DSL - Interview Questions

DSL stands for Digital Subscriber Line. It is a copper loop transmission technology that satisfies bottleneck problem often associated with the last mile between the network and service providers.

The dissipation of the power of a transmitted signal as it travels over the copper wire line. In-home wiring also contributes to attenuation.

These are unterminated extensions of the loop, which cause additional loop loss with loss peaks surrounding the frequency of the quarter wavelength of the extension length.

The interference between two wires in the same bundle, caused by the electrical energy carried by each of them.

DSL Home is an initiative taken by DSL-Forum. To define requirements related to home devices like residential gateways, VoIP devices and local & remote management of home devices.

Voice, video, data, including IPTV, video on demand, content on demand, et

DSL Home remote management protocol (TR-69) and its extensions are access agnostic.

The TR64 protocol is used for DSL local management.

TR111 allows TR69 remote management for the devices in the Home Network (HN).

TR-98 and TR-133: Configuration and Management of Service differentiation (QoS) parameters in the CPE devices through TR-69 and TR-64 respectively.

• TR-104 Data model for VoIP services is available for Video services too.

• TR-106 defines the common data model template. Defines the baseline object structure and set of accessible parameters for a TR-69 device.

• TR-122 defines Voice ATA Requirements.

• WT-135 is the object model for the STB device.

• WT-140 is the Object Model Network Storage Device.

• WT-142 is the framework for TR-069 enabled PON device.

The following services are offered by DSL Home TR-69 −

• Remote management of the devices in a secure manner (uses SSL/TLS based security)
• Real-time provisioning of services via auto-configuration
• Status and performance monitoring
• Diagnostics
• Access Control
• Notification
• Firmware upgrade

The following services are offered by TR-064.

• Adopts the UPnP v1.0 architecture and extends the UPnP IGD v1 specification (with some restrictions).

• A management application (TR-64 control point) runs on a PC and it pushes the service provider and the customer specific configuration to a CPE. This happens when the CPE adds to the network.

• More useful during the initial installation of new CPE devices and when there is WAN side connectivity issues.

SNMP stands for Simple Network Management Protocol. Use of SNMP requires opening of SNMP port through NAT as most of the home gateways use NAT and the devices being managed could be behind NAT. In SNMP, the request to get/set any parameters is always initiated by the manager hence the port has to be opened on the CPE to get the request. In TR-69, a TR-69 session is initiated by CPE and the server uses the same session to send get/set requests. That does away with opening of the port explicitly in NAT environment.

TR-69 also defines a way where ACS can send the request to CPE and this part is taken care by TR-111 part2 transparently. Most of the SNMP implementations existing today do not implement SNMPv3, hence the messages exchanged over SNMP is not very secure. In TR-69, the security is taken care through the SSL/TLS or HTTP based authentication schemes. Most of the TR-69 implementations as of today implement SSL/TLS.

Following are the components of the DSL System

• Transport system
• Local Access Network
• Multi Service DSLAM
• DSL Modem/ Router
• POTS Splitters and Microfilters

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, OC-12, STS-1 and STS-3.

The local access network uses inter-CO local carrier network as a foundation. To provide connectivity between multiple service providers and users of multiple services, additional hardware may be required. Frame Relay switches, ATM switches and / or routers may be provisioned in the access network for this purpose.

Increasingly, ILECs and PTO are looking for ATM equipment to fulfill this role, and nextgeneration DSLAM include ATM switching to accomplish it.

Residing in the CO environment (or in a space of near virtual collocation), the DSLAM is the cornerstone of DSL solution. Functionally, the DSLAM concentrates the data traffic from multiple DSL loops on the base network for connection to the rest of the network.

The DSLAM provides backhaul services for the packet, cell and / or circuit-based applications through concentration DSL ON 10Base-T lines, 100Base-T, T1 / E1, T3 / E3 ATM or outputs.

The criterion for assessment modem / DSL Router is the customer site equipment to connect the service user to DSL loop. The end point of DSL is generally 10/100Base-T, V.35, ATM, or T1 / E1, with new generations of consumer products also support methods such as USB, IEEE 1394 (Firewire) and factor internal PCI form.

Additionally, CPE parameters are being developed with additional ports designed to support specific applications, such as RJ11 ports for support of voice (for e.g. IADs for service VoDSL), ports Video for video services based on DSL, and new networking interfaces such as Home Phoneline Networking Alliance (HomePNA) or wireless network such as 802.11 wireless Ethernet interfaces.

POTS splitters option lie in both CO and service user’s slots, allowing the copper loop to be used for media transmission simultaneously for DSL high-speed data and the single line telephone service, when the DSL variant used these services.

Microfilter is a filter "low pass" that allows voice-band services to be transmitted while filtering the high frequencies used by DSL and eliminates interference.

The following table explains the ADSL standards available as of today.

The following points explain what ADSL technology is −

• Discrete Multi-Tone (DMT) modulation used by all ADSL standards for physical layer.

• Divide the frequency band into many small channels.

• QAM modulation on each channel.

