Fix your broadband

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Fix your broadband


In the UK, almost all broadband connections are made through ADSL from the local telephone exchange or over cable using the existing cable TV infrastructure.
There are a few remote areas where these are not available, and where the only options are satellite or wide-area wireless networks, but for simplicity we’ll just be looking at ADSL and cable in this feature.

Internet to MODEM

The most obvious performance measure for these broadband connections to the home is the overall speed of the broadband link, as quoted by the supplier in Kbits/sec or Mbits/sec. But that is only a nominal figure which users should not expect to achieve all the time, and some may not achieve at all.

This is partly because of physical circumstances – the distance the user is from the ADSL exchange, for example, or the quality of the cabling into the user’s house – and partly because of technical and marketing decisions taken by the broadband provider.

These decisions involve things such as the bandwidth of the internet connection installed at the provider’s distribution centre, commonly called the backhaul capacity, and the number of simultaneous users expected to share either that connection or the more local link to the distribution centre, usually called the contention ratio.

In an ADSL network, each user is individually connected over the local telephone system to the local exchange, where a unit called a Digital Subscriber Line Access Multiplexer (Dslam) routes the traffic from multiple users into a single asynchronous transfer mode (ATM) link that connects to the wider internet.

In most cases all this equipment and traffic is handled by BT, which sells capacity to ISPs to sell on to users; in some cases, though, local loop unbundling (LLU) has meant ISPs can install their own equipment in BT exchanges to route traffic themselves and, hopefully, provide cheaper and faster broadband connections.

The ‘asymmetric’ in the ADSL term means that the bandwidth of the connection is much higher for data travelling from the Dslam to the user than for data travelling in the opposite direction. A typical link might run at 8Mbits/sec for downloads and only 256Kbits/sec for uploads. This reflects most people’s internet usage, but is not ideal for, say, those trying to run their local machines as web servers.

Contention ratios

It is only on the other side of the Dslam at the exchange that ADSL contention ratios come into play. The internet backhaul capacity from the exchange to the nearest or most convenient internet point of presence (PoP) is set by BT or the provider handling the equipment at the exchange, and is not often quoted to users.

In the early days, the standard BT backhaul bandwidth was 4Mbits/sec for up to 400 home users at 512Kbits/sec, and the company offered a contention ratio of 50:1; in effect, each user was sharing a single 512Kbits/sec connection with up to 49 others.

In practice, though, the wider backhaul pipe meant that most users could get their full bandwidth or close to it for most of the time, as internet use is typically ‘bursty’ and not continuous. It was still possible for greedy users to saturate the connection with large continuous downloads, but they were quickly spotted and warned off.

Today, BT and its resellers do not quote contention ratios. The backhaul capacity provided at each exchange varies depending on the demand and size of the exchange, and can be beefed up if required. However, it’s a fair assumption that home users are still faced with a contention ratio of about 50:1, and business users with about 20:1, unless they pay specifically for something better.

In cable systems, the contention starts straight away. Broadband users in a cable segment – which can be about 2,000 households – connect over a single cable to a Cable Modem Termination System (CMTS) at the supplier’s local distribution centre, and share its bandwidth. That is not such a problem as the cable they share between the local cable hub and the CMTS is a high-capacity fibre-optic one, and the cable between the hub and each house is a high-capacity coaxial type.

The details of these standards, such as MCNS/Docsis in the US and Eurodocsis or DVB/Davic in Europe, are significant for cable companies and their suppliers rather than for users.

But they do fix the maximum speed possible per channel between the CMTS and a user’s cable modem; in the UK, using Eurodocsis 2.0, this is 54Mbits/sec. Of course, this bandwidth is used by all those on the cable segment, so it might be shared between thousands of simultaneous users.

Cable companies manage their systems by capping the connection rate for each cable modem, downloading a configuration file at start-up that fixes the maximum permissible upload and download speeds.

This is why they can offer a variety of speeds and upgrade services without having to change cable modems. In all cases, however, the speeds are asymmetric, so a nominal four or 10Mbits/sec cable connection will have an upload speed fixed at a maximum of 384Kbits/sec.

Upload saturation

This can cause a problem, because of limitations in the TCP standard that forms part of TCP/IP networking. If the upload bandwidth is saturated – which is quite possible on a cable segment with hundreds of users – the download speed drops to about the same speed as the capped upload speed for all users on the cable segment.

The same should apply to ADSL networks, but this does not happen because each user has a dedicated path to the Dslam. This explains why cable companies treat continuous upload use as ‘abuse’.

