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 high-speed 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: video-on-

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