aa.net.uk Broadband - Broadband you can work with

Skip to Navigation / Skip to Content

Broadband Latency

LatencyNot all broadband services are created equal. More importantly, the requirements of customers using broadband can be very different depending on what you are doing. On-line interactive games have very different requirements to reading email and checking facebook.

This page details some of the key factors relating to interactive on-line games, like World of Warcraft and League of Legends, and why it can be important to pick an ISP that has a good technical understanding of these requirements.

Speed

One of the main things you will see in any advert for broadband is the speed of the service (or bandwidth), so much so that OFCOM and the ASA have a set of rules on how ISPs advertise service speeds. Technically, we are not talking about actual speed here (e.g. miles per hour), but how much data can be transfered in a time, typically megabits per second.

Obviously speed can be an important factor in any Internet connection, but as long is it is enough for what you need any extra is no help. Basically, these days, the biggest user of bandwidth is streaming video (on-line TV), and that takes a few Mb/s (mega bits per second). Whether you have 10Mb/s or 100Mb/s you can still steam high quality video live.

For on-line games the actual amount of data while playing is low - you exchange details of locations of players and NPCs, and the actions they are taking. It is nothing like streaming video.

Speed is also a factor if you are downloading a lot of data - and this is where it can be relevant. On-line games, like WoW, can have large patches of several hundred MB (mega bytes, a byte being 8 bits). The time taken to download a patch will relate directly to the speed of the service.

What is relevant here is that you should not simply be seduced by the claimed speed of a link. For a start, the speed you get may be very different. Even current advertising rules are designed so that 90% of potential customers will be disappointed if they expect the claimed speed. ISPs don't control the whole of the Internet, so even when your link itself has the speed claimed, that does not mean you will get that speed in transfers from specific servers on the Internet. Some times services that use different technology can be much better for on-line gaming than services which are simply faster.

WoW latencyLatency

There is a key metric which you rarely see mentioned by any ISP. That is latency (often referred to in online games as ping time). This is how long it takes to get data from one end of a link to the other, normally measured as a round trip time (getting data from A to B and then back to A), and normally measured between the customer and the ISP. Many games show a latency measure as well, but that will be all the way to the game server and back, and is often using higher level protocols. That does not make it an invalid measurement, just that it will be slightly different to ping or LCP latency measures but still based on the same underlying round trip time. Latency is usually measured in milliseconds (1/1000 of a second).

The thing you want is a consistent and low latency. Latency means delay, and delay means getting killed in the game! There are several factors that affect latency.

Distance

One small factor in latency is the speed of light - how long it actually takes for signals to travel the distance. If the link from you to the ISP was a single fibre then the distance, and speed of light, would be the key factor in latency of the service. However, in practice, the main distance factor is the number of bits of equipment involved in getting the traffic to you. Even so, customers in Scotland see higher latency to London than those in London.

There are many bits of equipment involved: routers; switches; LNS; BRAS; DSLAM; modems; each of which adds some delay. Some of these change the signalling system, and some change the protocol used to carry the data. Each of these has an impact on latency.

Line speed

The actual speed of the line is a factor as well. The latency is not simply the time taken to get the packet from A to B, but also the time it takes the whole packet to arrive. A longer packet takes more time, and this time is added to the latency. For example, a 1kB (1000 byte) packet (that's 8000 bits) will take 1ms on an 8Mb/s line. On an 800kb/s line it will take 10ms. On an 80Mb/s line it will take 100us (100 micro seconds). Given that latency of a service is often of the order of 10ms before you start, you can see that speed is mainly a factor when it very is low, down below 1Mb/s.

Interleaving

On broadband lines there is a key factor that affects the latency. Lines use a technique called Forward Error Correction which allows small errors in the signal to be corrected without having to resend the data. This only works for small errors (e.g. one bit), but sometimes lines have interference (called Impulse Noise) which affects lots of bits at a time (a click or pop on the line, in effect). To fix this a technique called interleaving is used. It spreads each packet out over a longer time, interleaving it with other packets. When there is impulse noise the effect is a small error in a whole series of packets, each of which can be corrected.

