DSL Broadband Internet Service

DSL, Digital Subscriber Line, is a high-speed Internet service over ordinary phone lines using broadband modem technology. DSL technology allows Internet and telephone service to work over the same phone line without requiring customers to disconnect either their voice or Internet connections.

DSL works by transferring voice and data simultaneously over the existing twisted-pair copper telephone lines by using different frequency ranges on the same line. A DSL modem at the customer residence sends data over the phone line to a DSLAM at the CO. The DSLAM is what terminates and aggregates incoming ADSL lines. The DSLAM redirects voice to the PSTN and data to the Internet backbone connection.

The customer CPE is a DSL modem, which may or may not also be a router. The Speedstream 4200 is an example of a CPE that is both a DSL modem and router, but capable of acting as a pass through modem that may be used with a separate router. The CPE can use either FDM or Echo Cancellation to divide an allocate the available bandwidth of the line. POTS are low pass filters that are used on the customer line to separate low frequency voice signals from high frequency data signals, so that one doesn’t interfere with the other. Voice speech is below 4KHz so the filter blocks anything above that.

Modulation is used to modify the data signal transmitted over the phone line. QAM is a modulation method used and can be implemented in either one of two different competing standards, CAP or DMT. CAP is less common and not considered as "standard" as DMT. Many providers started using CAP and then migrated to DMT. DMT allows for the connection of customers living further from the DSLAM location, and is capable of higher speed. The telcos that switched from CAP to DMT also implement PPPoE with the change. The Cisco 675 was a common CAP only DSL modem/router while the Cisco 678 is capable of both CAP and DMT (with limited speed) after a firmware upgrade. The Speedstream 4200 is a common DMT modem/router used in moden ADSL installations and is made by the Siemens Corporation.

terminology

• DSL - Digital Subscriber Line
• CPE - Customer Premises Equipment
• ADSL - Asymmetric Digital Subscriber Line
• CO - Central Office (service provider)
• DSLAM - DSL Access Multiplexer
• PSTN - Public Switched Telephone Network
• FDM - Frequency Division Multiplexing
• POTS - Plain Old Telephone Service
• CAP - Carrierless Amplitude Phase
• DMT - Discrete MultiTone
• QAM - Quadrature Amplitude Modulation
• DWMT - Discrete Wavelet MultiTone
• R-ADSL - Rate-Adaptive Digital Subscriber Line
• HDSL - High Bit-Rate Digital Subscriber Line
• VDSL - Very High Bit-rate Digital Subscriber Line
• PPPoE - Point-to-Point Protocol over Ethernet