Vision

The BT Master Socket

Firstly, a small explanation regarding a standard BT ‘master’ phone socket.

The telephone line coming in to the premises normally consists of a single twisted pair. This is then fed to a “master socket” that has the ring capacitor fitted. The line is fed to pins 2 & 5, and the ring capacitor to pin 3 (the 470k resistor helping to charge the capacitor to line voltage to stop ring ‘blips’ when plugging in a telephone).

Extending the line from the master socket to an extension socket has a basic requirement of 3 wires (pins 2 & 5, and 3). At first it appears that all that is required is a 3-core cable, something along the lines of a small mains cable or similar.

The 1.8µF capacitor, however, ‘joins’ pins 2 & 3 audio wise. Using a 3-core extension cable will therefore result in twice as much coupling capacitance to the outside world via pins 2 & 3 with respect to pin 5 making the circuit unbalanced and therefore susceptible to interference.

Standard (non-broadband) extensions

The trick to twisted pairs is each signal must have a return. This applies equally to a telephone line. However, as shown, extending a BT socket means extending two signals i.e. the main telephone signal and the ring signal. This requires a slight diversion to what is often done in practice.

In this solution two twisted pairs are used. The upper pair carries the main telephone signal to pin 2 with the return connected to pin 5. The ring signal on pin 3 is treated in the same manner and taken to the extension socket on the lower pair with the return again being done on pin 5.

Each telephone line extended requires two twisted pairs (thus two telephone lines will need 4 pairs). There is no short-cut to this and it is suggested, for the sake of saving frustration, that no short-cut be even considered.

Broadband extensions

Upsetting the twisted pair run from exchange to ADSL modem is a sure-fire way to reduce the speed the modem can operate at. The trick with twisted pairs, at these kinds of speeds (anything from 1Mbps up), is to maintain an even impedance. Using the standard ’3-wire’ extension method and even the double twisted pairs as shown above will upset the impedance sufficiently to reduce the speed on multi-Mbps ADSL links.

However, there is one thing on our side in this instance; The ADSL filters themselves. Each of these filters has a ‘ring capacitor’ fitted i.e. each and every filter is, in effect, a master socket.

Therefore, all that is required is a single pair to run between sockets. Each socket will have a filter providing the required ‘ring signal’ at the filter itself (yes, the ring circuit within the ‘master’ BT socket actually becomes defunct!).

My personal arrangement is one better. I run two services within the house. A single filter is attached to the master socket. The data port is taken to the ADSL modem/router and the telephone port feeds all the extensions in the house (including the fax-modem) thus ensuring the least possible loading on the ADSL signal (because too many filters is as bad as impedance variations on the line).

Basics of ADSL and telephone

Telephone wires were originally designed to carry “Commercial Speech” between your home and the telephone exchange. This uses a band of frequencies from 300 to 3400 hertz. this system is called PSTN (public switched telephone network).

ADSL uses frequencies very much higher than this speech band to carry fast data traffic. ADSL systems use typically frequencies between 25 kHz and around 1.1 MHz.

Because PSTN and ADSL systems operate at different frequencies, they can be carried though the same wire pair at the same when the operating conditions are right. Voice calls operate between 300Hz and 3.4KHz, and include also DC power (0-72V DC at on-hook condition, typically 0-60 mA current and lower voltage at on-hook) and rign voltage (typically 40-80 V AC at 20-25 Hz freuquency). The voice telephone system is matched to 600 ohm (or close to it) impedance at voice frequencies. ADSL technology operates between 26KHz and 1.1 MHz and is designed for around 100 ohms impedance. Because the two frequency spectrums do not overlap, it follows that both data and voice can be present at the same time on a single pair of copper wire. The different impedances have hostorical and technical reasons. The impedance of telephoen wiring is typically around 100-120 ohms at the frequencies ADSL system uses. The cable impedance is somewhat higher at voice frequency range, considerably higher than 100 ohms, and where where historical 600 ohms impedance comes to picture (cable might not be exactly 600 ohms for voice, but that’s what devices are designed for).

Why splitters / filters are needed

When ADSL and PSTN work at the same line at the same time, the electronics inside a normal telephone can be problem for high frequency ADSL signals: the ADSL signals can be attenuated (high capacitance on telephone input, possible resonances inside telephone, impedance mismatch) and ADSL signals can be heard as noise on some telephones (phone electronics demodulates high frequency signal outside it’s operating range to voice frequency noise). In order to keep these systems apart and stop them interfering with each other it is necessary to separate the two components from the telephone line in your home.

