Understanding Contention

As people should be aware, the ADSL product line sold by PlusNet is a contended service of either 20:1 or 50:1. These are the only two contentions currently possible under the BT IPStream package, which PlusNet uses and as provided to them by BT.

DataStream ADSL providers have far more control over the connection including contentions of 1:1. DataStream does not allow for different speeds, instead, the provide has almost full control over the process used in Step 2 below.

People get very confused on what contention is and isn't, even those people that do know, can still be in the dark to many of the facts.

Cold hard fact:
The word Contention means to "To compete in order to win something".

To extend that, it means that when you are using your ADSL connection, you are competing with other people, in order to gain certain aspects of the service.

Be your account a 20:1 or 50:1, this means you are competing with either 19 or 49 other users/connections, in order to get the full 512Kb, 1Mb or 2Mb line speed of the product you are on.

That is only a basic concept of contention, or at least the contention that ISPs report. At this point, many will still be confused, or have a pretty good grasp. To understand it further, we have to look at exactly how your ADSL connection traverses the BT network, from your premises to your selected ISP.

This should give you a better understanding of what ISPs call ADSL contention, but it is likely to disturb others of you, when you understand how deep it actually goes.


Step 1 - Your phone line

Your line connects to the exchange DSLAM equipment, which is designed to handle your incoming ADSL signal. It then converts it into something more presentable and capable of travelling the longer distances of the BT network.

There is no contention at this point, as it is required that each line is connected to its very own port of the DSLAM. That is to say, contention here is 1:1 or one line into one port.


Step 2 - Linking your exchange to the BT POPs

This is the real start of the contention ballgame.

Your data, and that of other ADSL customers, are transferred onto the BT ATM network using backhaul links from the exchange.

The size of the exchange, the number of 20:1 and 50:1 customers (ie, is it mainly business or residential customers?) and the existing connections to the rest of the BT network, will all help determine what type and size of connection this is.

Many people assume that because 50 users are sharing bandwidth, that means there are 50 users connected to a single 512Kb backhaul (50:1 account type).

This is not true. If it were like this, then there is a very extreme chance of one single user using the full potential of the connection in one go, leaving others chewing dust and crawling.

Instead, BT provide 4Mb/s backhauls as standard. As noted above, the size and type will depend on the exchange. A little calculation is needed here to show what happens. Let's base our example on a 512Kb/s 50:1 IPStream Home connection.

First, let's work the 4Mb into Kb:
4Mb/s = 4 * 1024 Kb/s = 4096Kb/s

Next, how many 512Kb/s ADSL lines should this supply:
4096 / 512 = 8

This means, that only 8 customers could be supplied at a contention ratio of 1:1, ie, no customer could affect another user's speeds.

Because this is a 50:1 product, the following applies
50:1 * 8 = 400
For those who are not sure, to perform maths using ratios, you calculate how many times the right
goes into the left. 50:1 is the same as 50/1. As such, the value is 50

That is to say that up to 400 customers will share that single 4Mb/s backhaul. The 50:1 advertised ratio is still maintained, only instead of working with a single block of 50 customers, this backhaul handles 8 lots of 50:1 (IE 8 lots of 50) customers.

There is a chance that a single customer can quickly fill the whole bandwidth pipe (as noted above), by using this method of increased pipe size, lighter users can use the bandwidth that the heavy user isn't. This gives people a much better and fair chance.

There is still a chance that a few customers could quickly fill and saturate the whole bandwidth; however, unlike 50 users on one 512Kb/s connection, they would all need to be using there connection to the max at the same time. Although there may be people like this on a single exchange, you are more likely to see them doing this at different times of the day.

At least, that is the reasoning behind why they supply backhauls like this and it works very well in most parts.

That being said, contention is contention. ADSL is sold as a contended product and from time to time, or in the future, it may be a much larger problem.

