Perform precise and detailed comparison of Digital Subscriber Line (DSL) and Cable modem technology. Take the three sample applications and look at the actual set of transmissions that would be necessary for each technology to do each of these applications. Try to get some sample timings for each transmission or make some reasonable assumptions about how long each would take. Then determine the actual time it would take for the application to complete with each technology.
This paper will examine the present state of the two
technologies, DSL and Cable. We will review cost, speed, types,
availability and security of these services as we consider these to be
the principle criteria by which users choose between them. The results
of a simple survey conducted, are presented and summarized. Finally, we
offer our conclusions and sources of information for further research.
Before examining these details, some background on these technologies may
be of use to the reader.
CABLE MODEM TECHNOLOGY:
What is Cable modem :
A cable modem is an external device that allows your
computer to connect to the Internet through a cable TV wire, instead of
a telephone line (or another system). basically you just connect the Cable
Modem to the TV outlet for your cable TV, and the cable operator connects
a Cable Modem Termination System (CMTS) in his end (the Head-End). Cable
modems translate radio frequency (RF) signals to and from the cable plant
into Internet Protocol (IP), the communications protocol spoken by all
computers connected to the Internet. Cable modems are designed to take
advantage of the "broadband" cable infrastructure enabling peak connection
speeds over 100 times faster than traditional dial-up connections.
Cable modem speeds vary widely, depending on the cable
modem system, cable network architecture, and traffic load. In the downstream
direction (from the network to the computer), network speeds can be anywhere
up to 27 Mbps, an aggregate amount of bandwidth that is shared by users.
Few computers will be capable of connecting at such high speeds, so a more
realistic number is 1 to 3 Mbps. In the upstream direction (from computer
to network), speeds can be up to 10 Mbps. However, most modem producers
have selected a more optimum speed between 500 Kbps and 2.5 Mbps. An asymmetric
cable modem scheme is most common. The downstream channel has a much higher
bandwidth allocation (faster data rate) than the upstream, primarily because
Internet applications tend to be asymmetric in nature. Activities such
as World Wide Web (http) navigating and newsgroups reading (nntp) send
much more data down to the computer than to the network. Mouse clicks (URL
requests) and e-mail messages are not bandwidth intensive in the upstream
direction. Image files and streaming media (audio and video) are
very bandwidth intensive in the downstream direction.
How does cable modem works:
Current Internet access via a 28.8-, 33.6-, or 56–kbps
modem is referred to as voiceband modem technology. Like voiceband modems,
cable modems modulate and demodulate data signals. However, cable modems
incorporate more functionality suitable for today's high-speed Internet
services. In a cable network, data from the network to the user is referred
to as downstream, whereas data from the user to the network is referred
to as upstream. Cable modems can be part modem, part tuner, part
encryption/decryption device, part bridge, part router, part network interface
card, part SNMP agent, and part Ethernet hub. Typically, a cable modem
sends and receives data in two slightly different fashions. In the downstream
direction, the digital data is modulated and then placed on a typical 6
MHz television channel (bandwidth), somewhere between 50 MHz and 750 MHz
(frequency). Currently, 64 QAM (or 256- QAM) is the preferred downstream
modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal
can be placed in a 6 MHz channel adjacent to TV signals on either side
without disturbing the cable television video signals.
The upstream channel is more tricky. Typically, in
a two-way activated cable network, the upstream (also known as the
reverse path) is transmitted between 5 and 42 MHz (frequency) with a bandwidth
of 2Mhz. This tends to be a noisy environment, with RF interference and
impulse noise. Additionally, interference is easily introduced in the home,
due to loose connectors or poor cabling. Since cable networks are tree
and branch networks, all this noise gets added together as the signals
travel upstream, combining and increasing. Due to this problem, most manufacturers
use QPSK or a similar modulation scheme in the upstream direction, because
QPSK is more robust scheme than higher order modulation techniques in a
noisy environment. The drawback is that QPSK is "slower" than QAM.
The upstream and downstream data rates may be flexibly
configured using cable modems to match subscriber needs. For instance,
a business service can be programmed to receive as well as transmit higher
bandwidth. A residential user, however, may be configured to receive higher
bandwidth access to the Internet while limited to low bandwidth transmission
to the network. The normal upstream data-rate is 3Mbit/s (~400KB/s) where
as data-rate for download is 27-56MBit/s (4-7 Mbyte/s). Incase of transmission,
cable modem transmits bursts of data in timeslots (TDM) and reserved contention
timeslots. On the other hand, it receives continuos stream of data.
A subscriber can continue to receive cable television
service while simultaneously receiving data on cable modems to be delivered
to a personal computer (PC) with the help of a simple one-to-two splitter
(see Figure 1). The data service offered by a cable modem may be shared
by up to sixteen users in a local-area network (LAN) configuration.
Figure 1: Cable Modem at the subscriber Location.
Because some cable networks are suited for broadcast
television services, cable modems may use either a standard telephone line
or a QPSK/16 QAM modem over a two-way cable system to transmit data upstream
from a user location to the network. When a telephone line is used
in conjunction with a one-way broadcast network, the cable data system
is referred to as a telephony return interface (TRI) system. In
this mode, a satellite or wireless cable television network can also function
as a data network.
At the cable headend, data from individual users is
filtered by upstream demodulators (or telephone-return systems, as appropriate)
for further processing by a cable modem termination system (CMTS). A CMTS
is a data switching system specifically designed to route data from many
cable modem users over a multiplexed network interface. Likewise, a CMTS
receives data from the Internet and provides data switching necessary to
route data to the cable modem users. Data from the network to a user group
is sent to a 64/256 QAM modulator. The result is user data modulated into
one 6-MHz channel, which is the spectrum allocated for a cable television
channel such as ABC, NBC, or TBS for broadcast to all users (see Figure
2).
Figure2 : Cable modem termination system and cable
headend transmission.
A cable headend combines the downstream data channels
with the video, pay-per-view, audio, and local advertiser programs that
are received by television subscribers. The combined signal is then
transmitted throughout the cable distribution network. At the user
location, the television signal is received by a set-top box, while user
data is separately received by a cable modem box and sent to a PC.
A CMTS is an important new element for support of
data services that integrates upstream and downstream communication over
a cable data network. The number of upstream and downstream channels in
a given CMTS can be engineered based on serving area, number of users,
data rates offered to each user, and available spectrum. A CMTS can talk
to all the Cable Modems(CM's), but the Cable Modems can only talk to the
CMTS. If two cable modems need to talk to each other, the CMTS will have
to relay the messages.
Another important element in the operations and day-to-day
management of a cable data system is an element management system (EMS).
An EMS is an operations system designed specifically to configure
and manage a CMTS and associated cable modem subscribers. The operations
tasks include provisioning, day-to-day administration, monitoring, alarms,
and testing of various components of a CMTS. From a central network
operations center (NOC), a single EMS can support many CMTS systems
in the geographic region.
The cable modem has two interfaces: a standard "F"
port connector and a 10Base-T Ethernet RJ-45 port. The PC attaches to the
cable modem via the 10Base-T Ethernet cable. The "F" connector attaches
to the same cable that provides the video signal to your television/VCR.
When attached to a two-way cable system, the modem will send and receive
data via the cable. The downstream (network to home) channel will
occupy a single 6MHz (standard CATV) channel within the Radio Frequency
(RF) spectrum -- 54 to 750MHz.
