1. Lec # 19 Data Communication
Muhammad Waseem Iqbal

2. Lecture Objectives Introduction to DSL Devices used in DSL Technology
DSL Equipment
DSL Modem
DSLAM
How DSL works?
DSL Modulation
CAP
DMT
DSL Advantages and Disadvantages
DSL vs. T1/E1 lines

3. Introduction to DSL
Digital Subscriber Line provides high speed Internet access using regular telephone lines.
It has the ability to move data over the phone lines typically at speeds from 256kbps to 1.5Mbps – up to 25 times quicker than the fastest analog modems available today (56kbps)
Digital Subscriber Line (DSL) technology is a modem technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers

4. DSL
DSL is a technology that allows high speed data transmission over normal telephone lines at low cost.
There is minimal load on the CO
The ISPs can provide multimedia services using the much higher speeds than offered by normal modem connections.
There are many flavors of DSL, the higher the frequency the greater the effect of distance and crosstalk.

5. Low Data Rates using Dial up Modems
Why ordinary modems supports low data rates?
Telephones were invented for carrying human voice not for data
Human voices can be carried in a frequency range of 300 to 3,400 Hertz
At the end point of local loop in the end office, the wire runs through a filter that attenuates all frequencies below 300 Hz and above 3400Hz, almost 4KHz band
Data are thus also restricted to this narrow bandwidth.
Standard phone service limits the frequencies that the switches, telephones and other equipment can carry.

6. DSL Design Considerations
A few Design considerations were:
Service must work over existing infrastructure (local loops)
It must not effect the customers telephone and fax machines
It must be much faster than 56 Kbps
It should always be on, with just a monthly charge but no per-minute charge.

7. What DSL does?
When a DSL customer subscribes, the incoming line is connected to a different kind of switch at the end office, one that does not have this filter, thus making the entire capacity of local loop available.
In most cases, the wires themselves have the potential to handle frequencies of up to several-million Hertz.
Modern equipment that sends digital (rather than analog) data can safely use much more of the telephone line's capacity, and DSL does just that.

8. Introduction to DSL
Digital Subscriber Line (DSL) gives you broadband access over your existing copper telephone wires
A DSL connection to the Internet is a high-speed, ‘always on’ (you don’t need to dial up your ISP each time you want to connect)

9. DSL standardization
The ANSI T1E1.4 working group are responsible for overseeing the development of the various DSL technologies.
The European Technical Standards Institute (ETSI) also contribute from the European point of view.

10. How does DSL Technology Works?
ADSL works by splitting the phone line into 2 frequency ranges .
Voice Traffic (below 4KHz)
Data Traffic (above 4KHz)
Upstream
Downstream
This makes it possible to use the line for phone calls and data network access at the same time.

11. DSL Equipment ADSL uses two pieces of equipment:
DSL Modem / Transceiver
At customer end
point where data from the user's computer or network is connected to the DSL line.
DSLAM (DSL Access Multiplexer)
At the service provider end (end office)
DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet.

12. DSL Modem
DSL Modem
At the customer's location, there is a DSL transceiver, which may also provide other services.
Most residential customers call their DSL transceiver, a DSL modem.
The DSL transceiver or modem can connect to a customer's equipment in several ways, though most residential installation uses USB or 10BaseT Ethernet connections.

13. DSL Modem
At the customer's location, there is a DSL transceiver, which may also provide other services.
Most residential customers call their DSL transceiver a DSL modem. The engineers at the telephone company or ISP call it an ATU-R (ADSL Termination Unit - Remote).
The DSL transceiver or modem can connect to a customer's equipment in several ways, though most residential installation uses USB or 10BaseT Ethernet connections.
An DSL modem performs the same task as a conventional modem in that, it converts the digital computer signals into analog signals that can be sent down a telephone line.

14. DSL modem

15. DSLAM
A DSLAM is a network device, usually at a telephone company central office, that receives signals from multiple customer DSL connections and puts the signals on a high-speed backbone line using multiplexing techniques.
Depending on the product, DSLAM multiplexers connect DSL lines with some combination of ATM, Frame Relay, or IP networks.

16. DSLAM
A DSLAM is a network device, usually at a telephone company central office, that receives signals from multiple customer DSL connections and puts the signals on a high-speed backbone line using multiplexing techniques.
It also known as ATU-C (ADSL Termination Unit - Central Office)
Depending on the product, DSLAM multiplexers connect DSL lines with some combination of ATM, Frame Relay, or IP networks.

17. DSLAM

18. ADSL Equipment Configuration

19. PPPoE Protocol used in DSL technology
PPPoE stands for Point to Point Protocol over Ethernet.
PPP is usually used over serial communications like dial-up modem connections.
Many DSL Internet service providers now use PPP over Ethernet because of its added login and security features.

20. xDSL
DSL is also known as xDSL, with the ‘x’ standing for various kinds of DSL technologies.
These technologies differ in the connect speed and connection (asymmetric or symmetric) they provide
The term xDSL refers to a number of similar yet competing forms of DSL technologies
E.g. ADSL, SDSL, HDSL, HDSL-2, G.SHDL, IDSL, and VDSL.
xDSL promises to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing Telco infrastructure.
DSL Connection lets you use your telephone lines for making and receiving calls and for Internet access simultaneously

21. DSL Technologies SDSL HDSL HDSL-2 G.SHDSL
ISDN Digital Subscriber Line (DSL)
VDSL

22. ADSL vs. SDSL
A DSL service can be symmetric, in which the downstream and upstream speeds are identical (SDSL), or asymmetric in which the downstream speed is faster than the upstream speed (ADSL).

23. ADSL
Asymmetric Digital Subscriber Line (ADSL) technology is asymmetric.
It allows more bandwidth downstream—from central office to the customer site—than upstream from the subscriber to the central office.
This asymmetry makes ADSL ideal for Internet/intranet surfing, video-on-demand, and remote LAN access.
Users of these applications typically download much more information than they send.

24. ADSL
ADSL service has a maximum distance of 18,000 feet (5,460 m) between the DSL modem and the DSLAM
For speed and quality of service reasons, many ADSL providers place an even lower limit on the distance.
At the upper extreme of the distance limit, ADSL customers may experience speeds far below the promised maximums, whereas customers close the central office or DSL termination point may experience speeds approaching the maximum

25. ADSL Frequency Bands
The ADSL divides the 1.1 MHz spectrum available on the local loop into 3 frequency bands:
POTS
Upstream
Downstream
This is achieved using FDM
ADSL is a physical layer standard

26. SDSL
Symmetric Digital Subscriber Line (SDSL) is a rate-adaptive version of HDSL and, like HDSL, is symmetric.
It allows equal bandwidth downstream from an NSP's central office to the customer site as upstream from the subscriber to the central office.
SDSL supports data only on a single line and does not support analog calls.

27. DSL Advantages
Static IP Address (this is an assigned number that is the signaling method used for all communication over the Internet)
Surf and talk on the phone at the same time
Up to 140 times faster than analog modems
“Always On” connection (No waiting to “dial in”)
Unlimited Internet Access
Your home has its own dedicated connection
Your connection is highly reliability
Your connection is highly secure

28. DSL Disadvantages Expensive Distance dependence Access Asymmetry
Limited availability
Very new technology
Low or no CIR  (Committed Information Rate).