BT try to upgrade in advance of full capacity, and in most cases, do not even fill a single pipe to the maximum levels.

The same applies to other 50:1 products and to 20:1, though BT do use separate backhauls for 50:1 and 20:1 customers. This is mainly for quality control.

Look at it this way, to mix 20:1 accounts and 50:1 accounts would see the ratio decrease for 50:1 users, and increase for 20:1 users. As you can see, it keeps people happy (or tries to).

With the introduction of the new 1Mb 50:1 home product, many people are further confused if contention is going to kick in on them more. Provided BT are performing their calculations correctly, then contention will be no more of an issue.

It is possible for a 4Mb/s backhaul to handle a mixed number of customers and the different 50:1 account types. The following could be one example of how this is done. Note: This is unconfirmed. BT does not let slip on all that much detail.

200 users on 512Kb/s 50:1 would take up 1/2 (2Mb/s) of the backhaul
100 users on 1Mb/s 50:1 would take up 1/2 (2Mb/s) of the backhaul

That totals to 300 users, yet contention levels of 50:1 are still being maintained.

The problem with 1Mb ADSL is the increased chance of backhaul saturation. Where four 512Kb/s users use 1/4 of the 4Mb/s backhaul capacity, it only takes 2 1Mb/s to use the same.

As you can see, this has the potential for the effects of contention to kick in more quickly. This will depend on how people user their newer and faster connections.

Under a worst case scenario, customers should see speeds reduced to a certain level. Because products are sold with contention, ISPs and BT are under no obligation to fix things when speeds are under these levels.

That being said, this should never happen and if it were to, you would likely see a boycott of ADSL. See the end of this FAQ for a few hints on what to do when you have a connection under that which you expect it to perform.

Here is a list of each product type available under IPStream and the minimum you should ever see with contention:

• 512Kb/s 50:1
  • 10.24Kb/s (1.28KB/s)

• 1Mb/s 50:1
  • 20.48Kb/s (2.56KB/s)

• 512Kb/s 20:1
  • 25.6Kb/s (3.2KB/s)

• 1Mb/s 20:1
  • 51.2Kb/s (6.4KB/s)

• 2Mb/s 20:1
  • 102.4Kb/s (12.8KB/s)

Whereas modem connections are contended on the ability to gain an initial connection, you can see here, ADSL is contended on the speed once connected.

These are worst cases though, so this should never happen.


Step 3 - POP to ISP handover

Once your traffic is on a backhaul to one of the POPs (Point Of Presence), it usually means that the contention will be maintained throughout the network, until it is time to be handed to the ISP.

This is another point where contention can be inflicted on your traffic; however, calculating contention levels here is near impossible without exact customers figures, something that ISPs do not like to give out.

An ISP may have multiple paths through which your connection may reach it, however, once your connection is established (authentication of your DSL connection and your IP address has been assigned to you), you are stuck with a path until you make a new connection to the ISP (though that may not guarantee a change).

PlusNet use the 155Mb/s type of connection from the BT network (The connection is known as BT Central 155Mb/s L2TP).

This allows up to 8000 connection sessions at any one time. As you can see, 155Mb/s divided between 8000 sessions provides 19.84Kb/s per session, meaning these connections are contended too.

If all connections are 512Kb/s, then that would mean a contention level of 25.81:1. However, these links handle all customer connections, so the higher speed 1Mb & 2Mb connections are un-accounted for here.

Without the exact figures of customers an ISP has on each product, it is impossible to tell what contention levels, if any at all, are kicking in. The only way an ISP can minimise this, is to provide an upgrade plan that will provide over-capacity.

As ISPs do not like to provide information on their customer levels, it is near impossible to tell if contention is a factor here. However, PlusNet provides a services utilisation graph to tell you how much of the capacity is in use for today and backdating one week.

PlusNet have between 4000 to 8000 customers per pipe. However, many customers through the use of PCI or USB modems, disconnect from the internet when not in use. As such, there is a average 3000 active customers connected at a time.

