Download TEL 355: Communication and Information Systems in Organizations

TEL 355: Communication and
Information Systems in
Organizations
Asymmetric Digital
Subscriber Line (ADSL)
Professor John F. Clark
Modem Basics
• Short for Modulate/Demodulate
• Converts digital signals to analog for
transmission over the phone network
• Converts analog signals back to digital for
reception by another computer
• All modems use compression to achieve
higher line speeds and error checking to
examine packets and request retransmission
Definition of ADSL
• Modem technology that converts existing
2X phone lines into access paths for highspeed communications
• It’s asymmetric – downstream speeds are
faster than upstream speeds
• Increases dial-up line speeds by 1000-fold
• Can transmit up to 24 Mbps one way
Asymmetric Digital Subscriber Line
• An evolving high-speed transmission technology –
developed by Telcordia
• Developed with video-on-demand as goal
• Is now ANSI standard T1.413, ITU standard
G.992.1 (8 Mbits/s) and G.992.2 (ADSL Lite)
• ILECs seek to extend the life of network
– Several billion miles of 2X in the local loop
– With ADSL, is valuable in convergence scenario
– CLECs are interested in competing with ILECs
Asymmetric Digital Subscriber Line
• Does not really refer to a line, but to a pair of
modems – one on each end creates a digital
subscriber line
• ADSL can provide up to
– 1.5 Mbps downstream on 2X lines to 18,000 ft.
– 6.1 Mbps downstream on 2X lines to 12,000 ft.
– 8 Mbps downstream on 2X lines to 6600 ft.
• Simultaneous analog telephone service is not an
option – POTS is supplied in the C channel of the
DSL line
T1 and E1
• Early 60s – Bell Labs digitized voice into a
64 kbps stream and multiplexed 24
elements in a channel resulting in 1.544
Mbps, or DS1/T1
• European systems modified the approach
and multiplexed 30 elements for a line rate
of 2.048 Mbps, or E1
• Not suitable for residences because they
require new wire installations and frequent
repeaters and corrupt adjacent 2X pairs
High Data-Rate (HDSL)
• The most mature of the xDSL technologies
• An improved method of transmitting T1/E1
over 2X lines
• Four-wire method uses less bandwidth and
does not require repeaters
• Advance modulation techniques transmits
1.544 Mbps up to 12,000 feet
• Supplies approx. 70% of T1 service in U.S.
Symmetrical DSL (SDSL)
• Single line version of HDSL – often all that
are available, also known as HDSL2
• Transmits up to 2.3 Mbps over single 2X
both ways up to about 10,000 feet
• Supports POTS and T1/E1 simultaneously
• At the same distance, ADSL is capable of
transmitting over 6 Mbps
• Aimed at the corporate user with high
bandwidth needs upstream and downstream
Asymmetric (ADSL)
• Specifically designed for the home user or
small business customer
• Asymmetric channels allow greater data
rates and longer line lengths
– Transmits two separate data streams
– much more bandwidth devoted to the
downstream channel
– best option for most online multimedia: videoon-demand, audio streaming, LAN access
The Market for xDSL Modems
• The first 14.4 kbps modem cost $14,400!
• Fortunately those days are over, but xDSL
market maturity will follow the same plan:
• Consumers will be less interested in
standards and become more interested in
ease of installation and use, plug and play
features, small size and power requirements,
and best speeds over 2X
ATM vs. IP 10/100baseT
• The great debate: what’s the best network
interface for the PC?
• Is it Asynchronous Transfer Mode (ATM) protocol
or the IP-based 10/100baseT Ethernet protocol?
• The two are actually similar and are often used in
conjunction with one another – the differences lie
in the software and hardware
• This is done deal nowadays – who won?
IP Advantages
•
•
•
•
10/100baseT is essentially self-learning
Inexpensive Ethernet cards are common
10/100baseT is a bigtime industry standard
LAN networks are everywhere – connection
is ubiquitous and expertise abounds (sort of)
• PC software and OS drivers are all equipped
to deal with it
ATM Advantages
• Streaming video support is a proven factor
• Mixing of services such as video, telephony
data is much easier
• Traffic speeds conform to standard
telephony transport rates (marginal reason)
• New PC software and drivers will work
with ATM (or not – it depends)
• An idea whose time never came
ATM vs. IP 10/100baseT
• Both usually connect to ATM backbones
• At the moment, IP is winning – Internet use
rules, equipment is available and inexpensive
• But ATM may still be a factor – standards for
most services are complete
• Another but: it will be very hard to overcome
installed base of cheap IP, especially as IP
telephony standards evolve
The Future?
• Standards weren’t an issue with early
modem development, but they are in xDSL
– ATM vs. IP 10/100baseT (IP is the clear
winner)
– Cellular Array Processor (CAP) vs. Discrete
Multitone (DMT seems to be winning)
• Remember, Betamax had better quality
picture, but VHS won the market
Local Prices: Verizon 9/2000
• DSL Bronze Plus: 768k down and 128k up
– your average service at $32.50
• DSL Silver Service: 384k up and down –
for added oomph in transmission at $53.00
• DSL Gold Service: 768k up and down – for
webhosting at $68.00
• DSL Platinum Service: 1.5 Mbps down and
768k up at $95.00; Multiuser at $215.00
Local Prices: Verizon 9/2001
• DSL Bronze Plus: 768k down and 128k up
– your average service at $49.95 per month
• DSL Enhanced Bronze Plus: 1.5 Mb down
and 128k up
– For telecommuters at $59.95 per month
• DSL Silver Service: 384k up and down
– for gamers and conferencers at $69.95 per
• DSL Silver Plus: 1.5 Mb down and 384k up
– Professional model for offices at $79.95
Local Prices: AT&T 5/2007
• FastAccess DSL Lite – 256 Kbps down and 128
Kbps up
– Free modem and $24.95 a month
• FastAccess DSL Ultra – 1.5 Mbps down and 256
Kbps up
– Free modem, $75 cash back and $32.95 a month
• FastAccess DSL Xtreme – 3 Mbps down and 384
Kbps up
– Free modem, $75 cash back and $37.95 a month
• FastAccess DSL Xtreme 6.0 – 6 Mbps down and
512 Kbps up
– Free modem, $75 cash back and $42.95 a month