Advanced modulation techniques allow data to travel as fast as 36Mbps
on a single 6MHz analog carrier. To avoid consuming too much of the CATV
spectrum on the "upstream" return, several cable modem manufacturers have
designed their products to use less than a full 6MHz carrier out of the
home. The cable runs from your neighborhood to a central location, referred
to as the headend. Additional equipment is installed there that
communicates to all the cable modems in subscribers' homes.
Cable Data System Features:
Beyond modulation and demodulation, a cable modem
incorporates many features necessary to extend broadband communications
to wide-area networks (WANs). The network layer is chosen as Internet protocol
(IP) to support the Internet and World Wide Web services. The data link
layer is comprised of three sublayers: logical link control sublayer, link
security sublayer conforming to the security requirements, and media access
control (MAC) sublayer suitable for cable system operations. Current cable
modem systems use Ethernet frame format for data transmission over upstream
and downstream data channels. Each of the downstream data channels
and the associated upstream data channels on a cable network form an extended
Ethernet WAN. As the number of subscribers increases, a cable operator
can add more upstream and downstream data channels to support demand for
additional bandwidth in the cable data network. From this perspective,
growth of new cable data networks can be managed in much the same fashion
as the growth of Ethernet LANs within a corporate environment.
The link security sublayer requirements are further
defined in three sets of requirements: baseline privacy interface (BPI),
security system interface (SSI), and removable security module interface
(RSMI). BPI provides cable modem users with data privacy across the
cable network by encrypting data traffic between the user's cable modem
and CMTS. The operational support provided by the EMS allows a CMTS to
map a cable modem identity to paying subscribers and thereby authorize
subscriber access to data network services. Thus, the privacy and security
requirements protect user data as well as prevent theft of cable data services.
Cable Data Network Architecture:
To offer high-speed Internet services, a cable operator creates a data
network that operates over its hybrid fiber/coax (HFC) plant. The following
diagram provides a high-level look at a typical large market cable network,
including a regional cable headend (typically serving 200,000 to 400,000
homes), which feeds distribution hubs (each serving 20,000 to 40,000 homes)
through a metropolitan fiber ring. At the distribution hub, signals are
modulated onto analog carriers and then transported over fiber-optic lines
to nodes serving 500 to 1,000 homes. From the node, these signals are carried
via coaxial cable to a home or business.
Regional Cable Headend :
The regional cable headend serves as the local data network operations
center. A carrier-class IP switch or router interfaces with a backbone
data network, such as those operated by @Home or Road Runner, offering
connectivity to remote content servers, as well as the global Internet.
This switch/router also connects to cable modem termination systems
(CMTS) housed in the distribution hubs (hyperlink). Many cable operators
are beginning to deploy high-capacity packet transport solutions over fiber
rings connecting the CMTS units in their distribution hubs, such as Packet
Over SONET (POS), at up to OC-12 speeds (622 Mbps).
Content and application servers are typically at the regional cable
headend, as are network management and operations support systems. If the
cable operator were offering IP telephony, voice calls would be directed
by the headend router to a IP telephony gateway, and then onto the public
switched telephone network (PSTN).
Distribution Hub
The hub is the interchange point between the regional fiber network
and the cable plant. At the hub, the cable modem termination system (CMTS)
coverts data from a wide area network (WAN) protocol, such as POS, into
digital signals that are modulated for transmission over HFC plant, and
then demodulated by the cable modem in the home or business. The CMTS unit
provides a dedicated 27 Mbps downstream data channel that is shared by
the 500 to 1,000 homes served by a fiber node, or group of nodes. Upstream
bandiwdth per node typically ranges from 2 Mbps to 10 10 Mbps.
Home Environment
A splitter at the side of the home segments coaxial cable lines serving
the cable modem and TV outlets. Cable modems currently connect to an Ethernet
card in the PC with Category 5 cabling and RJ-45 connectors. Forthcoming
cable modem products will also offer Universal Serial Bus (USB) connections.
Business Environment
In a business environment, the cable modem interfaces with a local area
network (LAN) through an Ethernet hub, switch or router, providing access
to multiple users through a single cable modem.
Cable data network standards:
A cable data system is comprised of many different
technologies and standards. To develop a mass market for cable modems,
products from different vendors must be interoperable. To accomplish
the task of interoperable systems, the North American cable television
operators formed a limited partnership, Multimedia Cable Network System
(MCNS), and developed an initial set of cable modem requirements (DOCSIS).
MCNS was initially formed by Comcast, Cox, TCI, Time Warner, Continental
(now MediaOne), Rogers Cable, and CableLabs. The DOCSIS requirements are
now managed by CableLabs. Vendor equipment compliance to the DOCSIS
requirements and interoperability tests are administered by a CableLabs
certification program.
The OSI layer for a DOCSIS cable modem looks like this.
| OSI | DOCSIS |
| Higher Layers | Applications DOCSIS |
| Transport Layer | TCP/UDP Control |
| Network Layer | IP Messages |
| Data Link Layer | IEEE 802.2 |
| Upstream Downstream | |
| Physical Layer | TDMA (mini-slots)
TDM(MPEG)
5 - 42(65) MHz 42(65) - 850 MHz QPSK/16-QAM 64/256-QAM ITU-T J.83 Annex B(A) |
Some of the details of cable modem requirements are
listed below.
Physical Layer
At the cable modem physical layer, downstream data channel is based on North American digital video specifications (i.e., International Telecommunications Union [ITU]–T Recommendation J.83 Annex B) and includes the following features:
64 and 256 QAM 6 MHz–occupied spectrum that coexists with other signals in cable plant concatenation of Reed-Solomon block code and Trellis code, supports operation in a higher percentage of the North American cable plants variable length interleaving supports, both latency-sensitive and latency-insensitive data services contiguous serial bit-stream with no implied framing, provides complete physical (PHY) and MAC layer decoupling.
Upstream Data Channel:
The upstream data channel is a shared channel featuring the following:
MAC Layer
The MAC layer provides the general requirements for many cable modem subscribers to share a single upstream data channel for transmission to the network. These requirements include collision detection and retransmission.
Privacy of user data is achieved by encrypting link-layer
data between cable modems and CMTS. Cable modems and CMTS headend controller
encrypt the payload data of link-layer frames transmitted on the cable
network. A set of security parameters including keying data is assigned
to a cable modem by the Security Association (SA).
All of the upstream transmissions from a cable modem
travel across a single upstream data channel and are received by the CMTS.
In the downstream data channel a CMTS must select appropriate SA based
on the destination address of the target cable modem. Baseline privacy
employs the data encryption standard (DES) block cipher for encryption
of user data. The encryption can be integrated directly within the MAC
hardware and software interface.
Network Layer
Cable data networks use IP for communication from
the cable modem to the network. A network address translation (NAT)
system may be used to map multiple computers that use a single high-speed
access via cable modem.
Transport Layer
Cable data networks support both transmission control
protocol (TCP) and user datagram protocol (UDP) at the transport layer.
Application Layer
All of the Internet-related applications are supported
here. These applications include e-mail, ftp, tftp, http, news, chat, and
signaling network management protocol (SNMP). The use of SNMP provides
for management of the CMTS and cable data networks.
Operations System
The operations support system interface (OSSI) requirements
of DOCSIS specify how a cable data network is managed. To date, the requirements
specify an RF MIB. This enables system vendors to develop an EMS to support
spectrum management, subscriber management, billing, and other operations.