This is where the Filter / Splitter comes in. The ADSL POTS Splitter / filter allows taking the full advantage of the 1.1MHz copper line frequency spectrum, by stopping the telephone and ADSL systems from interfering with each other.

An ADSL filter is normally a small plastic box with a short lead that plugs into your phone socket and two outputs, one for your ADSL Modem and another for a telephone. Some filters have only one telephone output in them. ADSL filter select the band of frequencies for each of the outputs, phone or ADSL, and send just the correct band to the appropriate socket. The phone output gets only telephone frequencies (from DC to 3.4 kHz) and the ADSL output gets the higher freuquencies well (above 25 kHz).

For good system performance it is very important that all your other telephony equipment is separated from the ADSL signals by the use of a splitter / filter — this equipment includes telephones, answering machines, “normal” computer modems, etc, etc.

Tips:

• All phones or other equipment must pass through a filter.
• Make sure that the ADSL signal is only passing through one Filter / Splitter.
• It can be the same Filter / Splitter for all of the phones.

How real life ADSL filters at home work

The signal to telephone output is generally just low-pass filtered so that voice frequencies (frequencies up to 3.4 kHz) get nicely though, but higher frequencies gets filterted. This filtering generally consista of LC low-pass filter designed to some suitable operating frequency between 4 and 20 kHz (between voice and ADSL bands). This kind of filter causes that the high frequencies of the ADSL signal will be severely attenuated (usually by at least 30dB with a good filter) so the signal reaching your telephone equipment does not contain such amount of high frequency signals that could cause noise. The telephone LC filter is also designed in such way that the filter impedance towards the line that carries ADSL signals is high at the high frequencies, meaning that those telephone equipment and cables related to them look like they are look to high freuquency signals that they would ne “disconnected from the main line”. line.

The ADSL POTS splitter is simply a series of coupled inductors and parallel capacitors forming a low pass filter that attenuates the higher frequency ADSL data and permits only the voice frequencies to reach the telephone. The series inductor shows high impedance to high freuqencies, so the ADSL signals on the line are not attenuated.

General design specifications for an ADLS filter should be somethign like this:

• Return loss at voice frequencies (against 600 ohms) would be should be good enough.
• Should not alter voice band freuqncy response too much
• Should not have too high series resistance (commercial filters seems to have between 50 and 100 ohms for whole loop resistance)
• Filter must pass the POTS tip-to-ring dc voltages (typically o-72V)
• Filter must pass ring voltages well (40V to 80V rms at any frequency from15.3Hz to 68Hz with a dc component in the range from 0V to 72V)
• Filter must
• All requirements must be met in the presence of POTS loop currents (usually around 0-40 mA, can be up to 120 mA in some cases)

The ADSL output from filter (if it has such thing) is generally unfiltered line signal (normal home ADSL devices are not to be bothered with line voltage and voice signals.

ADSL splitters at the central office

When the operator install ADSL system to the central office, they install ADSL splitter filters on the central office end of the telephone wire. The filters at the central office have basically the same functional needs as the home units, they need to be able to keep different signals separate, and separate those two signals to different outputs. Typical central office ADSL splitter filter is a device that ghas many filters built into one package. For each outgoing line there is one PSTN connection (goes to centeral office telephone central equipment) and one for ADSL connection (goes to DSLAM rack that terminates ADSL connections). Typical ADSL splitter in central office has series capacitors (blocks telephone line DC well, attenuates ring signal sonciderably, but passes ADSL signals well) between line and ADSL output going to DSLAM.

Because ADSL splitter filter connects directly to the subscriber’s loop media, it must also provide some surge protection from externally induced voltage which could damage any attached equipment or endanger humans interacting with the installed equipment. The ADSL splitters in the central ofice typically include overvoltage protection components to protect the ADSL DSLAM agains overvoltages on the line. Some filtered ADSL outputs provide protection from the high frequency transient and impedance effect that occur during POTS operations (ringing transients, on-hook, off-hook transient and so on).

Here are some specifications related to ADSL splitters at central office:

• Ref. 1 : ETS 300 001 Attachment to Public Switched Telephone Network
• Ref. 3 : ITU-T K21 Resistibility of subscribers terminal to over-voltage and over-currents

Commercial ADSL filter example: ELEXI ADSL filter

The following photos, circuit diagram and text try to describe how ADSL filter sold by ELEXI (product number 440072 “ADSL puhelinsovitin USA/USA GP4C”) works. This ADSL fitler is picked as example because I happened to own one and the construcion of this filter is quite simple (=easy to understand).

