Mumbai University > Electronics and Telecommunication > Sem 6 > Computer Communication and Telecom Network

Marks: 10M

Year: May 2015

DSL:

Digital subscriber line (DSL) technology is one of the most promising for supporting high speed digital communication over the existing local loops (telephone line).

DSL technology is a set of technologies, each differing in the first letter (ADSL, VDSL, HDSL, and SDSL). The set is often referred to as xDSL, where x can be replaced by A, V, H, or S.

ADSL:

• The first technology in the set is asymmetric DSL (ADSL). ADSL, like a 56K modem, provides higher speed (bit rate) in the downstream direction (from the Internet to the resident) than in the upstream direction (from the resident to the Internet).
• That is the reason it is called asymmetric. Unlike the asymmetry in 56K modems, the designers of ADSL specifically divided the available bandwidth of the local loop unevenly for the residential customer. The service is not suitable for business customers who need a large bandwidth in both directions. Figure 1 shows how the bandwidth is divided:

• Voice: Channel 0 is reserved for voice communication.
• Idle: Channels 1 to 5 are not used, to allow a gap between voice and data communication.
• Upstream data and control: Channels 6 to 30 (25 channels) are used for upstream data transfer and control. One channel is for control, and 24 channels are for data transfer. If there are 24 channels, each using 4 kHz (out of 4.312 kHz available) with 15 bits per Hz, we have 24  4000  15, or a 1.44-Mbps bandwidth, in the upstream direction.
• Downstream data and control: Channels 31 to 255 (225 channels) are used for downstream data transfer and control. One channel is for control, and 224 channels are for data. If there are 224 channels, we can achieve up to 224  4000  15, or 13.4 Mbps.
• Because of the high signal/noise ratio, the actual bit rate is much lower than the above mentioned rates. The bit rates are as follows: Upstream: 64 kbps to 1 Mbps Downstream: 500 kbps to 8 Mbps

• Figure 2 shows an ADSL modem installed at a customer’s site. The local loop connects to the filter which separates voice and data communication. The ADSL modem modulates the data and creates downstream and upstream channels.

Other DSL Technologies:

• ADSL provides asymmetric communication. The downstream bit rate is much higher than the upstream bit rate. Although this feature meets the needs of most residential subscribers, it is not suitable for businesses that send and receive data in large volumes in both directions.
• The symmetric digital subscriber line (SDSL) is designed for these types of businesses. It divides the available bandwidth equally between the downstream and upstream directions.
• The high bit rate digital subscriber line (HDSL) was designed as an alternative to the T-1 line (1.544 Mbps). The T-1 line (discussed later) uses alternate mark inversion (AMI) encoding, which is very susceptible to attenuation at high frequencies.
• This limits the length of a T-1 line to 1 km. For longer distances, a repeater is necessary, which means increased costs.
• The very high bit rate digital subscriber line (VDSL), an alternative approach that is similar to ADSL, uses coaxial, fiber-optic, or twisted-pair cable for short distances (300 to 1800 m).
• The modulating technique is discrete multitone technique (DMT) with a bit rate of 50 to 55 Mbps downstream and 1.5 to 2.5 Mbps upstream.

HFC Network:

• The second generation of cable networks is called a hybrid fiber-coaxial (HFC) network. The network uses a combination of fiber-optic and coaxial cable.
• The transmission medium from the cable TV office to a box, called the fiber node, is optical fiber; from the fiber node through the neighborhood and into the house, the medium is still coaxial cable. One reason for moving from traditional to hybrid infrastructure is to make the cable network bidirectional (two-way).
• Bandwidth: Even in an HFC system, the last part of the network, from the fiber node to the subscriber premises, is still a coaxial cable. This coaxial cable has a bandwidth that ranges from 5 to 750 MHz (approximately). The cable company has divided this bandwidth into three bands: video, downstream data, and upstream data, as shown in Figure 3.

• Video Band: The downstream-only video band occupies frequencies from 54 to 550 MHz. Since each TV channel occupies 6 MHz, this can accommodate more than 80 channels.
• Downstream Data Band: The downstream data (from the Internet to the subscriber premises) occupies the upper band, from 550 to 750 MHz this band is also divided into 6-MHz channels. The downstream data can be received at 30 Mbps. The standard specifies only 27 Mbps.
• Upstream Data Band: The upstream data (from the subscriber premises to the Internet) occupies the lower band, from 5 to 42 MHz this band is also divided into 6-MHz channels. The upstream data band uses lower frequencies that are more susceptible to noise and interference. Theoretically, downstream data can be sent at 12 Mbps (2 bits/Hz × 6 MHz). However, the data rate is usually less than 12 Mbps.