What is CATV Network:
A CATV network is designed and used for cable TV distribution.
With an upgrade of the system, it is normally possible to allow signals
to flow in both directions. Higher frequencies flow toward the subscriber
and the lower frequencies go in the other direction. This is done by upgrades
to the amplifiers in the cable distribution network etc.
Most CATV networks are Hybrid Fibre-Coax (HFC) networks.
It has tree like structure. The single (head end) is the root and the (many)
cable modems are at the leaves. The signals run in fiber-optical cables
from the Head-End center to locations near the subscriber. At that point
the signal is converted to coaxial cables, that run to the subscriber premises.
One CMTS will normally drive about 1-2000 simultaneous Cable Modem users
on a single TV channel. If more Cable Modems are required, the number of
TV channels are increased by adding more channels to the CMTS.
Cable Modem types:
A number of different Cable Modem configurations are
possible. These three configurations are the main products nowadays.
External Cable Modem
The external Cable Modem is the small external box that connect to your computer normally through an ordinary Ethernet connection. The downside is that you need to add a (cheap) Ethernet card to your computer before you can connect the Cable Modem. A plus is that you can connect more computers to the Ethernet. Also the Cable Modem works with most operating systems and hardware platforms, including Mac, UNIX, laptop computers etc.
Another interface for external Cable Modems is USB,
which has the advantage of installing much faster. The downside is that
you can only connect one PC to a USB based Cable Modem.
Internal Cable Modem
The internal Cable Modem is typically a PCI bus add-in
card for a PC. That might be the cheapest implementation possible, but
it has a number of drawbacks. First problem is that it can only be used
in desktop PC's. Mac's and laptops are possible, but require a different
design. Second problem is that the cable connector is not galvanic isolated
from AC mains. This may pose a problem in some CATV networks, requiring
a more expensive upgrade of the network installations. Some countries and/or
CATV networks may not be able to use internal cable modems at all for technical
and/or regulatory reasons.
Interactive Set-Top Box
The interactive set-top box is really a cable modem
in disguise. The primary function of the set-top box is to provide more
TV channels on the same limited number of frequencies. This is possible
with the use of digital television encoding (DVB). An interactive set-top
box provides a return channel - often through the ordinary plain old telephone
system (POTS) - that allows the user access to web-browsing, email etc.
directly on the TV screen.
TYPICAL CABLE MODEM INSTALLATION:
When installing a Cable Modem, a power splitter and
a new cable is usually required. The splitter divides the signal for the
"old" installations and the new segment that connects the Cable Modem.
No TV-sets are accepted on the new string that goes to the Cable Modem.
The transmitted signal from the Cable Modem can be
so strong, that any TV sets connected on the same string might be disturbed.
The isolation of the splitter may not be sufficient, so an extra high-pass
filter can be needed in the string that goes to the TV-sets. The high-pass
filter allows only the TV-channel frequencies to pass, and blocks the upstream
frequency band. The other reason for the filter is to block ingress in
the low upstream frequency range from the in-house wiring. Noise injected
at each individual residence accumulates in the upstream path towards the
head-end, so it is essential to keep it at a minimum at every single residence
that needs Cable Modem service.
Data-interface
On any kind of external cable modem (the majority
of what is in use today), you obviously need some kind of data-interface
to connect the computer and the cable modem.
Ethernet
On most external modems, the data-port interface is
10 Mbps Ethernet. Some might argue that you need 100 Mbps Ethernet to keep
up with the max. 27-56 Mbps downstream capability of a cable modem, but
this is not true. Even in a very good installation, a cable modem can not
keep up with a 10 Mbps Ethernet, as the downstream is shared by many users.
The 1st version of the MCNS standard, that dominates the US market,
specified 10 Mbps Ethernet as the only allowable data-interface. The DVB/DAVIC
standard is totally open, allowing any type of interface. Other types of
interfaces are being incorporated in the MCSN standard to allow for a wider
range of cable modem configurations.
USB (Universal Serial Bus)
Among others, Intel recently announced that they are
working with Broadcom on cable modems with USB interface. This is expected
to bring down the installation hassle for the many users with less computer
skills. Obviously you do not need to open the box to install an Ethernet
card, if the computer has an USB interface. If the computer does not have
an USB interface, you will need to install that (and you are back to about
the same hassle-level as with the Ethernet interface).
Cost
The installation cost is a significant issue, as this
is something that needs to be done in the house of every subscriber. The
CATV operators and equipment manufactures needs to try really hard to push
down the installation cost, to keep the whole operation profitable.
Cable modem Components:
Tuner
The tuner connects directly to the CATV outlet. Tuner
converts TV channel to a fixed lower frequency (6 – 40 MHz). Normally a
tuner with build-in diplexer is used, to provide both upstream and downstream
signals through the same tuner. The tuner must be of sufficiently good
quality to be able to receive the digitally modulated QAM signals. Companies
like ALPS, Sharp, Temic and Panasonic are strong suppliers here.
A new concept of a silicon tuner is in the works. This is basically
a tuner on a chip, and is expected to cut the cost down quite a bit compared
to a more conventional tuner module.
Demodulator
In the receive direction, the IF signal feeds a demodulator.
The demodulator normally consists of A/D converter, QAM-64/256 demodulator,
MPEG frame synchronization, Reed Solomon error correction. Demodulator
performs A/D, demodulation, error correction and MPEG syncronization.
The clear leader here is Broadcom, with a single chip demodulator. Other
companies are Stanford Telecom wit a combined demodulator and burst modulator,
but also companies like SGS Thomson, VLSI Technologies, LSI Logic and Fujitsu
play a role here. The demodulator component is required both in a cable
modem and in the more mature product, the digital (receive-only) set-top
box, so many companies have developed products for this part of the game.
Burst modulator
In the transmit direction, a burst modulator feeds
the tuner. The burst modulator does Reed Solomon encoding of each burst,
modulation of the QPSK/QAM-16 on the selected frequency and D/A conversion.
The output signal is feed though a driver with variable output level, so
the signal level can be adjusted to compensate for the unknown cable loss.
In short, burst modulator performs R-S encoding, modulation, frequency
conversion, D/A conversion etc.
The burst modulator is unique to the cable modem (and
some two-way set-top boxes), so less component are available here. Broadcom
leads the pack, with Stanford Telecom, Analog Devices, SGS Thomson and
others playing catch-up Combined demodulator and burst modulator chips
are also available as the integration race drives more and more functions
into a single chip.
MAC
A Media Access Control mechanism sits between the
receiver and transmit paths. This can be implemented in hardware or split
between hardware and software. The MAC is pretty complex compared to an
ethernet MAC, and in reality no MAC's are able to handle all of the MAC
layer function.
MAC extracts data from MPEG frames, filters data for other cable modems, runs the protocol, times transmission of upstream bursts etc.
For DOCSIS cable modems, Broadcom and Libit (now Texas Instruments)
are known to have MAC ASIC's available as a standard products. Connexant
is also in the market with a MAC that rely more on software to handle the
various functions, supposedly giving more flexibility. Other companies
are known to be working on various MAC chips for both DOCSIS and DVB/DAVIC,
with different partitions of what goes in software and hardware. Some cable
modem manufacturers even develop their own MAC apparently in an attempt
to be more competitive or to differentiate their products.
