2.1 Asymmetric Digital Subscriber Line (ADSL) Downstream Speeds
2.2 Asymmetric Digital Subscriber Line (ADSL) Upstream Speeds
2.3 ADSL Implementation


Digital Subscriber Line (DSL) refers to a group of technologies that utilize the unused bandwidth in the existing copper access network to deliver high-speed data services from the distribution centre, or central office, to the end user. DSL technology is attractive because it requires little to no upgrading of the existing copper infrastructure.  Figure 1.0 shows the various components in a DSL network.

Figure 1. Components of a DSL system showing connections with overhead cable.

The local loop connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3.4 kHz upper limit of the Plain Old Telephone Service (POTS). Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4,312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured.  Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until additional channels are deemed unusable.

Figure 2. Allocation of Frequency in a  Digital Subsriber Line (DSL)

 Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. If the modem establishes more usable channels, then this  equates to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances).

The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers conntinuously monitor the quality of each channel and will add or remove them from service depending on whether they are meet the set quality threshold.


The most commonly used DSL system is the Asymmetric Digital Subscriber Line which evolved over time and was guided by the following standards. The maximum download speed with the latest standard, i.e ADSL4 is 52 Mbps.

(a) ANSI T1.41  First standard developed in1998
(b) G.992.1       8.1/0.8 Mbps (down/up) using 256 bins
(c) G.992.2       1.5/0.5 Mbps using 128 bins
(d) G.992.5       ADSL2plus = 24 Mbps at 5,000 feet (Developed in 2003)
(e) ADSL4         52 Mbps proposed quad spectrum (ADSL2++)

The  frequency band  in the copper cable is divided into upstream and down stream bands as shown in Figure 3.

Figure 3. Asynchronous Digital Subscriber Line (ADSL) Frequency Bands
The ANSI T1.413 was the first ADSL standard  and was developed by the American National Standards Institute Telecommunication (ANSI) Committee and labelled “Network and Customer Installation Interfaces – Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.” The standard divides the useful bandwidth of the standard two-wire copper medium used in the Public Switched Telephone Network (PSTN), which is 0 to 1,104kHz, into 256 separate 4.3125kHz wide bins called sub-carriers (224 down-stream and 32 up stream. A sub-carrier is associated with a discrete frequency, or tone, indicated by:

fn = 4.3125kHz x n, where n = 1 to 256

Each sub-carrier is essentially a single distinct data channel. The frequency layout can be summarised as:
(a) 300 Hz - 4 kHz carry voice.
(b) 4–25 kHz form  unused guard band.
(c) 25–138 kHz constitute 25 upstream bins (7-31).
(d)138–1104 kHz, constitute 224 downstream bins (32-255)

Typically, a few bins around 31-32 are not used in order to prevent interference between upstream and downstream bins either side of 138 kHz. The unused bins constitute a guard band to be chosen by each Digital Subscriber Line Access Multiplexer (DSLAM manufacturer.

2.1 Asymmetric Digital Subscriber Line (ADSL) Downstream Speeds

The downstream band (138–1104 kHz), carries 224  downstream bins (carriers) denoted as number 32-255. Bin 256 and bin 64 are not available for user data, thus limiting the total number of available downstream bins to 254. Each bin supports 15 bit frame size which are transmitted at 4,000 frames/second. The maximum down link data rate is therefore 15 X 4,000 X 254 = 15.24 Mb/s.

2.2 Asymmetric Digital Subscriber Line (ADSL) Upstream Speeds

In the upstream direction, a maximum of 32 carriers are used to modulate data. Sub-carrier 32 and sub-carrier 16 are not used to carry user data. Each carrier supports 15 bit frame size which are transmitted at 4,000 frames/second. The maximum up-stream data rate is therefore 30 X 15 X 4,000 = 1.8 Mb/s.

2.3 ADSL Implementation

Figure 4 shows a typical ADSL implementation. The right side shows a Didital Subsriber Line Multiplxer (DSLAM) which resides in the telephone exchange. The left side shows the customer premises equipment with an optional router. The router manages a local area network (LAN) off of which are connected some number of PCs.  A wireless router connects laptops and other wireless access devices. The filter blocks the rest of the signal exept 0-4KHz to pass through to the telephone.

Figure 4. Asymmetric Digital Subsriber Line (ADSL) installation.

Figure 5 shows the various ADSL components and the conectors used in a customer premises.

Figure 5. ADSL installation in a customer's premises

Prof. James Kulubi May-Aug 2015