Photo 1: ELEXI ADSL filter outside viewe

Photo 2: ELEXI ADSL filter opened

Photo 2: Close-up of the circuit board iside the filter. The telephone line connecting wires go to top right and the wires going to telephone connector leave at bottom left.

Figure 1: Circuit diagram of the filter circuit board electronics.

The phone line to central office (that carries ADLS and PSTN signals) goes to the left side of this filter and the phone goes to right side. This filter consists of an LC low pass filter made of the series connected coil (total impedance of 10.260 mH per wire) and the 22 nF capacitor across the wires. Those form the actual filter. There are resistors after this LC filter to make the filter properties have proper impedance matching for voice frequencies (should be around 600 ohms). This filter seemed to have one 1A fuse on the output to protect the circuit agains some catastrophic damage (what this would protect agains..).

The total series resistance this kind of filter cause to one telephone wire was around 45 ohms, meaning that having this filter adds around 100 ohms to the telephone wire resistance. The series resistors and internal coil resistance reduces the impedance mismatches that just putting a capacitor across telephone, wires would cause. If we take a look at the circuit construction with only resistances and capacitances in it, it is pretty close to a simple model of 0.5 km of 0.5 mm telephone cable wire. The coil impedance at voice frquencies would be at 20-200 ohms, causing some mismatch and attenuation at high telephone frequencies. The capacitor has impedance of aroudn 250-2500 ohms at voice frequencies. Return loss at voice frequencies (against 600 ohms) would be acceptable with this kind of circuit.

The coils in this circuit are built to ferrite cores, boppin cores for highest inductance values and the 40 uH coils are wound to small ferrite toroids. My quess is that the coil part is constructed from three separate coils to make the filter to work well at high freuquencies. The 10 mH coil has lots of turns, so at high freuquencies it’s perfomrance might not be best (potential coil resonances etc.), but at those high freuquencies those smaller value coils have already attenuted the signal enough. Getting three different coils in series gives best performance on all freuquencies.

GETTING THE MOST FROM YOUR ADSL CONNECTION

The first time you connect to the Internet with your all-new ADSL modem, you may find that you are not surfing or downloading at the rate that you had hoped the connection would give you. This guide will hopefully help you to optimize your broadband connection.

Maybe you are unsure as to what downloading speeds you should be getting. Well, if you have a 500Kbps connection, you should be getting close to 60K/s when downloading. This does depend greatly on the site you are downloading from, but I have generally managed to obtain decent download speeds of 57-60K/s.

If you work it out: 500000 / 8 = 62500 (62.5K)

So 60K/s is pretty good really. In fact, the actual reason that you could never obtain a true 62.5K/s download is because of transfer loss. This can be as much as 13%. TCP/IP loss can be up to 3% and the rest is ATM loss, which can be up to 10% (ATM = a protocol for transferring data between two points).

If you’re not getting this sort of speed then it is probably because your RWIN and MTU settings are incorrectly set for broadband optimization. There are other reasons of course, such as contention, but incorrect optimization is the most probable.

A simple way to check this would be to visit www.dslreports.com and use the ‘online tweak tester’ in their tools section. This will inform you if there is a problem with your RWIN or MTU settings.

For example: My test told me that my RWIN XXX is too low, and that my MTU is too small.

These are pretty common results for a first time test, but are easily rectified using the DRTCP program that is also available on DSLReports website. For convenience, I have added the link here: www.dslreports.com/front/drtcp.html.

Tip for using DR TCP: When changing a setting, you might notice that the “Apply” button does not highlight. You need to hit “TAB” to activate it and then click on “Apply”. The settings do not become apparent until after a reboot.

RWIN in more detail: If you are experiencing packet loss, then you may want to lower your RWIN setting. There is a method for determining your maximum required setting for the RWIN, but it is dependant on latency (ping), and seeing as ping is not constant, it is different for each user/system.

<Connection Rate> * <(Latency * 1.5)> / &lt8> = <RWIN setting>

One point that I’m sure you’re all aware of. Latency is measured in milli-seconds, this means that a ping of 150 would translate to 0.150 for the latency variable.

The only thing I can suggest is that you find a service that you use with a high ping and then calculate your RWIN setting from there. The other option is to calculate the ping for your favorite games server and work out the RWIN for that, you could then set it when you want maximum throughput for that particular period. Otherwise, it’s too variable to detail.