Interface
The data that pass through the MAC goes into the computer
interface of the Cable Modem, be it Ethernet, USB, or PCI bus.
CPU
The microprocessor is not explicitly shown on the
diagram, but for external cable modems a CPU is required. For external
cable modems with Ethernet interface, the Motorola embedded PowerPC series
of microprocessors are popular, but other RISC based architectures are
also used.
Single devices combining MAC, demodulator, burst modulator, processor,
ethernet/PCI/USB interfaces and more are emerging, in effect integration
of a cable modem in a single chip. There will still be some additional
parts for memory, tuner, analog stuff, power supply etc. so we are still
no-where near the true single-chip cable modem.
DOWNSTREAM DATA FORMAT:
Downstream data format
| MPEG Paybad | SYNC Byte | MPEG Header | MPEG Paybad | SYNC Byte |
- Reed-Solomon error correction
- Corrects 6 errors in 204 bytes
- MPEG-TS ( Transport Stream)
- MPEG-PS (Program Stream)
-MAC messages
-ATM cells (DVB/DAVIC)
- Data addressed to one, many or all cable modems
UPSTREAM DATA FORMAT:
Upstream data format
| ATM paybad | Gap | U.W. 16 bit | ATM header | ATM Paybad | Gap | U.W. 16 bitb |
- Reed-Solomon error correction
- Prepended unique word
- One ATM cell per burst (DVB/DAVIC)
- MAC message or data as payload
- 18 time-slots per 3 ms (DVB/DAVIC)
- Reserved time-slots for longer data
- Contention time-slot for small data (initiate)
- Ranging time-slots are 3 slots
Cable modem history:
1. Propriety systems (Ist generation systems)
2. MCNS (USA mainly). Developed for cable modem only. Specifies external
cable modem only, but may add internal cable modem also.
3. DAVIC/DVB (Europe manily). Used for set-top box and now also cable
modem.
4. IEEE 802.14 be the standard of the future (3rd generation systems).
The many advantages to using cable modem technology include:
Speed:
The peak Internet connection speed using a cable modem is over 100 times faster than a standard 28.8 modem. This gives you true multimedia capabilities, as well as access in seconds to things on the Internet that normally would take minutes. Cable modems will be able to receive data at up to 10 Mbps and send data at speeds up to 2 Mbps (some up to 10 Mbps).
Always-on service:
Cost:
Cable Internet access is as cost-effective as dial-up services, even though dial-up services are significantly slower. For one fixed monthly fee, you get unlimited Internet access without having to pay the "hidden costs" of an extra telephone line or additional usage charges. Often, the cost of a second phone line is the difference between the cost of cable modem service and dial-up service.
Disadvantages of cable modems:
One of the most cited disadvantages of cable modems
is the shared bandwidth to the cable head-end. You and your neighbors share
the same cable which should be able to carry about 30 MBit/sec total bandwidth.
However, in my experience this is not the real bottleneck. The access point
to the Internet (gateway) as well as the Internet itself turn out to be
slower than the local cable loop. You should see speeds 'as advertised'
(multiple MBit/sec) as long as you stay within the local system.
Another problem of shared bandwidth is security. There
are still some cable modem systems in existence that do not encrypt/filter
traffic within the local cable loop. In this case, you will be able to
look in on all the traffic sent by your neighbors (so will they be able
to look in on your traffic).
The main disadvantage in my opinion is that you don't
have a choice of ISPs. Cable TV lines do not have 'common-carrier' status
as do phone lines. However, there are some efforts underway to change that.
Who manufactures cable modems?
Many different companies make cable modems. @Home's preferred partners include 3Com, Askey, Cisco, Com21, General Instrument, Motorola, Nortel Networks, RCA, Samsung, and Zenith.
What computer hardware do I need to support a cable modem?
|
Windows 95/98/NT4.0
|
Minimum
|
Recommended
|
| CPU | Pentium or higher | Pentium 166 equivalent or higher |
| RAM | 16MB | 32MB |
| Disk Space | 125MB | 125MB |
|
Macintosh
|
Minimum
|
Recommended
|
| Operating System | 7.6.1 or higher | 7.6.1 or higher |
| CPU | PowerPC 601 | PowerPC 603 or higher |
| RAM | 24MB | 32MB |
| Disk Space | 50MB | 50MB |
Cable modem technology offers high-speed access to the Internet and World Wide Web services. Cable data networks integrate the elements necessary to advance beyond modem technology and provide such measures as privacy, security, data networking, Internet access, and quality-of-service features. The end-to-end network architecture enables a user cable modem to connect to a CMTS which, in turn, connects to a regional data center for access to Internet services. Thus, through a system of network connections, a cable data network is capable of connecting users to other users anywhere in the global network.
DIGITAL SUBSCRIBER LINE TECHNOLOGY:
What is DSL:
DSL - Digital Subscriber Line technology is a copper loop transmission technology that converts existing copper telephone wire into a high-speed data highway with broadband speeds at a fraction of the cost of other broadband technologies. xDSL is a generic name for the various versions of DSL technologies such as ADSL, HDSL and RADSL etc.
DSL technology achieves broadband speeds over the most universal network media in the world: ordinary phone wire. DSL circuits are full duplex, meaning that data can flow in both directions at the same time. Traditional analog telephone conversations, faxes and modem transmissions are limited to a 3,400hz analog voice channel. The maximum possible modem speed using this analog voice channel more recently approaching the 56kbps range. Poor line quality and distance often make it impossible for a modem to function at the speed it was designed for.
DSL transmits a broader range of frequencies over existing copper telephone
wire to achieve speeds over 50 times faster than a 28.8k modem, over 30
times faster than a 56k modem and over 12 times faster than 128k ISDN.
This significant increase in speed is possible because DSL uses a dedicated
secure copper wire circuit that does not go through analog telephone switching
equipment and because digital data (not an analog signal) is being transmitted.
( DSL leverages the Copper Loop )
Voice communications only uses a certain portion of the analog signaling spectrum. An uncompressed analog voice signal is marked at four kilohertz of the spectrum to run up to distances of 18,000 to 22,000 feet. In digital, this converts to about 64 Kbps. However, using typical Category 3 telephone wire, it is possible to actually use up to one megahertz of the spectrum at 18,000 feet. So, theoretically you might be able to drive the line up to 16 Mbps. In practice, this would almost never happen due to interference. Every single electric or electronic device, not to mention the many natural phenomena that come into play, generates this interference. There is, however, another likely prospect. If you reduce distance, the chances of signal degradation go down. Some DSL types take advantage of this; but as the line gets shorter, the CO equipment has to be closer to the consumer. This limits the possible density ratio of the supporting CO system because close proximity also means less customers served by the reach of the system.
The theory behind DSL is in many ways similar to traditional T-1 and 56 Kbps digital data service technologies. The difference lies in actual signal modulation and data framing. Signal modulation is the set of frequencies defining bits and bytes of data as they are transferred.
Incase of home or small business is close enough to a telephone company central office that offers DSL service, one may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected.
Over any given link, the maximum DSL speed is determined by the distance between the customer site and the Central Office. Most ISP's offer Symmetric DSL data services at seven different speeds--144 Kbps, 160 Kbps, 200 Kbps, 416 Kbps, 784 Kbps, 1.04 Mbps and 1.54 Mbps, and now even faster up to 6.0 Mbps--so customers can choose the rate that meets their specific business needs. At the customer premises, a DSL router or modem connects the DSL line to a local-area network (LAN) or an individual computer. Once installed, the DSL router provides the customer site with continuous connection to the Internet.