For those that wish to know, this makes the average contention of 9.68:1 on these pipes. This is much less than the possible figure above.


Step 4 - External connectivity

Once within the ISPs network, your data is yet to flow onto the Internet. This is again another point that contention can be seen.

However, ISPs would usually have a peering arrangement that would provide them with near limitless capacity making it almost a non-issue. You would have to find out these arrangements to know if this is a possible factor.

As proof of concept, to know if there is contention, you would see faster speeds from systems within your ISPs network, and slower from the Internet.

Slow speeds from the Internet are by no means proof, though. What happens with your data on the Internet is out of the control of your ISP.

I should note in the above, most of this is a worst case scenario, meaning that if you see problems, they should be no worse than this. If they are, you have a definite complaint, provided you can prove that your own equipment (ADSL modem, splitters and phones) and wiring are not to blame.

You may like to consult the "ADSL Troubleshooting" and "ADSL Wiring and filters FAQ" guides for help on diagnosing issues, prior to to submitting a fault to PlusNet

Most calculations above, have been done using a 512Kb/s service. Contentions and speed figures will vary depending on the number and type of connections.

Understanding Line Loss and Measurements

What causes line loss?

The telephone line from your local Telephone Exchange to your house is made up of a twisted pair of wires within a cable

Everything that carries electricity has what is known as "Resistance". This is measured in "Ohms" and resistance impedes the flow of current in a conductor.

Because your connection is two wires twisted together it also has some "Capacitance" between the wires. A capacitor is an electrical component and there is more current passed through it as the frequency gets higher.

Also your pair of wires has yet another characteristic called "Inductance" and inductance allows less current to flow through it as the frequency gets higher.

So as you can see from the above the electrical characteristics of your phone line are quite complex.

To sum up:

Resistance reduces the current so increases the loss.
Capacitance effectivly short circuits the line more as frequencies increase so increasing loss.
Inductance resists current flow more as frequencies get higher thus increasing the loss.

Obviously the longer your line then the greater the effect of the above characteristics and the greater the loss.


How is line loss measured?

Loss is measured by comparing the power level of the signal sent from one end of the line with that received at the other end. The difference between these levels is expressed in Decibels (dB).
The decibel scale is logarithmic and works as follows.

If the power received was 1/2 the power sent, then that would calculate as
10 x log base10 of 0.5 = -3.010

So a loss of 3 dB is equal to almost exactly half the power being received.
Similarly, if you were to look at log tables and calculate other figures you would find that losses of:

10 dB = 1/10th of the power
20 dB = 1/100th of the power
30 dB = 1/1000th of the power.

If you were to carry on until you got to 60 dB loss you would find you are only receiving one millionth of the power that was originally sent out.


How do you find out what your line figures are?

Many ADSL Modems and Routers have a function in the set-up options that will actually measure the losses for you and give you an on-screen display.

If you have one that doesn't give you this feature then you could contact Customer Support and ask to be told the figures for a "Whoosh" test on your line .

One point to bear in mind with the Whoosh test is that the activation of your line actually adds about another 4 dB to the original loss. BT take this view when doing a test so if a loss figure of say 64 dB is produced from the test you will still be just within the limit of 60 dB (for 1meg service).


What do the figures mean?

Let's look at some typical figures, and here I will use the ones shown by my own equipment a D-link DSL-504 Router using 512K 50:1 ADSL.

These are as follows:

• Attenuation Downstream: 28dB. This means I am receiving just under 1/500th of the signal sent from the exchange.
  
  
• Attenuation Upstream: 31dB. This means the Exchange is receiving just less than 1/1000th of the signal sent from my modem.
  
  
• SNR: 19dB. SNR stands for Signal to Noise Ratio and is basically the difference between the level of the signal being received compared with the natural noise level on the line.
  
