[Miscellaneous links]
Morning, Jim!
Here are a few you may not have seen:
bctel.com
You have to hand it to BCTEL, they really are ahead of the game.
And this which I've marked on my calendar:
<<<BC TELECOM has requested a decision from the CRTC by August 27.>>>
Did you see this at InterOp?
newbridge.com
New on Siemens' homepage:
ssc.siemens.com
ssc.siemens.com
This from GTE. Think it could portend more than just an email announcement?
news.com
And an article I missed earlier compares DMT from MOT and ADI. I don't know why they didn't include the Amati/TI C6X
June 23, 1997, Issue: 959 Section: Communications Design
ADSL boosts file transfers among PCs
By David Connolly, Manager, Hamilton Hallmark Technology Support Center, Irvine, Calif.
With processor speeds, memory configurations and disk-drive capacities rising and costs dropping, the average home or business computer's ability to handle multimedia applications has grown by leaps and bounds. As a result, Internet content providers have developed ever more elaborate offerings with beefy files that take far too long to download at 28.8 or even 33.5 kbits per second. For example, a 10-Mbyte video clip containing less than four minutes of video takes a minimum of 46 minutes to download at 28.8 kbits/s.
An ADSL modem could transfer that amount of data in less than 10 seconds.
Asynchronous digital-subscriber-line technology originally was conceived as a way of delivering video on demand. The downstream channel would deliver an MPEG data stream, and the upstream channel would carry VCR commands and control streams. Built on the idea that more data moves out from the network's central office than back to it, ADSL optimizes the downstream data rate at the expense of the upstream. Hence the term "asymmetric."
ADSL supports three channels: downstream (simplex), upstream plus control (full-duplex) and POTS (plain old telephone service). It works over the existing local cooper loop from the telco's central office to the subscriber premises, without the expensive line conditioning often needed for ISDN and T1 services.
Comparing channels
An ADSL modem uses the spectrum from 26 kHz to 1.1 MHz. The full-duplex channel uses roughly 26 to 120 kHz, while the downstream channel uses the remainder to 1.1 MHz. POTS still uses the bottom 4 kHz, and is separated from the digital data by a passive low-pass filter. Both the downstream and full-duplex channels can carry more than one bearer channel.
The American National Standards Institute's T1E1.4 committee has approved ADSL standards up to 6.1 Mbits/s (simplex), utilizing discrete-multitone (DMT) modulation, and has assigned it the standards number T1.413. The European Telecommunications Standards Institute has okayed an annex to this spec, making T1.413 an international standard.
The core of the T.413 standard is the DMT modulation technique. The voice-band analog POTS is split off via transformer coupling and a passive low-pass filter. The advantages are that use of the line for POTS does not affect DSL, and the passive splitter assures that POTS lifeline services are not affected.
The basic idea is to divide the available bandwidth into a large number of subchannels, typically 256 (some are lost as guard channels, so the number of channels actually used for data transmission is 249 or so). Each subchannel is 4-kHz wide. When the modems are powered up and connected to the line, the individual line's ability to carry data is analyzed on a subchannel-by-subchannel basis. Evaluating the frequency response and impairments of each subchannel determines the amount of data that each can carry, after which the aggregate data is assigned. If a subchannel cannot carry any data, it can be disabled. This process is continuous, so if the data capacity of a particular subchannel deteriorates for some reason, some or all of its bits can be assigned to other subchannels.
Data is modulated onto the subchannel using an inverse discrete Fourier transform, to implement 64-QAM (quadrature amplitude modulation). The maximum number of bits for this modulation scheme is 15 bits per Hz (60 kbits/s maximum per channel, theoretically). If a particular subchannel on a particular line cannot carry this much data, the algorithm might, for example, assign only 8 bits/Hz. In this way, each individual line will be automatically configured to carry the maximum amount of data it can support.
Asymmetry is achieved by assigning more channels to the downstream data than to the full-duplex variety. A typical split might be 224 channels for the simplex data stream and 25 channels for the full-duplex.
