There are both technical
and marketing reasons why ADSL is in many places the most common type offered to
home users. On the technical side, there is likely to be more crosstalk from
other circuits at the DSLAM end (where the wires from many local loops are close
to each other) than at the customer premises. Thus the upload signal is weakest
at the noisiest part of the local loop, while the download signal is strongest
at the noisiest part of the local loop. It therefore makes technical sense to
have the DSLAM transmit at a higher bit rate than does the modem on the customer
end. Since the typical home user in fact does prefer a higher download speed,
the telephone companies chose to make a virtue out of necessity, hence ADSL. On
the marketing side, limiting upload speeds limits the attractiveness of this
service to business customers, often causing them to purchase higher cost Leased
line services instead. In this fashion, it segments the digital communications
market between business and home users. [edit] How ADSL works [edit] On the wire
Currently, most ADSL communication is full-duplex. Full-duplex ADSL
communication is usually achieved on a wire pair by either frequency-division
duplex (FDD), echo-cancelling duplex (ECD), or time-division duplexing (TDD).
FDD uses two separate frequency bands, referred to as the upstream and
downstream bands. The upstream band is used for communication from the end user
to the telephone central office. The downstream band is used for communicating
from the central office to the end user. Frequency plan for ADSL. The red area
is the frequency range used by normal voice telephony (PSTN), the green
(upstream) and blue (downstream) areas are used for ADSL.With standard ADSL
(annex A), the band from 25.875 kHz to 138 kHz is used for upstream
communication, while 138 kHz – 1104 kHz is used for downstream communication.
Each of these is further divided into smaller frequency channels of 4.3125 kHz.
These frequency channels are sometimes termed bins. During initial training, the
ADSL modem tests each of the bins to establish the signal-to-noise ratio at each
bin's frequency. The distance from the telephone exchange and the
characteristics of the cable mean that some frequencies may not propagate well,
and noise on the copper wire, interference from AM radio stations and local
interference and electrical noise at the customer end mean that relatively high
levels of noise are present at some frequencies, so considering both effects the
signal-to-noise ratio in some bins (at some frequencies) may be good or
completely inadequate. A bad signal-to-noise ratio measured at certain
frequencies will mean that those bins will not be used, resulting in a reduced
maximum link capacity but with an otherwise functional ADSL connection. The DSL
modem will make a plan on how to exploit each of the bins sometimes termed "bits
per bin" allocation. Those bins that have a good signal-to-noise ratio (SNR)
will be chosen to transmit signals chosen from a greater number of possible
encoded values (this range of possibilities equating to more bits of data sent)
in each main clock cycle. The number of possibilities must not be so large that
the receiver might mishear which one was intended in the presence of noise.
Noisy bins may only be required to carry as few as two bits, a choice from only
one of four possible patterns, or only one bit per bin in the case of ADSL2+,
and really noisy bins are not used at all. If the pattern of noise versus
frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin
allocations, in a process called "bitswap", where bins that have become more
noisy are only required to carry fewer bits and other channels will be chosen to
be given a higher burden. The data transfer capacity the DSL modem therefore
reports is determined by the total of the bits-per-bin allocations of all the
bins combined. Higher signal-to-noise ratios and more bins being in use gives a
higher total link capacity, while lower signal-to-noise ratios or fewer bins
being used gives a low link capacity. The total maximum capacity derived from
summing the bits-per-bins is reported by DSL modems and is sometimes termed sync
rate. This will always be rather misleading as the true maximum link capacity
for user data transfer rate will be significantly lower because extra data is
transmitted that is termed protocol overhead, a reduced figure of around 84-87%
at most for PPPoA connections being a common example. In addition some ISPs will
have traffic policies that limit maximum transfer rates further in the networks
beyond the exchange, and traffic congestion on the Internet, heavy loading on
servers and slowness or inefficiency in customers' computers may all contribute
to reductions below the maximum attainable. The choices the DSL modem make can
also be either conservative, where the modem chooses to allocate fewer bits per
bin than it possibly could, a choice which makes for a slower connection, or
less conservative in which more bits per bin are chosen in which case there is a
greater risk case of error should future signal-to-noise ratios deteriorate to
the point where the bits-per-bin allocations chosen are too high to cope with
the greater noise present. This conservatism involving a choice to using fewer
bits per bin as a safeguard against future noise increases is reported as the
signal-to-noise ratio margin or SNR margin. The telephone exchange can indicate
a suggested SNR margin to the customer's DSL modem when it initially connects,
and the modem may make its bits-per-bin allocation plan accordingly. A high SNR
margin will mean a reduced maximum throughput but greater reliability and
stability of the connection. A low SNR margin will mean high speeds provided the
noise level does not increase too much, otherwise the connection will have to be
dropped and renegotiated (resynced). ADSL2+ can better accommodate such
circumstances, offering a feature termed seamless rate adaptation (SRA), which
can accommodate changes in total link capacity with less disruption to
communications. Vendors may support usage of higher frequencies as a proprietary
extension to the standard. However, this requires matching vendor-supplied
equipment on both ends of the line, and will likely result in crosstalk problems
that affect other lines in the same bundle. There is a direct relationship
between the number of channels available and the throughput capacity of the ADSL
connection. The exact data capacity per channel depends on
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