  Here, the higher the figure the better. In my case 19dB means that the signal I am receiving is almost 100 times stronger than the noise level.
  
  Noise on your line is caused by many things. Some examples are other wires in the cable running alongside your wires, interference from power cables, radio signals, higher than normal resistance joints in the telephone wires and damp in the wires or cables.

Other figures you may see.

Upstream power 10 dBm
Downstream power 12 dBm

These relate to the output power from the transmitters at your modem and the exchange.

The dBm notation means decibels relative to one milliwatt (the "m" in the figure) so in the above case the powers are 10 milliwatts and approx 18 milliwatts (13 dBm would be 20 milliwatts).


What levels are acceptable for ADSL service?

Now you know what the figures above mean, you may be amazed that ADSL works at all!!

To get a reliable service your line needs to meet the following criteria:

1. 512K Service. There is now no upper limit and BT will attempt than make it work on any line if possible.
   
   
2. 1 Meg Services. You need a line loss of less than 60 dB, and so, typically, will be no more than 6.0 kilometres from the exchange.
   
   
3. 2 Meg Service. You need a line loss of less than 45 dB which means you will be 3.5 Kilometres or less from the exchange.
   

The distances from the exchange are based on the average signal losses versus line lengths and you may find that even if you are under the distance you may still have too much loss for the chosen service.

Conversely you may also find that even if you are outside the distance you may still get the product.

At the end of the day, it is the important line tests that are done after you apply for ADSL that matter.


Effects of too high a line loss.

Line losses can alter over a period due to factors such as temperature, rainfall, corrosion in cable joints, etc.

If you have had a good ADSL service but find you start getting frequent disconnections, you need to check your line is still within the limits above to meet the type of service you have.

So, if you have the original figures, keep a note of them so you can make a comparison later if you start getting problems.

Remember from the above dB notation, an increase of 3dB in the loss figure means that you are only getting 1/2 of the signal you had when it was working fine.

Also very important is the Signal to Noise Ratio. Remember that higher figures are better here.

I have not as yet found any article that determines the minimum Signal to Noise Ratio as acceptable for your ADSL to work correctly. However it is generally accepted that anything less than 10 -12 dB will cause problems and I would worry if my line had a figure of, say, 10dB and normally I would expect 15 dB or better.


Improving your loss figures.

Unfortunately there is little you can do about the actual line itself other than get it maintained by BT. But you can take some steps to ensure that you are not adding more than the minimum loss yourself:

1. Use good quality Splitter / Filters.
   
2. Use good quality extension cables.
   
3. Ensure that where you plug into cable sockets that the pins are clean and bright.
   It has been known for the connections here to corrode with time and you could try pulling out and re-inserting the plug into the BT socket several times to polish the connections.

Hopefully the above information will have been useful to those who wanted to know more about their ADSL connection.

Networking Basics

Here are some of the fundamental parts of a network:

• Network - A network is a group of computers connected together in a way that allows information to be exchanged between the computers.

• Node - A node is anything that is connected to the network. While a node is typically a computer, it can also be something like a printer or CD-ROM tower.

• Segment - A segment is any portion of a network that is separated, by a switch, bridge or router, from other parts of the network.

• Backbone - The backbone is the main cabling of a network that all of the segments connect to. Typically, the backbone is capable of carrying more information than the individual segments. For example, each segment may have a transfer rate of 10 Mbps (megabits per second), while the backbone may operate at 100 Mbps.

• Topology - Topology is the way that each node is physically connected to the network (more on this in the next section).

• Local Area Network (LAN) - A LAN is a network of computers that are in the same general physical location, usually within a building or a campus. If the computers are far apart (such as across town or in different cities), then a Wide Area Network (WAN) is typically used.

• Network Interface Card (NIC) - Every computer (and most other devices) is connected to a network through an NIC. In most desktop computers, this is an Ethernet card (normally 10 or 100 Mbps) that is plugged into a slot on the computer's motherboard.

