ADSL Broadband : Interview Question and Answers

Question 1: What is adsl?

Answer : Asymmetric Digital Subscriber Line is a high-speed Internet access service that uses your telephone line to send and receive Internet data at much faster speeds than a conventional dial-up connection. With ADSL you are connected to the Internet 24 hours a day (“Always On”). You no longer need to dial up to open your Internet connection. You can also make and receive telephone calls while online. This is because the ADSL signal operates on a digital frequency, while the voice signals (telephone conversation), operate on an analogue frequency. Voice data and Internet data can, therefore, travel along the line simultaneously. The "asymmetric" in ADSL refers to the fact that the data being received by your computer from the Internet (downstream data / downloading), travels at a faster speed than data travelling from your computer to the Internet (upstream data / uploading).

 Most general users will download more data than they will upload, so the slower upstream speed is not a problem. For example, when you click on a hyperlink, your browser sends a few characters upstream to request the page that the hyperlink points to, and the page then comes downstream (downloads) to your PC so you can see it.

Question 2 : How Does Adsl works?

Answer : The telephone system was originally designed to carry the voice along copper telephone wires. The voice is converted into small electric pulses which travel along the telephone wires. These electric pulses are then converted back into sound at the other end, so the other person can hear you. This happens in both directions so you can have a conversation.

ADSL also works through the copper wires of your telephone line. However, Internet data travels along the wires at a different frequency to the voice signals. When ADSL is installed a micro-filter is plugged into your phone connection which separates the frequency of voice signals (telephone line) from that of your Internet data (broadband connection) and it is this that allows you to surf the web and talk on the phone at the same time.

Question 3 : What is the speed of ADSL?

Answer : ADSL is fast. Compared to a dial-up modem, which does  56kbits per second downstream, ADSL is up to ten times faster. That means that a large download that might take ten minutes on a dial-up modem will happen in around a minute on ADSL.

Question 4 : What are the reasons to get low speed broadband?

Answer : 

·  Quality of your phone line - areas with better copper wiring can achieve somewhat faster DSL speeds.

·  Length of the phone line between the residence and the phone company hub (exchange hub). DSL technology is "distance sensitive" - its performance decreases significantly as you get further away from this hub.

·  Service glitches. While normally a constant, DSL speed can suddenly drop if the service provider has technical difficulty with their network.

Question 5 : What are the factors that you can control directly to increase your DSL speed from your end?

Answer : 

• Spyware on computer(s). Even when the DSL network may be functioning at full speed, spyware programs may be consuming the bandwidth.  Anti-spyware programs should be run regularly on networks to prevent this problem.
• Wireless router. Routers sit between your computers and the Internet connection. If not functioning properly, a router can greatly limit the DSL speed. Temporarily connecting a computer directly to the Internet can help identify this situation.
• Slow wireless network connection. In extreme cases, a very slow Wi-Fi connection between a computer and a wireless home network will not keep pace with the speed of the DSL Internet connection. Improving the quality of the Wi-Fi connection will solve this problem.
• Old computer(s). Very old computers lacking sufficient processing power or memory cannot keep pace with a high-speed DSL connection. You can verify this problem by comparing the DSL speed between computers in your home or buy a new computer.

Question 6 :What is line attenuation?

Answer : 

Attenuation is the loss of signal over distance.

* 20bB. and below = Outstanding * 20dB-30dB. = Excellent * 30dB-40dB. = Very Good * 40dB-50dB. = Good * 50dB-60dB. = Poor and may experience connectivity issues * 60dB. and above = Bad and will experience connectivity issues Line attenuation also affects your speed. * 75 dB+: Out of range for broadband * 60-75 dB: max speed up to 512kbps * 43-60dB: max speed up to 1Mbps * 0-42dB: speed up to 2Mbps+

Question 7 : What is SNR?

Answer :  Signal-to-noise  means  the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power, often expressed in (dB).

6dB or below noise margin is bad, it will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems (if no large variation)
20dB-28dB is excellent
29dB or above is outstanding

Question 8 : What is SLA?

Answer :  Service Level Agreement, a contract between an Provider and the Client which commits the Provider to a required level of service. An SLA should contain a specified level of service, support options, penalty provisions for services not provided, a guaranteed level of system performance as relates to downtime or uptime, a specified level of customer support and what software or hardware will be provided and for what fee.

Question 9 : What is the difference between PPPoE and Bridge Mode?

Answer : PPPoE mode

1. User id and password stored inside the Modem.

2. Multiple PCs can be connected. For example most of the basic ADSL Modems having at least one ADSL port and one USB port. In PPPoE mode, can connect one PC to Ethernet port and one PC to USB port which enable simultaneous internet usage in both the PCs.

3. PPPoE mode is more secured.

4. NAT can be enable in PPPoE mode.

Bridge Mode

1. In bridge mode user id and password to be entered in the dialer of computer.

2. Only single PC can be connected.

3. Less secured because all the ports are open need good firewall to avoid virus infection.

Question 10 : What is Last Mile?

Answer : In telecommunications technology, connectivity between the  customer's home to the  telephone company via Copper cable called last mile.

Question 11 : What is leased Line?