I have no packet loss problems, and have never had a problem with my RWIN set to 65535.

MTU in more detail: The MTU setting is a definition of how much data you can transmit in one go before it has to be cut-up or fragmented. Every connection has a limit and can be determined using a simple command in DOS.

The value you should use for your MTU setting is dependant on the MTU value for your ISP since all packets (data) will be travelling through their servers.

To determine the maximum MTU value for your ISP, open a DOS window and type :

ping -f -l [packetsize] [www.your_isp_url.com]

Where [packetsize] is the amount of data you want to send (range is 0 – 1500) and [www.your_isp_url.com] is the url of your ISP.

Your ISP’s MTU is determined from the larest packetsize value that does not return the error “Packet needs to be fragmented but DF set” -28 (20 bytes for the IP and 8 bytes for the ICMP header). This is dependant on how the server is configired, but essentially the result is the same.

For example:

C:\WINDOWS>ping -f -l 1472 192.168.1.10

Pinging 192.168.1.10 with 1472 bytes of data:

Reply from 192.168.1.10: bytes=1472 time=2ms TTL=128
Reply from 192.168.1.10: bytes=1472 time=1ms TTL=128
Reply from 192.168.1.10: bytes=1472 time=1ms TTL=128
Reply from 192.168.1.10: bytes=1472 time=1ms TTL=128

Ping statistics for 192.168.1.10:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 1ms, Maximum = 2ms, Average = 1ms

The maximum packet size I could send to my gateway system without it fragmenting was 1472. This means that an MTU value of 1500 is fine for my ethernet connection (1472 + 28 = 1500).

As you can see from the example above, you can determine MTU value for you LAN systems also, as this is dependant on the potential bottle neck that is the gateway system. All packets have to pass through that, so the MTU value for your gateway limits the MTU value for your LAN systems.

Summary:

What are RWIN (TCP Receive Window) and MTU?

What is Rwin?

RWIN (or TCP receive window) is the amount of data that your PC can accept without acknowledging the user.
When a sender sends a packet to the user, it requires an acknowledgement from the receiving system. If this ACK is not received, it will wait for a certain amount of time, and then retransmit. This is how TCP is made reliable.
This start-stop process slows down transmission, but to enable a speedier process, you can set the size of the receiving window so as to sustain a continuous data transfer.
By default, this window is too small for many types of DSL and Cable (8192 for Windows 95/98/98SE/NT and 16384 for Windows ME/2000).
Increasing the Rwin setting, will allow more information to be transferred non-stop, up to a point, and then after this point, no difference will be noticed for the particular connection. The point will vary depending on bandwidth * delay. This is why you should allocate more than you actually need. I use a value of 65535.

What is MTU?

MTU is short for Maximum Transmission Unit, the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packet before being sent.
In order to transmit the most amount in one go, you should set your MTU to a high value. I use a value of 1500.
If your MTU is low, then it will take more packets to transmit the same amount of data as a higher valued MTU, thus taking more time.

How do I use DrTCP and what does it do?

Firstly, I would like to start by explaining that Dr TCP is not a patch… it is purely a shortcut to registry editing. It does nothing without user intervention.

All information has been grabbed from www.dslreports.com and assumes that you are using Win98/98SE/ME/2000.

TCP Receive Window: This is where you set the Rwin. This is the single most important tweak, and raising the value from the Windows default will greatly improve download speeds. My Rwin is set to 65535.

Windows Scaling: 65535 is the highest value that you can set your Rwin to, without having to use windows scaling. Scaling is needed to enter any number higher than 65535. Most users do not need a higher Rwin that 65535, and so I recommend that this setting be set to default ( off).

Time Stamping: This setting may or may not improve performance. If you have a line where latency varies a lot, time stamping may be beneficial…….experiment with it to make sure. I have my setting at default ( off).

Selective Acks: This improves the speed of lines that tend to lose packets (packet loss), by re-transmitting only packets that were lost, if any. I have my setting at default ( on).

Path MTU Discovery: This automatically sets your MTU to suit the type of line that you have (dial-up or broadband). The highest MTU that you can set is 1500. I have mine set to default ( on).

Black Hole Detection: This discovers routers on the web that cause MTU Discovery to work sub-optimally. I have mine set to default ( off).

Max. Duplicate ACKs: This allows for faster re-transmission of packets when lost. I leave this setting blank. (blank = 3 for Win98/98SE/ME and blank = 2 for Win2000).