How DSL actually works:
Traditional phone service (sometimes called "Plain Old Telephone Service"
or POTS) connects your home or small business to a telephone company office
over copper wires that are wound around each other and called twisted
pair. Traditional phone service was created to let you exchange voice
information with other phone users and the type of signal used for this
kind of transmission is called an analog signal. An input device
such as a phone set takes an acoustic signal (which is a natural analog
signal) and converts it into an electrical equivalent in terms of volume
(signal amplitude) and pitch (frequency of wave change). Since the telephone
company's signalling is already set up for this analog wave transmission,
it's easier for it to use that as the way to get information back and forth
between your telephone and the telephone company. That's why our computer
has to have a modem - so that it can demodulate the analog signal and turn
its values into the string of 0 and 1 values that is called digital
information.
Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN, one can receive up to 128 Kbps.) The ability of computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.
There are several varieties of DSL, but all achieve their high speeds the same way: By sending data over previously unused frequencies in phone lines. Regular voice signals travel over phone lines at frequencies ranging from 0 kHz to 4 kHz. Standard modems use the same frequencies as voice. But DSL uses frequencies between 25 kHz and 1 MHz. That extra bandwidth also means it can send more data. This broadband connection requires special hardware at both ends. On your end, a DSL modem (also called a bridge or router) modulates digital information from your computer to send it along phone lines. These signals are then translated by a Digital Subscriber Line Access Multiplexer located at the phone company's nearest central office. The DSLAM separates the voice from the data signals, sending the latter to an ISP (which is frequently the phone company itself) and from there to the Internet at large.
Digital Subscriber Line is a technology that assumes digital data does
not require change into analog form and back. Digital data is transmitted
to your computer directly as digital data and this allows the phone company
to use a much wider bandwidth for transmitting it to you. Meanwhile, if
you choose, the signal can be separated so that some of the bandwidth is
used to transmit an analog signal so that you can use your telephone and
computer on the same line and at the same time.
Splitter-based vs. Splitterless DSL:
Most DSL technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless DSL, "DSL Lite," G.Lite, or Universal ADSL and has recently been made a standard.
DSL Modulation Schemes:
There are many ways to alter the high-frequency carrier signal that results in a modulated wave. For ADSL, the most talked-about xDSL technology, there are two competing modulation schemes: carrierless amplitude phase (CAP) modulation and discrete multitone (DMT) modulation. CAP and DMT use the same fundamental modulation technique—quadrature amplitude modulation (QAM)—but differ in the way they apply it.
Quadrature Amplitude Modulation: (QAM):
QAM, a bandwidth conservation process routinely used in modems, enables two digital carrier signals to occupy the same transmission bandwidth. With QAM, two independent message signals are used to modulate two carrier signals that have identical frequencies, but differ in amplitude and phase. QAM receivers are able to discern whether to use lower or higher numbers of amplitude and phase states to overcome noise and interference on the wire pair.
Carrierless Amplitude Phase (CAP) Modulation:
Generating a modulated wave that carries amplitude and phase state changes
is not easy. To overcome this challenge, the CAP version of QAM stores
parts of a modulated message signal in memory and then reassembles the
parts in the modulated wave. The carrier signal is suppressed before transmission
because it contains no information and is reassembled at the receiving
modem (hence the word “carrierless” in CAP). At start-up, CAP also tests
the quality of the access line and
implements the most efficient version of QAM to ensure satisfactory
performance for individual signal transmissions. CAP is normally FDM based.
CAP, a single carrier system, has several advantages: it is available today at 1.544 Mbps (T1) speeds, and it is low on the cost curve due to its simplicity. It has the disadvantage that it is not a bona fide American National Standards Institute (ANSI) or European Telecom Standards Institute (ETSI) standard.
Discrete Multi-Tone (DMT) Modulation:
DMT offers a multicarrier alternative to QAM. Because high-frequency signals on copper lines suffer more loss in the presence of noise, DMT discretely divides the available frequencies into 256 subchannels, or tones. As with CAP, a test occurs at startup to determine the carrying capacity of each subchannel. Incoming data is then broken down into a variety of bits and distributed to a specific combination of subchannels based on their ability to carry the transmission. To rise above noise, more data resides in the lower frequencies and less in the upper ones.
DMT’s main advantage is the fact that it is the ANSI, ETSI, and ITU
standard. But DMT also has drawbacks: it will initially be more costly
than CAP, and it is very complex. A variant of DMT, discrete wavelet multi-tone
(DWMT), goes a step further in complexity and performance by creating even
more isolation between subchannels. When fully developed, DWMT could become
the ADSL protocol of choice for long-distance transmission in environments
with high interference.
Other versions of DMT, including Synchronized DMT and “Zipper” are
being proposed for use with VDSL.
Factors Affecting the Experienced Data Rate:
DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without repeaters is 5.5 km (18,000 feet). As distance decreases toward the telephone company office, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24 gauge wire carries the same data rate farther than 26 gauge wire. If you live beyond the 5.5 kilometer range, you may still be able to have DSL if your phone company has extended the local loop with optical fiber cable.
DSL Components:
The existing Copper Wire Infrastructure, basically the traditional ILEC/PTO network was designed to carry voice traffic and performs this function extremely well. However, in general, the existing telephone network is not particuarly adept at carrying high-speed data.
In figure1, we show a traditional ILEC/PTO network configured to support
low speed data (e.g. 28.8 kbps) as well as higher speed data. At the customer
location, a standard analog modem is utilized for the provisioning of low-speed
connectivity to the local access network, whereas a Digital Service Unit
(DSU) or Network Termination Unit (NTU) is utilized for higher speed digital
connections such as 56/64 kbps or T1/E1 services.
fig 1: (Adding
data to traditional Voice network)
As we move from the low-speed analog world to the higher speed digital world, we notice an important change has taken palce in the topology at the CO. Whereas analog modem traffic can be carried through the telephone switch (providing worldwide dialing capability), high-speed data will typically bypass the switch altogether. This is largely because telephone switches are not designed to carry high-speed data.
In figure2, we can trace the path of the high-speed data ciruits across the local loop, through the DACS and transmission system, bypassing the telephone switch. Since the DACS is used as the basis of transport, Time Division Multiplexing (TDM) technology is used throughout the network.
In general, it can be said that low-speed data services based on traditional modem technologies integrate well into the POTS network, since the telephone switch is integral to the solution, whereas higher speed services must be configured as a dedicated network, bypassing the switch entirely. This concept will be extended as we explore specific configurations of DSL-based services.
DSL technology, when deployed in the local loop, enables high-speed access service without repeaters. When DSL-based services are provisioned, data received in the CO bypasses the telephone voice switch, is concentrated, and handed off to the inter-CO DACS network. As we will show in the following section, a Digital Subscriber Line Access Multiplexer (DSLAM) can be utilized to group data channels before the handoff. Furthermore, we will show how packet and cell multiplexing technology in addition to TDM can be introduced into the DSLAM resulting in much greater bandwidth efficiency.