Another capacity limitation is the backhaul bandwidth at the cable distribution centres, which is fixed by the companies and out of the user’s vision and control. But as with ADSL, it can be varied by the supplier, according to overall demand.

For ADSL, there is not much you can do to optimise the connection between the exchange and your home. Whether you buy an ADSL modem or take one as part of a bundled deal with a service provider, it will negotiate the maximum possible data rate with the Dslam, establish a connection using point-to-point protocol over ATM (PPPoA) encapsulation, and then transfer the data packets through a suitable interface socket.

Cable connections are different, as here the cable modem is supplied by the cable company and can be regarded as part of the infrastructure.

A cable modem will typically have just one interface connector, most often Ethernet but sometimes USB, and is intended for connection to a single PC; any additional equipment you connect will not be supported by the cable company. But the cable modem is effectively an Ethernet bridge, and once a PC is connected to it you can configure some of its characteristics and possibly improve the performance of the connection.

One thing you definitely can’t do is ‘uncap’ the connection speed, although there is a pervasive urban myth that this is possible. The myth arose in pre-Docsis days, when a particular brand of cable modem with particular firmware could be set to bypass the speed cap on a particular cable network.

With Docsis, the modem start-up routine contacts the CMTS, which responds with the address of a configuration file. The modem goes to the address, downloads the file with the speed limit settings, the IP addresses other details it needs, applies the settings and authenticates them with the CMTS; only then does it establish communication. The user cannot bypass or affect this process until it is complete.

Modems and routers

In the simplest case, your broadband connection comes in through a basic ADSL or cable modem and is connected by a cable to a single PC.

There does not seem to be much that can be done here, but if you have the choice of an Ethernet or USB connection, choose Ethernet every time. It is more reliable, does not slow down the PC system overall, does not need to be shared with other USB devices such as printers, works automatically on any system, and allows extra networking and security devices as needs change.

With a single PC connected to a cable or ADSL modem via its Ethernet socket, the modem is effectively acting as a transparent bridge to pass the IP address issued by the ISP on to the PC itself.

The ISP’s DHCP server allocates an IP address to the connection once communication has been established with the modem, and this address is then applied to the user’s PC to make it visible on the internet. That is why it is essential to run security software on a PC connected in this way, as the modem’s transparency offers no security at all.

Router security

Security is one reason many users choose to add an Ethernet router between the modem and the PC, even when only a single machine is to be connected, or – for ‘wires-only’ ADSL – choose to buy a combined modem and router. The router can be thought of as a non-transparent bridge, where the IP address issued by the ISP is applied only to the router, and a private IP address is applied to the PC.

In this way only the router is visible to the wider internet, and the PC connected to it is invisible and untouchable. The router can handle more than one PC simultaneously; it gives private IP addresses to any connected PCs and lets them share the single public IP address. Using a standard method called Network Address Translation (Nat), the router sorts out which incoming data packets should go to which machine, and routes them over the right connection.

For the user, this is transparent. Each PC plugged into the router gets its private IP address – usually in the standard range 192.168.x.x – from a DHCP server in the router itself, and automatically picks up a Domain Name Server (DNS) address.

Any PC that works when it’s plugged into a broadband modem directly will work without any hardware and software changes when it’s plugged into a router instead; only its apparent IP address will change, to one in the 192.168.x.x or 10.x.x.x ranges reserved for private networks.

One problem that cable modem users may face is Media Access Control (MAC) registration. The MAC address is a unique identifier for every Ethernet device, and is hardwired into the hardware; your cable company may need to be notified of the MAC address belonging to the PC you are connecting to the cable modem, or might automatically pick up its MAC address the first time you connect.

If you add a router between this PC and the modem, its MAC address will also need registration before it can work. You can register the router’s MAC address at the ISP’s website, but many routers allow you to bypass this procedure by simply giving it the same MAC address as the PC that was originally connected, and is therefore already registered.

As with all other router settings, this is controlled through a web interface built into the router. Launch a browser, enter the private address – its exact form will be in the router manual – and you will reach the configuration pages. This may not be necessary as router installation programs often give you the option to clone the MAC address of the PC when you first set up the router.

A wireless access point is just another kind of Ethernet bridge, transferring data packets from point to point over the network using radio signals rather than signals in a twisted-pair cable. The standard to look for is 802.11, often called Wifi, which comes in various speed variants.