• Media Access Control (MAC) address - This is the physical address of any device -- such as the NIC in a computer -- on the network. The MAC address, which is made up of two equal parts, is 6 bytes long. The first 3 bytes identify the company that made the NIC. The second 3 bytes are the serial number of the NIC itself.

• Unicast - A unicast is a transmission from one node addressed specifically to another node.

• Multicast - In a multicast, a node sends a packet addressed to a special group address. Devices that are interested in this group register to receive packets addressed to the group. An example might be a Cisco router sending out an update to all of the other Cisco routers.

• Broadcast - In a broadcast, a node sends out a packet that is intended for transmission to all other nodes on the network.

Network Topologies

Some of the most common topologies in use today include:

• Bus - Each node is daisy-chained (connected one right after the other) along the same backbone, similar to Christmas lights. Information sent from a node travels along the backbone until it reaches its destination node. Each end of a bus network must be terminated with a resistor to keep the signal that is sent by a node across the network from bouncing back when it reaches the end of the cable.

Bus network topology

• Ring - Like a bus network, rings have the nodes daisy-chained. The difference is that the end of the network comes back around to the first node, creating a complete circuit. In a ring network, each node takes a turn sending and receiving information through the use of a token. The token, along with any data, is sent from the first node to the second node, which extracts the data addressed to it and adds any data it wishes to send. Then, the second node passes the token and data to the third node, and so on until it comes back around to the first node again. Only the node with the token is allowed to send data. All other nodes must wait for the token to come to them.

Ring network topology

• Star - In a star network, each node is connected to a central device called a hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network. A hub does not perform any type of filtering or routing of the data. It is simply a junction that joins all the different nodes together.

Star network topology

• Star bus - Probably the most common network topology in use today, star bus combines elements of the star and bus topologies to create a versatile network environment. Nodes in particular areas are connected to hubs (creating stars), and the hubs are connected together along the network backbone (like a bus network). Quite often, stars are nested within stars, as seen in the example below:

A typical star bus network

The Problem: Traffic

In the most basic type of network found today, nodes are simply connected together using hubs. As a network grows, there are some potential problems with this configuration:

• Scalability - In a hub network, limited shared bandwidth makes it difficult to accommodate significant growth without sacrificing performance. Applications today need more bandwidth than ever before. Quite often, the entire network must be redesigned periodically to accommodate growth.

• Latency - This is the amount of time that it takes a packet to get to its destination. Since each node in a hub-based network has to wait for an opportunity to transmit in order to avoid collisions, the latency can increase significantly as you add more nodes. Or, if someone is transmitting a large file across the network, then all of the other nodes have to wait for an opportunity to send their own packets. You have probably seen this before at work -- you try to access a server or the Internet and suddenly everything slows down to a crawl.

• Network failure - In a typical network, one device on a hub can cause problems for other devices attached to the hub due to incorrect speed settings (100 Mbps on a 10-Mbps hub) or excessive broadcasts. Switches can be configured to limit broadcast levels.

• Collisions - Ethernet uses a process called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to communicate across the network. Under CSMA/CD, a node will not send out a packet unless the network is clear of traffic. If two nodes send out packets at the same time, a collision occurs and the packets are lost. Then both nodes wait a random amount of time and retransmit the packets. Any part of the network where there is a possibility that packets from two or more nodes will interfere with each other is considered to be part of the same collision domain. A network with a large number of nodes on the same segment will often have a lot of collisions and therefore a large collision domain.

While hubs provide an easy way to scale up and shorten the distance that the packets must travel to get from one node to another, they do not break up the actual network into discrete segments. That is where switches come in. In the next section, you'll find out how switches assist in directing network traffic.

The Solution: Adding Switches

Think of a hub as a four-way intersection where everyone has to stop. If more than one car reaches the intersection at the same time, they have to wait for their turn to proceed.