Given below we are presenting a DSL network reference diagram. Several types of data networking equipment are required for the deployment of high-speed DSL-based services. In Figure, we show a multiservices DSLAM located at the CO and a DSL Remote Transceiver Unit (ATU-R) located at the home or the remote office. Transmission speeds of 7 Mbps and beyond are possible depending upon a number of factors including equipment, loop length, and condition of the loop.
fig 2:
(ADSL based Services Reference Diagram)
Now let's describe each specific component and its function within a comprehensive DSL solution.
Transport System
This component provides the carrier backbone transmission interface for the DSLAM system. This device can provide service specific interfaces such as T1/E1, T3/E3, OC-1, OC-3, STS-1, and STS-3.
Local Access Network
The Local Access Network utilizes the local carrier Inter-CO network as a foundation. In order to provide connectivity between multiple service providers and multiple services users, additional equipment may be required. Frame Relay switches, ATM switches and/or routers can be provisioned within the access network for this purpose. More and more, ILECs/PTOs are looking to ATM equipment to fill this role.
Now consider the concept of an Access Node or AN. The AN is the place where switches and/or routing equipment are physically located. Depending upon the scale of the desired access network and the costs associated with transport, we can expect to find one or more ANs per Local Access Network thus creating an overlay structure on top of the Inter-CO network. In some cases, the AN is integrated within the DSLAM.
Digital Subscriber Line Access Multiplexer (DSLAM)
To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections.
Residing within the CO environment, the DSLAM is the cornerstone of the DSL solution. Functionally, the DSLAM concentrates the data traffic from multiple DSL loops onto the backbone network for connection to the rest of the network. The DSLAM provides backhaul services for packet, cell and/or circuit-based applications through concentration of the DSL lines onto 10Base-T, 100Base-T, T1/E1, T3/E3 or ATM outputs.
In addition to concentration functions and depending upon the specific service being provisioned, a DSLAM will provide added functions. The DSLAM may, in some cases, be required to open data packets in order to take some action. For example, in order to support dynamic IP address assignment using the Dynamic Host Control Protocol (DHCP), each packet must be viewed in order to direct packets to the proper destination (this is referred to as a DHCP-relay function).
Key features to consider in a DSLAM include:
Feature: Multiservices Support
Benefit: Investment Protection. As the DSL market grows, application
diversity will grow too. Multiservices DSLAMs negate the need for multiple
independent chassis, but rather utilize circuit cards within the DSLAM
to support various services and applications.
Feature: DSL Line Code Support
Benefit: Deployment flexibility. The DSLAM should support a
variety of DSL line codes (e.g., CAP, DMT, QAM) and line protocols.
Feature: DSL Line Aggregation
Benefit: Economy of scale and space saving benefits. Multiple
DSL lines aggregated onto a single output for network connection is more
efficient and cost effective than a 1:1 ratio.
Feature: Scalability
Benefit: Flexibility to support from a few to many service
users at competitive price points.
Feature: Maintainability
Benefit: NEBS compliance for ease of deployment and ongoing
maintenance.
Feature: SNMP
Benefit: Standards oriented for compatibility with various NMS
platforms and reliable end-to-end network management.
DSL Transceiver Unit (ATU-R)
The remote transceiver unit is the customer site equipment for the service users' connection to the DSL loop. The ATU-R connection is typically 10Base-T, V.35, ATM-25, or T1/E1. Multiport device support for voice, data, and/or video is also possible.
ATU-Rs are available in a number of different configurations depending upon the specific service being provisioned. In addition to providing basic DSL modem functionality, many ATU-Rs contain additional functionality such as bridging, routing, TDM multiplexing or ATM multiplexing.
Bridged endpoints serve the marketplace well with their ease of installation and maintenance. Any bridged implementation device should feature a learning filter in order to keep unwanted traffic from traversing the network.
Routed endpoints offer IP flexibility at the customer site. With an IP aware endpoint, subnets can be created and maintained allowing efficient segmentation of the remote LAN as well as Multicast and Unicast downstream recognition. Multiple service domains can also be utilized by the remote LAN users at the same time. Multiple service domains become important when you have a large group of users who need to access different service providers like the corporate LAN and the Internet through different ISPs.
Protocol transparent endpoints behave very much like a DSU/CSU. They provide an interface to the DSL link for existing routers and/or FRADS. The routers and FRADs handle all of the connected LAN's traffic management while the ATU-R passes all traffic to the upstream DSL link. Channelized TDM endpoints can operate like DSU/CSUs for traditional T1/E1 service. They also provide interface to routers, FRADs, multiplexers, PBXs or any other device accustomed to a traditional service.
The ATU-R should be designed so that it can be installed with little or no configuration necessary. Additionally, many service providers have mandated that the ATU-R be installed by the service user necessitating plug-and-play characteristics.
The ATU-R should be highly manageable by the service provider. Features to look for are:
- Ability to provide Layer 1 and 2 management statistics such as signal-to-noise
ratio.
- Ability to provide Layer 3 MIB statistics such as packet counts.
- Devices that are fully manageable by the service provider without
the need of on-site personnel.
- Ability to be remotely downloaded with new software as required.
POTS Splitters
This optional device resides at both the CO and service user locations, allowing the copper loop to be used for simultaneous high-speed DSL data transmission and single line telephone service. POTS splitters usually come in two configurations -- a single splitter version designed for mounting at the residence and a multiple splitter version designed for mass termination at the CO. Note that while many DSL line coding schemes support a single channel of POTS service, others do not.
POTS splitters can be either passive or active. The active POTS splitter requires an external power source for voice and DSL to operate over a single copper pair. The passive POTS splitter requires no power and will typically have a higher MTBF than its active counterpart. While the passive POTS splitter supports lifeline services such as 911 in the event of a DSLAM or ATU-R power loss, the active POTS splitter must have power backup in order to provide these critical services in the event of a power loss.
END-TO-END Network management component:
Perhaps one of the most essential elements of a comprehensive DSL system
is the Network Management System (NMS). Business-critical applications
require reliable network management support, and this key attribute
should be factored into any DSL-based services implementation plan.
The DSL system and its management components should be securely partitionable to allow management by the service provider and the commercial or residential service user. Given that the size of a DSL network may vary from small to extremely large, the management system must also be scalable to accommodate these differences without a loss of management functionality.
CO (Central Office) Locations:
Because DSL technologies are limited to finite lengths of copper wire, it is often interesting to know how far you are from your central office (the local Telco wire center that houses the other end of your telephone local loop).
Finding this information is problematic. Perhaps you know of a Telecom facility in your local neighborhood. Alternatively here are some other options: Mapquest.com has a locator tool (telephone area code search) where, given an area code and the prefix, it will locate on a map the CO for the area code, prefix combination.
A database that covers the entire US is available at www.telcoexchange.com, where there is a Digital Line Pricing Tool. Fill out the form for this tool, specify DSL as the desired connection type, and the tool will spit out the name of the CO for the specified number. Beware, however, that the distance calculations provided by the tool may be way off.
www.getspeed.com will also provide some CO distance calculations. The US West and the www.telcoexpress.com databases express central office locations in V&H (vertical and horizontal) coordinates, which is a coordinate system developed at Bell Labs and used by most Telecom companies in North America to express CO locations.
DSL Pros and Cons:
DSL pros
A major benefit of DSL for home users is that it is able, in most places, to utilize copper lines. This allows it to make use of infrastructure in place in over 80 percent of homes throughout the world. The ratio of use varies, but copper lines are still the number-one point of connection between home users and the outside world -- even more so than television.