TTL: Time To Live is the amount of hops (servers) that a transmission packet will take before all packets are lost. If you were receiving packets from 20 hops away, and your TTL was set to 19 or less, then all packets would be lost before they reach you. I leave this setting blank (blank = 128 in Win98/98SE/ME/2000).

Adapter Settings: This is where you set your MTU. I have mine set to 1500 for both NIC and Dial-up.

ICS Settings: If you use Internet Connection Sharing (a Microsoft program), then you should set the ICS MTU to the same as that of the PC. This is grayed out if ICS is not being used.

When you are happy with your settings, you need to hit the Apply tab (you may need to hit tab to highlight it). Next click on Exit and then reboot the PC. A reboot is necessary to activate the settings.

Internet Explorer – Registry Tweak

Registry tweaks should not be done unless you are comfortable using Regedit. To avoid mistakes, you should back up your registry before applying the tweak.

This particular tweak is not just for broadband, but will definitely give a major improvement to 56K connection also.

For those willing to give it a try, all you have to do is click on this link for instructions.

For those interested in what it does: Internet Explorer 4 & 5 are designed to comply with Hypertext Protocol 1.1. This dictates that a web browser should draw no more than two streams of data from a web server at any one time. However your browser is capable of getting many more times than that , and if you increase the number it’s looking for, you vastly speed up the whole process.

DHCP Stall/Freeze

If you have a network card installed on your PC, the chances are that your ADSL modem may try to talk to the network card, this can cause your PC to freeze for a while. I have only recently discovered that disabling the card in Device Manager is not the only method of preventing the stall.

In-built DHCP services tend to become active about once every 10 minutes. They look for an IP Address of a DHCP server from the network card. Obviously, this DHCP server does not exist. This may cause your PC to freeze up. It is especially noticeable when surfing.

Here’s how to rectify this:

Essentially what you need to do is to define an IP Address instead of letting it obtain one automatically. This way, it never needs to search because it thinks it has one already.

• Open Control Panel
• Open Network
• Look for the fist entry in the scroll box marked TCP/IP, that is associated with a hardware ethernet card. e.g. : TCP/IP -> Realtek XXXXX etc.
• Select it and press Properties
• Select “IP Address” from the tab
  Is “Obtain IP Address Automatically” checked? If so, change it to “Define IP Address” and enter an IP Address within the range of 192.168.1.1 and 192.168.1.254, and a Subnet Mask of 255.255.255.0, into the IP Address and Subnet Mask fields respectively. These numbers are harmless and should not cause a problem unless you actually use a DHCP server, in which case you should set the addresses accordingly.
  OK out and close down control panel.

Now see if you get any stalls.

(Info grabbed from DSLReports FAQ)

P.S. DHCP = Dynamic Host Connection Protocol. It basically allows workstations to connect to a network and automatically obtain an IP Address, Subnet Mask etc.

What Is “Contention Ratio”?

Normally people panic when they here stories about contention ratios and how you could end up surfing at a lower speed than a 56K modem.

This guide will hopefully reassure those who have concerns, especially those on a 50:1 contention ratio service.

An ADSL connection into your premises (office or home) is technically a dedicated line between you and your telephone exchange. However, from your telephone exchange to the ISP’s network, your data would be traveling over a network that is shared between you and other ADSL users.

The speed that you would get would fluctuate depending upon how many users are connected to the network (“contending” for the bandwidth available) at any point in time.

The contention ratio reflects the amount of bandwidth actually available to all the customers versus the maximum bandwidth all of the customers could attempt to use at the same time.

Each Pipe carries a 10Mb capacity, this is the contended bandwidth, it’s not 50 people connected to a 512K pipe. It 50 people connected to a 10Mb pipe.

A contention ratio of 1 would mean that you would always be able to send or receive at the maximum data rate because there is no other user sharing the bandwidth with you, a contention ratio of 50 means that it is possible that you can only send or receive at 1/50th of the pipe capacity because you are sharing the bandwidth with 50 other people.

If you do the sums :

10,000,000 / 50 = 200,000

So the worst-case scenario would be 200K/sec connection.

Statistically, most users do not use their full bandwidth most of the time, so the worst case seldom if ever occurs. The general rule is: the lower the contention ratio, the more likely you are to continue to get fast throughput during busy times. However, because of the “bursty” nature of Internet traffic, it is unlikely that the worst-case scenario will come about.