• Different bits assigned to each channel in terms of SNR.

The working Group T1E1 ANSI established a standard known as ANSI TR59 RADSL. The FCC has specifically cited RADSL as a technology that is spectrally compatible with voice and other DSL technologies in the local loop.

Beyond the bandwidth of 144 Kbps provided by IDSL, there are new technologies that have emerged that can be better-classified office / small office and residential home (SOHO) possibilities. These technologies offer operating ranges between 128 Kbps and 2.048 Mbps.

For symmetric applications, Multirate SDSL (M / SDSL) has emerged as a valuable technology to meet the requirements of carriers to deliver Time Division Multiplex (TDM) services on an almost ubiquitous base. Based on the single pair SDSL technology, M / SDSL supports changing rate of command line transceiver and thus the operating distance of the transceiver.

This version of the CAP supports eight separate rates for a service 64 Kbps / 128 Kbps to 29 kft (8.9 km) 24-gauge wire (5mm) and 15 kft (4.5 km) at a speed of 2 Mbps in full. With a capacity of AutoRate (similar to RADSL), symmetric applications can now be universally deployed.

Following are the benefits of RDSL −

• No POTS splitter is required at the customer premises.

• In addition to ADSL systems, which can generally reach distances below 18 000 feet from the central office, the ReachDSL systems extend well beyond services 20,000 feet, with some power plants above 30 000 feet.

• Lower product cost – Since ReachDSL products utilize "off the shelf" rather than customized Digital Signal Processors (DSPs).

• Dynamic bandwidth allocation – Allows the service to be customized for different applications.

Following are the benefits of ADSL2 −

• ADSL provides up to 8Mbps/800Kbps data rate (possibly 12M/1.2M)

• Reach of 18-20kf 26AWG (about 6000m)

• No seamless rate change

• No power saving mode when there is no user activity

• No 1-bit per bin and partial byte per symbol

• Fixed 64Kbps overhead channel rate (Framing Structure3)

Following are the benefits of ADSL2+ −

• ADSL2+ provides up to 24Mbps/1Mbps data rate

• Seamless rate adaptation when SNR change

• Power Management greatly reduce power consumption

• 1-bit per bin and partial byte per symbol improves reach

  • Reach of 20-22kf 26AWG (about 7000m)

• Variable overhead channel rate meets user need

• Loop diagnostic function during training

ADSL2 and ADSL2+ delivers next generation features to improve the DSL deployment business case −

• Higher Rates
• Extended Reach
• Improved Stability
• Power Management
• Enhanced Spectral Compatibility

The following points explain the features of ADSL2+ −

• Doubles downstream spectrum from 1.1MHz to 2.2 MHz with downstream bin number increased from 256 to 512.

• Maximum downstream data rate increase from 8Mbps to 24Mbps.

• Improved performance at short loop length (more on this under Spectrum Management topic).

• Wider range for SRA and Power Management - from 32Kbps to 24Mbps.

VDSL stands for Very High Bit Rate Digital Subscriber Line. It is the technology that has the highest rate of DSL. Operating at speeds up to 52Mbps, VDSL is the next generation of DSL technology with higher throughput and requirements for implementing simpler than ADSL.

VDSL began its life being called VADSL, but was renamed VDSL by the ANSI working group T1E1.4. The main reason T1E1.4 decided VDSL on VADSL was that, unlike ADSL, VDSL is both symmetric and asymmetric. VDSL is nearly ten times faster than ADSL and is over thirty times faster than HDSL. The tradeoff for increased speed loop length: VDSL has a shorter reach in the loop.

The following table describes the different variants of VDSL today.

VDSL is designed to offer a multitude of asymmetric broadband services, including digital television broadcasting, video on demand (VoD), high-speed Internet access, distance learning and telemedicine, to name a few.

The delivery of these services requires the downstream channel to have a higher bandwidth that the channel upstream and is asymmetrical. For example, HDTV requires 18 Mbps for video content downstream. Upstream, however, it does not require the transmission of signaling information (e.g., change of channel or program selection), which is of the order of kbps.

VDSL is also designed to provide symmetrical services for small and medium business customers, business enterprise, high-speed data applications, video conferencing and tele-applications, etc. Symmetric VDSL can be used to provide short-haul T1 replacements NXT1 rate and supports a host of other business applications.

The following table describes the service and rate comparison between ADSL and VDSL technology.

Following are the key features of VDSL2.

DMT modulation −

• Same as ADSL
• Bandwidth increased from 30 MHz ( 14x ADSL2+)
• Up to 4096 tones (8x ADSL+!)

Worldwide Versatile Standard −

• 8 Profiles defined for different services
• Different band plans for different regions
• Variety of PSDs to optimize spectral compatibility

Support for a variety of Services −

• Integrated Quality of Service features
• ATM as well as Ethernet payload
• Channel bonding for extended reach or rate

Discrete Multi-Tone (DMT) is a method of separating a Digital Subscriber Line (DSL) signal, so that the usable frequency range is separated into 256 frequency bands (or channels) of 4.3125 KHz each.