The downside is that, whilst the overall speed is the same, interleaving adds latency. Typically around 8ms is added to all communications on the line, though there are different levels of interleaving (or Impulse Noise Protection) which cause even higher latency. The latency added is however consistent, which can, in itself, be an important factor for on-line games.

This is a slight challenge: with interleaving the line has extra latency all the time, but without it you get errors. When there is an error then the packet has to be resent, which adds a lot more latency but randomly and inconsistently. Usually interleaving is a better choice, but it does depend in the interference. In some cases interference is only at certain times of day, and if that is not when you are playing, then interleaving is best turned off.

There is often one other option, which is running the line at a lower speed. This can remove the interference if it is only at certain frequencies, and as we explained above, the speed is not necessarily a key factor.

As an ISP we have some controls on most types of lines where we can turn interleaving on or off, or cap the line speed. This is not possible on all services. In some cases our customers have direct and instant control of the settings (TT lines). In other cases nobody has control (BT FTTC lines), though we can cap such lines to 40Mb/s.

Technology

Different technology is a key factor in latency of the service. As explained above, DSL lines can have interleaving, however, apart from that they have very good, low, consistent latency. FTTC is the best of these with latency as low as 8ms, but ADSL can be down near the 10ms level. What is nice, as you can see from the graph above, is that it is consistent.

Cable services (which we don't sell) can have low latency too, but they operate on a broadcast domain. This means that there can be extra delays, typically around 18ms, added randomly. This makes the latency inconsistent, which is a nuisance.

Satellite services are the worst for latency, adding 500ms to a second depending on the service. This is usually consistent but stupidly high even for simple interactive typing. It makes on-line gaming unusable.

Congestion

Finally, a key factor in latency is congestion. When any link gets full it will create a queue of traffic. The main links that can get full and affect your service are your own line. If you download or upload something whilst playing that can fill the line. Thankfully we have techniques to help, at least with the download side. We can set the line rate on our end to avoid any queues, and drop only large (i.e. non interactive) packets when hitting that limit. This means the download is slowed down without affecting the interactive traffic, and means no delays to any traffic. The best answer is not to download or upload whilst playing though.

Congestion within the network, and in particular within the back-haul between you and the ISP, is a bigger issue when it happens. You generally have no control over this (and neither does the ISP). In our case we monitor the back-haul networks and chase the carriers to fix congestion issues, but most ISPs have no way to even see these issues.

Finally, congestion at the ISP in their links to carriers can be an issue. With cable networks there can also be congestion within their network. These are in control of the ISP, and this is the key difference between a good (and usually more expensive) ISP like ourselves, and cheaper ISPs. We aim not to be the bottleneck and so not have congestion on our carrier links. Not all ISPs have the same policy.

For the HordePacket Loss

Another key factor in any Internet service is packet loss. This means a packet does not get from A to B, basically. The main cause of packet loss is a full link - firstly the packets are queued (adding latency) and then when the queue is full the packets are dropped. Only then does the protocol slow down the communications. Packet loss on a full link when downloading a lot of data is normal.

Again, we are a bit smarter on this, not queuing traffic, and dropping the larger (non-interactive) packets first.

A bigger issue is when there is packet loss because of random interference. This causes data to be resent, which adds much larger delays. It makes the service seem sluggish and slow. It is not good for on-line gaming. Sometimes interleaving can help, but usually the solution is to find and fix the underlying cause (typically a line fault or some sort of interference).

By monitoring every line every second we can identify this type of problem and take steps to get the fault fixed.

Why AAISP?

It should be pretty clear that we take latency and packet loss and speed seriously - the key thing we do is monitor every line every second. The graph shown above is our normal Constant Quality Monitoring (CQM) graph which we have for every line and you can see yourself for your service. We have a history of every line to help diagnose faults.

Oh, and we also have an alliance questing guild on Thunderhorn (AAISP Broadband) and should have a Horde one (AAISP Darkside) shortly.