Digital Subscriber Line (DSL)

• After traditional modems reached their peak data rate, telephone companies developed another technology called DSL to provide higher-speed access to the internet.

• DSL is a family of point-to-point technologies to provide high-speed data transmission over traditional telephone lines. These technologies are ADSL, VDSL, HDSL and SDSL.

Using existing local loops

• DSL uses existing local loops. But the question arises how does DSL reach a data rate that was never achieved with traditional modems??

• The answer is that twisted-pair local loop is actually capable of handling bandwidths up to 1.1 MHz. But the filters installed at the end of the line by the telephone company limits the bandwidth to 4 KHz (sufficient for voice communication.

• Therefore conversion for POTS to DSL usually requires just changing the equipment and removing the filter at the exchange and not the wiring of the local loop. This is the factor which makes DSL technology so attractive.

Equipment used in DSL

• Only the equipment at the subscriber end and at the exchange are replaced for DSL. The wiring remains the same.

• At the subscriber’s end:• Line Splitter: It splits up the voice and data

transmission for telephone and DSL modem respectively. It may consist of just a low-pass and bandpass filter. Thus voice transmission continues even if the data equipment fails.

DSL Equipment

• DSL Modem: It usually uses Ethernet 10BaseT data format so that it can be connected to an Ethernet port of a PC, router or hub etc. DSL modem does not convert analog to digital signals or vice versa. It just converts the incoming data into Ethernet format and vice versa. So in reality, its not a modem in a strict sense. It just converts one standard into another.

Equipment at the Local Carrier’s End

• Main Distribution Frame (MDF): Using local loop, many customers get connected to the Main Distribution Frame/Facility (MDF) which acts as a splitter. It sends voice traffic to the Voice Telephone Network and data traffic to DSLAM.

• DSLAM: DSL Access Multiplexer (DSLAM) converts the data into the ATM (Asynchronous Transfer Mode) data format to be transmitted to the ATM switch which distributes it to the ISPs (Internet Service Providers) for access to the Internet.

High-Bit-Rate Digital Subscriber Line (HDSL)

• HDSL was designed as an alternative to the T-1 line (1.544 Mbps).

• Standard T1 line uses alternate mark inversion coding (AMI) which occupies a bandwidth of 1.5 MHz and is very susceptible to attenuation. This limits the length of the line to 1km and repeaters have to be used for longer distances.

• HDSL uses the 2B1Q coding scheme to provide a data rate of 2 Mbps over 2 twisted pair lines within a bandwidth that extends only up to about 196 kHz. This enables a range of about 3.7 km to be achieved.

HDSL• A long string of 0s in the case of AMI might de-

synchronize the receiver. Another advantage of 2B1Q is that it also includes timing information in the data being transmitted which means that in 2B1Q, there are transitions in the signal that alert the receiver to the beginning, middle or end of the pulse. If the receiver’s clock is out of synchronization, these alerting points can reset the clock.

Bipolar AMI Encoding

Symmetric Digital Subscriber Line (SDSL)

• SDSL uses the same coding scheme as HDSL i.e. 2B1Q.

• As the name suggests, its symmetric (like HDSL) i.e. it divides the available bandwidth equally between the downstream and upstream directions.

• SDSL was developed to provide the same type of service as HDSL but over a single twisted-pair line.

Asymmetric Digital Subscriber Line (ADSL)

• ADSL, as the name suggests is asymmetric, which refers to the fact that ADSL provides more capacity downstream (from the carrier’s central office to the customer’s site) than upstream (from customer to carrier).

• Typically while using Internet, the user requires far more capacity for downstream than for upstream transmission. Thus, ADSL provides a perfect fit for the Internet requirement.

• ADSL is carrier-based unlike HDSL or SDSL which use coding.

ADSL Design

• Frequency division multiplexing– Lowest 25kHz for voice

• Plain old telephone service (POTS)

– Use echo cancellation or FDM to give two bands (upstream and downstream).

– Use FDM within bands

• Range 5.5km

ADSL Channel Configuration

Echo Cancellation in ADSL

• Echo cancellation is a signal processing technique that allows transmission of digital signals in both directions on a single transmission line simultaneously. In essence, a transmitter must subtract the echo of its own transmission from the incoming signal to recover the signal sent by the other side.

• When echo cancellation is used, the entire frequency band for the upstream channel overlaps the lower portion of the downstream channel. This has 2 advantages:

Echo Cancellation in ADSL• Advantages1.The higher the frequency, the greater the attenuation. With the use of

echo cancellation, more of the downstream bandwidth is in the “good” part of the spectrum.

2.The echo cancellation design is more flexible for changing upstream capacity. The upstream channel can be extended upward without running into the downstream: instead the area of overlap is extended.

• DisadvantageThe need for echo cancellation logic on both ends of the line adds tothe cost and complexity of the equipment.

Discrete Multitone (DMT)• Standard modulation technique for ADSL which

combines QAM and FDM.• Multiple carrier signals at different frequencies• Some bits on each channel• 4kHz subchannels• Send test signal and use subchannels with better signal

to noise ratio• 256 downstream subchannels at 4kHz (60kbps)

– 15.36Mbps– Impairments bring this down to 1.5Mbps to 9Mbps

DMT Transmitter

Very-High-Bit-Rate DSL(VDSL)

• An alternative approach to ADSL

• Uses DMT

• Bit rate of 50-55 Mbps downstream and 1.5-2.5 Mbps upstream.