Imagine that each vehicle is a packet of data waiting for an opportunity to continue on its trip.

Now imagine what this would be like with a dozen or even a hundred roads intersecting at a single point. The amount of waiting and the potential for a collision increases significantly. But wouldn't it be amazing if you could take an exit ramp from any one of those roads to the road of your choosing? That is exactly what a switch does for network traffic. A switch is like a cloverleaf intersection -- each car can take an exit ramp to get to its destination without having to stop and wait for other traffic to go by.

A vital difference between a hub and a switch is that all the nodes connected to a hub share the bandwidth among themselves, while a device connected to a switch port has the full bandwidth all to itself. For example, if 10 nodes are communicating using a hub on a 10-Mbps network, then each node may only get a portion of the 10 Mbps if other nodes on the hub want to communicate as well. But with a switch, each node could possibly communicate at the full 10 Mbps. Think about our road analogy. If all of the traffic is coming to a common intersection, then each car it has to share that intersection with every other car. But a cloverleaf allows all of the traffic to continue at full speed from one road to the next.

Fully Switched Networks

In a fully switched network, switches replace all the hubs of an Ethernet network with a dedicated segment for every node. These segments connect to a switch, which supports multiple dedicated segments (sometimes in the hundreds). Since the only devices on each segment are the switch and the node, the switch picks up every transmission before it reaches another node. The switch then forwards the frame over the appropriate segment. Since any segment contains only a single node, the frame only reaches the intended recipient. This allows many conversations to occur simultaneously on a switched network.

Image courtesy Cisco Networks An example of a network using a switch

Switching allows a network to maintain full-duplex Ethernet. Before switching, Ethernet was half-duplex, which means that data could be transmitted in only one direction at a time. In a fully switched network, each node communicates only with the switch, not directly with other nodes. Information can travel from node to switch and from switch to node simultaneously.

Fully switched networks employ either twisted-pair or fiber-optic cabling, both of which use separate conductors for sending and receiving data. In this type of environment, Ethernet nodes can forgo the collision detection process and transmit at will, since they are the only potential devices that can access the medium. In other words, traffic flowing in each direction has a lane to itself. This allows nodes to transmit to the switch as the switch transmits to them -- it's a collision-free environment. Transmitting in both directions can effectively double the apparent speed of the network when two nodes are exchanging information. If the speed of the network is 10 Mbps, then each node can transmit simultaneously at 10 Mbps.

Mixed Networks

Most networks are not fully switched because of the costs incurred in replacing all of the hubs with switches.

A mixed network with two switches and three hubs

Instead, a combination of switches and hubs are used to create an efficient yet cost-effective network. For example, a company may have hubs connecting the computers in each department and then a switch connecting all of the department-level hubs.

Routers and Switches

You can see that a switch has the potential to radically change the way nodes communicate with each other. But you may be wondering what makes it different from a router. Switches usually work at Layer 2 (Data or Datalink) of the OSI Reference Model, using MAC addresses, while routers work at Layer 3 (Network) with Layer 3 addresses (IP, IPX or Appletalk, depending on which Layer 3 protocols are being used). The algorithm that switches use to decide how to forward packets is different from the algorithms used by routers to forward packets.

One of these differences in the algorithms between switches and routers is how broadcasts are handled. On any network, the concept of a broadcast packet is vital to the operability of a network. Whenever a device needs to send out information but doesn't know who it should send it to, it sends out a broadcast. For example, every time a new computer or other device comes on to the network, it sends out a broadcast packet to announce its presence. The other nodes (such as a domain server) can add the computer to their browser list (kind of like an address directory) and communicate directly with that computer from that point on. Broadcasts are used any time a device needs to make an announcement to the rest of the network or is unsure of who the recipient of the information should be.

The OSI Reference Model consists of seven layers that build from the wire (Physical) to the software (Application).

