Technology DSL modem

1 technology

1.1 dsl concept
1.2 data transmission
1.3 data rates , access
1.4 filters

technology
dsl concept

the public switched telephone network, network of switching centers, trunk lines, amplifiers , switches transmits telephone calls 1 phone another, designed carry voice frequency signals, , therefore limited bandwidth of 3.4 khz. before dsl, voice-band modems transmitted information through telephone network audio frequencies within bandwidth, limited them data rate of 56 kbit/s. however, copper wires connect telephones local switching center (telephone exchange), called subscriber loop, able carry wider band of frequencies, several megahertz. capacity unused in normal phone service. dsl uses these higher frequencies send digital data between dsl modem , local switching center, without interfering normal telephone service. @ local switching center data transferred directly between customer s phone line , internet lines, dsl signals not travel through telephone network itself. not necessary dial telephone number initiate connection; dsl connection on whenever modem on.

data transmission

the device @ local switching center communicates dsl modem called digital subscriber line access multiplexer (dslam), connected directly internet. local switching center must equipped these devices offer dsl service.

with adsl, modem , dslam communicate protocol called discrete multitone modulation (dmt), form of frequency division multiplexing. modem uses frequencies above 8 khz, avoid interfering normal phone service. bandwidth of line between 8 khz , 1 mhz divided 247 separate channels, each 4 khz wide. separate carrier signal carries information in each channel. system acts 247 separate modems operating simultaneously. bits of incoming digital data split , sent in parallel on channels. each data stream sent using error-correcting code allow minor bit errors due noise corrected @ receiving end. of channels unidirectional, carrying download data dslam modem, on low frequency end bidirectional, carry smaller quantity of upload traffic. modem monitors transmission quality on each channel, , if impaired shift signal other channels. modem shifting data between channels searching best transmission rates. interference or poor quality lines not interrupt transmission, cause data rate of modem degrade.

for example, when downloading web page, packets of web page data travel on optical fiber internet lines directly dslam @ neighborhood telephone exchange. @ dslam split many 247 parallel data streams. each modulated onto separate carrier signal , sent through separate frequency channel on subscriber s telephone line dsl modem. modem demodulates carrier, extracting data stream each carrier signal, performs error correction, puts data again in proper order, , sends computer on ethernet line.

data rates , access

most consumer dsl lines use 1 of several varieties of asymmetric dsl (adsl). asymmetric means more of bandwidth of line dedicated downstream (download) data upstream (upload) data, so, download rates faster upload rates, because users download larger quantities of data upload. because telephone lines never designed carry such high frequency signals, dsl distance-sensitive. farther away switching center modem is, longer telephone wires, weaker signal, , lower data rate modem can achieve. users in cities, close switching centers, may have access higher rate service, 8 mbit/s. available adsl service today has rates of 1.5 mbit/s download , 64 640 kbit/s upload. distance limit adsl 18 000 feet (5.5 km or 3.4 miles). however, other devices installed in telephone lines phone company, such loading coils , bridge taps, block signal, , may disqualify given phone line dsl service.

filters

to prevent dsl signal entering phone lines telephones, answering machines, faxes , other devices cause interference, dsl modems come low pass filters must plugged phone lines going voiceband devices on same line. filter blocks frequencies above 4 khz, blocks dsl signal while allowing voice frequency signals through. filter must not inserted in phone line going dsl modem, because block communication between modem , switching center.