DSL requires fewer pairs of copper lines than T-1 connections, making it competitive with the T-1 market. As noted, it's use of fewer pairs of lines means that new infrastructure does not always have to be implemented when the lines are put into use. This may prove useful in meeting the ever-increasing phone and data wiring needs of today's professional users, particularly in the downtown sector of cities, where the infrastructure of older buildings must be revamped to meet modern communication needs.
One of the biggest drawbacks of ISDN in this area is the overhaul needed for most telephone systems to accommodate its digital services. Some variants of DSL allow use of the same physical line for both DSL and analog voice communications. The same pairs of wires can be used, saving infrastructure and installation costs while providing a level of backward compatibility with most telephones. Another drawback of ISDN is that digital telephones require a separate power source. DSL draws power for the analog voice from existing pairs of copper line, which can be split with a passive electrical device and remain in operation during a general power failure. In emergency power failure situations, this is extremely useful.
DSL can separate analog data modem connections and voice switches. With the growth of the Internet, the number of voice lines used by modems has sky rocketed. More and more LECs complain that they are running out of voice lines as a result. Typically, voice services last only a fraction of the time used by modem users. The deployment of telephone switches and services is balanced by a fine calculation of the ratio of population to actual use. Typically, there are seven to eight users for every available port on a given voice switch. With modem users, this ratio goes way down, and the very expensive switches quite often get used up. With people using data service systems separate from voice lines, these switch ports can again be made available.
DSL can also provide much higher bandwidth than analog modem connections. This will primarily benefit users. The wide range in speeds will allow different applications of the technology, but even at the most basic level, DSL technologies are at least twice as fast as today's hybrid anadigi 56-Kbps modems. In fact, some variants offer such high bandwidth that new services such as digital television are under consideration as a result.
Strategically, DSL is moving customers towards a digital communications network while keeping one foot in analog territory. From the sales point of view, this will open up new markets and services that were not available before, or were available only at high cost. As DSL becomes more widely deployed, the marginal costs of manufacturing and distributing such digital equipment will go down.
DSL cons
No new technology moves forward without problems, so there are downsides to DSL technologies as well. These downsides are most relevant to service providers (SPs), but any costs incurred will inevitably be passed on to consumers.
DSL comes in many varieties, some of which are incompatible. This means that any SP wishing to deploy DSL has to choose one or two methods and stick with them. Because deployment is often on a large or very large scale (from portions of towns to whole states and countries), SPs can't afford to make such decisions lightly. And once an SP has made its decision, its users are confined to utilizing compatible systems, even if they'd prefer more variety. An additional problem lies in the fact that the industry is still so new that no serious effort has been made to standardize products. Not all DSL products made by different vendors are compatible, even those of the same type.
DSL is expensive to the SP and the consumer. Because SPs have to deploy DSLf, they must make sure that they can provide the service as required in all areas of their infrastructure. Preliminary inspection of existing infrastructure can cost tens of millions and the installation of new lines in place of older, degraded-quality lines is easily a billion dollar prospect; and this is in the U.S. alone. For consumers, DSL equipment is expensive compared to modems; although customer premises (CP) DSL devices are falling below $500, central office (CO) devices are relatively expensive and do not yet have high enough density ("modems" per enclosure) to make them effective.
DSL is not a switched service. This means that the copper lines have to go directly between the CP device and the CO device without any repeaters or line amplifications intervening. Because most DSL lines are limited to a maximum of 18,000 feet, SPs have to place CO equipment closer to their users and install additional haul lines to the main switch COs. This often means using more expensive fiber lines for these higher bandwidth congregations of the various individual customer connections.
The non-switched nature of DSL also means that only larger SPs can afford to provide it, and large companies are often not quick to change. In the U.S., SPs are supposed to offer a portion of their infrastructure to competitors, but they don't do it easily. The infrastructure is most often installed and owned by the LEC; the cost of installation is amortized over very long-term (several decades) leases, and the LEC generates a profit from it only after many years. ISPs who see DSL as a great new service opportunity often cannot provide it because provisions from LECs are either slow or lacking. US West, for example, recently completely closed down its LADS (local area distribution services) lines. It had sold unconditioned lines, great for DSL, direct to businesses; these lines were historically used by burglar alarm companies to wire individual houses to their monitoring systems. Several ISPs began to buy LADS lines to operate DSL networks.
DSL provides consumers with higher bandwidth data services, which in turn means that SPs have to upgrade their data service backbone to support additional needs. Long-haul backbones and interconnections are significantly pricey endeavors.
One last element of DSL affects SPs directly. DSL competes with the existing and very fruitful data services market in T-1s and such. In the competitive U.S. telecommunications marketplace, home user and personal services make up the majority of connections, but are actually financially supported by the revenue from services provided to businesses. To interfere with the gross profit margins made on T-1s to support personal services does not make immediate financial sense for SPs. The business services must likewise grow to maintain that balance or the major SPs will suffer seriously. This explains why most of the major DSL deployments in the U.S. today are still targeted at the business market.
In international markets, where PTTs owned by the state government decide on service policy, the chances for DSL implementation are much higher. Even the government, however, has to financially justify its services. Government policy makers aren't all that different than those of competitive business SPs, but governments usually try to plan for nationwide services which can be implemented at one time.
Types of DSL:
ADSL
The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line.
Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL. In some areas, they may need upgrading.
CDSL
CDSL (Consumer DSL) is a trademarked version of DSL that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end. Rockwell, which owns the technology and makes a chipset for it, believes that phone companies should be able to deliver it in the $40-45 a month price range. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.
HDSL
The earliest variation of DSL to be widely used has been HDSL (High bit-rate DSL) which is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).
IDSL
IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.
RADSL
RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.
SDSL
SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.
UDSL
UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL.
VDSL
VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments is not yet determined. A number of standards organizations are working on it.
FreeDSL
A service offering and not a technology, FreeDSL is a company offering free ADSL hardware and setup with no monthly charge for service. For the service, users must agree to provide personal information for demographic use and to have a small navigational bar containing advertising always visible while connected. To get the free DSL modem, you need to refer 10 people to the FreeDSL site. There may be other requirements. FreeDSL reportedly plans to offer optional premium services at a future time.
G.Lite or DSL Lite
G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call "the truck roll." G.Lite, officially ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most widely installed form of DSL.
x2/DSL
x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."