A hub or a switch will pass along any broadcast packets they receive to all the other segments in the broadcast domain, but a router will not. Think about our four-way intersection again: All of the traffic passed through the intersection no matter where it was going. Now imagine that this intersection is at an international border. To pass through the intersection, you must provide the border guard with the specific address that you are going to. If you don't have a specific destination, then the guard will not let you pass. A router works like this. Without the specific address of another device, it will not let the data packet through. This is a good thing for keeping networks separate from each other, but not so good when you want to talk between different parts of the same network. This is where switches come in.

Packet-switching

LAN switches rely on packet-switching. The switch establishes a connection between two segments just long enough to send the current packet. Incoming packets (part of an Ethernet frame) are saved to a temporary memory area (buffer); the MAC address contained in the frame's header is read and then compared to a list of addresses maintained in the switch's lookup table. In an Ethernet-based LAN, an Ethernet frame contains a normal packet as the payload of the frame, with a special header that includes the MAC address information for the source and destination of the packet.

Packet-based switches use one of three methods for routing traffic:

• Cut-through
• Store-and-forward
• Fragment-free

Cut-through switches read the MAC address as soon as a packet is detected by the switch. After storing the 6 bytes that make up the address information, they immediately begin sending the packet to the destination node, even as the rest of the packet is coming into the switch.

A switch using store-and-forward will save the entire packet to the buffer and check it for CRC errors or other problems before sending. If the packet has an error, it is discarded. Otherwise, the switch looks up the MAC address and sends the packet on to the destination node. Many switches combine the two methods, using cut-through until a certain error level is reached and then changing over to store-and-forward. Very few switches are strictly cut-through, since this provides no error correction.

A less common method is fragment-free. It works like cut-through except that it stores the first 64 bytes of the packet before sending it on. The reason for this is that most errors, and all collisions, occur during the initial 64 bytes of a packet.

How DSL Works

When you connect to the Internet, you might connect through a regular modem, through a local-area network connection in your office, through a cable modem or through a digital subscriber line (DSL) connection. DSL is a very high-speed connection that uses the same wires as a regular telephone line.

Here are some advantages of DSL:

• You can leave your Internet connection open and still use the phone line for voice calls.
• The speed is much higher than a regular modem
• DSL doesn't necessarily require new wiring; it can use the phone line you already have.
• The company that offers DSL will usually provide the modem as part of the installation.

But there are disadvantages:

• A DSL connection works better when you are closer to the provider's central office.
• The connection is faster for receiving data than it is for sending data over the Internet.
• The service is not available everywhere.

In this article, we explain how a DSL connection manages to squeeze more information through a standard phone line -- and lets you make regular telephone calls even when you're online.

Telephone Lines
If you have read How Telephones Work, then you know that a standard telephone installation in the United States consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.

To understand DSL, you first need to know a couple of things about a normal telephone line -- the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second -- see How Telephones Work for a great demonstration of this). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases.

The use of such a small portion of the wire's total bandwidth is historical -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.

Asymmetrical DSL
Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).

Other types of DSL include:

• Very high bit-rate DSL (VDSL) - This is a fast connection, but works only over a short distance.

• Symmetric DSL (SDSL) - This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same.

• Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line.

Distance Limitations
Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.

You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:

• Bridge taps - These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
• Fiber-optic cables - ADSL signals can't pass through the conversion from analog to digital and back to analog that occurs if a portion of your telephone circuit comes through fiber-optic cables.
• Distance - Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office.

Next, we'll look at how the signal is split and what equipment DSL uses.

Splitting the Signal and DSL Equipment

The CAP System
There are two competing and incompatible standards for ADSL. The official ANSI standard for ADSL is a system called discrete multitone, or DMT. According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. An earlier and more easily implemented standard was the carrierless amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.

CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4 KHz (kilohertz) band, as they are in all POTS circuits. The upstream channel (from the user back to the server) is carried in a band between 25 and 160 KHz. The downstream channel (from the server to the user) begins at 240 KHz and goes up to a point that varies depending on a number of conditions (line length, line noise, number of users in a particular telephone company switch) but has a maximum of about 1.5 MHz (megahertz). This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.

The DMT System
DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 KHz wide.

One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and then attaches a modem to each one. You get the equivalent of 247 modems connected to your computer at once! Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels, for upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.

Filters
CAP and DMT are similar in one way that you can see as a DSL user.

If you have ADSL installed, you were almost certainly given small filters to attach to the outlets that don't provide the signal to your ADSL modem. These filters are low-pass filters -- simple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass (LP) filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.

ADSL uses two pieces of equipment, one on the customer end and one at the Internet service provider, telephone company or other provider of DSL services. At the customer's location there is a DSL transceiver, which may also provide other services. The DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections.

The Transceiver
Most residential customers call their DSL transceiver a "DSL modem." The engineers at the telephone company or ISP call it an ATU-R. Regardless of what it's called, it's the point where data from the user's computer or network is connected to the DSL line.

Photo courtesy Allied Telesyn DSL modem

The transceiver can connect to a customer's equipment in several ways, though most residential installation uses USB or 10 base-T Ethernet connections. While most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, the devices used by businesses may combine network routers, network switches or other networking equipment in the same platform.

The DSLAM
The DSLAM at the access provider is the equipment that really allows DSL to happen. A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. DSLAMs are generally flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation -- both CAP and DMT, for example -- in the same type of DSL. In addition, the DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers.

The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable-modem users generally share a network loop that runs through a neighborhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added -- until the total number of users begins to saturate the single, high-speed connection to the Internet. At that point, an upgrade by the service provider can provide additional performance for all the users connected to the DSLAM.

For information on ADSL rates and availability in the United States, go to Broadband Reports. This site can provide information on ADSL service companies in your area, the rates they charge, and customer satisfaction, as well as estimating how far you are from the nearest central office.

Application Architecture

A possible ADSL system is illustrated in figure. A flexible way to connect various servers to corresponding application's device is to use ATM-switches. Local ATM-switch is connected to an access module in a telephone central office. The access module is used to connect the ATM network to phone-lines. In the access module ATM data stream from server is decomposed and routed to the corresponding phone-lines.

Figure: ADSL System Architecture

There is a large number of different kind of servers that can be accessed by an ADSL system. Those servers shown in figure are not only future but also today's technology.

An employee using a work-at-home-server can take full advantage of the high-speed capabilities of an ADSL-system in many ways, e.g., running licensed software, downloading CAD, documents etc.

Video-on-Demand-service is one of the most interesting aspect of ADSL. By using MPEG-coded video it is possible to deliver video-quality movies over existing copper-loops to customers. A video-quality can be achieved by only 1.5 Mbps data rate. Together with pure VoD-services there might exist combined movie/information/advertizer-services in which commercial and non-commercial information providers and advertizers can deliver their information.

ADSL

DSL is a newly standardized transmission technology facilitating simultaneous use of normal telephone services, data transmission of 6 Mbit/s in the downstream and Basic-rate Access (BRA).

ADSL can be seen as a FDM system in which the available bandwidth of a single copper-loop is divided into three parts.The baseband occupied by POTS is split from the data channels by using a method which guarantees POTS services in the case of ADSL-system failure (eg. passive filters).

Introduction

Asymmetric Digital Subscriber Lines (ADSL) are used to deliver high-rate digital data over existing ordinary phone-lines. A new modulation technology called Discrete Multitone (DMT) allows the transmission of high speed data.

ADSL facilitates the simultaneous use of normal telephone services, ISDN, and high speed data transmission, eg., video.

DMT-based ADSL can be seen as the transition from existing copper-lines to the future fiber-cables. This makes ADSL economically interesting for the local telephone companies. They can offer customers high speed data services even before switching to fiber-optics.