The original was 80.2.11b, which runs at 11Mbits/sec; the most popular now is 802.11g at 54Mbits/sec, and some manufacturers are offering high-end units meeting the draft 802.11n specification at 540Mbits/sec. On top of that, individual manufacturers have their own high-speed specifications based on 802.11g, offering double the data rate, say, as long as you use their hardware.

The safest choice is still 802.11g, which guarantees interoperability between hardware from the widest possible range of makers and will also work with older equipment.

Throughput hints
It’s worth talking here about network speeds and their overall impact on throughput. The key point to remember is that broadband traffic comes in packets, and that the thing to avoid is a queue of undelivered packets building up at a bottleneck anywhere in the system that is under your control.

For example, an 8Mbits/sec broadband connection might be presented at the modem through a 10Mbits/sec Ethernet port. This is then plugged into a 100Mbits/sec Ethernet port on a router, which then connects over a 54Mbits/sec 802.11g wireless link to a PC.

All this seems fine, as the broadband link is the slowest in the chain, and everything else is faster. The other connections will be waiting for packets and then sending them as fast as they can. Besides, there might be four PCs sharing the 8Mbits/sec link, meaning that only a maximum of 2Mbits/sec is required for each Ethernet connection. But things are not quite so clear cut.

No network runs at its maximum rated speed for any individual point-to-point connection; there are always overheads that slow things down, dropped packets that need to be retransmitted, and network latencies.

For example, an 802.11g network’s real-life throughput is only about half its maximum 54Mbits/sec rating. It’s always good practice to over-specify at each link in the network if you can, and provide as much clear bandwidth as possible. And that’s particularly true as broadband speeds are increased.

Wireless networks are easy to set up and use, and most problems arise because of signal, strength and security. An 802.11g network works at a frequency of 2.4GHz – the same as thome.

The positioning of the access point and its aerials can be crucial to getting the coverage you want, and there may be hotspots and dead spots in particular rooms or positions. The effect of low signal strength is low throughput, as many packets will need to be retransmitted to make sure they get through the unreliable link, and the signal strength should be checked first if wireless performance is disappointing at particular points in the house.

Better coverage can be obtained using new wireless technology, particularly the Multiple Input Multiple Output (Mimo) extension to 802.11g. This uses ‘smart’ antennas to extend wireless range and throughput, and many manufacturers now offer Mimo routers alongside their standard models.

You will now also see ‘draft N’ routers on the market, which meet the unratified draft 802.11n standard and again promise better coverage using Mimo techniques and much faster throughput. Either of these options can work well to overcome wireless ‘dead spots’ in the home.

Encryption

Wireless security is essential, as without it any PC within range could steal your internet bandwidth and, potentially, monitor your private traffic. The original security standard was Wired Equivalent Privacy (Wep), which was part of 802.11b, although now the sterner Wifi Protected Access (WPA) standard is more common. Both require you to enter a code at the access point and at every connected PC, and no machine without that code can connect to the network.

There are various sources of possible trouble here. One is that there are various levels of security in both Wep and WPA, using different lengths of hexadecimal digits, words and phrases, and if you choose different levels at different points in the network, they will not talk to each other.

It’s worth being particularly rigorous about settings, writing everything down, and typing very carefully. In particular, it’s galling to connect wirelessly to a router/access point, change its security setting, and then find that the settings you enter on the PC don’t work. The only solution then is to connect the PC to the router via an Ethernet cable, and put things right from there.

The PC end The purpose of any broadband connection is to deliver information and provide services to the user on the PC screen. And once the connection has been made, the PC is the tool used to monitor, optimise and generally make the most of the broadband opportunity.

The first stage is connection. Although most connected machines will be PCs, Macintosh, Linux or any other sort of system can be used alongside or instead of PCs in any broadband-sharing network. The Ethernet packets are not hardware or operating system-specific, and can be universally understood.

Most modern desktop and notebook PCs will come with a 10/100Mbits/sec Ethernet port as standard, and connecting this to a router just involves plugging in the appropriate cable. Connections from PC to router use straight-through cable, as do most connections from router to modem, while direct PC-to-PC links require a crossover cable, where the transmit and receive pins are exchanged at the different ends.

However, you should check the modem and router manuals to make sure that you use the right cables and plug them into the right sockets. As a cable modem is a transparent bridge, its output should go into the Wan port of the router, while the PC cable should go into one of the Lan ports. In an ADSL system the router’s Wan port will connect directly to the telephone socket, and the PC cable to a wired Lan port as before.