A DSL Summary Table:
| DSL Type | Description | Data Rate
Downstream; upstream |
Distance Limit | Application |
| IDSL | ISDN Digital Subscriber Line | 128 Kbps | 18,000 feet on 24 gauge wire | Similar to the ISDN BRI service but data only (no voice on the same line). |
| CDSL | Consumer DSL from Rockwell | 1 Mbps downstream; less upstream | 18,000 feet on 24 gauge wire | Splitterless home and small business service; similar to DSL Lite |
| DSL Lite (same as G.Lite) | "Splitterless" DSL without the "truck roll" | From 1.544 Mbps to 6 Mbps downstream, depending on the subscribed service | 18,000 feet on 24 gauge wire | The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business |
| G.Lite (same as DSL Lite) | "Splitterless" DSL without the "truck roll" | From 1.544 Mbps to 6 Mbps , depending on the subscribed service | 18,000 feet on 24 gauge wire | The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business |
| HDSL | High bit rate digital Subsriber Line | 1.544 Mbps duplex on two twisted-pair lines; 2.048 Mbps duplex on three twisted-pair lines | 12,000 feet on 24 gauge wire | T1/E1 service between server and phone company or within a company; WAN, LAN, server access |
| SDSL | Symmetric DSL | 1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single duplex line downstream and upstream | 12,000 feet on 24 gauge wire | Same as for HDSL but requiring only one line of twisted-pair |
| ADSL | Asymmetric Digital Sunscriber Line | 1.544 to 6.1 Mbps downstream; 16 to 640 Kbps upstream | 1.544 Mbps at 18k feet;
2.048 Mbps at 16k feet; 6.312 Mbps at 12k feet; 8.448 Mbps at 9k feet. |
Used for Internet and Web access, motion video, video ondemand, remote LAN access |
| RADSL | Rate-Adaptive DSL from Westell | Adapted to the line, 640 Kbps to 2.2 Mbps downstream; 272 Kbps to 1.088 Mbps upstream | Not provided | Similar to ADSL |
| UDSL | Unidirectional DSL proposed by a company in Europe | Not known | Not known | Similar to HDSL |
| VDSL | Very hight Digital Subscriber Line | 12.9 to 52.8 Mbps downstream; 1.5 to 2.3 Mbps upstream; 1.6 Mbps to 2.3 Mbps downstream | 4,500 feet at 12.96 Mbps;
3000 feet at 25.83 Mbps; 1000 feet at 51.84 Mbps |
ATM networks;
Fiber to the neighbourhood |
Four Quick Facts About DSL:
DSL is fast! - DSL modems are much faster than analog modems. Different varieties of DSL provide different maximum speeds, from twice as fast to approximately 125 times faster than a 56.6K analog modem. The only speed limit with DSL is the speed of the Internet and all the different computers attached to it.
DSL doesn't tie up your phone line! - DSL doesn't interfere with phone calls, even though it uses your regular phone line. What this means is that you can be on the Internet and you can pick up the phone and make a phone call on the same line. With DSL, you won't have to worry about missing calls, or logging off the Internet to order a pizza, and then logging back on when you're done with the call.
DSL is always on! - Your DSL connection is always there. There's no need to dial up and listen to your modem squawk every time you want to do something online. And there's no frustration about the line dropping when you're in the middle of browsing or downloading. Want to check your e-mail? Set up your computer to check for new e-mail and notify you when you receive something instead of logging in and checking it yourself. Want to look at just one web page? Just open your browser and look. With DSL, you are always online!
DSL is reliable! - Phone company networks are among the most reliable in the world, experiencing only minutes of downtime each year.
DSL hardware information:
If you are going the ADSL route, there are two types of modems that will work: internal and external. Internal modems are cards that are installed inside your computer via a plug-in card. External modems can be connected to your computer via a USB port, Ethernet jack, or a parallel connection. If your computer is already configured to operate on a local area network (LAN), it will have an interface that can connect to an external modem.
It is important to make sure that the DSL modem (sometimes referred to as a "terminal adapter") works with the DSL provider's equipment. This should not be much of an issue to concern yourself with now. In every case to date, the DSL modem is included in the package with the high-bandwidth service sold to you by the DSL provider. This will probably change in the future as more manufacturers produce equipment based on universal standards.
If you have more than one computer at home, they can all be connected to one DSL modem using a home network. One option is to buy an Ethernet hub and connect all your computers to it, much like a small-office local area network (LAN). You can then connect the hub to your DSL modem and all the computers can access the DSL connection. There are some DSL modems that include an Ethernet hub. One disadvantage of this approach is that you would need to string special wiring throughout your home to connect the computers. Another option is to use one computer as a "gateway" to other computers in the home via home networking technology.
There are three types of home networking that don't require any new wiring in the house: power line, phone line, and wireless. Power line technology uses the electrical wiring and outlets of your home to create a network. Phone line networking does the same thing using the telephone wiring and outlets-and it does not interfere with phone calls on the same wires. Wireless technology accomplishes the task using two-way radio waves transmitted through the house. Overall, using a PC as a gateway has the disadvantages of requiring some technical expertise, requiring the gateway PC to be turned on for other PCs and networked devices to use the Internet connection, and lacking reliability as PCs often crash or lock up. A new category of ADSL equipment known as residential gateways is emerging. These dedicated devices act as a bridge between the Internet and the home LAN. It is a specific purpose, stand alone box that acts as a DSL modem and a home networking hub for multiple PCs. The main advantages of a residential gateway will be their ease of use and reliability. More intelligent residential gateways will provide additional capabilities such as enhanced telephone features and entertainment services. 2Wire is currently developing the next generation of advanced residential gateways.
DSL Service provider:
Choosing a Service Provider is difficult task. You can choose one of the following.
1. Traditional Telephone Companies
2. New, Competitive Telephone Companies
3. Internet Service Providers
Because DSL uses phone lines, many of the same companies that provide Internet access with an analog modem will be offering DSL service. There are three types of providers: 1) traditional telephone companies, 2) new, competitive telephone companies and 3) ISPs (Internet service providers).
Traditional telephone companies include all the companies that used to provide telephone service in one area on a monopoly basis. These include huge companies such as Southwestern Bell, Pacific Bell, Bell Atlantic, BellSouth, USWest, Ameritech and GTE, as well as very small companies that serve a single service area. The larger phone companies as well as many of the smaller ones offer DSL in at least part of their service area, with coverage areas increasing every month.
The new, competitive telephone companies (created after The Telecommunications Act of 1996 was passed) that compete to offer local phone services will also offer DSL service. Today, businesses, rather than individuals, are the principal users of these companies' services.
Internet service providers (ISPs) provide access to the Internet. The ISPs that offer DSL usually don't own the equipment that makes the service possible. Instead, they buy the service from a traditional phone company or one of the newer competitive ones. The distinctions between telephone companies and Internet service providers are already blurred because ISPs can also be telephone companies. Also, many telephone companies sell Internet access. The terms NSP (network service provider) and USP (universal service provider) are coming into use to describe these companies that sell many different communication services.
What computer hardware do I need to support a DSL?
| Windows 95/98/NT4.0 | Minimum | Recommended |
| CPU | Pentium or higher | Pentium 166 equivalent or higher |
| RAM | 16MB | 32MB |
| Disk Space | 125MB | 125MB |
| Macintosh | Minimum | Recommended |
| Operating System | 7.6.1 or higher | 7.6.1 or higher |
| CPU | PowerPC 601 | PowerPC 603 or higher |
| RAM | 24MB | 32MB |
| Disk Space | 50MB | 50MB |
Conclusion:
xDSL technology—with its ability to support voice, content-rich data, and video applications over the installed base of twisted-pair copper wires—is inherently suited to meet user demands for broadband, multimedia communications. The most promising of the xDSL technologies for integrated Internet access, intranet access, remote LAN access, video-on-demand, and lifeline POTS applications in the near term is ADSL or R-ADSL (a rate-adaptive version of ADSL). During the past year, ADSL has concluded trials by more than 40 network service providers throughout the world, primarily in North America and northern Europe.
Service introduction began in 1997, but ADSL service is still being
rolled out in many areas. In the meantime, xDSL technologies and standards
will continue to evolve, as will user demand for these emerging services
relative to other local access service alternatives.