The digital channels are connected directly to the WAN via routers, and therefore do not go through a switch. At the central office, POTS is connected to the switch in the normal way.
A typical ADSL modem consists of the following functional blocks:
- Passive low-pass-filter POTS splitter: Transformer coupling and passive low-pass filter to separate the analog POTS signals from the digital ADSL signals.
- ADSL line driver/receiver: Provides required drive levels and reception of raw signals.
- Analog front end: Contains analog and mixed-signal processing elements, including analog-to-digital and digital-to-analog conversion, and filtering.
- DMT processor: Performs processing required for DMT modulation/demodulation.
- Modem interface: Connects to external equipment, typically V.35, Ethernet, etc.
- Modem controller: Provides for configuration, training (measuring the line) and operation of DMT processor and modem interface.
Controller memory: Local controller memory.
For carriers to be able to offer ADSL services, strict constraints must be set on modem price, power and size. Satisfying these constraints will require highly integrated, low-cost, high-performance chip sets. Two of the leading suppliers for this market are Motorola Inc. and Analog Devices Inc.
For example, Motorola has announced a two-chip set that provides all the ADSL-specific modem functions: the MC14560 ADSL transceiver and the MC03AX1456CO/RT line driver. The MC14560 ADSL transceiver packs all the basic functions to perform transmit and receive operations in a Category 2 ADSL modem. It can handle data rates up to 8 Mbits/s downstream and 1 Mbit/s full-duplex. When combined with a controller and line-driver interface, it can be configured as either a central-office or remote-terminal ADSL transceiver unit (ATU-C or ATU-R, respectively)
The 14560 uses a digital interface to transfer input and output data. The transceiver can support up to seven independent data streams or bearer services, including four simplex ports and three duplex ports for bidirectional transfers. The digital interface provides framing, data interleaving and deframing trellis encoding/decoding. An on-chip DMT processor sports embedded DSPs and peripherals to perform modulation, demodulation and echo cancellation for the system. This device handles both transmit and receive functions.
The 14560's analog front end performs D/A conversions on the data samples from the DMT processor, as well as the filtering and signal conditioning required for transmission on the line interface. On the receive side, it filters the analog input, performs A/D conversion and provides the samples to the DMT processor. The analog front end also contains a special host-processor interface consisting of an 8-bit data bus, 5-bit address bus, chip select, read-write signal and a single interrupt line. The interface is directly compatible with the 8-bit parallel data and address interface on Motorola microcontrollers. Depending on the application, the host processor could be from Motorola's MC68302, MC68360 or MPC860 families.
Completing the Motorola solution is the MC03AX1456 line driver, a single chip to interface the MC14560 transceiver with the line transformer. The MC03AX1456CO line driver is designed for the central office (ATU-C) and the MC03AX1456RT for the remote terminal (ATU-R). The line driver provides three differential ports: a transmit and a receive port to interface to the transceiver, and a bidirectional line port. The transmit port incorporates a high-pass filter and programmable gain stage, while the receive port provides programmable high-frequency boost, programmable gain and a high-pass filter.
For its part, Analog Devices has announced a five-chip set for implementing an ADSL modem. The AD20mps910 chip set consists of the AD6435 modem interface, the AD6436 DMT coprocessor, the AD6437 analog front end, the AD816 line driver and receiver, and the ADSP-2183 DSP.
The modem-interface device forms the interface between the ADSL chip set and external systems. The ADSL transceiver includes an internal V.35 interface, and Ethernet or ATM interfaces can be implemented.
The DMT coprocessor performs core DSP functions to implement DMT modulation and demodulation. The AD6437 analog front end is a single-chip implementation of all analog and mixed-signal operations required in an ADSL system, including A/D, D/A, filters and amplifiers.
The AD816 incorporates the line driver, receiver and amplifiers to interface to the line interface. Finally, the ADSP-2183 general-purpose 16-bit DSP polishes off a variety of general control tasks, including training (line characterization) and system-control operations.<<<<
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