Once the cables are connected, the connection rate is automatically negotiated between the Ethernet ports, so that if you plug a cable from a 10/100Mbits/sec socket into a 10Mbits/sec socket on a cable modem, say, the system will automatically set itself to work at the lower speed.

One problem some users may notice is that this can go wrong with certain models of cable modem, so that the half-duplex port on the modem is wrongly recognised as a full-duplex one. This results in the PC Ethernet interface being set up wrongly, and the efficiency of the Ethernet interface drops dramatically because of multiplying packet collisions and resends. However, this is unlikely to happen in modern setups, or when a router is placed between PC and modem.

For wireless connections, you will need a PCI or USB wireless interface for desktop machines, and a PC Card wireless interface for notebook PCs that do not come with 802.11g built in. Configuring these cards is usually straightforward, although there can be some confusion when the interface manufacturer’s software tries to take over from Microsoft’s built-in wireless configuration routines in Windows XP.

This can lead to users not being able to find the configuration settings, as it just produces a message saying that ‘Windows cannot configure this wireless connection’. In this case you need to find the manufacturer’s own configuration utility and use that.

If a wireless card is within range of one or more access points, choose the right wireless network from the list, enter the security code, and click the Connect button. Then, once connection has been made, the channel, signal strength and other status indicator are made available.

At this point, you should be able to use the browser, email and other standard internet tools over your broadband connection. But what do you do if you can’t?

There’s a logical sequence of steps to try. First, check the hardware and the physical connections; are the modem and router switched on and showing the appropriate indicator lights, and are all the cables firmly plugged into the right sockets?

For wireless networks, is the signal strength good enough, and is the configuration and status utility showing an established connection? In Windows XP, the Network Connections control panel allows you to check that your broadband connection, whether Ethernet or wireless, has been successful.

Address allocations

Next, check the IP address that your PC currently has. In Windows XP, you can do this through Network Connections. If you right-click the broadband connection in the panel and choose Properties, and then the Support tab, the PC’s IP address will be displayed. Alternatively, selecting the connection and scrolling down the left-hand pane of the window will show the current IP address at the bottom.

This address will generally have one of five values. If the PC is connected directly to a working ADSL modem or cable modem with a working broadband connection, then it will be the public IP address assigned by the ISP to the modem and hence to your machine. The address depends on the ranges assigned to the ISP for distribution.

If the PC is connected to a working router, then the address will be the private IP address assigned by the router’s DHCP server, and will be in the 192.168.x.x or 10.x.x.x range. If it’s connected to a working cable modem, but with no cable connection, then the address will have the form 192.168.100.x; this is issued by DHCP in the cable modem so that the PC can at least reach the configuration pages on 192.168.100.1.

If the PC could not pick up an IP address from DHCP at the ISP or at the router, then the address will have the form 169.254.x.x, as an automatically assigned private address provided by Windows. And if the address is 0.0.0.0 then the Ethernet network is not working, the Windows DHCP client is not working, the automatic private IP address routine is disabled, or any combination of these things.

The internet connection will work in only the first two cases, and if the cables and hardware seem to be operational, then you should try to force the PC to acquire a new IP address. This can be done by rebooting the PC, but a quicker way is to open a Command Prompt window and type IPCONFIG /RENEW .

This command will try to acquire a new IP address from the DHCP server, either at the router or the ISP. It may solve the problem if it was simply a glitch, or if you turned on the PC and the cable modem, say, at the same time and the PC booted before the modem was ready.

In this case the boot sequence might be complete, and the private 169.254.x.x address assigned, before the public IP address from the ISP was accessible. As a rule, power up in the order modem; router; PC, and let each one run through its diagnostics before turning on the next.

If you have a 192.168.x.x. address from the router but still have no internet access, the cause may be a lost connection between the router and the ISP. Even without an internet connection, you can access the router or cable modem’s configuration pages in the normal way and check the status of the ISP connection.

For example, in some cases an ADSL connection could have been dropped by a glitch in the phone line or at the exchange, and the modem’s automatic reconnect routines could have failed. Checking the account name and password entered in the modem or Wan section of the configuration pages, and clicking the Connect button, can solve problems like this quickly, although you may prefer to reboot the modem and see if it comes back up with a proper connection.

Checking cable modem settings

To check the configuration of a cable modem, plug a PC into its interface socket, launch a browser, and enter the address 192.168.100.1; this is the default IP address for Docsis modems, and should take you to the configuration pages.

There should be settings for US or European frequency standards, such as NTSC and Pal – since the channel spacing is 6MHz for the former and 8MHz for the latter – and for the current channel frequency, for example.