Answer : A permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect long distant offices. Unlike normal dial-up connections, a leased line is always active. The fee for the connection is high. Because the connection doesn't carry anybody else's communications, the carrier can assure a given level of quality.

What is the Function of BRAS or BNG

• Aggregates the circuits from one or more link access devices such as DSLAM
• Provides layer 2 connectivity through either transparent bridging or PPP sessions over Ethernet or ATM sessions
• Enforces quality of service (QoS) policies
• Provides layer 3 connectivity and routes IP traffic through an Internet service provider’s backbone network to the Internet
• Release Public ip to the subscriber

Question 12 : What it the Function of DSLAM?

Answer : The DSLAM equipment collects the data from its many modem ports and aggregates their voice and data traffic into one complex composite "signal" via multiplexing. Depending on its device architecture and setup, a DSLAM aggregates the DSL lines over its Asynchronous Transfer Mode (ATM), frame relay, and/or Internet Protocol network (i.e., an IP-DSLAM using PTM-TC [Packet Transfer Mode - Transmission Convergence]) protocol(s) stack.

The DSLAM acts like a switch since its functionality is at Layer 2 of the OSI model. Therefore it cannot re-route traffic between multiple IP networks, only between ISP devices and end-user connection points. The DSLAM traffic is switched to a BRAS where the end user traffic is then routed across the ISP network to the Internet.

Question 13 : Explain Broadband Call Flow?

Answer : 

1. When the PPPoE client is dialed, the PPPoE client broadcasts a PPPoE Active Discovery Initiation (PADI) message to BNG.
2. The BNG that receive the PADI message respond with a PPPoE Active Discovery Offer (PADO) message advertising the domains and contexts they support if configured to advertise.
3. The PPPoE Client sends a PPPoE Active Discovery Request (PADR) message to the BNG.
4. The BNG receives the PADR message and prepares to bring up the PPP session.  Then responds with PPPoE Active Discovery Session-Confirmation (PADS) message advertising PPPoE session ID.
5. The PPPoE Client sends PPP LCP Configuration Request message advertising to BNG.
6. The BNG responds with PPP LCP Configuration Ack if PPP parameters are acceptable.
7. The BNG then sends PPP CHAP Challenge to PPPoE Client.
8. The PPPoE Client responds with PPP CHAP Response.
9. The BNG sends  Access-Request packet to AAA Server with the session’s PPP username and CHAP password.
10.   AAA sever reply Access-Accept to BNG and BNG will release public ip to the subscriber.

Question : 14 What is the difference between PPPoE and PPPoA?

Answer : Point to Point Protocol over Ethernet (PPPoE) and Point to Point Protocol over ATM (PPPoA) are both technologies that offer a provider the opportunity to roll out broadband services. This document will show why PPPoE should be chosen over PPPoA.

Advantages of a both PPPoE & PPPoA based broadband service

• End user authentication to the network, forces the end user to authenticate to AAA server before being allowed access to the network
• Billing options, gives the provider the ability to offer different billing options, by time, by data, unlimited or by services purchased
• IP address conservation, a provider can limit the number of IPs a specific user can receive or can force a user off the connection at will
• Trouble shooting, a provider can easily tell what users are on or off on a per user basis
• Scalable, all authentication, authorization, and accounting can be handled for every user using existing AAA server
• Invisible to end user, both can be integrated in the Customer Premise Equipment CPE making the connection process invisible to the end user – though we do recommend they stay off the CPE but it is easily done and PPPoA is traditionally only on the CPE
• Service Selection, both can be used to offer multiple services and service selection

Negatives of PPPoA

• Only a single session per CPE can be established. In PPPoE we offer the ability to log in to multiple services or create multiple sessions all at the same time over the same line
• CPE setup and access, in general PPPoA must be configured on the CPE itself, PPPoA software is not available on platforms. Either the CPE must support PPPoE or an ATM network interface must be installed in the PC. ATM network interface cards are expensive and both can be difficult for an end user to configure. Once an end user has the ability to configure the CPE it opens up the problem for incorrect configurations making trouble shooting by the provider very difficult resulting in increased support costs

Positives of PPPoE

• Brand management, by having the customer use software to log on and off they force the user to see the providers brand, logo or company name. Word of mouth is the strongest form of advertising
• Increased revenue opportunities, with PPPoE we can offer the ability to log in to multiple services or create multiple sessions all at the same time over the same line. What do we mean by multiple services? How about a special account for a child that gives them G rated access only? How about a dedicated movie or music server?
• Ease of support, PPPoE software has troubleshooting and help files built in to the application. If an error occurs the error message can be present to the end user with a possible resolution preventing the customer from ever having to call the provider for help. In PPPoA everything is done in the CPE meaning when a problem occurs the only notification is lights on the CPE which tell a user nothing
• End User Familiarity, client side software present users with the familiar look and feel of dial-up which if purchasing a broadband service means they are almost 100% familiar with the dial-up look and fee reducing their learning curve to use the new service
• Works in existing environment, if a provider already offers an Ethernet based service PPPoE can be implements without changing the existing CPE, as mentioned above PPPoA needs an ATM interface