With some cable modems it is possible to choose a different default upload channel, restart the modem, and see if upload speeds improve; this works because some CMTS units have multiple upload channels, and choosing a different one might help if the default channel is being over-used. However, there is no guarantee that your CMTS will have multiple channels available, or that your new choice will be any better.

The other main settings to check are the signal levels and the signal-to-noise ratios quoted by the modem. If the signals are too weak, or the signal-to-noise ratio too low, throughput can suffer packet loss, continual retries, loss of DHCP-issued IP address, and other bad things.

The signal-to-noise ratio should be 30dB or higher, while the received signal power level should be between -15 and +15dBmV; anything outside those limits should be referred to the cable company for advice.

A rather crude but effective piece of software called Docsdiag can be used to examine the current configuration of a cable modem, although what you do with that information is up to you. This works with most standard Docsis cable modems, although you will need to read the documentation carefully.

But the most significant thing that users can do to check the broadband connections from their suppliers is to make sure that the speed they are getting is the speed they are paying for, or at least something near enough to it.

Troubleshooting Nat and Wifi
Standard internet services only work because the router knows about them and has set up something called ‘port forwarding’ to let traffic through the Network Address Translation (Nat) system unhindered.

By default, the router keeps all ports closed apart from things like port 80 for web browsing, port 110 for email, and port 21 for FTP file downloads. These are automatically left open so that any machine on the private network can use these services. This, incidentally, is why you still need to install browser security updates and anti-virus software on each connected machine; the router cannot protect you against threats over these open ports.

Applications such as online games and some virtual private networks will need other ports to be opened to work properly once a router has been installed, and this has to be done explicitly by the user through the router’s configuration pages.

Once you’ve found the right page – it might be called ‘port redirection’ or ‘port forwarding’ – all you need to do is enter the application name, the port or range of ports you need to open for that application, and the private IP address of the connected machine for which those ports should be accessible. More advanced port clearing and blocking can be done through the router’s firewall configuration.

The simplest solution to Nat or port forwarding problems is to reset the router by turning off the power, waiting for a minute, and then turning it back on. If the problem is with an individual machine, though, you might be able to restore connectivity without disturbing other users by rebooting the PC so that it re-establishes its Ethernet connection and acquires a new private IP address over DHCP.

One possible solution to poor wireless network signals is to try a different wireless channel. The 802.11g standard allocates multiple channels within the 2.4GHz band, and picking a different channel can minimise interference from nearby devices such as cordless phones or from other 802.11g networks in wireless range that are using the same channel.

Also ensure your network is secure by choosing a strong encryption standard from those offered by both the router and the wireless network interface. Most modern cards now offer the stronger WPA standard as well as Wep; you should choose the toughest encryption that is supported by both your router and your wireless card.

TCP optimisations

The trouble with online speed tests is that the internet does not involve fixed links from point to point, and the networks and servers have to respond to numbers of simultaneous users making varying requests for data.

The route that packets take between any two points can vary depending on network loads, and so introduce different delays for identical requests. However, you can monitor the speed at which data is being sent and received over your network adapter and so over your broadband link using the built-in Performance Monitor in Windows XP. Run the Monitor and check the Network Interface option.

The two Windows settings that might make a difference to performance are the TCP Receive Window (RWin) size and the Maximum Transmission Unit (MTU) size. The first sets the maximum amount of data that can be received before the sender requires an acknowledgement that a packet has successfully been sent.

If RWin is too low, then only a few packets can arrive before an acknowledgement signal must be sent, and over a long-latency network like ADSL or cable this can cause a break in sending data.

Increasing RWin so that more packets can be sent in one block means that there are fewer acknowledgement delays, although making it too large means that if there is an error all the packets in that large window must be resent, losing earlier speed gains.

MTU is the maximum size of each data packet, and for Ethernet this is a maximum of 1,500 bytes; however, for ADSL in the UK BT recommends that the setting should be 1,458.

These settings are assigned in the Windows Registry, and the easiest way to change them is by running a free utility such as DrTCP or TCPOptimizer after running the Java-based ‘tweak test’ atwww.dslreports.com/tweaks and checking its recommendations. The changes may not make any difference, and may make things worse; the only way is to try them and see if downloads are any faster t han they were before.

There are marginal tweaks for all web browsers, particularly the extremely configurable Firefox, and those interested in optimising Firefox performance should look at the Firetune utility. This automates the setting of many Firefox parameters to match your machine type, connection speed and pattern of Internet use.